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The underwater configuration analysis of a towing rope is crucial for estimating the rope tension and the state of the towed body. In the case where the rope inclination angle significantly increases, the partial hydrodynamic force coefficients of a rope segment should display apparent variations with the local inclination angle. However, constant drag coefficients ignoring the angle-dependent drag effect are widely used in numerical studies. To address the importance of the angle-dependent force coefficients for analyzing the underwater configuration of a towing rope, Large Eddy Simulations are carried out to obtain the drag forces of a cylindrical rope segment with varying inclination angles and Reynolds numbers. To model the rope deformation, a tensile experiment is also conducted to establish the nonlinear tension-strain relationship of the fibre rope. Parametric studies are performed to investigate the difference in the results calculated by both constant and angle-dependent hydrodynamic force coefficients. It was found that the application of constant drag coefficients leads to significant underestimation of the rope tension near the towed body, especially when the rope experiences large inclination angles. Therefore, the present study highlights the importance of using the angle-dependent hydrodynamic coefficients to predict the configuration and tension of towing ropes. • The underwater configuration of a towing rope is analyzed based on the drag coefficient validated by the inclination angle. • Large Eddy Simulations are carried out to obtain the drag coefficients at various inclination angles and Reynolds numbers. • Experiment is performed to establish the tension-elongation relationship of the synthetic fibre rope. • The rope tension predicted by the angle-dependent coefficients may be much larger than the one by constant coefficients. [ABSTRACT FROM AUTHOR]

This paper aims to assess the impact responses of hyperelastic materials that have the potential to be effective in reducing loads and drag during water impact. Using experimental and numerical methods, the mechanisms of energy conversion and stress distribution induced by the deformation behaviors of hyperelastic spheres impacting rigid and water surfaces are investigated. By comparing the impact events of rigid spheres, the elastic waves propagating inside the hyperelastic spheres over a single deformation cycle are analyzed to evaluate the impact resistance. The results show that the energy loss into the fluid is responsible for more kinetic energy loss in hyperelastic sphere during water impact. The drag reduction effect of hyperelastic spheres during water entry depends on the deformation behavior. Additionally, an analysis is conducted on the strain energy density of hyperelastic spheres during water entry. The initial change in kinetic energy is studied as a function of the dimensionless ratio of material shear modulus to impact hydrodynamic pressure. • Energy dynamics in hyperelastic spheres impacting rigid and water surfaces are elucidated. • The drag reduction in water of hyperelastic spheres is contingent on a specific range of material properties. [ABSTRACT FROM AUTHOR]

Research on the resistance and maneuvering characteristics is a key requirement in amphibious vehicle design to ensure safety and efficiency. To tackle the challenges of evaluating amphibious vehicles' hydrodynamic performance, as well as the lack of research and systematic approaches, a comprehensive performance evaluation method based on the RANS method and MMG model is introduced in this study, and the influence of a stern hydrofoil is evaluated. Firstly, the numerical calculation model is established based on the RANS method and validated by towing tests. The resistance characteristics of vehicles with and without stern hydrofoil are evaluated. The hydrofoil has a 9.37% drag reduction rate under the rated condition. Then, the static drift tests, circular motion tests, and static heel tests are simulated to obtain vehicles' hydrodynamic coefficients. Subsequently, the four-degree-of-freedom MMG models are established to predict the vehicles' maneuvering performance and evaluate the effect of stern hydrofoil. The results obtained from CFD and predictions are compared, confirming the capability of the MMG model. The stern hydrofoil can improve turning performance but affects course-keeping and yaw-checking abilities. • An amphibious vehicle's resistance and maneuvering characteristics are investigated. • The RANS method and towing tests are adopted for hydrodynamic analysis. • A 4 DOF MMG model is established to analyze the maneuvering performance. • Stern hydrofoil has a drag reduction effect of 9.37% at cruising speed. • Stern hydrofoil improves turning performance but hurts course-keeping and yaw-checking ability. [ABSTRACT FROM AUTHOR]

The planing trimaran has unique rapidity and stability. In this paper, airfoils are added to the planing trimaran to provide additional aerodynamic and hydrodynamic lift, achieving the purpose of reducing the resistance of the planing trimaran. By solving Reynold-Averaged Navier-Stokes (RANS) equations and combining SST k-Omega turbulence model with Volume of Fraction (VOF) method, a numerical simulation method is established to analyze the motion characteristics of the planing trimaran. The attack angle and the height of airfoils are mainly studied in this paper. NACA4404 and a special airfoil named 2# airfoil are selected as the research objects. The range of attack angle is 0–8°. The drag reduction effect of the airfoil does not change linearly with the increase of the attack angle in the high speed range. NACA4404 and 2# airfoil have the best drag reduction effect at 2°and 4°respectively. The range of airfoil height is 0.8h–1h ("h" is the wing height from baseline). For NACA4404, lower wing height is beneficial to reduce the resistance, especially in the low speed range. The 2# airfoil has poor performance in the low speed range after reducing the height, and can increase the resistance by up to 16.3%. • Adding the airfoils to the planing trimaran has great effect on reducing total resistance. • Properly changing the attack angle and the height of the wing can help reduce the resistance of the planing craft. • As the attack angle of the wing increases, there is an optimal attack angle that maximizes the drag reduction effect. [ABSTRACT FROM AUTHOR]

In this study, we investigate the effects of ventilation parameters and motion characteristics on the drag reduction characteristics of microbubbles during the pitching motion of underwater vehicle. Based on the Population Balance Model (PBM), a numerical method is established that takes into account the phenomenon of microbubbles aggregation and breakage. The effects of the pitching frequency and ventilation volume coefficient on drag and lift are conducted in this study. The results show that the numerical method can accurately predict the distribution and force characteristics of microbubbles for the vehicle in pitching motion. Added mass force of the water phase is weakened when the vehicle is surrounded by microbubbles during the pitching motion. This causes a gradual decrease in the phase difference between the drag and lift coefficients under different pitching frequencies. Moreover, the drag rate of the microbubble is inversely related to the pitch angle. Namely, the drag reduction rate of microbubbles decreases with an increase in pitch angle. The drag coefficient decreases significantly when the vehicle is covered by the microbubbles. This leads to the phase of the drag coefficient slightly lagging behind the pitch angle. Furthermore, the lift coefficients of vehicle are similar for different ventilation flow rates. • The drag and lift coefficients' amplitudes and the phase difference increase with a higher pitching frequency. • The ventilation microbubbles have a significant drag reduction effect on the underwater vehicle with unsteady motion. • The effect of the ventilation volume coefficient on the lift coefficients is minimal. • The introduced microbubbles achieve drag reduction and provide essential normal force for the underwater navigation. [ABSTRACT FROM AUTHOR]

To improve the endurance and speed of an autonomous underwater vehicle (AUV), we consider adopting the slip over a solid-liquid interface, which has the effect of drag reduction. However, the simulation of motion of an AUV with the slip remains a challenge due to the fluid-structure interaction and multiscale problems involving the moving boundary. To solve these problems, the present work proposes a novel and competitive mesoscopic method. Firstly, the Discrete Unified Gas Kinetic Scheme (DUGKS) coupled with the immersed boundary method is improved by correcting the boundary force, avoiding any additional computational time or implementation effort. Its effectiveness and accuracy are validated by two numerical tests involving the moving boundary. Furthermore, a fluid-solid interaction force is introduced to this method to model the slip condition due to its easy and effective implementation. We perform the experiment of motion of an AUV model in water to further validate the proposed method, which shows that it can predict acceptable results. Then, the method is applied to study the effect of the slip on the motion of the AUV model. The greater slip induced by the fluid-solid interaction force has a greater effect on reducing the drag and improving the velocity. It is indicated that increasing the fluid-solid interaction force can increase the amplitude and decrease the oscillation frequency of the drag curve. Overall, the slip can improve the endurance and speed of the AUV model. • An immersed boundary-discrete unified gas kinetic scheme is improved by correcting the boundary force. • A fluid-solid interaction force is introduced to IB-DUGKS to model the slip condition. • The experiments shows that the proposed method can predict acceptable results. • The proposed method is applied to study the effect of the slip on the motion of the AUV model. [ABSTRACT FROM AUTHOR]

