Aerodynamic Forces Research Articles

A Hybrid RANS/LES Model for Predicting the Aerodynamics of Small City Vehicles

Electric city vehicles are vital for reducing pollution in urban areas due to their zero emissions and high energy efficiency, significantly improving air quality and reducing the carbon footprint. This study investigates the aerodynamic behavior of simplified city vehicle models using computational fluid dynamics (CFD) simulations based on Reynolds-Averaged Navier-Stokes (RANS) and Detached Eddy Simulation (DES) turbulence model. The models are tested at speeds of 10 m/s, 15 m/s, and 20 m/s, with a grid independence study to ensure reliable results. ANSYS Fluent is used for the simulations, comparing the results from RANS and hybrid RANS/LES or DDES in terms of aerodynamic forces and flow patterns around the vehicle. Results show that the drag coefficient (Cd) decreases with increasing speed for both RANS and DDES models. At 10 m/s, the drag coefficients are 0.541 for RANS and 0.524 for DDES, a 3.14% reduction. At 15 m/s, the drag coefficients are 0.539 for RANS and 0.518 for DDES, a 3.89% reduction. At 20 m/s, the drag coefficients are 0.538 for RANS and 0.514 for DDES, a 4.46% reduction. Flow visualizations show that DDES simulations capture more detailed and complex flow structures, particularly in the wake region, compared to the smoother RANS patterns. These findings are essential guidelines for optimizing vehicle design to enhance aerodynamic performance and contribute to the development of more efficient, environmentally friendly urban transportation solutions.

Experimental and computational investigations of flexible membrane nano rotors in hover

With reduced size, the flight Reynolds number of the nano rotor decreases, leading to a sharp drop in the aerodynamic efficiency of the nano rotor. Therefore, improving the aerodynamic performance of the nano rotor at low Reynolds numbers through flow control methods becomes imperative. In this study, from the perspective of bionics, flexible materials are employed in nano rotor design. Seven different layouts of flexible membrane rotor blades are designed and fabricated. The influence of leading and trailing edge flexibility, as well as membrane occupancy ratio on rotor blade propulsion characteristics, is explored through propulsion performance tests in hover.Results show that among several layouts proposed in this study, the layout with both reinforced leading and trailing edges of the flexible membrane nano rotor blade exhibits excellent propulsive performance. However, the propulsion performance of the flexible membrane rotors doesn't vary linearly with the ratio of membrane area to the whole rotor area. The appropriate ratio or flexibility can increase the propulsive performance of the rotor, especially at medium and high collective angles. At 7000 RPM and a 20° collective angle, the Figure of Merit of the flexible membrane nano rotor increases up to 4.2% when comparing with the nano rotor without membrane. This improvement becomes more significant with higher collective angles. The structural natural vibration characteristics of each flexible membrane with different layouts are analyzed through modal tests, ensuring the accuracy of the finite element model for flexible membrane rotors. Numerical analysis of the fluid-structure coupling of a flexible membrane rotor with different layouts indicates that rotor structural vibrations is highly consistent with fluctuation of aerodynamic parameters. The deformation of the flexible membrane under aerodynamic forces enhances local blade camber, but reduces angles of attack. This, subsequently, minimizes the size of laminar separation bubbles and the intensity of the blade tip vortices at high collective angles. Consequently, rotor power coefficients decrease, and overall aerodynamic performance improves.

Modelling aerodynamic forces and torques of spheroid particles in compressible flows

In the present study, we conduct numerical simulations of compressible flows around spheroid particles, for the purpose of refining empirical formulas for drag force, lift force, and pitching torque acting on them. Through an analysis of approximately a thousand numerical simulation cases spanning a wide range of Mach numbers, Reynolds numbers and particle aspect ratios, we first identify the crucial parameters that are strongly correlated with the forces and torques via Spearman correlation analysis, based on which the empirical formulas for the drag force, lift force and pitching torque coefficients are refined. The novel formulas developed for compressible flows exhibit consistency with their incompressible counterparts at low Mach number limits and, moreover, yield accurate predictions with average relative errors of less than 5%. This underscores their robustness and reliability in predicting aerodynamic loads on spheroidal particles under various flow conditions.

