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Unmanned aerial vehicle (UAV) design necessitates significant effort in prototyping, testing, and design iterations. To reduce design time and improve wing performance, an automated design and optimization framework is proposed utilizing open-source software, including OpenVSP: VSPAERO & Parasite Drag Tool, XFOIL, and Python. This study presents a preliminary UAV wing design methodology, emphasizing weight estimation, drag analysis, stall prediction, and endurance optimization. The maximum takeoff weight of the UAV was calculated after estimating the empty weight using a linear regression from data from 20 existing similar UAVs. The wing and engine sizing were determined using the matching plot technique. A solver with low-fidelity models, combining the Vortex Lattice Method (VLM) and analytical expressions, was used to predict the drag coefficient and maximum lift coefficient of the designed wing. An optimization process using a genetic algorithm was applied to maximize endurance while satisfying requirements such as rate of climb, stall, and maximum speeds. The optimized wing was analyzed with computational fluid dynamics (CFD), and its aerodynamic characteristics were compared with those obtained using VLM and the suggested aerodynamic solver. According to the CFD results, the proposed aerodynamic solver estimated the drag coefficient at zero angle of attack with an error of 17.2% compared to 63.1% using the VLM classic method. The error on the maximum lift coefficient estimation was limited to 5.3%. In terms of optimization, the framework showed an increase in the endurance ratio of up to 2% compared to the Artificial Neural Network method coupled with XFLR5. The primary advantage of the suggested framework is the utilization of open-source software, giving a cost-effective and accessible solution for small and medium-sized startups to design and optimize UAVs to achieve mission objectives. [ABSTRACT FROM AUTHOR]

The combined implementation of porous medium and hybrid nanofluid with heaters and coolers can be an effective technique to improve the efficiency of several types of electric equipment. In this regard, the present study has been conducted to analyze the forced convection heat transfer of Al2O3‐CuO water‐based hybrid nanofluid in porous channel with pairs of heaters and coolers of various shapes. The circular and semi‐circular heaters and coolers with distinct orientations are considered. The Peclet number 25≤Pe≤200, Darcy number 10−6≤Da≤10−1, porosity 0.1≤ε≤0.9, and volume fraction of hybrid nanoparticles 0.02≤ϕ≤0.08 are chosen as the governing parameters. The governing equations are solved by using the finite element method based commercial software COMSOL Multiphysics. The acquired results exhibit that the heat transfer from heaters and coolers is enhanced by decreasing ε and Da for all the cases and values of Pe and ϕ. The lowest heat transfer has been obtained by circular heaters and coolers (case 1). Moreover, the semi‐circular heaters and coolers with curved facing towards channel inlet (case 2) and flat surface towards the bottom channel wall (case 4) show higher heat transfer compared to other cases. The average Nusselt number for case 4 is around 3.63% higher from case 2 at the highest values of the considered parameters. Case 4 shows the minimum drag coefficient and maximum heat transfer at the highest values of the governing parameters. [ABSTRACT FROM AUTHOR]

Satellite drag coefficients are crucial for determining the neutral mass densities that affect spacecraft operations in the thermosphere. Many studies typically utilize a constant drag coefficient of 2.2 to calculate the neutral density. However, due to the variability of space environment, uncertainties in the drag coefficient can lead to significant systematic discrepancies in neutral density measurements. Satellite drag coefficient may fluctuate in the thermosphere under various geomagnetic activities and altitudes. For the first time, we calculate the spherical satellite drag coefficient using data from the "Orbital Atmospheric Density Detection Experimental Satellite," referred to as the QX satellite. Our findings reveal that the drag coefficient can be estimated by thermospheric temperature and density, which are dependent on geomagnetic activity and altitude. At an altitude of ∼510 km, drag coefficients are adjusted to around 2.425, instead of the constant value of 2.2. Furthermore, the drag coefficient may decrease due to the significant influence of increasing geomagnetic activity, such as geomagnetic storms, on thermospheric density and temperature. These estimates of the drag coefficient can also be used to reduce discrepancies when deducing the ballistic coefficient. Consequently, using the estimated drag coefficient can accurately determine the QX‐derived neutral density, which agrees well with the density from Swarm‐B satellite. Plain Language Summary: Satellite drag coefficient is a key factor in advancing space engineering and understanding the impacts of space weather conditions on satellites in the thermosphere. It helps in calculating the neutral mass density associated with atmospheric drag force and estimating the satellite orbital lifetime. In this study, it is the first time calculating the spherical satellite drag coefficient using data from QX satellite, collected from 28 July 2022 to 30 June 2023. We investigate the response of the drag coefficient to thermospheric temperature and density under different geomagnetic activities and altitudes. These relationships allow us to estimate the spherical satellite drag coefficient, which in turn helps us improve the accuracy of determining the ballistic coefficient and the corresponding neutral mass density. Compared with the results of the drag constant of 2.2, the QX‐derived densities determined using the estimated drag coefficients, align well with the in‐situ observations from Swarm Precise Orbit Determination data. Overall, our results enhance the understanding of how the spherical satellite drag coefficient changes in response to variations in the thermosphere under different space weather conditions. We also suggest a method for estimating the drag coefficient, which has valuable applications in the calculation of neutral mass density. Key Points: We put forward a new method for determining drag coefficients using temperature and density affected by geomagnetic activity and altitudeGeomagnetic activity increases, associated with the enhancements in temperature and density, leading to a reduction of the drag coefficientUsing the estimated drag coefficients allows for accurate determination of QX‐derived density, which aligns well with Swarm‐derived density [ABSTRACT FROM AUTHOR]

The purpose of this study is to estimate the effect of roughness layer thickness on the heat transfer and drag coefficients of ellipsoidal particles. Using an OpenFOAM-based particle-resolved direct numerical simulation (PR-DNS) method, we calculated the drag coefficient and Nusselt number for an isolated axisymmetric nonspherical particle with a rough surface in a uniform flow. The PR-DNS results indicate that the drag coefficient varies linearly with the effective roughness S e f at different angles, which can be expressed as C D = k S e f − 1 + C D 0 . The changes in k are consistent with the Happel and Brenner equation. Furthermore, the influence of roughness on the heat transfer efficiency factor can be represented by E f = S e f − 6 5 . The models for the drag coefficient and Nusselt number are valid within the ranges 1.25 ≤ A r ≤ 2.5 , 1 ≤ S e f ≤ 2 , and 10 ≤ R e p ≤ 200 , thereby extending the applicability of the equations developed for smooth particles. These newly developed correlations for the drag coefficient and Nusselt number can be utilized for non-isothermal flows of particle mixtures containing materials with various rough-surfaced ellipsoids. [ABSTRACT FROM AUTHOR]

To address the issues of tedious optimization processes, insufficient fitting accuracy of surrogate models, and low optimization efficiency in drone propeller airfoil design, this paper proposes an aerodynamic optimization method for propeller airfoils based on DBO-BP (Dum Beetle Optimizer-Back-Propagation) and NSWOA (Non-Dominated Sorting Whale Optimization Algorithm). The NACA4412 airfoil is selected as the research subject, optimizing the original airfoil at three angles of attack (2°, 5° and 10°). The CST (Class Function/Shape Function Transformation) airfoil parametrization method is used to parameterize the original airfoil, and Latin hypercube sampling is employed to perturb the original airfoil within a certain range to generate a sample space. CFD (Computational Fluid Dynamics) software (2024.1) is used to perform aerodynamic analysis on the airfoil shapes within the sample space to construct a sample dataset. Subsequently, the DBO algorithm optimizes the initial weights and thresholds of the BP neural network surrogate model to establish the DBO-BP neural network surrogate model. Finally, the NSWOA algorithm is utilized for multi-objective optimization, and CFD software verifies and analyzes the optimization results. The results show that at the angles of attack of 2°, 5° and 10°, the test accuracy of the lift coefficient is increased by 45.35%, 13.4% and 49.3%, and the test accuracy of the drag coefficient is increased by 12.5%, 39.1% and 13.7%. This significantly enhances the prediction accuracy of the BP neural network surrogate model for aerodynamic analysis results, making the optimization outcomes more reliable. The lift coefficient of the airfoil is increased by 0.04342, 0.01156 and 0.03603, the drag coefficient is reduced by 0.00018, 0.00038 and 0.00027, respectively, and the lift-to-drag ratio is improved by 2.95892, 2.96548 and 2.55199, enhancing the convenience of airfoil aerodynamic optimization and improving the aerodynamic performance of the original airfoil. [ABSTRACT FROM AUTHOR]