Abstract A study of the hydrodynamic performance of a deep-vee ship was performed to determine the influence of the use of stern flaps and interceptors on drag-resistance, trim, and sinkage properties of the said ship. Factors leading to changes in the resistance and attitude of the ship were analyzed in terms of the stern pressure distribution and transom flow fields. Analysis results demonstrate that both the stern flap and interceptor serve to reduce the amount of drag resistance, running trim, and sinkage of the ship, and that the drag reduction rate attained was of the order of 3–9% in the Froude number (Fr) range of 0.334–0.584. Further, resistance performance of the deep-vee ship equipped with stern flaps was observed to be better in the Fr range of 0.334–0.5; however, when Fr value exceeded 0.5, ship equipped with interceptors demonstrated an superior drag-reduction effect. The study focused on evaluation of the combined effect of the use of stern flaps and interceptors in an attempt to enhance the energy-saving capacity of ships. Results obtained via experimental and numerical analyses demonstrate that deployment of a reasonable combination of stern appendages causes significant improvement in the drag-reduction effect and trim optimization. Highlights • CFD and EFD technologies used to study the influence of the interceptors, stern flaps and their combinations. • The main reason of ship resistance change was obtained. • The action mechanism of appendages was explained from the view of transom stern flow field. • A reasonable combination of stern appendages can improve the effect of drag reduction and trim optimization. [ABSTRACT FROM AUTHOR]

Abstract A novel fouling marine flow cell was designed and fitted with a clear plastic lid to allow real-time imaging of biofilms using optical coherence tomography (OCT). Marine biofilms were grown under controlled shear flow on coupons coated with 6 different biocidal antifouling coatings (SPC1, SPC2, SPC3, SPC4, CDP1 and CDP2, AkzoNobel) and one inert coating which contained no biocidal actives (NB-D) for 8 weeks. One set of coupons coated with NB was statically immersed in sea water during the same time period (NB-S). Biofilm removal was assessed by increasing the flow velocity while OCT simultaneously measured the biofilm cross-sectional area (CSA). The highest initial removal rates were observed for NB-S, NB-D, and SPC2. Percent biofilm cross-sectional area reduction (%CSAred) was higher on SPCs (>60%) compared to CDPs (<50%). SPCs had the highest percent reduction in biofilm surface area coverage (%SACred >60%) compared to the CDPs (<20%). The marine biofilm flow cell combined with OCT can be used to screen for coating-specific differences in biofilm growth and removal in real time rather than traditional before and after surface area coverage measurements. Future testing will focus on how the biofilm-coatings interactions interact with biofilm mechanical and structural properties to produce drag. [ABSTRACT FROM AUTHOR]

Abstract SST (shear stress transport) k-ω and Newmark-β methods are used to comprehensively understand vortex-induced vibration (VIV) characteristics of a cylindrical structure with a mass ratio of 2.6 in a range of reduced velocity from 2.0 to 14.0. The details of drag and lift forces, cross-flow and streamwise displacements, vortex pattern, trajectory, and frequency of VIV are presented and compared systematically with the experimental work of Jauvtis and Williamson that first captured the super upper branch in VIV with the maximum value of 1.5 D (diameter). In this study, the numerical simulation results successfully captured the initial branch, the lower branch, and the super upper branch. Very few research studies have successfully simulated the super upper branch by numerical methods. The vibration amplitude corresponding to the super upper branch is stable and the maximum value of the super upper branch is 1.46 D, which is fairly consistent with the results of the Jauvtis and Williamson experiment. This research also successfully captured the law of trajectory under different reduced velocities. With the reduced velocity increasing, the trajectories switch from an irregular shape to a regular "Figure 8″ shape and then enter into an irregular movement, finally again into a regular movement of a Figure 8 shape or crescent. In the range of the super upper branch, the vibration trajectories gradually change from a Figure 8 shape to a crescent shape with the increase of the transverse vibration amplitude. This work has successfully captured the different vortex patterns corresponding to each branch under different reduced velocities, and found the transitional forms of 2S to 2T, 2T to 2P, and 2P to 2S, respectively. Highlights • The numerical simulation results successfully captured the super upper branch. • The details of VIV are presented and compared systematically with the experimental work of Jauvtis and Williamson. • With the reduced velocity increasing, the trajectory switches from regular shape to regular shape three times. • This work has successfully captured the different vortex patterns corresponding to each branch. [ABSTRACT FROM AUTHOR]

Effective drag-reduction technology is important for enhancing navigation speed, extending endurance, and enabling efficient energy usage of underwater vehicles. In this study, we investigated the effectiveness of a body of revolution to influence the turbulent boundary-layer properties by performing numerical simulations and varying a periodic sinusoidal force in a series of annular slots. The selected control parameters included the normalized periodic force amplitude and periodic force frequency; the effects of these parameters on drag reduction have not been demonstrated in previous studies. The surface pressure and velocity analyses revealed that frictional drag accounted for 71.7% of the total drag reduction, and it was the dominant contributor to the drag-reduction effect. Unsteady vortices were generated in the boundary layer, and a retarded fluid region was created on the surface. A clockwise vortex was generated downstream of the annular slots during blowing, reducing the pressure drag on the surface of the revolution body. The clockwise vortex weakened when suction was applied. The results of this research might provide ideas for solving the drag-reduction problem in the design of revolving-body-type underwater vehicles. • This study researched a periodic force in incompressible-fluid environment for a revolution body with tandem annular slots. • This study discussed the effect of the control positions on the drag-reduction. • A simple model was developed to correlate the periodic force amplitude and frequency with the instantaneous drag. • The drag-reduction mechanism of the periodic force was analyzed by visualizing the boundary layer and surface-drag variation. [ABSTRACT FROM AUTHOR]

The flow of a weakly conductive fluid (i.e. seawater) can be controlled by Lorentz force, which holds promising applications in drag reduction. In this paper, the drag reduction on a stationary sphere in the weakly conductive fluid with the Lorentz force on the sphere surface are numerically investigated at Re = 300. The relations among the vortex structures, the hydrodynamic forces, and the effects of drag reduction are discussed before and after the application of the Lorentz force. The results indicate that the fluid near the surface of the sphere is accelerated with the application of Lorentz force. Therefore, the flow separation on the rear surface of the sphere is suppressed, and the periodic shedding mode of hairpin vortices is replaced by the steady double-thread wake structure. Meanwhile, the increase of momentum near the sphere improves the capability to overcome the adverse pressure gradient. Therefore, the pressure on the leeward side of the sphere increases, and then the effect of drag reduction is achieved. Moreover, the effects of drag reduction are distinct with the different locations (θ m) of the applied Lorentz force, which reaches the maximum drag reduction rate together with the maximum drag reduction efficiency for θ m = 90°. • The relations are revealed among the characteristic structures, drag reduction, and the parameters of Lorentz force. • The increase of the pressure on the leeward side of the sphere by Lorentz force is the key of drag reduction. • The optimal location is found based on the control mechanism with the different locations of the applied Lorentz force. [ABSTRACT FROM AUTHOR]

Amphibious vehicles have the unique ability to maneuver on land and water, due to their distinctive structural characteristics, their hydrodynamic performance differs significantly from that of ships. With the growing demand for high-speed navigation of amphibious vehicles, research into resistance and methods to drag reduction has become a hot issue. In this paper, drag reduction technology for amphibious vehicles is reviewed based on a thorough analysis of existing research. Various amphibious vehicles from different countries are introduced, outlining their different movement states on the water and classified based on their drag reduction mechanisms. The article explores the working principle and effects of various drag reduction technology from two perspectives: universal technology and amphibious technology. Research indicates that applying fouling and biofouling effect, surface drag reduction, and air injection as universal technology can effectively reduce the sailing resistance of amphibious vehicles. Due to the limitations of universal technology in some special military amphibious vehicles, amphibious technology is discussed as a supplement, such as planing hull, retractable walking mechanism, and drag reduction accessories. Finally, a summary of drag reduction technology for amphibious vehicles is provided, including pertinent design recommendations and an introduction to potential research areas for future development. • Traditional amphibious vehicles are weak in speed, maneuverability, and drag reduction methods. • Turbulence improvement, air injection, hydrodynamic lift, and multi-hulled designs have been validated in the laboratory. • Universal drag reduction technologies can be promoted to amphibious vehicles. • Planing hull, retractable walking mechanism, drag reduction accessory are efficient amphibious drag reduction technologies. • Future works should focus on structural, manufacturing, shape optimization, materials, and control. [ABSTRACT FROM AUTHOR]

Biofouling on netting is an important problem, which can increase hydrodynamics and reduce water exchange across the netting. In this study, drag effect of netting panels with artificial hard and flexible fouling was investigated by physical model tests. The results showed that fouling can cause a rapid increase in netting drag, and that the drag amplification factor of a netting caused by severe hard fouling can be up to 30 times higher than that of a clean netting. With increasing fouling, the drag coefficients for artificial hard fouling can up to 54% higher than those flexible fouling. The formulae proposed to predict the drag coefficients of netting panels with severe hard fouling, can provide a conservative drag coefficient for netting cage design to evaluate the cage safety. • The hydrodynamic effect of netting with different types of biofouling is studied. • The blocking effect of netting with different levels of biofouling is analyzed. • The prediction formulae for drag coefficient of netting compensates for the inadequacy of the existing empirical formulae. [ABSTRACT FROM AUTHOR]

