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In response to the optimization problem of the motion law of the pectoral fins of biomimetic robotic fish, this paper uses computational fluid dynamics (CFD) methods to achieve the CFD optimization of two types of rigid biomimetic pectoral fins with three degrees of freedom (DOF): elliptical and 8-shaped swing curves. By analyzing the hydrodynamic characteristic curve of the pectoral fins, pressure distribution upstream and downstream of the fin surface, velocity vector of the surrounding flow field during pectoral fin swing, and three-dimensional vortex structure, it is found that when the pectoral fins are pushed forward in an elliptical swing curve, the resistance peak value decreases after two optimizations. However, at the same time, a certain amount of thrust is lost. When the pectoral fin advances in an 8-shaped swinging curve, after two optimizations, the pectoral fins resistance decreases while maintaining an enormous thrust, comprehensively improving the propulsive effect of the pectoral fin on the robotic fish. By substituting the optimized 8-shaped pattern into the robotic fish model, it is found that the flapping angle of 35 ° has better drag reduction performance, and the average swimming speed of the robotic fish is the fastest at this angle. • After secondary optimization, the elliptical pectoral fin swing curve with better drag reduction performance and smoother curve was obtained • Optimize the 8-shaped swing curve to comprehensively enhance the propulsion effect of pectoral fins on robotic fish • Substituting the optimized 8-shaped swing pattern into the robotic fish model to achieve self-propelled motion, it was found that p h i F L A = 35 d e g r e e had the best performance [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]

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]

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]

Abstract: Flow around rectangular cylinders with different aspect ratios has been investigated using the two-dimensional (2D) Unsteady Reynolds-Averaged Navier–Stokes (URANS) equations with the k– Shear Stress Transport (SST) turbulence model. The aspect ratios (R=B/H, where B and H are the breadth and height of the cross-sections in the streamwise and cross-stream directions, respectively) of the cross-sections vary from 1 to 0.05. The Reynolds number () based on the free stream velocity and H is 21,400. The objective of the present study is to evaluate the validity of 2D URANS simulations with k– SST model for the flow around rectangular cylinders. The drag forces acting on the cylinders with high aspect ratios (i.e. R=1, 0.8 and 0.6) are well predicted by the present simulations; however, the drag forces are overpredicted for low aspect ratios (i.e. R=0.4, 0.2, 0.1 and 0.05). The vortex shedding frequencies are not sensitive to the aspect ratio, and the values calculated in the present simulations are in good agreement with the published results. The effects of aspect ratios on the vortex formation have also been discussed. [Copyright &y& Elsevier]

Abstract: The phenomenon of drag reduction by the injection of micro-bubbles into turbulent boundary layer has been investigated using an Eulerian–Eulerian two-fluid model. Multiple-size group (MUSIG) based on population balance models, which resolve a wide range of bubble sizes taking into account the bubble break-up and coalescence have been used for this purpose. The simulated results are compared against the experimental findings of Madavan et al. [1984. Reduction of turbulent skin friction by micro-bubbles. Physics of Fluids 27, 356–363] and also other numerical studies explaining the sophisticated phenomena of drag reduction. For the two Reynolds number cases considered, the buoyancy with the plate on the bottom configuration is investigated, as from the experiments it is seen that buoyancy seem to play a role in the drag reduction. Numerical model employed in the investigation comprises of a micro-bubble laden flow wherein two independent sets of Reynolds averaged Navier–Stokes (RANS) transport equations were used to describe both the phases of the flow. The shear stress transport (SST) turbulence model is used as the turbulent closure for the primary phase and a zero equation turbulence model is used for the micro-bubbles. Change in the mean streamwise velocity profiles, void fraction, turbulence modification and other results are presented and discussed with corresponding change in the gas injection rates. The complex mechanism of drag reduction are scrutinised and explained in context to our numerical findings. Special attentions have been also devoted to divulge the effect of bubble coalescence and break-up caused by random collision and turbulent impact. Numerical results showed good agreement for the skin-friction coefficients against experimental data throughout various air injection rates. The MUSIG model was found to be one of the best candidates to resolve the bubble dynamics in micro-bubble-induced drag reduction problems. [Copyright &y& Elsevier]

Abstract: In this study, two- and three-dimensional numerical simulations were performed to investigate the effect of the flow structure in the wake of a square cylinder placed near a plane wall by applying a fully implicit finite-difference method to the Navier–Stokes equations. The gap ratio between the cylinder and the wall, G/D, was varied from 0.2 to 4 for the Reynolds numbers of 175, 185 and 250. The role of the 3D structure on the lift and drag coefficients and Strouhal number was investigated. The results were compared with those of the 2D numerical simulations. The deviations of the 3D flow structure of the cylinder–wall pair from that of a single cylinder were also reported. At Re=250, B type secondary vortices were determined in the wake region. At Re=175 and 185, transition from A type vortex to fully periodic B type vortices was observed when the cylinder was brought closer to the wall. [Copyright &y& Elsevier]