The moving blades in a circular path have many industrial applications, in more modern turbo machines, such as jet engine compressors, the flow conditions are completely incompressible. On the other hand, the 2D study of the flow around these blades, which shows many characteristics of the flow and simplifies the matter, is usually unavoidable. In this regard, the simulation methods LES and RANS, in order to simulate the flow around the NACA0012 airfoil, different modes of fixed and rotating airfoil with different angles of attack and 3D impeller mode have been implemented. The lift coefficient, drag coefficient, torque, and mass flow of S-A, RNG, SST, RSM, and LES models are compared. The net mass rate will be different in the above methods. In RANS methods, the value of the net mass rate is negative; that is, loss of mass rate occurs, but in the LES method, the value of the net mass rate is positive. The highest net mass rate is related to the LES method, and in RANS models, the reverse flow is observed. According to the results, the effects of body forces in energy equation or thermal dissipation under circular motion are important in comparison with experimental data. A comparison of fixed and rotating airfoil lift coefficient diagrams with the LES model shows that the lift coefficient in a fixed airfoil is two times relative to a rotating airfoil. Also, the torque on the impeller, compared with different turbulent models, varies from 88381 to 172116. [ABSTRACT FROM AUTHOR]

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The increasing frequency of wind turbine failures due to extreme weather conditions necessitates the implementation of new solutions to enhance their operational reliability. This paper presents an automatic rotor drop system specifically designed for wind turbines equipped with the Onipko rotor. The system aims to protect turbines from damage caused by critical wind speeds, reducing maintenance costs and extending the equipment's lifespan. The unique design of the Onipko rotor allows it to operate at wind speeds as low as 0.1 m/s. However, its high drag coefficient and lack of aerodynamic optimization make it susceptible to mechanical stress and structural instability under strong gusts, requiring additional protective measures. The paper presents a calculation of the critical wind speed at which protective measures must be initiated. Through mathematical modeling, this study demonstrates the effectiveness of the rotor drop system in ensuring safe operation at wind speeds reaching 23.5 m/s. The optimization of the PI controller parameters provides a rapid response and stability, significantly enhancing the resilience of wind turbines to adverse weather conditions. [ABSTRACT FROM AUTHOR]

Biofouling is a serious problem in marine aquaculture facilities, exerting several negative effects on cage structures. In this study, different materials of nets were placed in the Fujian Sea area of China, and the main biological adhesion species were determined. The drag force of different materials of fouled nets was studied by a physical test in a flume tank. The drag force coefficient of a clean polyethylene terephthalate (PET) net was 0.53. The drag force coefficients of ultrahigh-molecular-weight polyethylene (UHMWPE) and polyethylene (PE) nets were 161.2% and 133.5% higher, respectively, compared with those of PET nets. Crustaceans, mollusks, and algae were the main organisms that adhered to the nets. Compared with the clean nets, the drag force of PET, UHMWPE, and PE nets increased by 1.29–5.06 times, 1.11–2.85 times, and 0.55–2.46 times, respectively. Based on backpropagation (BP) neural network training, the relationship between biological characteristics (average adhesion thickness and density) and the drag force of three kinds of net materials was determined. The drag force of the biofouled net at various time points throughout the year can be predicted based on this model, which can guide the cleaning and maintenance of nets in cage structures. [ABSTRACT FROM AUTHOR]

An adaptive Free-Form Deformation parameterization method based on a spring analogy is presented for aerodynamic shape optimization problems. The proposed method effectively incorporates the gradients of the objective and constraint functions, achieving automatic control point adjustment based on variances in design variable components. To evaluate the performance of the adaptive FFD parameterization method, two 2D airfoil optimization design problems are examined. The optimization of the RAE2822 airfoil with 12, 18 and 24 design variables demonstrates superior results for the adaptive method compared to uniform parameterization. The adaptive method requires fewer iterations and achieves lower objective function values. Additionally, the optimization design from NACA0012 to RAE2822 airfoil with 18 design variables shows that the adaptive parameterization method achieves a lower drag coefficient while satisfying the optimization objective. This validates the method's capability to finely adjust airfoil shapes and capture more optimal design points by exerting stronger control over local shapes. The proposed adaptive FFD parameterization method proves highly effective for optimizing aerodynamic shapes, offering stability and efficiency in the early stages of optimization, even with a limited number of design variables. [ABSTRACT FROM AUTHOR]

This paper reports the flow features and distributions of concentration and temperature of a micropolar type nanofluid (water-based) past a stretchable and shrinkable wedge, influenced by variable magnetic force, nonlinear sort thermal radiation and chemical reaction. Along with the consideration of multiple convection, the model of Buongiorno is stated. The Brownian motion and thermophoresis have been kept in the analysis. Suitable similarity alteration is approached to renovate the foremost equations to dimensionless ordinary differential equations (ODEs). Associated conditions became nondimensional forms according to this conversion. Then the numerical solutions of the reduced governing equations with boundary conditions are obtained by adopting the RK-4 technique with shooting criteria. The language MAPLE 17 assisted in developing this solution. Significant upshots of prime parameters on the fluid transmission, mass and heat transport properties are represented with suitable tables and graphs. In tabular form, we have reckoned the physical quantities of heat, mass transfer rates and drag friction coefficients to fulfill the engineering interest. This study acquaints that the material parameter negatively influenced nanofluid's angular velocity. The fluid's temperature improves with thermal and mass Biot numbers, but this response goes opposite for the parameter of wedge angle. Chemical reaction and wedge angle parameters amplify mass transport. This study can be beneficial in the blowing of chilled air by AC panels, the abstraction of crude oils, the nuclear power hub, the working of warships, making flaps on the wings of aeroplanes for advanced lift, submarines, the extraction of polymers and several other sectors in advanced science and industrial developments. [ABSTRACT FROM AUTHOR]

The article in the Journal of Fluid Mechanics delves into the bouncing behavior of particles settling through a density transition layer. It highlights the formation of an upward jet after the wake detaches from the particle, affecting the bouncing motion primarily through the lower layer's Reynolds number. The study sheds light on the intricate fluid dynamics and forces at play during particle settling in stratified fluids, emphasizing the importance of the lower Reynolds number in determining bouncing behavior. The research contributes valuable insights into the complex interactions between particles and irregularly shaped particles in similar scenarios, offering a deeper understanding of fluid dynamics in stratified environments. [Extracted from the article]

Self‐supporting concrete shell structures are highly efficient in distributing loads, which can result in very reduced thicknesses (ultra‐thin), giving them remarkable slenderness. Due to their geometric complexity, it is difficult to predict how they interact with wind action. The main aim of the present study is to assess the mean surface pressure coefficient distribution in a shell with a triangular plan shape and three supports for different angles of wind incidence. To determine the distribution of surface pressure coefficients and lift and drag force coefficients, an experimental study was carried out in a wind tunnel, and a numerical simulation study was performed through computational fluid dynamics. The experimental and numerical results were analyzed and compared, and making it possible to identify the most critical surface zones and wind incidences when the shell is under the wind action. [ABSTRACT FROM AUTHOR]