A numerical investigation of the drag on submarines close to the seabed was conducted. The objective was to obtain velocity corrections to account for the additional drag caused by the blockage effect. A simulation model to estimate drag of a submarine hull was built using the open source code OpenFOAM. Numerical errors due to grid refinement were assessed and simulations were validated through comparisons with experimental data. The analyzed Reynolds numbers ranged from 5 × 1 0 6 to 2. 5 × 1 0 7 . Results showed good agreement with experimental drag coefficients for open water conditions, with maximum deviation of 3.25% for the lowest Reynolds number. To assess the effect of the proximity to the bottom wall, four model-to-wall distances were considered. Results showed a prominent effect on the pressure drag components and were used to propose two types of velocity correction for the blockage effect. The proposed correlations successfully corrected the pressure coefficient towards the open water values. • Accurate prediction of drag on submarines. • The wall proximity significantly affects the pressure drag coefficient. • The correlations were successful in correcting the pressure drag due to wall proximity. [ABSTRACT FROM AUTHOR]

The wave interference and the corresponding effects on the wave drag of the leader–follower ship fleet are researched here, via a 4-point model and the Neumann-Michell potential flow computation. A simple analytical relation of the dominant wake angle is given, which considers both the wave interferences between the ship bow and the stern, and the wave interferences between the leader ship and the follower ship. Moreover, a simple relation of the destructive interference region (where the wave drag of the follower ship decreases) for the wave drag is given based on the research on the effects of the ship distance ℓ 2 and the Froude number F on the wave drag of the leader ship or the follower ship. An interesting finding is that the destructive interference relation is invalid at low Froude numbers or very high Froude numbers, since the constructive interference of the divergent waves is apparent at low Froude numbers and the transverse waves is relatively small at high Froude numbers. These findings can be used to decrease the wave drag of ship fleets or ship formations. • A simple analytical relation of the dominant. • A simple relation about the decrease of the follower ship's wave drag is given. • The destructive interference relation is invalid at low or very high Froude numbers. [ABSTRACT FROM AUTHOR]

The structural design of fixed offshore platforms is mainly based on characterizing inertial and drag loads due to waves, currents, and wind. These structures incorporate sacrificial anodes as corrosion protection systems. The sacrificial anodes increase the exposed area of the tubular elements of the jacket. Therefore, designers need to determine hydrodynamic coefficients for tubular elements with anodes to calculate the load increment due the anodes. This work focuses on evaluating the effects of anodes on the drag coefficients for tubular elements, considering current velocities from 0.25 m/s to 2 m/s, in 0.25 m/s increments, tubular element diameters from 0.508 m to 3.048 m, in 0.508 m increments, and three different positions of the anode, back (P1), front (P2), and lateral (P3) to the tubular element, concerning the direction of the current. This work uses CFD analysis, considering RANS and the k-ω model. The results show that the drag coefficient can be calculated as a function of the tubular element's diameter and the anode's position. Correlations were obtained for calculating an equivalent drag coefficient or an equivalent hydrodynamic diameter for any position of the anode as a function of a diameter ratio and the angle of its position in the tubular element concerning the flow direction. • Drag coefficients were calculated for tubular elements with sacrificial anodes. • CFD analysis, RANS and k-ω model to obtain drag coefficients. • Drag coefficient increases considerably with the use of sacrificial anodes. • Correlations to evaluate the equivalent drag coefficients. • Correlations to evaluate an equivalent hydrodynamic diameter. [ABSTRACT FROM AUTHOR]

Propagation speed of acoustic waves in liquid can be significantly degraded when gas bubbles exist. To reveal the influence of gas bubbles on propagation characteristics of leakage acoustic waves in liquid pipelines, numerical simulation is utilized in the present paper. It can be divided into three stages when leakage acoustic waves propagate through a single gas bubble and the influence of each stage on propagation characteristics of acoustic waves is discussed. In gas-liquid two-phase flow, there are two basic mechanisms of gas resistance on wave propagation: elastic resistance effect and drag resistance effect. The results show that the influence of elastic resistance effect on sound speed is much greater than that of drag resistance effect. When propagating through a long gas bubble with the radius of 2 mm and the length of 7 mm, the elastic resistance effect causes a time delay of 0.00038 s while the drag resistance effect only causes a time delay of 0.00002 s. The results of this paper contribute to better understanding of the differences in acoustic wave propagation characteristics between different flow patterns in gas-liquid two-phase flow, and lay a foundation for the utilization of acoustic leakage detection methods in gas-liquid two-phase flow pipelines. • The resistance effect of gas bubbles on sound speed mainly exists in the process acoustic waves approaching the gas bubble. • In gas-liquid two-phase flow, there are two basic mechanisms of gas resistance on wave propagation: elastic resistance effect and drag resistance effect. • The influence of the elastic resistance effect caused by gas compressibility is much greater than the drag resistance effect caused by gas-liquid interface pressure balance effect on sound speed. [ABSTRACT FROM AUTHOR]

The accurate and efficient prediction of the flow and temperature fields on water surface modulated by internal waves induced by underwater vehicles in a density-stratified ocean can serve as the basis for indirect detection. The disturbance of an underwater object on density-stratified water can be considered as the mixing of two incompressible fluids: freshwater and saltwater. Furthermore, the modulation effect of internal waves on the convergence and divergence of flow field and thermal skin layer is captured at the air–water interface. Therefore, in this study, a CFD solver called "interMixing_skin_Foam" based on OpenFOAM is proposed. It can manage three incompressible fluids, two of which disperse, and the interface between the immiscible fluids (air–water) is captured using the VOF technique. In addition, the energy equation is implemented to investigate the thermal signature variations at the air–water interface. Comparison analysis of experimental results and those obtained from the CFD solver "twoLiquidMixing_skin_Foam" developed in a previous study, wherein only two incompressible fluids were considered, is performed for standard cases in literature. Subsequently, the influence of the air–water interface on the flow and temperature fields is studied for a submerged sphere travelling horizontally in linearly and strongly density-stratified water. • The thermal signature variations of internal waves by underwater target at the air–water interface were investigated. • A solver based on OpenFOAM platform for flow and temperature fields of three incompressible fluids was developed. • The viscosity drag effect, surface tension, and movable free surface have significant influence on thermal signatures. • In strongly density-stratified water, the upper fresh-water layer could suppress the thermal signatures. [ABSTRACT FROM AUTHOR]

To investigate the drag reduction characteristics of a flexible plate, a numerical simulation analysis was performed on the relationships of flexibility and thickness of a flexible plate on the effect of drag reduction. To accurately demonstrate the mutual interaction between flexible and fluid flows and capture the moving boundary of the plate, a bidirectional fluid–structure interaction (FSI) method was developed. The numerical results demonstrated that the flexible plate played a positive role in drag reduction. The drag reduction effect could be improved by increasing the flexibility of the plate or decreasing its thickness. Moreover, the pressure-difference resistance acting on the flexible plate was negative, indicating that the existence of pressure resistance was favorable for drag reduction. Further, the numerical results revealed that the real contribution to the drag reduction lay at the middle part of the flexible plate, i.e., the part with the significant deformation, whereas the parts with small deformation exerted little effect on the drag reduction. The conclusions from this study can provide guidance for future investigation of FSI problem and flexible surface-drag-reduction technology. • The numerical method for solving the problem of bi-directional fluid-structure interaction was established. • The effect of flexible plate's thickness and elastic module on drag reduction effect is reported. • The correlation between flow mechanism and drag reduction effect of flexible plate is well established. [ABSTRACT FROM AUTHOR]