Investigation of a Tube-Launched Unmanned Aerial Vehicle with a Variable-Sweep Wing

Foldable wings are designed for tube-launched unmanned aerial vehicles (UAVs), aiming to improve portability and meet launch platform requirements. However, conventional tube-launched UAVs cannot operate across the wide speed ranges required for the performance of multiple missions, due to the fixed configuration of their wings after launch. This study therefore proposes a tube-launched UAV which can change wing-sweep angle to expand the flight speed range and enhance the UAV’s agility. A computational aerodynamics method is employed to assess the transient aerodynamic performance of the UAV during the sweep morphing process. The simulation results indicate that the transient aerodynamic forces generate a dynamic hysteresis loop around the quasi-steady data. The lift and drag coefficients exhibit maximum relative deviations of 18.5% and 12.7% from the quasi-steady data for the sweep morphing period of 0.5 s. The hysteresis effect of the flow structure, rather than the additional velocity resulting from wing-sweep morphing, is the major contributor to the aerodynamic hysteresis loop. Compared to the conventional tube-launched UAVs, the proposed tube-launched UAV with a variable-sweep wing shows a wider flight speed range, from 22.59 to 90.12 m/s, and achieves an 82.84% increase in loitering speed. To verify the effectiveness of the wing-sweeping concept, a prototype was developed, and a flight test was carried out. The test data obtained from flight control system agree well with the simulation data, which demonstrates the feasibility and effectiveness of the variable-sweep wing in widening the speed range for tube-launched UAVs. This work can provide a reference for the design of tube-launched UAVs for wide speed range flight.

High-dimensional nonlinear flutter suppression of variable thickness porous sandwich conical shells based on nonlinear energy sink

Nonlinear energy sink (NES) is widely applied in engineering field due to the advantages of light weight, high robustness, unidirectional energy transfer, rapid and broadband vibration isolation. In this paper, nonlinear energy sink is utilized to suppress vibration of the variable thickness porous sandwich conical shells for the first time, and the high-dimensional nonlinear flutter suppression characteristics of the system of simply supported variable stiffness truncated porous sandwich conical shell coupled NES under aerodynamic force and thermal stress are investigated. By applying the first-order shear deformation theory (FSDT), Hamilton's principle and Galerkin technique, the high-dimensional nonlinear ordinary differential flutter suppression equations of the system appended with NES are established. The accuracy of the theoretical approach is ensured by the comparison of frequency results, while the NES dissipated kinetic energy ratio and the comparison of NES performance with other suppression systems are presented to prove the effectiveness of NES on nonlinear flutter suppression. The time history diagrams and limit cycle oscillation (LCO) amplitude curves, which reflect the high-dimensional nonlinear flutter suppression effect of NES, are obtained by employing the Runge-Kutta method. The effects of aerodynamic pressure, the parameters and positions of single NES, and the positions of parallel NES and series NES on the high-dimensional nonlinear flutter suppression characteristics of the system attached with NES are discussed in depth. Finally, the optimal high-dimensional nonlinear flutter suppression scheme is arrived.

On Robust Stabilization of Motion of a Quadrotor with Slung Load

One of the important tasks that can be performed by quadrotors is the transportation of various payloads. If the load is suspended from the quadrotor and can move relative to the quadrotors body, this motion must be taken into account in the control system, in particular, in order to suppress and prevent unwanted oscillations of the load. In the present paper, a mechanical system is considered that consists of a quadrotor and a load suspended to it with a weightless rod. It is assumed that the cross-sectional area of the load is large enough so that the aerodynamic forces acting on it cannot be neglected. It is known that for a number of shapes of payload casing, this aerodynamic force is not reduced only to the drag force, but also contains a component orthogonal to the drag force, that is, the lift force. In order to these forces, the quasi-steady approach is used. At the same time, accurate information about the aerodynamic capabilities for each particular payload is, generally speaking, not available. However, it is known that the values of aerodynamic coefficients for a fairly wide class of body shapes are lie in a specific range. Here we discuss the problem of developing the quadrotor control that would ensure robust stabilization of the ascent and descent of the system in the conditions of incomplete information about the aerodynamic load. A method is proposed for constructing the quadrotor control for robust stabilization of uniform vertical ascent and descent of the system as a whole. It is shown that this control ensures stabilization of the target mode within a fairly wide range of system parameters. Restrictions on the target speed of quadrotor motion are determined, violation of which makes the robust stabilization impossible.