Urethane surfactants (REQ) were synthesized with octadecanol ethoxylate (AEO) and isocyanate methacrylate (IEM). Subsequently, reactive-carbamate-surfactant-modified nanomicrospheres (PER) were prepared via two-phase aqueous dispersion polymerization using acrylamide (AM), 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and ethylene glycol dimethacrylate (EGDMA). The microstructures and properties of the nanomicrospheres were characterized and examined via infrared spectroscopy, nano-laser particle size analysis, scanning electron microscopy, and in-house simulated exfoliation experiments. The results showed that the synthesized PER nanomicrospheres had a uniform particle size distribution, with an average size of 336 nm. The thermal decomposition temperature of the nanomicrospheres was 278 °C, and the nanomicrospheres had good thermal stability. At the same time, the nanomicrospheres maintained good swelling properties at mineralization < 10,000 mg/L and temperature < 90 °C. Under the condition of certain permeability, the blocking rate and drag coefficient gradually increased with increasing polymer microsphere concentration. Furthermore, at certain polymer microsphere concentrations, the blocking rate and drag coefficient gradually decreased with increasing core permeability. The experimental results indicate that nanomicrospheres used in the artificial core simulation drive have a better ability to drive oil recovery. Compared with AM microspheres (without REQ modification), nanomicrospheres exert a more considerable effect on recovery improvement. Compared with the water drive stage, the final recovery rate after the drive increases by 23.53%. This improvement is attributed to the unique structural design of the nanorods, which can form a thin film at the oil–water–rock interface and promote oil emulsification and stripping. In conclusion, PER nanomicrospheres can effectively control the fluid dynamics within the reservoir, reduce the loss of oil and gas resources, and improve the economic benefits of oil and gas fields, giving them a good application prospect. [ABSTRACT FROM AUTHOR]

This paper conducts a fundamental study on the hydrodynamic performance of a floating offshore wind turbine (FOWT). This includes investigating the impacts of platform motion, turbine operation, and environmental conditions on hydrodynamic wake development behind the FOWT. Using underwater particle image velocimetry in a wave basin, the fluid flow behaviour behind the floating wind substructure model is compared for different test configurations. The FOWT is examined in three configurations: I. Zero degrees of freedom (fixed), II. Six degrees of freedom (floating), and III. Floating with turbine in operation (full system). The results reveal significant hydrodynamic wake turbulence differences between fixed and floating systems, with 95% and 89% increases in turbulent kinetic energy (TKE) and turbulence intensity (TI), respectively. Turbine loading in configuration III results in additional damping and reduces eddy shedding, resulting in a 26% and 31% decrease in TKE and TI, compared to the floating configuration II. It is found that neglecting turbine operation overestimates substructure drag terms by at least 30%, while fixing the model underestimates drag terms by 54%. This study highlights a trade-off between model accuracy and dynamic complexity when estimating viscous effects on FOWT. It also provides a valuable set of high-fidelity validation data for the development of numerical tools for FOWT analysis. [ABSTRACT FROM AUTHOR]

The wind drag coefficient, C d , has a great influence on the numerical results obtained from shallow lakes. To analyze the modeling impacts of C d on wind-driven currents, a series of numerical simulations of Lake Taihu were conducted at three grid resolutions (800 m × 800 m, 400 m × 400 m, and 100 m × 100 m) using the empirical formulae of Flather (F76), Large and Pond (LP81), Large and Yeager (LY04), Andreas (A12), and Gao (G20). The G20 formula produced the optimum results of all the formulae for both the water level and velocity simulations; however, the grid resolution was found to have a significant influence on simulation in G20 cases. Thus, the G20 formula is only recommended when using a high-resolution grid to meet the accuracy requirements of analyzing wind-driven currents in the numerical modeling of Lake Taihu. A combination of the A12 formula and a coarse grid is preferred when taking computational efficiency into consideration. [ABSTRACT FROM AUTHOR]

This study investigates the effects of positive horizontal acceleration of the freestream velocity on a pitch‐oscillating VR‐12 airfoil using computational fluid dynamics. The shear stress transport k–ω model, coupled with a low‐Reynolds number correction, was employed for Re <105 during dynamic stall. The flow equations were solved in two‐dimensional, incompressible form using the finite volume method. The study examined various parameters, including positive acceleration values of the inflow and the angle of attack of the airfoil, to determine their impact on lift and drag coefficients, as well as the Cl/Cd ratio. Additionally, the maximum lift coefficient was analyzed under different inflow and airfoil motion conditions. The results indicate that aerodynamic force coefficients and the Cl/Cd ratio are influenced by both the attack angle and the acceleration of the inflow. Furthermore, inflow acceleration affects the onset of dynamic stall conditions. Generally, inflow acceleration modifies the lift coefficient of the airfoil during the upstroke, while having minimal effect on the drag coefficient, except near dynamic stall points. The findings also suggest that, for a specific airfoil, the sequence of factors with the greatest influence on lift force generation before static stall occurs is as follows: asymmetric airfoil oscillation, symmetrical airfoil oscillation, accelerated inflow, constant velocity inflow, and static airfoil. [ABSTRACT FROM AUTHOR]

In the present study, the optimum wing and its aerodynamic performance investigation of an Active aeroelastic wing (AAW) structure for an unmanned aerial vehicle that operates at low subsonic speed is presented. AAW technology is a new design for UAV wing structures which ultimately increases its efficiency. The focus is mainly to design a seamless flexible wing structure. The shape changes made from root to tip wing twist enhance the aerodynamic performance of the UAV. Because of the shape change, the weight carrying capacity of the UAV can be increased. In the fluid domain, numerical analysis has been done for both the wings. The simulation was performed with max. velocity of 60 m/s and Reynolds number of 81300 based on incompressible Navier Stokes equations computational fluid dynamics (CFD) and vortex lattice approach in XFLR5. The investigation compares the AAW by changing its angle of twist with the normal wing by deflecting its flap. The final goal is to compare the AAW and normal wing computationally by varying their speed, to arrive the best lift based and drag based result for both the wings. Drag breakdown analysis on an AAW of UAV is done by using classical approach, XFLR5 and CFD aerodynamic coefficients estimation methods. [ABSTRACT FROM AUTHOR]

Acceleration of objects in fluids widely exists in biological propulsions and contains rich unsteady fluid mechanisms. In this paper, the instantaneous drag force on accelerating normal flat plates (circular, elliptical, square, and rectangular plates) in a wide range of dimensionless acceleration ( a * = 1 6 ∼ 2) is measured, and the underlying mechanism for force generation is investigated. At first, we find that the drag force coefficient generally scales with the square root of a * when a * < 1.0 , coinciding with the scaling law given by Reijtenbagh et al. (PRL. 2023,130,174001). However, the drag force coefficient more linearly scales with a * rather than a * when a * > 1.0 , thereby indicating acceleration plays two distinct roles on the force generation depending on a * . Moreover, two scaling laws are built to quantitatively describe the two distinct roles of a * on the drag force generation. Based on fluid impulse, the drag force is largely contributed by the added mass of the accelerating plates (added mass force) and vorticity generation fed by the shear layer on the edge of the plates (vortex creation force). When a * < 1.0 , the vortex creation force scales with a * and almost contributes to the total drag force. When a * > 1.0 , the added mass force scaling with a * contributes to most of the drag force. Furthermore, the two force generation mechanisms associated with acceleration ( a * ) are independent, and a criterion based on the energy ratio is proposed to identify the transition of the two force generation mechanisms. The present results uncover the role of acceleration in force generation and explain the inconsistencies of using one quasi-steady model in describing the force on accelerating plates. [ABSTRACT FROM AUTHOR]

In the feed industry, counterflow coolers are widely used cooling devices. The flow characteristics and pressure loss should be focused on during the design and optimization stages. Due to the complex accumulation of feed particles, it is difficult to accurately analyze the flow field behavior inside the cooler through experiments. Therefore, a detailed study was conducted with a cooler from Buhler using a simulation approach. First of all, the significant influence of the porous jump boundary of flap outlet in the simulation model on the flow field and temperature distribution was revealed. Second, limitations of the Ergun equation in estimating the drag coefficients of the porous medium and the porous jump boundary were explored. Finally, a rapid calculation model for pressure loss was established based on simulation data. Within a wide range of resistance coefficient inputs, the relative error between the model calculation and the corresponding simulation results is very small. Using this model to input a series of resistance coefficients, combined with a small number of overall pressure loss measurements for comparison, the resistance coefficient of the porous medium of the new material can be determined, solving the problem in determining this coefficient through typical measurement experiments at low flow velocity and the lack of effective estimation formulas. This research can provide practical references for improving cooling equipment and enhancing cooler efficiency, and the rapid calculation model can be extended to any fluid machinery containing porous media materials that satisfy Darcy–Forchheimer's law. [ABSTRACT FROM AUTHOR]