The hydrodynamics properties of sharkskin have been studied for the past few decades, demonstrating the drag-reducing effect of the sharkskin surface denticle structure. This study explores the influence of the sharkskin denticles structure on the hydrodynamic performance of airfoil and the drag reduction mechanism using time-resolved particle image velocimetry (TR-PIV) by arranging the sharkskin denticles on the National Advisory Committee for Aeronautics (NACA) 0012 airfoil. The change in lift and drag of the airfoil was measured by a six-dimensional force sensor. The effects of angle of attack, sharkskin denticles location, Reynolds number, and arrangement on the hydrodynamic performance of the airfoil were mainly analyzed. The results show that for the single-row sharkskin denticles structure, the closer the bionic structure is arranged to the trailing edge of the airfoil, the better the drag reduction effect is. The maximum drag reduction is up to 25.4% by arranging the bionic denticles at 0.38L of the airfoil, when Re = 1.62 × 105 (U = 0.8 m/s) and α = 0°. For the double-row sharkskin denticles structure, the staggered arrangement is less effective in reducing drag than the aligned arrangement due to the influence of form resistance. The velocity field of the turbulent boundary layer on the airfoil surface was obtained by TR-PIV. The experiment results show that the sharkskin denticles structure can change the boundary layer of the airfoil, and the static flow area formed at the tail can reduce the disturbance of the airfoil by the turbulent flow. • The bionic drag reduction experiment was carried out from macroscopic force and microscopic flow field. • The angle of attack, sharkskin denticles location, and Reynolds number effect on drag reduction. • Sharkskin denticles can improve the lift-to-drag ratio and performance of the airfoil. • The maximum drag reduction is up to 25.4% by arranging the bionic denticles at 0.38L of the airfoil. [ABSTRACT FROM AUTHOR]

The study of the ship airwake is critical as it explores the effect of unsteady wake flow on helicopter operations. This paper studied the near-wake flow topology of a generic ship at Re = 8 × 104 to assess the capability of a hybrid RANS/LES (Reynolds-averaged Navier–Stokes/large-eddy simulation) approach, known as IDDES (improved delayed detached-eddy simulation). The impact of computational parameters, including the mesh grid, residual level, time-step size, momentum discretization scheme and transient formulation, on the wake flow was investigated. The numerical results were validated by using the previous experimental data and LES results. The results show that except of the mesh resolution all other computational parameters varied in the current study do not have significant effect on the global drag forces, but showing large differences on the prediction of the local wake flow structures. This point has not been evidently reported in the previous work. The finer grid resolution is sufficient to produce an accurate qualitative and quantitative prediction of the flow structures, while using a poor grid resolution (coarse mesh) leads to inaccurate prediction of the flow topology. The recommended parameters for the time-step size (7.5 × 10−5s) and residual level (1 × 10−4) provide sufficient accuracy of wake predictions, showing good agreement with reference studies. For the convective term of the momentum equation in IDDES, the bounded central differencing scheme is proposed for its discretization, while the bounded second-order implicit is proposed to be adopted as the transient formulation. • The ability of IDDES to predict the bi-stable phenomenon of a generic ship has been evaluated. • The effect of computational parameters on the ship flow topology has been studied. • IDDES results show good agreement with available numerical and experimental data. • Consistency of global quantities can not guarantee the same trend for the local flow topology. • Optimum computational parameters are proposed for the study of the bi-stable ship wake flow. [ABSTRACT FROM AUTHOR]

A reliable load-to-response prediction for flexible risers under drag forces can protect the multiple-riser system from collision and optimize its spatial arrangement. To investigate the drag coefficients for double risers in tandem, a series of experiments for two unequal-diameter flexible cylinders in different initial gap spacings was performed under uniform flows with Reynolds numbers up to 11,328. Based on the reconstructed mean drag displacement from measured strains, a distributed drag force amplified by vortex-induced vibrations can be identified reversely. Meanwhile, a modified wake model was developed to estimate the mean wake velocity for the downstream cylinder. With this mean wake velocity, the drag coefficients of the downstream cylinder were obtained. The features of drag coefficients and vortex/wake-induced vibrations were found to be closer to the corresponding features of an isolated cylinder. Furthermore, an empirical drag coefficient model for double unequal-diameter cylinders in tandem was proposed, which accounts for the drag amplification effect by vibrations, tandem spacing, upstream-to-downstream cylinder diameter ratio, mean wake velocity, etc. The results show that this empirical drag coefficient model can obtain the prediction results with relatively higher precision. • A series of tests were performed to study the drag coefficients of double unequal-diameter flexible cylinders in tandem. • A modified wake model was proposed to predict the wake velocity, considering the VIV of the upstream cylinder. • A new drag coefficient prediction model for double unequal-diameter flexible cylinders in tandem was proposed. [ABSTRACT FROM AUTHOR]

Research on aerodynamic performance and reduced wind drag acting on hull and large windward areas of high superstructures, such as passenger ships, container ships, roll on - roll off car ferries, and other ships, remains essential marine transportation. In this study, the wind drag acting on a passenger ship's above water hull was computed using ANSYS-Fluent, a commercial Computational Fluid Dynamics (CFD) code. Using CFD, the wind drag components, friction viscous wind drag, and pressure viscous wind drag acting on the hull of the original passenger ship were investigated. Based on the pressure and velocity distributions around the hull, several new hull forms with different frontal accommodation shapes have been proposed to reduce wind drag and improve the aerodynamic performances of the ship. By comparing the CFD results, a new above water surface hull with reduced wind drag was indentified in this study. • Effects of mesh and turbulent viscous model on aerodynamic performances of a passenger ship investigated. • A bow cover affects the aerodynamic performance of the ship. • Wind drag acting on a passenger ship decreases by up to 38.71% and 42.41%, with proposed a flat plate and streamlined bow cover. [ABSTRACT FROM AUTHOR]

In this study, two superhydrophobic blade surfaces of propellers were prepared using nanosecond laser processing and superhydrophobic coating. Numerical calculations of the open water characteristics indicated that superhydrophobic propellers provide higher efficiency over the entire range of operating conditions. Towing tank tests showed that superhydrophobic propellers generate greater thrust but require more torque from an advanced coefficient of 0.3. The increased thrust was caused by the increased pressure differential between the pressure and suction sides. The change in torque was mainly attributed to the gradual loss of the air layer and the increased shape resistance of the rough structure. Within an advance coefficient of 0.3, the laser-processed superhydrophobic propeller exhibited a more apparent drag reduction effect, and the efficiency was improved by a maximum of approximately 2.8%. In contrast, the propeller with superhydrophobic coating has a stable drag reduction effect owing to the stable air layer on its surface during the entire testing process, with an average efficiency improvement of approximately 2.0%. The results confirmed that the superhydrophobic surface, especially the rough structure, significantly affected propeller efficiency. This research extends the practical engineering applications of superhydrophobic surfaces in vessel propulsion. • Superhydrophobic propeller surfaces were prepared by laser processing and coating. • Propeller with laser-processed blades performs well at low advance coefficients. • Propeller with coated blades has a good improvement of open water efficiency. • The superhydrophobic surface roughness has a significant effect on drag reduction. • Reasonable rough structure is beneficial to decrease the turbulence resistance. [ABSTRACT FROM AUTHOR]

Fluid flow around and drag of two cylindrical cruciform patterns, conventional (T0) and rotated 45º in its own plane (T45), were numerically investigated by solving three-dimensional Reynolds-averaged Navier-Stokes equations within the subcritical flow regime over angles of attack (AOA) from 90º to 0º. Firstly, the drag for a one-cruciform element was assessed, followed by analysis of a four-cruciform assembly (‘square mesh’) to take into account the wake effect of tandem elements. For the one-cruciform element, T45 experienced a less prominent streamline separation and consequently lower drag between 90° and 45° AOA, while T0 experienced progressively lower drag below 45° AOA owing to re-establishment of smoother streamlines caused by the gradually reduced circulation momentum from the adjacent vortex that rotates in the off-side direction. For the four-cruciform assemblies, T0 and T45 drag was essentially equal above 45º AOA; while below 45º, T45 had greater drag attributed to more prominent spanwise vortex downstream development. Overall, while the largest relative difference between the two orientations was 26.2% and 33.8% for the one- and four-cruciform configurations respectively at 0° AOA, for 30º and above AOA there were limited drag differences (generally below 10%). [ABSTRACT FROM AUTHOR]

Drag anchor is an economical foundation option for offshore floating structures. Although there are studies on ultimate pullout capacity of drag anchors, the drag-in installation process is still not fully understood. The approach using yield envelopes for drag anchor under combined loading for the installation behavior prediction is promising. However, more needs to be done to understand anchor bearing behavior during the installation process. The current study focuses on the capacity of drag anchor under uni-directional vertical, horizontal and moment loading in uniform clay and the behavior of anchor fluke under combined loading. Finite element analyses are conducted for “wished-in-place” anchor fluke. The effect of anchor embedment depth and soil overburden pressure is investigated. The horizontal and rotational capacity factors for different embedment ratios and overburden pressure ratios, which have not been covered in the existing studies on anchor capacity, are provided here. The effect of anchor/soil interface breakaway condition on the capacity under the three uni-directional loading is also studied. Based on the understanding of drag anchor capacity under uni-directional loading, its behavior under combined loading is characterized by the yield envelopes for both shallow and deep anchor behavior. The current study provides a good understanding for the capacity of drag anchors. The yield envelopes can be used for the prediction of anchor trajectory with consideration of both shallow and deep anchor behavior. [ABSTRACT FROM AUTHOR]