Semi-analytical solutions for steady-state bending-bending coupled forced vibrations of a rotating wind turbine blade by means of Green's functions

Wind power has received significant attention in recent years as a renewable energy source. Wind turbine blades, characterized by their long lengths, are prone to damage as a result of aeroelastic instability. This paper aims to derive semi-analytical solutions for steady-state forced vibrations of rotating wind turbine blades, considering the coupling of bending, bending, and unsteady aerodynamic loads. The novelty of this work lies in the use of the Green's function method to solve differential dynamic equations with variable coefficients, which are induced by aerodynamic forces. To model a wind turbine blade, the Euler-Bernoulli beam model is employed, and Greenberg's expressions are taken into consideration. Governing equations for coupled vibrations of the blade are then determined. In order to solve the governing equations, displacement solutions are decomposed into quasi-static displacements and dynamic displacements. The Laplace transformation and Green's function methods are utilized to obtain fundamental solutions of the governing equations. Subsequently, employing the principle of superposition, Fredholm integral equations for steady-state forced vibration of a rotating wind turbine blade are derived. To spatially discretize Fredholm integral equations, compound trapezoid formulae and central finite difference approximations are employed. This discretization process leads to the formation of a system of algebraic equations. Semi-analytical solutions for coupled forced vibrations of the rotating wind turbine blade are obtained by solving the algebraic equations. In the numerical solution part, validation of proposed solutions are verified by comparing them with numerical and finite element solutions in the literature. Influences of some important physical parameters, such as the rotating velocity, the setting angle, the cone angle, the inflow ratio, and damping, on vibration responses of blades are discussed.

Passive control of porous media on the aerodynamic forces and wake structures of wall-mounted short circular cylinders

This paper numerically investigates the aerodynamic forces and the three-dimensional wake characteristics of wall-mounted circular cylinders with and without porous media coatings using large eddy simulation at a Reynolds number of 3.2×104. Short cylinders with aspect ratios of 0.5, 1.0, and 3.0 are considered, with one end fixed to a bottom wall in the current work. The study focuses on aerodynamic coefficients, flow characteristics, and wake structures for cylinders both with and without porous coatings. The statistical results indicate that porous media significantly alter flow patterns behind the cylinders, suppress downwash flow from the free end, and reduce velocity fluctuations and turbulent kinetic energy within the wake. The porous coating enhances the leeward side's base pressure, leading to a reduction in drag on the cylinder surface. The analysis of flow structures reveals that the topology of the arch vortex behind solid cylinders is significantly dependent on the aspect ratio, whereas this dependency is negligible for porous cylinders. Porous coatings diminish the intensity of the tip and trailing vortices behind the cylinder. Finally, based on the time-averaged flow field, we proposed two conceptual models of topological correlation for wall-mounted short cylinders, both with and without porous coatings, which contributes to describing the geometric characteristics and interactions of vortex structures.

Mitigating crosswind response of a high-speed train passing the end of windbreak walls

The sudden change in wind speed at the end of a windbreak wall poses a significant threat to train safety, as it amplifies train’s responses. This study aims to investigate countermeasures to reduce the maximum unsteady crosswind response of high-speed trains at the end of windbreaks. Wind flow around windbreak walls was systematically examined by particle image velocimetry (PIV), considering both constant and varying porosities. Subsequently, a gust model was proposed to evaluate the wind distributions behind windbreak walls, validated by experimental results. Aerodynamic forces on high-speed train, incorporating windbreak walls with diverse porosities, were assessed by integrating the gust model and the quasi-steady theory. Ultimately, the unsteady crosswind responses were computed employing multibody simulations (MBS), accounting for both solid windbreak walls and those with varying porosities. A noteworthy finding was the reduction of approximately 25 % in the maximum unsteady crosswind response when windbreak porosity varied from 0 % to 80 %, thus affirming the efficacy of windbreaks featuring varying porosities in diminishing unsteady crosswind response. Overall, this study provides valuable insights into the design of windbreaks for high-speed trains, which can improve their safety and performance in strong wind regions.