This paper presents a novel framework for the active control of transonic airfoil flutter using synthetic jets through deep reinforcement learning (DRL). The research, conducted in a wide range of Mach numbers and flutter velocities, involves an elastically mounted airfoil with two degrees of freedom of pitching and plunging oscillations, subjected to transonic flow conditions at varying Mach numbers. Synthetic jets with zero-mass flux are strategically placed on the airfoil's upper and lower surfaces. This fluid–structure interaction (FSI) problem is treated as the learning environment and is addressed by using the arbitrary Lagrangian–Eulerian lattice Boltzmann flux solver (ALE-LBFS) coupled with a structural solver on dynamic meshes. DRL strategies with proximal policy optimization agents are introduced and trained, based on the velocities probed around the airfoil and the dynamic responses of the structure. The results demonstrate that the pitching and plunging motions of the airfoil in the limited cycle oscillation (LCO) can be effectively alleviated across an extended range of Mach numbers and critical flutter velocities beyond the initial training conditions for control onset. Furthermore, the aerodynamic performance of the airfoil is also enhanced, with an increase in lift coefficient and a reduction in drag coefficient. Even in previously unseen environments with higher flutter velocities, the present strategy is achievable satisfactory control results, including an extended flutter boundary and a reduction in the transonic dip phenomenon. This work underscores the potential of DRL in addressing complex flow control challenges and highlights its potential to expedite the application of DRL in transonic flutter control for aeronautical applications. [ABSTRACT FROM AUTHOR]

Harmonic oscillations of the walls of a turbulent plane channel flow laden with inertial particles are studied by point-particle direct numerical simulation to improve our understanding of the physical mechanism for friction drag reduction. We specify a high wall oscillation amplitude and choose particle parameters that feature a considerable drag-reduction effect. The particle effect on the drag modulation is investigated by varying the wall oscillation period ( T + ) across a wide range. We find that particles enhance drag reduction for T + ≤ 30 while attenuating it for T + > 30. Specifically, we observe drag increase near the optimal oscillation period, i.e., T + = 50 and 75. To explore the coupling mechanism of drag modulation by particles and spanwise wall oscillations, we examine the modifications of turbulence and particle statistics. Moreover, the self-sustaining cycle of near-wall turbulence is modulated by wall oscillations and/or particles. We find that the quasi-streamwise vortices are tilted and weakened by wall oscillations while significantly depopulated by particles. The modulated turbulence also affects the near-wall particle accumulation and clustering patterns, which results in different fluid–particle interactions compared with the non-actuated particle-laden channel flow. The mechanism of drag modulation is governed by the competition between reduced fluid Reynolds shear stress and particle stress. To give a quantitative view of the drag modulation, we compare the contributions of different stress components to the friction drag coefficient. Although the fluid Reynolds shear stress is reduced by particles, which corresponds to reduced turbulent kinetic energy production, the particle stress contribution remains significant, especially for the drag-increase case. Furthermore, the anisotropy invariant maps are provided, which show a striking resemblance of increased near-wall turbulence anisotropy as observed in other drag-reduced flows. This suggests that the constraint of increased turbulence anisotropy might be only a necessary but not sufficient condition for achieving drag reduction since this constraint is satisfied in our drag-increase case. [ABSTRACT FROM AUTHOR]

This study investigated the meandering of yawed wind turbine rotor wakes, focusing on the similarities across different yaw angle scenarios. Spectrum analysis of velocity fluctuations reveals that the meandering of the yawed rotor wake is symmetrical about the wake center, despite its skewness. The non-zero lateral force of the yawed rotor enhances meandering in the lateral direction compared to the vertical direction. However, the lateral profiles of meandering strength exhibit similarities across different yaw angle scenarios, indicating a consistent wake meandering mode. The wake meandering frequency increases with the yaw angle. A relationship involving wake meandering frequency, drag coefficient, and yaw angle is formulated for wind turbine rotor wakes under different yaw angles. This relationship is also applicable to thin plate wakes within a certain range of inclination angles/yaw angles. The present study reveals the similarity in wake meandering characteristics across different yaw angle scenarios, which is instrumental in improving our understanding of wake meandering and in developing analytical wake models for wind turbines. [ABSTRACT FROM AUTHOR]

Cavitation is a transient, highly complex phenomenon found in numerous applications and can have a significant impact on the characteristics as well as the performance of the hydrofoils. This study compares the evolution of transient cavitating flow over a NACA4412(base) (NACA stands for National Advisory Committee for Aeronautics) cambered hydrofoil and over the same hydrofoil modified with a pimple and a finite (circular) trailing edge. The assessment covers sheet, cloud, and supercavitation regimes at an 8° angle of attack and the Reynolds number of 1× 10 6 , with cavitation numbers ranging from 0.9 to 0.2. The study aims to comprehensively understand the role of the rectangular pimple in controlling cavitation and its impact on hydrodynamic performance across these regimes. Numerical simulations were performed using a realizable model and the Zwart–Gerber–Belamri (ZGB) cavitation model to resolve turbulence and cavitation effects. The accuracy of the present numerical predictions has been verified both quantitatively and qualitatively with available experimental results. The present analysis includes the time evolution of cavities, temporal variation in total cavity volume, time-averaged total cavity volume, distributions of vapor volume fractions along the chord length, and their hydrodynamic performance parameters. Results demonstrate that rectangular pimples have significant impacts in the different cavitation regimes. In the sheet cavitation regime (σ = 0.9), the NACA4412(pimpled) hydrofoil exhibits minimal cavity length and transient volume changes as compared to the NACA4412(base) hydrofoil. In the cloud cavitation regimes (σ = 0.5), cavity initiation occurs differently, starting from the pimpled location for the NACA4412(pimpled) hydrofoil, unlike the initiation just downstream of the nose in the case of base hydrofoil. In the supercavitation regimes (σ = 0.2), the cavity length remains comparable, but the NACA4412(pimpled) hydrofoil exhibits larger cavity volume evolution in both cloud and supercavitation regimes (σ = 0.5 and σ = 0.2) after initial fluctuations. Furthermore, hydrodynamic performance for the NACA4412(pimpled) hydrofoil shows 41%, 36%, and 17% lower lift coefficients, and 46%, 27%, and 9% lower drag coefficients in sheet, cloud, and supercavitation, respectively. [ABSTRACT FROM AUTHOR]

The present work investigates the unsteady flow around triangular prisms with vertex angles of 30 ° , 45 ° , 60 ° , and 90 ° for shedding Reynolds number between 50 and 150. The numerical simulations of flow around triangular prisms at different vertex angles and Reynolds number has been carried out using the open-source code OpenFOAM. The wake of the prisms in different cases has been examined using instantaneous and time-averaged velocity and vorticity fields. The energy dynamics in the wake are demonstrated using enstrophy. The paper explains the shedding around a prism and reports the differences in the wake due to the different vertex angles of the prisms employed in the present work. Strouhal number and force coefficients have been obtained and compared for different prisms. The coefficient of lift and drag phase plot indicates a higher spread for prisms with larger vertex angles at higher Reynolds number. The shedding frequency has a linear variation with Reynolds numbers for the prisms. The obtained results were compared with earlier works on square cylinders and 45 ° oriented square cylinder. [ABSTRACT FROM AUTHOR]

In the present study, a novel flow control technique based on jet focusing and vectoring from a synthetic jet array (SJA) for controlling the wake of a bluff body is proposed and demonstrated. A numerical investigation into the flow past a square cylinder modified by the SJA has been carried out at a free stream Reynolds Number of 100. The SJA consists of four independently controlled synthetic jet actuators operating at a peak velocity of eight times the free stream and fifteen times the natural vortex shedding frequency of the square cylinder. The SJA is operated in two different regimes; a focusing regime involving phase delay (Δ φ) with non-linear variation between the actuators and a vectoring regime with a linear phase delay without changing the geometric or operating parameters of the SJA. It has been found that jet focusing is able to reduce the coefficient of drag by as much as 43 % for Δ φ = 90 °. Focusing is also observed to reduce the fluctuations in the wake velocity with the maximum reduction in fluctuations also corresponding to Δ φ = 90 °. Jet vectoring is able to deflect the von Kármán vortex street in a singular direction along with shifting of the front stagnation point with maximum deflection for Δ φ = 60 °. Furthermore, vectoring leads to an asymmetry in the wake velocity field with the shifting of the velocity deficit region in the direction of the vectoring along with an asymmetry in the wake velocity fluctuations. This novel approach toward synthetic jet induced active flow control allows for greater manipulation of the flow field characteristics of bluff bodies than present methods with applications in areas of underwater and micro air vehicle maneuvering, automobile, and building aerodynamics among others. [ABSTRACT FROM AUTHOR]