In this paper, to reduce navigational resistance and energy consumption and to promote the efficiency of a novel track-propeller composite amphibious robot, a bionic fairing inspired by hammerhead sharks is proposed. Three types of fairing devices for underwater navigational resistance reduction are designed based on the bionic principle of hammerhead sharks. On this basis, the structural drag characteristics of four models, including an original model and three amphibious robot models with bionic fairings, are evaluated through computation fluid dynamics (CFD) analysis based on lattice Boltzmann method (LBM) theory. Experimental studies on the bionic fairing drag reduction of the amphibious robot model are conducted. In the experiments, the drag reduction effect of the bionic fairing devices is investigated considering two models and three speeds (0.5 m/s, 1.0 m/s, 2.0 m/s). Compared with the original model of the amphibious robot, the simulation and experimental results demonstrate that the bionic fairing devices inspired by the hammerhead shark head have a significant resistance reduction effect, the drag coefficient can be reduced by 23%, and the experimental results reveal a maximum drag reduction rate of 22%. This study provides an essential reference for the design of tracked amphibious robots with underwater navigation functions, reveals the drag reduction mechanism of the hammerhead shark head, and expands the bionic object of existing resistance reduction studies. • Present an amphibious robot that meet all-terrain and multi-sea conditions movement. • Present bionic fairing inspired by the shape of hammerhead shark for drag reduction. • The bionic fairing reduced the amphibious robot resistance in underwater navigation. • Reveal the drag reduction mechanism and expand the bionic object of existing studies. [ABSTRACT FROM AUTHOR]

This paper aims to analyse how hull geometry affects the ships sinkage. A 12 ship database is used to study hull forms effect on sinkage both experimentally and numerically, which sinkage towing test results – collected from previous experimental campaigns – were compared towards the ships form coefficients. The negative effect caused by a draft increase was studied and the bow shapes effect was analysed in detail with the experimental results of two pairs of ships in which the hull forms design only differed in the bow. Numerically, the dynamic pressure field diagrams for the different ships were analysed to study the pressure fluctuations, and the simulations were validated with the experimental results. From the study described above on sinkage, a strong relation between sinkage and the slenderness of ships was established leading to a predictive model of sinkage based on the slenderness of ships. By contrast, the bow shapes effect on sinkage suggested was rejected and it was concluded that the stern and the action of the propeller could have a key role in the sinkage of ships. Further research must be done to decrease CFD uncertainties at sinkage evaluation and to analyse the stern pressure fluctuations. • The sinkage of ships has significant effect on ship drag. • Sinkage mitigation can be tackled through hull form modification. • Correlation between numerical and experimental data on sinkage. [ABSTRACT FROM AUTHOR]

This paper studies the steady Coanda effect for reducing the aerodynamic drag of the Chalmers ship model (CSM) using Large Eddy Simulation (LES) with Wall-Adapting Local-Eddy Viscosity (WALE) model. The flow control mechanism is explored, and the analysis of energy efficiency is conducted to evaluate the net benefit of the flow control. Validating the numerical methods, the predicted aerodynamic drag of the ship and pressure coefficients distribution on the baseline CSM agree well with the experimental measurements and the maximum discrepancy is 4.2%. In creating the flow control models, the hanger base of the baseline CSM is modified with a Coanda surface and two different sizes of jet-blowing slots, 1% h (hanger height) and 2% h , respectively. A drag reduction of 5.34% is achieved by the 1% h slot-size case. The 2% h slot-size case further increases the drag reduction to 6.22% but has doubled power consumption. It is found that vectoring vorticity towards the low-speed area on deck is effective for enhancing the energization. Finally, the analysis of energy efficiency indicates that the net benefit is achieved in both flow control cases, and the case with the 1% h slot size is 11.9% more efficient due to a stronger Coanda effect. • The Coanda surface at the hanger base suppresses the low-speed area on deck. • The steady Coanda effect reduces drag by increasing the base pressure recovery. • The steady Coanda effect decreases the size of the high-turbulence region on deck. • Vectoring vorticity towards the low-speed area on deck better enhances mixing. • Net benefit is achieved with the power saving outweighing the power consumed by AFC. [ABSTRACT FROM AUTHOR]

Biofilm heterogeneity and adaptability complicates efforts to link biofilm structural and mechanical properties to frictional drag. As a result, rigid structures are typically used as the benchmark for studying biofilm-associated drag. Elastomeric sandpaper replicas were generated to be used as model systems for investigating the effect of roughness and elasticity on drag, over the Reynolds number range of approximately 2.0 × 104 to 5.2 × 104 Re using a marine biofilm flow cell. To control for roughness parameters and surface topography the replicas were created for sandpaper grit numbers: P40, P80 and P240 with average measured roughness (S a) of 108, 49 and 16 μm, respectively. Profilometry confirmed that there was no significant difference between the roughness of the rigid sandpaper sources and the material replicas. The marine biofilm flow cell was fitted with a clear lid, which allowed real-time visualisation of the replicas' surface topography using Optical Coherence Tomography. Pressure drop measurements, expressed as a friction coefficient, revealed that the elastomeric sandpaper replicas had a significantly higher associated drag, of up to 52%, when compared to the rigid counterparts. From statistical analysis it was confirmed that material mechanical properties, such as elasticity, and surface roughness both significantly affect drag. Elastic model systems can be used to enhance our understanding of biofilm physico-mechanics and their role in marine drag. • Created a novel method for creating an elastomeric biofilm model. • Derived mesoscopic structural parameters in-situ using OCT. • Roughness and elasticity had a significant shared and independent effect on drag. • Elastomeric replicas increased drag by up to 52% compared to rigid structures. • Importance of the effect of elasticity on drag is highlighted. [ABSTRACT FROM AUTHOR]

Flume testing of a scaled model under steady flow is an essential reference for accessing the flooding-induced drag force on a prototype pier, but is highly susceptible to the cross-sectional blockage effect. Round-ended bridge piers have a non-axisymmetric cross-section, of which the drag force varies significantly with inflow direction. A flume testing program was carried out to illustrate the cross-sectional blockage effect under different inflow directions. CFD numerical simulations were then conducted to understand the blockage effect on the drag force, flow field and hydrodynamic pressure of the round-ended pier with different blockage ratios and inflow directions. The major findings include: 1) the blockage effect affects the drag force of the scaled model and changes with inflow direction; and 2) the limit of occurrence of the cross-sectional blockage effect is defined by the critical blockage ratio and given as a function of inflow direction. An empirical approach was finally developed to correct the cross-sectional blockage effect on the drag force of the round-ended pier model under directional flow. The influences of the blockage ratio and inflow direction were included in the empirical calculation of the drag force on the prototype from the scaled model test. • Experimental and numerical studies are conducted to understand the blockage effect on the drag force and flow field of the scaled round-ended pier models. • The limit of occurrence of the cross-sectional blockage effect on round-ended pier model is investigated as a function of inflow direction. • An empirical approach is developed to estimate the drag force on the prototype pier from the scaled model testing. [ABSTRACT FROM AUTHOR]

Ventilated cavity flow is of a great practical interest for the features that may reduce the drag around the area near the cavity. The present paper demonstrates the prediction of ventilated cavity flows on the backward facing step, which establishes the prediction model of drag reduction by the numerical simulation and artificial intelligence algorithm. The results of the computational fluid dynamics are compared with available experimental data to validate the precision, which shows good agreement. This paper numerically analyzes the influence of the number, location and part ventilation volume of injectors on the drag reduction of ventilated cavity flows. With the same air-entrant coefficient, the number and location of injectors both have a significant impact on drag reduction performance. And the part ventilation volume of the injectors may also have a limited impact. Accordingly, the prediction model of drag reduction is established by the Error Back Propagation algorithm (BP algorithm) that has accurate algorithms and fast calculation to save time and computing resources. Meanwhile, the results indicate that BP algorithm can reasonably predict the performance of ventilated cavity flows, which means the model can optimize the drag reduction on ventilated cavity flows and it can be useful in engineering. • The effect of injectors on the drag reduction performance of ventilated cavity flows are investigated. • The influence of the injector location on the drag reduction performance are evaluated. • An optimization study is carried out based on the Error Back Propagation algorithm for ventilated cavity flows. [ABSTRACT FROM AUTHOR]