Effect of leading-edge and trailing-edge camber morphing on gust load for an elastic wing

This paper investigates the influence of the spanwise-distributed trailing-edge camber morphing on the dynamic stall characteristics of a finite-span wing at Re = 2 × 105. The mathematical model of the spanwise-distributed trailing-edge camber morphing is established based on Chebyshev polynomials, and the deformed wing surface is modeled by a spline surface according to the rib’s morphing in the chordwise direction. The Computational Fluid Dynamics (CFD) method is adopted to obtain flow-field results and aerodynamic forces. The SST-γ model is introduced and the overset mesh technique is adopted. The numerical results show that the spanwise-distributed trailing-edge morphing obviously changes the aerodynamic and energy transfer characteristics of the dynamic stall. Especially when the phase difference between the trailing-edge motion and the wing pitch is −π/2, the interaction between the three-dimensional (3-D) Leading-Edge Vortex (LEV) and Trailing-Edge Vortex (TEV) is strengthened, and the work done by the aerodynamic force turns negative. This indicates that the trailing-edge deformation has the potential to suppress the oscillation amplitude of stall flutter. We also found that as the trailing-edge camber morphing varies more complexly along the spanwise direction, the suppression effect decreases accordingly.

Numerical investigation of wingtip aerodynamic interference of two flapping wings on opposite sides

Aerodynamic interference occurs at the wingtips when flying organisms fly in a V formation. In this paper, the wingtip aerodynamic interference of two flapping wings on opposite sides at low Reynolds numbers (Re) is numerically investigated. The effects of streamwise spacing (L1), spanwise spacing (L2), and phase angle (γ) on aerodynamic performance are considered. The results show that, compared to a single wing, a favorable combination of L1 and L2 can improve the overall thrust by 24% while keeping the overall lift essentially unchanged. In an unfavorable case, overall lift and thrust decrease by 18% and 20%, respectively. The overall aerodynamic forces are dominated by the rear wing. Analyzing the essential flow characteristics reveals the double-edged role of downwash and upwash in force generation. Moreover, it is found that the rear wing can realize the upwash/downwash exploitation by flap phasing, turning an unfavorable situation into a favorable one. The key flow physics behind this transformation lies in the relationship between the direction of wing motion and the direction of fluid velocity induced by vortices. These findings provide valuable insights into the understanding of biological phenomena and the design of new flapping wing vehicles.

Water droplet transport on superhydrophobic surfaces induced by the dual synthetic jets

Droplet transport is very essential in many industrial applications. This article proposed the concept of employing dual synthetic jets in conjunction with superhydrophobic materials to facilitate the long-distance directional transport of water droplets. Experiments showed that the droplet transport speed could reach approximately 90 mm/s. In addition to linear transport, dual synthetic jets were also capable of achieving curved transport of droplets on superhydrophobic surfaces. High-speed photography captured the details of the droplet transport process. Additionally, simulations analyzed the water droplet's aerodynamic forces and the deformation and breakup mechanisms at the actuator's outlet. The research in this paper was anticipated to contribute to new methods for directed droplet transport on superhydrophobic surfaces. It is eliminated the need for pre-processing the surface to create a path, and there was also no need to incorporate conductive or magnetic substances within the droplets.