The drag coefficient ( C D ) of a sphere freely rolling without slipping on a rough plane is presented in this study. Increasing panel roughness has been found to increase C D , although lubrication theory predicts that the larger gap imposed by the rougher panel should yield a smaller drag. We propose that this increase in drag is due to the effects of rolling resistance, which increases with panel roughness. The total drag on a sphere is decomposed into fluid drag and drag due to rolling resistance, where the fluid drag is predicted using a combined analytical–numerical approach. It is shown that rolling resistance can be modeled as a resistive torque opposing the sphere motion, generated by the offset contact normal force from the sphere center plane. This coefficient of rolling resistance ( μ r ) can be predicted using the root mean square roughness ( R q ) of the panel. Additionally, μ r is observed to increase with sphere down-slope velocity and an empirical relationship between μ r , R q , and non-dimensional velocity ( U ∗ ) is given. A comparison of the drag predicted by the proposed model with measured data indicates good agreement for all the four panels considered. Consistent with previous literature, a non-linear relationship between μ r , R q , and U ∗ is proposed. Although increasing panel roughness leads to a smaller fluid drag due to the larger gap imposed by rougher panels, the drag due to rolling resistance increases more rapidly. This leads to an increase in total drag with increase in the panel roughness. Additionally, increasing panel roughness is observed to have a significant effect on the sphere wake, leading to irregular wake shedding and increase in the Strouhal number. [ABSTRACT FROM AUTHOR]

This paper presents a machine learning-based airfoil shape optimization framework designed to increase flutter resistance and reduce drag. Using the National Advisory Committee for Aeronautics airfoil as the base design and a Hicks–Henne bump function, we employ multi-objective Bayesian optimization and harmonic balance-based flutter prediction. The optimization process yields a Pareto front revealing trade-off relationships between the flutter speed index and drag coefficient. The optimized airfoils, resembling those of evolved marine animals, outperform the base design in terms of flutter resistance and drag. These results demonstrate the framework's potential to enhance aircraft performance and safety by addressing aeroelastic factors. [ABSTRACT FROM AUTHOR]

Reynolds‐averaged Navier–Stokes (RANS) and large‐eddy simulations (LES) of the flow around wings with a wavy leading edge (WLE) are conducted in order to assess the capabilities of a passive flow control strategy for drag reduction. The intended application is indoor track cycling with controlled flow conditions. A section of a single fork rod is investigated in order to make the numerical simulations feasible. The present study reveals that net drag reduction is possible by a nonsinusoidal modification of the leading edge of the wing. However, the drag reduction effect remains limited to a few percent. While RANS and LES yield the same drag coefficient for a reference case, RANS underestimates the drag reduction effect for a longer wing and the WLE cases, but exhibits otherwise a qualitatively similar trend as the LES. With the aid of RANS, an optimal geometry is obtained defined by the wavelength‐to‐chord length ratio of w/c≈0.7$w/c\approx 0.7$ and the amplitude‐to‐chord length ratio of a/c≈0.2$a/c\approx 0.2$. Corresponding LES results give an indication of the origin of drag reduction by a hampered vortex shedding. The generation of smaller and more streamwise oriented vortical flow structures at the trailing edge and behind the WLE wing is correlated with significantly reduced lift fluctuations and drag reduction. [ABSTRACT FROM AUTHOR]

As a result of increasing population density, problems in residential areas have emerged in cities in recent years. With the development of technology, engineers have turned to the construction of taller buildings to meet the increasing demand. As a result, heat island formation becomes inevitable if there is not enough distance between buildings. In this study, it is aimed to numerically investigate the heat island formation and wind effects in buildings. The Ansys Cfx software program was used for the modeling process. Six different building configurations were analyzed to investigate heat island formation. Building heights and inter-building distances were varied for different aspect ratios. As a result of the study, more heat islands formed when the distance between buildings was smaller. As a result of the study, more heat island formation was observed in the first four cases (C1-C4) C5 and C6 were found to be the most suitable building sequences. drag coefficients (Cd) were obtained in the range of 1.35 to 1.65 for different building sequences. As a result of the cooling effects of the wind on the building, a decrease of 2 to 5 degrees in the average temperature of the building was observed. The average heat transfer coefficient is (68 W/mK) when only concrete is used in buildings. The best insulation was realized when glass wool was used. [ABSTRACT FROM AUTHOR]

Offshore floating photovoltaic (FPV) systems are subjected to significant aerodynamic forces, especially during extreme wind conditions. Accurate estimation of these forces is crucial for the proper design of mooring lines and connection systems. In this study, detailed CFD simulations were performed for various PV panel configurations, and using these CFD simulation correlations were developed to estimate lift and drag forces as a function of the number of panels. These correlations provide valuable tools for designing large-scale FPV systems with multiple PV modules. Additionally, this study investigates the potential of using breakwaters to reduce aerodynamic forces on FPV systems. Breakwaters, typically used to mitigate wave impacts, can also serve as wind barriers, significantly reducing wind forces before they reach the FPV array. Aerodynamic simulations with and without a breakwater were conducted using CFD to assess this effect. The results show a substantial reduction in lift and drag coefficients, especially for angles of attack up to 10 degrees, demonstrating the effectiveness of the breakwater in protecting the FPV system. However, beyond this threshold, the effectiveness of the breakwater of 2 m reduces. These findings highlight the importance of strategic breakwater placement and heights and their role in enhancing FPV system resilience. The insights gained from this study are critical for optimizing breakwater design and placement, ensuring the structural integrity and performance of FPV systems in varying environmental conditions. The data generated will also contribute to future design improvements for floating PV systems. [ABSTRACT FROM AUTHOR]

Newtonian (power-law index n = 1) and pseudoplastic (n = 0.8 and 0.5) fluid flow past a rectangular cylinder placed close to the plane wall (the gap ratio G/a varying from 0.25 to 1) is studied numerically using Ansys Fluent when the body shape of the cylinder changes from a bluff body (the aspect ratio AR = 0.5, 1, and 2) to an elongated body (AR = 3 and 4). The 2-D unsteady, incompressible Navier–Stokes equations with varying viscosity are solved at a constant Reynolds number, namely, Re = 100, using a second-order upwind momentum and least square cell-based pressure solver. The instantaneous and time-averaged flow fields are analyzed and compared in 45 cases concerning AR, n and G/a. It has been observed that the body shape of the cylinder and pseudoplasticity play pivotal roles in changing the flow field variables along with aerodynamic forces for a relatively intermediate and high gap ratio (G/a = 0.5 and 1, respectively). The quantity increases and reduces with increase in the AR value of the cylinder. In the presence of high pseudoplasticity in fluid flow (n = 0.5), this trend becomes more prominent. Also for n = 0.5, multiple values of Strouhal number (St) are obtained for a cylinder with low AR (AR = 0.5 and 1) at a large gap ratio (G/a = 1) owing to differences in the celerity of the vortices produced from outer and inner shear layers of the cylinder after the interactions. When the rectangular cylinder is kept very close to the plane wall (G/a = 0.25), vortex suppression occurs irrespective of the cylinder size or flow nature which negates the changes in the flow field and the aerodynamic forces. [ABSTRACT FROM AUTHOR]