The drag of a prawn-trawl body is characterised by five design and three operational variables. The design variables comprise headline length, steepness of trawl side cut, width-to-depth mesh ratio of the trawl mouth (gape), vertical wing-end mesh count, and netting solidity; all of which effectively determine the planar twine-area of the trawl. The operational variables include towing velocity, horizontal spread, and vertical opening (headline height)—these determine the extent that the netting is exposed to relative water movement. The individual drag effects of the above variables (except for headline length, gape, and netting solidity) were systematically examined in a flume tank with prawn-trawl models built with low-stiffness full-scale netting; and the existing literature was consulted on the drag effects of gape, while drag was assumed to be proportional to twine diameter, mesh size −1 and headline length 2 . The developed equations in non-dimensional forms provide the basis for a drag-prediction model for a prawn trawl of any size, construction and operating conditions. Comparisons with previously published prediction equations showed considerable disagreement in some aspects, and suggest that using stiff, full-scale netting in past model experiments have produced significant model-to-full-scale prediction errors owing to the poor equivalence of twine bending-stiffness-to-netting-tension ratio. [ABSTRACT FROM AUTHOR]

An analytical method is proposed in this paper to calculate the maximum embedded depth of a dragged Hall anchor when passing through rock berm and to thus define a minimum buried depth of pipelines in rock berm to prevent pipelines from being damaged by dragging anchors. The movement of a Hall anchor in rock berm is interpreted based on the equilibrium conditions for resisting and driving moments acting on the anchor. To verify the accuracy of the proposed analytical method, model tests were carried out by using three scaled Hall anchor models and dragging them through rock berm. The comparisons between the two studies show that the average value of their differences for the stable embedded depth of a Hall anchor in sand and in rock berm are only 1.7% and 2.7%, respectively. The good agreements indicate that the proposed method is accurate enough to calculate the minimum buried depth of pipeline in rock berm during pipeline design. [ABSTRACT FROM AUTHOR]

The concept of cavitation drag reduction is widely known in many fields. However, the existence of cavitation is not beneficial to the protection of structures. In this study, an attempt was made to improve the protection efficiency of fluid-filled vessels by adopting the reverse approach of increasing drag forces by reducing cavitation: The vessels were filled with flowing fluids to reduce the pressure and change the shape of cavitation during penetration. Ballistic tests, numerical simulations, and theoretical analyses were conducted to evaluate the impact response of flowing-water filled square vessels impacted by a spherical projectile. The impact responses of flowing-water filled square vessels were analyzed and compared with static-water filled vessels, in terms of the deformation and failure of target panels as the vessel's walls, sidewall pressure time histories, and cavitation processes. Moreover, the mechanism of increasing drag forces due to the fluid flow was elucidated. The effects of fluid flow on the cavitation and pressure distribution were determined. This study elucidated the active attenuation mechanism of fluid flow on mitigating the damage of rear panel. The results show that fluid flow can increase the drag forces, leading to a higher protection efficiency of flowing-water filled vessels than static-water filled vessels. Owing to the effect of fluid flow, sidewall pressures are evidently lower than those in static fluid cases. Fluid flow can change pressure distributions in the vicinity of cavitation, which can alter not only the cavitation shape but also the action direction of cavitation jetting. The active attenuation effect of fluid flow predominantly contributes to mitigating the pressure accumulation on the inner surface of rear panel during penetration, resulting in lower pressures and subsequently less damage to the structures. This study sheds light on the benefits of flowing-fluid filling on increasing the drag forces as well as mitigating the cavitation pressures. • A reverse approach of increasing drag force with reducing cavitation was used. • The effects of flow on the drag force and cavitation were illuminated. • The active attenuation mechanism of flow was elucidated. • A fascinating phenomenon of pressure accumulation was uncovered. • The underlying benefits of flowing filling are shown. [ABSTRACT FROM AUTHOR]

From the viewpoint of the improvement of the air lubrication method for the ship's hull resistance reduction, an experimental study was conducted for two-phase flow in a horizontal rectangular channel with a length of 3.0 m, a gap of 0.05 m, and a width of 0.2 m, which simulates the external two-phase flow. The database on frictional drag and local two-phase flow parameters, including void fraction profile and gas chord length, was collected using a shear stress sensor and a double sensor probe. Measurement of two-phase flow structure was performed at three axial positions of z = 0.930, 1.43, and 1.93 m from the bubble injection port. The wall shear stress was obtained at z = 1.93 m. A total of 72 datasets are acquired a room temperature and at flow conditions of superficial liquid velocity < j f > ranging from 0.832 to 3.00 m/s and superficial gas velocity < j g > ranging from 0.0232 to 0.714 m/s. The effects of entry length, liquid and gas flow rates on the phase distribution characteristics beneath the wall were discussed. The bubble layer thickness was newly defined as the length scale. The bubble layer thickness was not so sensitive to the changes in the liquid flow rates and the entry length, and highly dependent on the gas flow rates in the present experimental conditions. The mean void fraction in the bubble layer was highly reliant on the liquid flow rate and decreased with the superficial liquid velocity. In contrast, the insignificant effect of gas flow rate on the mean void fraction was confirmed. A strong correlation between the drag reduction effect due to the air lubrication and the mean void fraction in the bubble layer was confirmed. The obtained data are expected to be used for the modeling of the interfacial area transport terms, development of the constitutive equations of the drift flux model, modeling of the drag reduction for the external two-phase flow and the benchmark tests of various CFD codes in the future study. • An experimental study was conducted for two-phase flow in a horizontal rectangular channel with a gap of 0.05 m and a width of 0.2 m, which simulates the external two-phase flow. • The database on frictional drag and local two-phase flow parameters, including void fraction profile and gas chord length, was collected using a shear stress sensor and a double sensor probe. • The effects of entry length, liquid and gas flow rates on the phase distribution characteristics beneath the wall were discussed. • The bubble layer thickness was newly defined as the length scale, and the averaged two-phase flow parameters in the bubble layer were discussed. • A strong correlation between the drag reduction effect due to the air lubrication and the mean void fraction in the bubble layer was confirmed. [ABSTRACT FROM AUTHOR]

In the present paper, the freefall hydrodynamic effects of torpedo anchors are studied through drop tests. A numerical computational fluid dynamics (CFD) model was used to simulate these drop tests relying on Reynolds-averaged-Navier-Stokes equations. A theoretical model hypothesis is necessary for these numerical simulations to define the inlet velocity, which was validated using physical experiments. The hypothesis uses the constant added-mass and drag coefficients, which are never constant in the field. Herein, the 1:50 scale model experiments (in a 4 m water pit) helped to capture the time-varying effect of the hydrodynamic forces (i.e., added mass and drag effects) of the dynamically installed torpedo anchors. In turn, the theoretical model is used in the numerical simulation of the CFD model to understand the hydrodynamic effects in the prototype. The parametric analyses are conducted with different anchor configurations as a part of the study. In particular, the effect of aspect ratio and anchor geometry (nose, fin configurations) are reported. Experimental findings report best hydrodynamic performance by the hemispheric nose followed by ellipse and ogive. In contrast, the conical nose shows inferior performance due to its significant lateral displacement. Numerical simulations also showed that long elongated cones (tip angle <60°) were more efficient. • Drop tests are performed in the 4 m deepwater pit on a 1:50 scaled modeled instrumented torpedo anchors. • Drop tests are also numerically simulated using computational fluid dynamics (CFD) simulations for prototype anchors. • The parametric analyses conducted on 11 anchor models varying aspect ratio, fin numbers & ratio, weight and nose shapes. • An increase in the anchor's density results in greater resistance to the lateral displacement. • An increase in aspect ratio increases the terminal velocity of the anchor with an increase in ballast weight. [ABSTRACT FROM AUTHOR]

The dynamic model of a flexible robotic chain by considering its motion in a fluid medium is developed in this study. The use of flexible manipulators in fluid applications offers several advantages, but the fluid-manipulator interaction forces, as well as the elasticity of each link, complicates the modeling process. To tackle this issue, the dynamic effects of the fluid on the performance of flexible arms are identified and expressed mathematically so that the resulting interaction during the robot motion can be modeled simultaneously. In this regard, the dynamic interaction between the links and the fluid as a result of link flexibility is modeled as a distributed load along the arm length. In addition to the structural damping of the links, the effect of fluid damping along with its added mass affects the performance of the considered robotic system. Thus, compared to a flexible robotic arm in the air or a rigid robotic arm in the fluid, the flexible deformation of the robot links during motion is affected by the fluid flow. The recursive Gibbs-Apple formulation is used to derive the equations of motion of an N-link robotic chain. Not only does this formulation reduce the computational complexity compared to similar algorithms, but it also allows us to consider the effect of forces exerted on the joints and robot links in the form of the Rayleigh dissipation function. The final equations of motion are simulated using MATLAB for a two-link flexible arm. Simulations of robot links are conducted for different elasticities, different initial conditions, and different media with different viscosities. According to the obtained results, the deformation of flexible manipulators decreases from 5 mm (in the air) to 3 mm (in the fluid), while the displacement in water is 10% of that in the air under similar conditions. [Display omitted] • The dynamic of flexible robotic chain by considering fluid interaction is developed. • The dynamic interaction between links and fluid modeled as a distributed load. • The fluid damping effects and its added mass considered as links structural damping. • The recursive Gibbs-Apple formulation is used to derive the motion equations. [ABSTRACT FROM AUTHOR]