Dynamically Linearized Time-Domain Approach for Limit-Cycle Oscillation Prediction for an Elastic Panel

A nonlinear aeroelastic model for a flexible panel is extended to consider the Euler equation as the aerodynamic model. A highly efficient method is presented to compute the generalized aerodynamic forces from a CFD simulation, and the results are compared to those results previously obtained using full potential flow aerodynamics and the linear piston theory. The flutter onset condition as well as limit cycle oscillation amplitudes are computed for a range of supersonic Mach numbers. Additionally, the effects of a shock wave and a viscous boundary layer (Reynolds-averaged Navier–Stokes) are considered in the computational fluid dynamics simulation and are implemented in the aeroelastic model via the generalized aerodynamic force obtained by the new approach. Conclusions are made based on the use and application of this more complete aerodynamic theory, including cases with shock impingement and near-transonic Mach numbers.

Wing-strain-based flight control of flapping-wing drones through reinforcement learning

Although drone technology has advanced rapidly, replicating the dynamic control and wind-sensing abilities of biological flight is still beyond reach. Biological studies reveal that insect wings are equipped with mechanoreceptors known as campaniform sensilla, which detect complex aerodynamic loads critical for flight agility. By leveraging robotic experiments designed to mimic these biological systems, we confirm that wing strain provides crucial information about the drone’s attitude angle, as well as the direction and velocity of the wind. We introduce a wing-strain-based flight controller that employs the aerodynamic forces exerted on a flapping drone’s wings to deduce vital flight data such as attitude and airflow without accelerometers and gyroscopic sensors. The present work spans five key experiments: initial validation of the wing strain sensor system for state information provision, control in a single degree of freedom movement environment with changing winds, control in a two degrees of freedom movement environment for gravitational attitude adjustment, a test for position control in windy conditions and a demonstration of precise flight path manipulation in a windless condition using only wing strain sensors. We have successfully demonstrated control of a flapping drone in various environments using only wing strain sensors, with the aid of a reinforcement-learning-driven flight controller. The demonstrated adaptability to environmental shifts will be beneficial across varied applications, from gust resistance to wind-assisted flight for autonomous flying robots.

Numerical Analysis of Small Caliber Spinning Projectile with Variety Boattail Configurations

Range extension has received great interest in the field of artillery and spinning projectiles. Reducing base drag is an effective way for rang extension, because it is a major component in transonic and supersonic Mach numbers. Boattailing is considered as one of the most implemented techniques to reduce base drag. This work uses variety of passive drag reduction techniques over M33 .50cal projectile. Multiple design variations on projectile boattail are considered. This includes varying boattail angle, use of riblets, and use of arced boattail. Computations are conducted using computational fluid dynamics by solving RANS equations. Simulations are performed over Mach numbers from 0.6 to 2.5. The aim of the simulations is to calculate most affecting aerodynamic forces and moments which include drag force, lift force, Magnus force, Magnus moment, overturning moment, and spin damping moment. Then these coefficients are compared for all design variations. Results are validated by comparing with experimental and numerical results from the literature. Furthermore, a mesh independence study is done for verification. It was found that every configuration has good performance at a certain speed range with regard to zero yaw drag and significant yaw drag reduction for riblets boattail for low supersonic speeds.

3D LES numerical investigation of vertical vortex-induced vibrations of a 4:1 rectangular cylinder

The aerodynamic characteristics of rectangular cylinders provide valuable references for many engineering structures. Aiming to investigate the characteristics of vortex-induced vibration (VIV) of this kind of bluff section cylinder, the three-dimensional (3D) large-eddy simulations (LES) were conducted on the flow around an elastically mounted rectangular cylinder with an aspect ratio of 4. A novel method for creating 3D meshes was introduced and validated by comparing the aerodynamic parameters and vibration amplitudes with those of experimental data. The mechanism of VIV was then investigated by examining the evolution properties (including the mean and fluctuating pressure, predominant frequency, and spanwise correlation) of distributed pressure at various amplitude-dependent VIV stages. Additionally, the contributing/inhibiting effect of wind load acting on specific regions around the cylinder’s surface on VIV was clarified by investigating the energy input or output. The results show that the vibration amplitude affects both static and fluctuating pressures, with a significant variation in these pressures observed during the amplitude-ascent stage. The distributed aerodynamic force is mainly due to self-excited forces resulting from the cylinder vibration during amplitude-ascent stage, whereas both self-excited forces and forcing forces are prominent during the amplitude-descent stage. Notably, the spanwise correlation coefficient displays its minimum value at the peak amplitude, even lower than that in the non-lock-in region, with the highest value occurring at the onset of the amplitude-ascent stage. The energy analysis revealed that the wind loads acting on the upstream of the cylinder's surface tend to enhance VIV, while those acting downstream tend to inhibit it.