The angle of attack plays a pivotal role in determining the performance of an aircraft wing, a critical component of its overall design. This angle, defined as the angle between the chord line of the wing and the relative wind direction, has a profound impact on the lift and drag forces experienced by the wing. When the angle of attack is low, the wing generates lift with minimal drag. However, at higher angles of attack, the wing encounters increased drag and may reach a stall condition. Understanding the influence of the angle of attack on an aircraft wing is crucial in both design and operation, significantly impacting the aircraft's capabilities in takeoff, climb, navigation, and landing. Therefore, a comprehensive comprehension of the relationship between the angle of attack and wing performance is imperative for ensuring safe and efficient aircraft operation. This study is dedicated to elucidating the effect of the angle of attack on aircraft performance, focusing on the variation in aerodynamic coefficients for two distinct airfoils. Employing Computational Fluid Dynamics (CFD) analysis via SolidWorks, the research examines NACA airfoil types, specifically NACA 2412 and NACA 4412, each featuring different cambers. The selected angles of attack for the investigation range from 0° to 20°, with a constant flow rate of 43 m/s. The findings reveal that the NACA 2412 airfoil exhibits a higher lift-to-drag ratio near to 20 compared to 6 in NACA 4412 airfoil. This insight provides valuable information for optimizing the aerodynamic performance of aircraft wings, contributing to the enhancement of overall efficiency and safety in aviation. [ABSTRACT FROM AUTHOR]

In order to improve the aerodynamic characteristics of the drone wings, CFD method was used to simulate and calculate the lift coefficient, drag coefficient, and lift-drag ratio under different relative inflow velocities, as well as the velocity and pressure fields under different attack angles. Modal calculations were conducted on the wing to obtain the first four modal shapes, providing a basis for analyzing flutter characteristics. An iterative calculation method of incompressible potential flow-boundary layer based on surface element method was combined with the software XFOIL to optimize the airfoil at low wind speeds. The results indicate that the airfoil is susceptible to stall at high angles of attack, with the pressure of the separation flow being nearly equivalent to that at the separation point. Subsequent to separation, there is an increase in differential pressure resistance, resulting in a marked rise in the drag coefficient. At the optimized angle of attack, the lift-drag ratio of the optimized wing increases by 12.58 %, while there is a decrease of 0.084 % in lift coefficient and an increase of 11.21 % in drag coefficient. [ABSTRACT FROM AUTHOR]

In this study, the parameterization of the drag coefficient is improved using remotely sensed wind and wave products during tropical cyclones (TCs) and implemented for wave simulation using a third-generation numeric model, namely, WAVEWATCH-III (WW3). The significant wave height and peak wavelength from SWIM(Surface Waves Investigation and Monitoring) measured wave spectra are collocated with wind speeds derived from the Haiyang-2 (HY-2) constellation, Soil Moisture Active–Passive (SMAP) radiometer, and Advanced Microwave Scanning Radiometer-2 (AMSR2) in 2019 − 2023. Cyclonic winds at a spatial resolution of 0.25° and intervals of 6 h are obtained by fusion remotely sensed winds from the HY-2 scatterometers and radiometers from SMAP and AMSR2. The parameterization of the drag coefficient during TCs is improved using a matching dataset for 100 TCs consisting of fusion winds and SWIM-measured wave spectra. The significant wave height (SWH) is simulated by WW3 using the optimized parameterization in the several input/dissipation source terms, i.e., ST2, ST3, ST4 and ST6. It is found that the accuracy of simulated SWH using optimized parameterization in switch ST2, ST3 and ST4 is worse than that using optimized parameterization in switch ST6. Moreover, taking two parameterizations in switch ST6 of WW3, validation against the measurements from altimeters and the SWH calculated by SWIM-measured wave spectrum during other 20 TCs yields a root mean square error (RMSE) of 0.60 m, and a correlation coefficient (COR) of 0.85 by optimized parameterization, which are better than the 0.68 m RMSE, and 0.80 COR obtained using existing parameterization. Furthermore, the variation in the bias (altimeter minus WW3) shows that the larger bias (> 2 m) at wind speed > 20 m/s or SWH > 6 m is significantly improved using the optimized parameterization formula and fusion remotely sensed winds. [ABSTRACT FROM AUTHOR]

This research seeks to explore the behavior of Ree–Eyring nanofluid over an extended surface influenced by an inclined magnetic field within a permeable medium. The local thermal non‐equilibrium between the particle, liquid, and solid phases is represented by a three‐temperature model. The problem is addressed numerically using bvp4c code in MATLAB software. The findings are displayed in the format of tables and graphs. The study shows that higher values of the interface heat transfer parameter led to a decrease and increase in the fluid phase and solid phase Nusselt number, respectively. The velocity and concentration distributions decrease with increasing porosity coefficient. Nevertheless, the fluid phase temperature distribution shows an opposing trend. Furthermore, increasing the non‐Newtonian fluid parameter leads to a raise in the surface drag coefficient. [ABSTRACT FROM AUTHOR]

Airfoil leading-edge fluid-blowing control is computationally studied to improve aerodynamic efficiency. The fluid injection momentum coefficient Cμ (the ratio of injection to incoming square velocities times the slot's width to airfoil's half chord length) varies from 0.5% to 5.4%. Both static and dynamic conditions are investigated for a NACA0018 airfoil at low speed and Reynolds number of 250 k based on the airfoil's chord length. The airfoil is dynamically pitched at a reduced frequency (the pitching tangential speed to the freestream speed ratio), varying between 0.0078 and 0.2. Reynolds-averaged Navier–Stokes (RANS) and unsteady RANS (URANS) is used in the simulations as based on the Transition SST and Spalart–Allmaras models, generally achieving good agreement with experimental results in lift and drag coefficients and in the pressure coefficient distributions along the airfoil. It is found that oscillating the airfoil can delay stall, as expected, in dynamic stall (DS). Leading-edge blowing control can also significantly delay stall both in static and dynamic conditions as long as sufficient momentum is applied to the control. On the other hand, for a small Cμ such as 0.5%, the leading-edge control worsens the performance and hastens the appearance of stall in both static and dynamic conditions. The airfoil's oscillation reduces the differences between pitch-up and pitch-down aerodynamic performances. Detailed analysis of vorticity, pressure, velocity, and streamline contours is given to provide plausible explanations and insight to the flow. [ABSTRACT FROM AUTHOR]

The movement of submerged flexible vegetation leads to an increase in resistance to the stream flow. In this study, a formula that can directly calculate the drag force on a highly flexible submerged vegetation, called Ceratophyllum, by using the vegetation swaying characteristics and the flow field information in a steady‐uniform open‐channel flow is derived. The drag force on submerged flexible vegetation is characterized by the time‐averaged flow velocity, turbulence intensity, and the additional force arising from the vegetation swaying. Based on the results of the numerical models in the previous studies (Wang et al., 2022a, 2022b, https://doi.org/10.1017/jfm.2022.598, https://doi.org/10.1017/jfm.2022.899), the drag coefficient is determined. It is revealed that the drag coefficient is influenced by a combination of factors, including the flow conditions, and the distribution and movement characteristics of vegetation. The drag coefficient decreases with an increase in velocity and is approximately linearly related to the cubic power of the bulk flow velocity. In the case of an inter‐plant spacing of 0.5 times the initial plant height, the drag coefficient ranges from 10.72 to 2.11, as the Reynolds number varies from 20,000 to 50,000. Besides, the vegetation distribution density and the relative submergence influence the drag coefficient. In this context, the drag coefficient decreases linearly with an increase in the inter‐plant spacing. For the Reynolds number equaling 50,000, the drag coefficient ranges from 2.11 to 2.02, when the inter‐plant spacing varies from 0.5 to 2 times the plant height, and from 2.47 to 1.79, when the flow depth varies from 1.5 to 3 times the plant height. Key Points: A formula for the drag force on submerged flexible vegetation in an open‐channel flow is proposedThe drag coefficient is inversely proportional to and linearly varying with the cubic power of the bulk flow velocityThe drag coefficient decreases with an increase in the inter‐plant spacing and the submergence depth [ABSTRACT FROM AUTHOR]