A pair of flexible cable models were fabricated to simulate the wake induced vibration (WIV) of two tandem stay cables. The center-to-center separation of the two cable models was fixed to be 4 D. The in-plane and out-of-plane accelerations of both upstream and downwind models were measured as the incoming wind speed increased from 3 m/s to 40 m/s (R e = 0.96 × 104–1.28 × 105). The response characteristics of WIVs, including the vibration amplitude, power spectral density, and non-dimensional trajectory, were investigated in detail. The results show that the downstream model begin to strongly vibrate when the wind speed is above a critical value, and the WIV is dominated by the in-plane oscillation, which is consistent with those observed from an elastic model test in previous reports. However, different from the previous experimental results, three critical reduced speeds were observed, and four reduced speed ranges were proposed according to the response characteristics of WIV. The reasons for the response characteristics of WIV are qualitatively analyzed. The wake stiffness and the drag force effect are the main reasons that the dominating frequency of WIV is always higher than the fundamental frequency and increases with the wind speed. • Wake induced vibration (WIV) was simulated using two flexible models in a wind tunnel. • The downwind model experienced large vibrations beyond a critical wind speed dominated by the in-plane vibration. • Three critical reduced speeds and four ranges were proposed according to the response characteristics of WIV. • Vibration frequency of downwind model increases with the increasing wind speed. [ABSTRACT FROM AUTHOR]

Understanding the dynamics of an FWT (Floating Wind Turbine) is essential for its design and operation. Since a truss structure can reduce the wave load/resistance on the floating foundation, it becomes more popular in industrial applications. In this regard, knowing the effect of slender members of the truss structure on the motion response characteristics of such an FWT is vital. The present work develops a time-domain method for modeling the dynamics of a floating truss-structure wind turbine with multiple rotors on the deck of the platform. A hybrid panel-stick model is built up incorporating the potential flow theory to calculate the wave inertia force and a Morison strip method to calculate the wave drag force. A systematic methodology, and the corresponding efficient tool, have been developed to deal with the floating trussed structure consisting of a set of slender cylindrical members in arbitrary lengths, diameters, orientations, and locations. The Morison dynamic solver is incorporated into the time-domain solver for the FWT dynamics. The proposed model is validated against a model experiment of a semi-submersible FWT with a triangular-shaped truss-structured platform, which was carried out in RIAM (Research Institute for Applied Mechanics), Kyushu University. Good agreements between the simulation results and the experimental data confirm the validity of the developed method. Further numerical simulations are performed in a set of wind and wave conditions to investigate the effect of wave drag force on the FWT dynamics. It is found that without the fluid viscosity, resonant responses are excited in the platform motions at frequencies that are close to the natural frequencies of the FWT system. Via a comparison between the parked conditions and operating conditions of the FWT, it is found that in the presence of steady wind, the translational surge or sway motion is significantly excited at its resonance frequency. This may be attributed to the work done by the wind to the FWT, which enhances remarkably the total kinetic energy of the platform and consequently increases the translational surge or sway velocity of the platform at the equilibrium position. Applying a hybrid panel-stick model will be effective in reducing all these non-realistic large resonant responses. • A hybrid panel-stick method is systematically developed for a complex trussed-structure semisubmersible in the time domain. • The experimental test in the wave tank confirms the validity of the developed method and tools. • Numerical results show that the wave drag force can effectively suppress the unrealistic resonant response. • It is found that in steady wind, the translational surge or sway motion is significantly excited at its resonance frequency. [ABSTRACT FROM AUTHOR]

Inspired by nature, more and more domestic and foreign scholars began to pay attention to the application of hydrophobic materials in various fields for military and civil applications. The suitable bionic super-hydrophobic surface (SHS) has high drag reduction effect because of the slip effect on the solid surface. This paper conducted numerical simulation to study the aerodynamic performance improvement of vertical-axis tidal turbine (VATT) with SHS on the airfoils. The influence of different inflow velocities, blade number and solidities of VATT with SHS are studied. The results show that the maximum energy efficiency can be increased by 16.5% at the inflow velocity U = 3 m/s. The improvement of power coefficient is found greatly influenced by the inflow velocity, blade number and solidity at small tip-speed ratios, while the increment of the energy efficiency is almost independent to these aerodynamic parameters at high tip-speed ratios. • A vertical-axis tidal turbine with super-hydrophobic surface is proposed. • Influences of inflow velocity, blade number and solidity on VATT with SHS are studied. • The maximum power coefficient improved by SHS on hydro-airfoils can be 16.5% at U = 3 m/s. • The improvement of the power coefficient for two-blade VATT is larger than 7% for all TSRs investigated at U = 1 m/s. • Increment of energy efficiency at high TSRs is almost independent to aerodynamic parameters for blades number N ≥ 3. [ABSTRACT FROM AUTHOR]

Antifouling and drag reduction are two critical issues for the shipping industry. Previous studies have revealed that biomimetic shell surfaces have distinct antifouling features. In this study, ten simplified surface microstructures with the same cross-section area are designed based on the antifouling scale of the shell surface morphology. Turbulent flow over the riblet structures is simulated using a finite volume based CFD solver with the Shear Stress Transport k-ω model to examine the effects of the riblet structures on skin friction with mean flow velocities ranging from 1 to 7 m/s. The corresponding dimensionless square root of the groove cross-section area is l g + ≈4.5–25.6. Considering the predicted drag performance and issues concerning riblet tip rounding and fabrication, an optimal geometry with both antifouling and drag reduction features has been identified. The present study confirms that the riblets lift off streamwise vortices, pushing the high turbulent kinetic energy region away from the surface, and impede the spanwise movement of streamwise vortices, leading to skin-friction reduction. Further, the investigation of the effect of the riblet interval on the drag behaviour indicates the existence of a balance point between the wetted perimeter and the total area of the riblet surface for minimizing drag. • Drag performance of antifouling surfaces are investigated numerically. • Three shapes of riblets with various configurations are examined. • A specific riblet design with a circular cross-section has both antifouling and drag reduction potentials. • For a given cross-section area of riblets, a correlation between drag reduction and wetted perimeter is identified. • A simplified drag reduction mechanism is proposed. [ABSTRACT FROM AUTHOR]

Undulating propulsion is promising solution for underwater vehicles used in subsea security systems, especially due to its low noise, better energetic efficiency, low water turbulence and living animals mimicry. The design of the biomimetic autonomous underwater vehicle (BAUV) for intelligence surveillance and reconnaissance (ISR), built in the SABUVIS project, is presented in the paper. The shape of the vehicle hull was inspired by living fish. Key aspects of the design and construction of BAUV needed to achieve tasks required to be performed by the robot are described. Two research questions are stated in the paper: how big drag forces act on a biomimetic underwater vehicle and how mechanical design influences the noise level generated by the vehicle. To answer these questions vehicle forward velocity, hull drag, added mass and generated noise were investigated. Forces acting on the BAUV hull while being pulled forward and sideways were measured. Noise generated by the vehicle was also measured for two types of propulsors' drives in order to select those with the lower sound pressure level. Quantitative results of drag and noise obtained in the research are important for designers of mechanical structures of useful, full scale biomimetic underwater robots and their control subsystems. • The hull of underwater vehicle design based on the shape of a real fish 3D scan. • Fish-like shaped hull of the vehicle is characterized by good drag coefficient. • The obtained drag coefficient values are smaller than for other fish-like vehicles. • Added mass can be as high as the mass of the biomimetic vehicle. • Noise of the biomimetic vehicle can be greatly reduced by proper design. [ABSTRACT FROM AUTHOR]

Air lubrication drag reduction has been proven to be a workable drag reduction technology in laboratory and for the full ship scales. The bottom air layer shape and coverage are the key factors affecting the drag reduction effect, which cannot be controlled well in actual sea conditions. This study presents the experimental investigation on the air-water mixed flow shape at the lower surface of a flat plate. The air layer shapes are captured by an image system under different air flow rates and velocities. Four distinct air-water mixed flow are classified, referred to as dispersed bubble flow (DBF), mixture flow (MF), air layer flow (ALF) and bifurcation flow (BF). The ALF regime should be considered as the best way for the drag reduction, because the air layer flow is relatively stable and the air layer coverage is large and uniform under flat plate. In addition, the attitude of the plate has a great influence on the air-water mixed flow shape on the lower surface of the flat plate. The tail trim of the plate would suppress the longitudinal diffusion of the air bubbles, and the heel will shift the air-water mixed shape. • Four distinct air-water mixed flow are classified: dispersed bubble flow (DBF), mixture flow (MF), air layer flow (ALF) and bifurcation flow (BF). • The ALF regime is relatively stable and the air layer coverage is large and uniform under the flat plate. • The attitude of the plate has a great influence on the air-water mixed flow shape on the lower surface of the flat plate. [ABSTRACT FROM AUTHOR]