The objective was to construct and test an economical, accurate, and open‐source Lagrangian drifter design suitable for lakes <200 km2. Lagrangian drifters are used to trace water currents in marine and freshwater settings and comprise of a low‐friction surface float containing instrumentation for location and environmental measurement, tethered to a high‐friction drogue at the depth of interest. Oceanic drifters are robust but expensive, and this design tailored to inland lake waterbodies fills a durability and cost gap for lake environments. Water‐following characteristics were tested using theoretical drag coefficient calculations, practical drag measurements, and comparison of wind and drifter vectors while deployed on two deep inland lakes (maximum area 175 km2) in the Finger Lakes region of New York, USA. The ratio of drag between float and drogue met or exceeded the minimum value of 40 recommended in the literature, and the vectors of wind and drifter during deployment were independent of one another, meaning the device accurately traced the movement of water currents at depth without undue influence of wind and waves. Each device cost USD $265 in 2021 and was built from materials readily available at hardware and sporting goods stores, allowing their use by research institutions and communities with smaller budgets. This design reliably measured lake currents at sampling depths that ranged from 2 to 30 m. We anticipate that this design will have application to a wide range of hydrodynamic and ecological research where empirical insights to physical processes like lake currents are sought by scientists and managers. Plain Language Summary: Lake currents are complex, yet often lack data for scientists to analyze them. We built and tested a low‐cost, accurate device that can follow water currents in lakes, made from easily available parts. Unlike similar devices for oceans which can be bulky and costly, this device is well‐suited for lakes. It works by floating on the water's surface and has a part that hangs below it in the water to track water movement at different depths. Tests in two New York lakes showed it does a great job of tracking water without being thrown off by wind or waves. Making each device costs USD $265, using parts found in regular stores, so it's affordable for smaller research teams or local communities. It's good for studying water movements from near the surface down to 30 m or even deeper, helping scientists and local managers learn more about how water moves in lakes. Key Points: An economical, open‐source Lagrangian drifter designed to collect current data on lakes <200 km2 was evaluated against existing designsThe new design was tested in deep inland lakes in the Finger Lakes region of New York, USA and is effective at tracking deep currentsThe ease and low‐cost of fabrication and launch/recovery should facilitate use of this design by less‐advantaged communities & researchers [ABSTRACT FROM AUTHOR]

The determination of critical shear stresses is fundamental to bedload sediment transport prediction in gravel‐bed rivers. Due to the heterogeneous shape and arrangement of the individual clasts in a riverbed, critical shear stresses typically show a large spatial variability, which is not adequately captured by the reach‐averaged description followed in common studies. In this regard, there is a general paucity of field data on this spatial variability of the critical shear stress, largely due to the lack of a standardized measurement method. In an attempt to fill this gap, we propose a field‐based workflow to estimate the frequency distribution of dimensionless critical shear stress (also named critical Shields number), which is based on the measurement of a series of variables related to the position, orientation and resistance to motion of individual clasts in a gravel‐bed river, combined with a probabilistic approximation to drag and lift coefficients. Following this workflow, the patch‐scale variability of particle incipient‐motion conditions was determined in a gravel bar of the Upper Cinca River, Spain. The results are consistent with what is known about sediment entrainment in gravel‐bed rivers. We consider this method to have great potential to advance our understanding of particle initiation of motion in gravel‐bed rivers as it provides valuable systematic field information. Plain Language Summary: Predicting the flow conditions required to entrain sediment in gravel‐bed rivers is essential for fluvial ecology, flood risk assessment, river restoration and sustainable river management, among many other issues. In this regard, sediment entrainment conditions exhibit large spatial variability due to the heterogeneous shape, weight and arrangement of individual grains in a gravel bed. Our ability to model and quantify bedload transport is limited by this variability and exacerbated by the lack of standardized protocols for characterizing it in the field. In this context, this work builds on existing theory to propose a field‐based workflow for characterizing the variability in the forces required to move gravel in a river bed. This field‐based workflow opens the door to the collection of field data that will allow progress in our understanding of bedload transport in natural rivers. Key Points: Critical shear stress is a key parameter to understand bedload transport. It is often described by section‐ or reach‐averaged values in field studiesWe propose a field‐based workflow to characterize the spatial variability in critical Shields stress in gravel‐bed riversThis workflow allowed us to approximate the critical Shields frequency distribution in two 1 m2 patches of the Cinca River (NE Iberian Peninsula) [ABSTRACT FROM AUTHOR]

In this paper, a novel surrogate model for shape-parametrized vehicle drag force prediction is proposed. It is assumed that only a limited dataset of high-fidelity CFD results is available, typically less than ten high-fidelity CFD solutions for different shape samples. The idea is to take advantage not only of the drag coefficients but also physical fields such as velocity, pressure, and kinetic energy evaluated on a cutting plane in the wake of the vehicle and perpendicular to the road. This additional "augmented" information provides a more accurate and robust prediction of the drag force compared to a standard surface response methodology. As a first step, an original reparametrization of the shape based on combination coefficients of shape principal components is proposed, leading to a low-dimensional representation of the shape space. The second step consists in determining principal components of the x-direction momentum flux through a cutting plane behind the car. The final step is to find the mapping between the reduced shape description and the momentum flux formula to achieve an accurate drag estimation. The resulting surrogate model is a space-parameter separated representation with shape principal component coefficients and spatial modes dedicated to drag-force evaluation. The algorithm can deal with shapes of variable mesh by using an optimal transport procedure that interpolates the fields on a shared reference mesh. The Machine Learning algorithm is challenged on a car concept with a three-dimensional shape design space. With only two well-chosen samples, the numerical algorithm is able to return a drag surrogate model with reasonable uniform error over the validation dataset. An incremental learning approach involving additional high-fidelity computations is also proposed. The leading algorithm is shown to improve the model accuracy. The study also shows the sensitivity of the results with respect to the initial experimental design. As feedback, we discuss and suggest what appear to be the correct choices of experimental designs for the best results. [ABSTRACT FROM AUTHOR]

A non-watertight Ahmed body is adopted to examine the influence of geometrical defects on the aerodynamic drag. The Ahmed body, a standard automotive test model, is widely used in wind tunnel experiments to study vehicle aerodynamics. However, in real-world applications, vehicles often exhibit deviations from the ideal geometry due to manufacturing tolerances, wear, and other factors. The aim of this study is to quantify the impact of such geometrical defects on the aerodynamic drag. The research employs the lattice Boltzmann method (LBM) software to analyze the flow field around the Ahmed body with and without the introduction of geometrical defects. These defects are modeled as small holes in the shape of the body. The simulations are performed under the varying location and size of the holes to explore the aerodynamics phenomena. The results indicate that the location and size of geometrical defects can significantly alter the aerodynamic drag of the Ahmed body. The holes located at the rear of the Ahmed body rarely effect the aerodynamic drag. The holes at the top or bottom are found to have the most pronounced effect. The study also reveals that the influence of defects varies with their size, with large areas leading to more substantial changes in the aerodynamic drag. The results demonstrate that non-watertight geometry with small defects can be used to produce a reasonable drag coefficient compared to the results of watertight geometry. [ABSTRACT FROM AUTHOR]

This study explores the flow dynamics and sediment entrainment mechanisms around a circular array of emerged cylinders in a shallow channel. Eddy resolving simulations reveal the formation of vortex street-type shallow wakes, influenced by cylinder diameter and submergence ratio. The research emphasizes the impact of flow conditions on wake instabilities, coherent structures, and sediment transport, shedding light on the complex interactions in shallow water environments. [Extracted from the article]

This study employs a two-dimensional and incompressible flow of Herschel–Bulkley visco-plastic materials in order to investigate the hydrodynamic forces that are acting on a barrier that is located close to the inlet of a channel. As the benchmark configuration, the flow domain that has been selected is a channel that still contains the impediment. The two important parameters of the Herschel–Bulkley Model (HBM) are the yield stress τ y and power law index n. Obtaining special situations within the HBM, such as Newtonian, power-law, and Bingham fluids, can be accomplished by assigning certain values to these parameters at the appropriate times. Utilizing a numerical strategy grounded in the Finite Element Method (FEM), we tackle the nonlinearity of the governing equations as well as the viscosity models. As a result of this nonlinearity, FEM becomes an essential tool. The generation of a refined hybrid mesh is done in order to guarantee accuracy in the computations. The stable finite element pair ( ℙ 2 ∕ ℙ 1 ) has been selected for discretization purposes. The discretized nonlinear system is linearized with Newton's method and subsequently, a direct linear solver PARDISO has been employed in the inner iterations. The pressure, velocity, and viscosity profiles are plotted for various values of n and Bingham number (Bn). In addition, the velocity behavior is observed along the y-direction in a channel through line graphs. Code validation is done as a special case Bn = 0 and a good agreement is found with the results available in the literature. Finally, a correlation analysis has been performed for the drag coefficient C d and lift coefficient C l . [ABSTRACT FROM AUTHOR]

Functionally graded material (FGM) combines two or more constituents in which one gradually varies toward the other along the structure, improving properties. The contribution of this study is to present the characterisation results of an FGM composed of two groups of cutting tool materials, Al2O3-ZrO2 (ceramic-based alumina) and WC–Co (cemented carbide), designed by a thermomechanical model for predicting the residual stress to avoid thermal cracking. The main result comes from the sample manufactured by pulsed electric current sintering (PECS) at 1300 °C, 50 MPa, and a dwell time of 7 min. Microstructure, density, and mechanical and tribological properties were characterised. Two additional experiments were included to support the observations of the main result. Furthermore, homogeneous commercial materials, cemented carbide and aluminium oxide ceramic, were used to compare with the FGM results. Microscopic analysis revealed an eight-layer structure with no cracks. The relative density result indicated a porosity of 5.27%, mainly concentrated in the first layers. Thus, the Vickers hardness values showed that the first layer, rich in alumina-based ceramic (817 HV), was softer than the last layer, rich in cemented carbide (1272 HV). Conversely, the fracture toughness values were higher in the first layer (8 MPa.m1/2) and decreased towards the last layer (5 MPa.m1/2). Variations in properties along the structure were also observed in the scratch test. Scratch hardness increased towards the final layer. The FGM sample exhibited microcutting as an abrasive wear mechanism for the first three layers and a mixture of microcutting and microploughing for the last three layers. Finally, the drag coefficient results indicated greater friction in the first layers because of their toughest characteristics. Although the sample did not achieve full densification, changes in properties were observed; these properties may be improved by adjusting the sintering parameters. [ABSTRACT FROM AUTHOR]

To achieve an aerodynamically efficient design of fuselage for bioinspired unmanned aerial vehicles (UAV), the development of a systematic method is of paramount need. This work presents a multiobjective optimization study for shape parameterization of a 2D fuselage of foldable flapping wing UAV in low Reynolds number (2e05) flight conditions. The novelty of the research work lies in integrating S1223 airfoil characteristics for predicting the efficient shape design of the fuselage. This paper offers four sections; (i) the validation of the ANSYS Fluent model with experiment, (ii) the exploration of design variables using Design of Experiment (Sparse Grid Initialization), (iii) the implementation of response surface method (Genetic Aggregation) to know about dimensional sensitivity among independent and dependent variables and (iv) the application of multiobjective optimization method i.e. NSGA II to optimize the drag and lift coefficient. To identify the superiority, a comparative study between the original and optimized fuselage is presented considering many parameters like pressure contours, velocity contours, pressure coefficients, and streamlines representations. It is evident from various results that the 2D optimum shape significantly minimizes the drag coefficient and increases the lift coefficient. This work also makes room for 3D shape optimization, which helps in prototype fabrication for real-time flight conditions. [ABSTRACT FROM AUTHOR]

Coastal lagoons are important wetland sites for migratory species and the local flora and fauna population. The Stagnone Lagoon is a coastal lagoon located on the west edge of Sicily between the towns of Marsala and Trapani. The area is characterized by salt-harvesting plants and several archaeological sites and is affected by microtidal excursion. Two mouths allow exchange with the open sea: one smaller and shallower in the north and one larger and deeper in the south. This study aims to understand the lagoon's hydrodynamics, in terms of circulation and involved forces. The circulation process appears to be dominated mainly by tide excursions and wind forces. Wind velocity, water levels, and water velocity were recorded during different field campaigns in order to obtain a benchmark value. The hydrodynamic circulation has been studied with a 2DH (two-dimensional in the horizontal plane) unstructured mesh model, calibrated with data collected during the 2006 field campaign and validated with the data of the 2007 campaign. Rapid changes in averaged velocity have been found both in Vx and Vy components, showing the strong dependence on seiches. This study tries to identify the main factor that domains the evolution of the water circulation. Sensitivity analyses were conducted to estimate the correct energy transfer between the forcing factors and dissipating ones. A Gauckler–Strickler roughness coefficient between 20 and 25 m1/3/s is found to be the most representative in the lagoon. To enhance the knowledge of this peculiar lagoon, the MIKE 21 model has been used, reproducing all the external factors involved in the circulation process. Nash–Sutcliffe coefficient of efficiency (NSE) values up to 0.92 and 0.79 are reached with a Gauckler–Strickler coefficient equal to 20 m1/3/s related to water depth and the Vy velocity component. The Vx velocity component NSE has never been satisfying, showing the limits of the 2D approach in reproducing the currents induced by local morphological peculiarities. Comparing the NSE value of water depth, there is a loss of up to 70% in model predictivity capability between the southern and the northern lagoon areas. This study aims to support the local decision-makers to improve the management of the lagoon itself. [ABSTRACT FROM AUTHOR]

Pulse opposing plasma synthetic jets (OPSJs) have potential for reducing the drag coefficient of hypersonic airfoils. This paper numerically investigated the effects and flow control mechanisms of a continuous nanosecond pulse OPSJ on the aerodynamic performance of a hypersonic airfoil. The results indicate that a continuous-pulse OPSJ reduces the airfoil drag coefficient by generating diffracted waves to shift the leading-edge shock wave. However, the flow control effectiveness of a pulse OPSJ decreases with the increase of the pulse sequence. The time-averaged lift-to-drag ratio of the airfoil decreased rapidly from 1.11 for the first pulse sequence to 0.94 for the third pulse sequence, followed by a gradual decrease. The thermal accumulation within the cavity triggers a blocking effect, impeding the heat exchange between the cavity and external fluid. This is the primary factor leading to the decreasing flow control effectiveness of continuous-pulse OPSJs. Furthermore, the results suggest that enhancing the mixing effect within the cavity can improve heat transfer between the cavity and the external flow, suppress thermal accumulation, and improve the flow control capability of continuous-pulse OPSJs in hypersonic flows. [ABSTRACT FROM AUTHOR]

This study investigates the potential of an optimized combination of active nose droop and active Gurney flap (CADAG) in a new flow control strategy to manage dynamic stall over a pitching blade section under variable Mach number flow. The optimization method employs the genetic algorithm coupled with a computational fluid dynamic (CFD) solver and artificial neural network. The base blade section belongs to a section positioned at r/R=0.865 of the rotor blade of the UH-60A helicopter in forward flight condition. A high relative angle of attack on the retreating side makes the flow susceptible to dynamic stall. A nose droop is employed to control the dynamic stall of the blade section, and a Gurney flap is used to maintain the balance of the generated lift of the blade during 360° of rotation. A comprehensive investigation is performed to determine the most significant parameters affecting the performance of the present combined active flow control. The ratio of the total generated lift to the drag is chosen as the objective function of the optimization. Results show that this ratio and the total generated lift in one rotation cycle increase by 193% and 13%, respectively, at the optimum condition of the present combined active flow control, while the ratio of the generated lift over the advancing side to the retreating side is equal to that of the base blade section. In addition, the dynamic stall hysteresis loop reduces significantly, and the maximum value of the drag coefficient and the negative aerodynamic damping decrease up to 87% and 83% compared to the base blade section, respectively. In general, the proposed innovative combined active flow control is an adjustable method to alleviate dynamic stall and improve the aerodynamic performance of rotary wings in different operation conditions. Practical Applications: The rotary blades are extensively used in rotorcraft, turbo engines, and wind turbines. Despite their massive use, they suffer from some essential issues, of which the most important one is the so-called dynamic stall. Dynamic stall is a complex phenomenon that limits the performance of the rotary blades. Due to the physics governing a rotary wing, such as a helicopter rotor blade, dynamic stall and flow separation are very common. Understanding dynamic stall physics and providing solutions to prevent it is still one of the main challenges of aerodynamic scientists. The present study introduces a novel adjustable method for practically alleviating the dynamic stall on helicopter blade section in forward flight conditions to improve its aerodynamic performance in different operational conditions. A comprehensive investigation is carried out to determine the key parameters affecting the proposed method. These findings can serve as a valuable tool for other researchers to develop various active flow control strategies. This article applies an optimization process using artificial neural networks, genetic algorithms, and CFD tools, which forms a comprehensive framework that can be easily extended to other applications. [ABSTRACT FROM AUTHOR]