Effects of microbubbles injection on friction drag in a turbulent boundary layer are studied using large-eddy simulation. A flat-plate turbulent boundary layer with a Reynolds number Re θ = 1430 is generated within a short distance of inflow by introducing disturbances. The parameters investigated include the gravity, the injection height, and the volume fraction of microbubbles. It is found that the mechanism for drag reduction is threefold, that is, buoyancy acting on microbubbles, the change of coherent structures, and hydrodynamic viscosity near the wall. Under different conditions, the dominant mechanism for drag reduction varies among them. With microbubbles injected above the flat plate, the primary mechanism is the lift of coherent structures from the wall and the decrease in the size and vorticity of the streamwise vortices. When microbubble volume fraction increases, the larger buoyancy acting on microbubbles leads to coherent structures shifted further away from the wall, resulting in higher drag reduction. With microbubbles injected beneath the flat plate, the dominant mechanism is a significant decrease in hydrodynamic viscosity as buoyancy pushes microbubbles gathering near the wall and forming a thin air layer. The drag reduction is higher than that of microbubbles injected above the flat plate. • Mechanism for drag reduction includes buoyancy, coherent structures, and viscosity. • Microbubbles above plate break large-scale coherent structures into small ones. • Buoyancy shifts coherent structures further away and leads to higher drag reduction. • Microbubbles gathering beneath plate decreases hydrodynamic viscosity significantly. [ABSTRACT FROM AUTHOR]

Floating structures have become the most feasible solution for supporting wind turbines when offshore wind project move to deeper water. In this paper, a hydrodynamic analysis of three different semisubmersible floating offshore wind turbines is carried out including second-order hydrodynamic effects. The three examined platforms are V-shaped semisubmersible, Braceless semisubmersible and OC4-DeepCwind semisubmersible and are used to support the NREL 5 MW reference wind turbine. The main objective of the present study is to investigate and compare the hydrodynamic response of the three different semisubmersible floaters in two water depths (100 m, and 200 m) under different load conditions. The effects of second-order wave loads on the platform motions and mooring tension are discussed and compared by using different methods including Newman's approximation and the full QTF (Quadratic transfer function) method. The drag effect on the structure motion response is also discussed in this paper. The comparison presented is based on statistical values and response spectra of floating platform motions as well as mooring tensions. The results show that the dynamic response of semisubmersible FOWTs (floating offshore wind turbines) is overestimated when ignoring the Morison drag effect on the columns of the semisubmersible FOWT. The second-order difference wave loads can excite the resonance of motion especially for the platform-pitch motion, which could cause structural failures. The full QTF method should be used to calculate the second-order wave force to better simulate the realistic dynamic response of semisubmersible FOWTs. • Hydrodynamic analysis of semisubmersible FOWTs is carried out including second-order hydrodynamic and viscous effects. • Dynamic response of semisubmersible FOWTs is overestimated when ignoring the viscous drag effect on the platforms. • Pitch motion for the semisubmersible FOWTs can be greatly excited due to the second-order wave loads. • Contribution of second-order wave load effects on the pitch motion increase when the water depth decreases. [ABSTRACT FROM AUTHOR]

In this study, the vertical water entry of two tandem spheres is experimentally evaluated through high-speed photography. It is found that because of the impact of the second sphere on the upward jetting formed by the surface closure of the first sphere, more kinematic energy is transmitted to the flow, forming a significantly large cavity behind the second sphere. As the second sphere descends into water, the cavity is elongated with a widely open water splash. For different impact velocities of the second sphere, the water splash shapes are obviously different owing to the pressure drop across the splash curtain. After the cavity deep seal, an interesting "multi-section cavity" appears behind the second sphere due to disturbance of the bubbly wake by the first sphere. When the second sphere penetrates the bubbly flow caused by the cavity collapse of the first sphere, it moves relatively faster than a single sphere with the same initial impact velocity by reason of the bubble drag reduction effect, which is also verified by the quantitative data of accelerations for the spheres by a digital image correlation method. Image 1 • The water splash and cavity by Sphere II in the tandem water entry are remarkable. • Movements of Sphere II are significantly affected by the bubbly wake of Sphere I. • An interesting "multi-section cavity" appears behind Sphere II after deep seal. [ABSTRACT FROM AUTHOR]

Analytical solutions for wave interaction with a vertical porous barrier are presented. The analytical solutions are derived using two different methods for taking the depth-average of the pressure drop across the porous barrier. Both solutions assume that the evanescent modes in the wave field can be neglected. The results from the analytical models are compared to results from an iterative boundary element method (BEM) model. The BEM model shows that neglecting evanescent modes is a reasonable assumption for long waves, but that for short waves the velocity through the porous wall from the evanescent modes can be up to 25% of the velocity from the progressive modes at the free surface. However, the effect of neglecting the evanescent modes has only a small effect on the depth-averaged velocity through porous wall and the analytical models derived using depth-averaged assumptions are shown to give good agreement with the BEM model for the reflection coefficient, horizontal force and overturning moment on the porous barrier. The analytical models are used to investigate the effects of the drag and inertia coefficients of the porous barrier on the behaviour of the solution. It is shown that for fixed values of the drag coefficient, wave frequency and amplitude, the solutions for the reflection coefficient lie on approximately semicircular arcs on the complex plane, with the position on the arc determined by the inertial coefficient. This places bounds on the size of the phase change in the reflected and transmitted wave that are possible. The analytical models are also used to derive the asymptotic behaviour of the solution in long and short waves. The implications of the results for more general cases of wave interaction with porous structures are discussed. • New analytical solutions presented for wave interaction with vertical porous barrier. • Analytical solutions agree well with iterative boundary element method solution. • Asymptotic solutions for long and short waves presented. • Bounds derived for maximum phase change in reflected and transmitted waves. [ABSTRACT FROM AUTHOR]

Characteristics of creatures that have been evolved over long time in nature provide numerous inspirations to mankind for solving engineering problems. Inspired by biological principles, the development of bio-inspired surfaces for reducing the frictional resistance in fluid, such as the micro-groove surface of shark skin, the self-cleaning and antifouling surface of lotus leaf has attracted even more attention over the last couple of years. Based on the development of biomimetic drag reduction technology in recent decades, the present paper explains the morphology in brief and reviews the development of mainly three types of biomimetic surfaces, including non-smooth surface, superhydrophobic surface and water jet surface. The drag reduction effect and mechanism are discussed as well to understand the biomimetic drag reduction technology comprehensively. Moreover, the manufacturing methods are briefly summarized. In the end, the existing issues and future research priorities are proposed. This paper may provide a better comprehension of the current research status of bio-inspired drag reduction techniques. • Biomimetics is taken as the main line and the development of mainly three types of bio-inspired drag reduction surface is reviewed. • The bottleneck of the current research is proposed by combing the main research results and the future research directions are pointed out. • The research status and the future solutions of bio-inspired surface application toward underwater drag reduction are clearly reviewed. [ABSTRACT FROM AUTHOR]

Optimal design of an immersed elastic body interacting in a viscous fluid medium is interested in many aspects of engineering and industry. In this research, fluid structure-interaction (FSI) analysis is used to recognize the importance of the flow and solid geometrical parameters on the vortex-induced vibration of a rigid cylinder and attached beam parallel to flow direction. The optimal design of the attached beam to suppress the vortex behind a circular cylinder has been numerically investigated. The finite element method (FEM) is used to produce 2D models of a beam attached to the cylinder to modeling the vortex induction to predict the optimal geometry for the beam and velocity regions. Turek and Hron's benchmark is utilized to confirm the FSI models, where a good agreement was shown. Numerical modeling of FSI of the circular cylinder is devoted to computing the vertical forces applied to it along with finding the critical values of parameters. Vorticity in fluid and stresses in the structure field for various beam length and velocity magnitude are examined and simulated. • The effects of beam on the lift, drag on cylinder and beam tip displacement were investigated. • Optimization of beam length was performed to reduce lift and beam vertical displacement. • Velocity range to optimal suppressing the vortex induced vibration is found. [ABSTRACT FROM AUTHOR]