Investigation of models to estimate flight performance of gliding birds from wakes.

Mathematical models based on inviscid flow theory are effective at predicting the aerodynamic forces on large-scale aircraft. Avian flight, however, is characterized by smaller sizes, slower speeds, and increased influence of viscous effects associated with lower Reynolds numbers. Therefore, inviscid mathematical models of avian flight should be used with caution. The assumptions used in such models, such as thin wings and streamlined bodies, may be violated by birds, potentially introducing additional error. To investigate the applicability of the existing models to calculate the aerodynamic performance of bird flight, we compared predictions using simulated wakes with those calculated directly from forces on the bird surface, both derived from computational fluid dynamics of a high-fidelity barn owl geometry in free gliding flight. Two lift models and two drag models are assessed. We show that the generalized Kutta-Joukowski model, corrected by the streamwise velocity, can predict not only the lift but also span loading well. Drag was predicted best by a drag model based on the conservation of fluid momentum in a control volume. Finally, we estimated force production for three raptor species across nine gliding flights by applying the best lift model to wake flow fields measured with particle tracking velocimetry.

Wall-Modeled Large-Eddy Simulation Method for Unstructured-Grid Navier–Stokes Solvers

This paper reports on the implementation and assessment of a wall-modeled large-eddy simulation (WMLES) methodology in an unstructured-grid, node-centered flow solver, FUN3D, that is developed and supported at the NASA Langley Research Center. Finite-volume (FV) and finite-element (FE) discretization schemes considered in the study provide formal second-order spatial accuracy. Large-eddy simulations (LES) resolve large-scale turbulent-flow features, and small-scale effects are modeled using the Vreman subgrid-scale model. At solid-wall boundaries, a shear-stress model is employed to provide a proper boundary-flux closure. The nonlinear equations are integrated in time using either an optimized backward difference formula or an implicit multistage Runge–Kutta temporal scheme. The implicit equations at each time step are solved by strong nonlinear iteration schemes. WMLES demonstrations are shown for two high-lift configurations, namely, the McDonnell Douglas 30P30N multi-element airfoil and a NASA High-Lift Common Research Model. Results show that the WMLES approaches implemented in the FV and FE discretization methods produce consistent solutions and are capable of capturing key aerodynamic characteristics and flow structures for high-lift configurations at a wide range of angles of attack, including maximum-lift conditions. In the 30P30N example, correct trends in the variations of integrated aerodynamic forces and moments, surface pressure distributions, and boundary-layer profiles are captured as the Reynolds number is increased.

Numerical simulation of jet break-up in the second-wind induced regime using the local front reconstruction method

In the second wind-induced regime, the jet deformations are influenced by aerodynamic instabilities acting on the jet surface. In addition, velocity relaxation in the liquid is an important mechanism with a strong influence on the violent break-up of laminar jets (i.e., the bursting). The exact interplay between the aerodynamic forces and the velocity profile relaxation is insufficiently described in the literature. Thus, we investigate numerically cylindrical liquid jets with fully developed and uniform velocity inlet profiles for different liquid and gas properties to elucidate the physical mechanisms governing the bursting of the jet. For this study, the gas-liquid interface is tracked using a front tracking technique: the Local Front Reconstruction Method (LFRM). In contrast with traditional front tracking techniques, LFRM allows for complex topological changes (merging and break-up) while keeping a sharp representation of the interface. The comparison of the obtained results with literature suggests that LFRM is a suitable technique for capturing the complex morphological deformations of a bursting jet. Additionally, we analyzed the influence of air and liquid properties on the jet deformations, which evidenced that velocity profile relaxation plays an important role in the complex process of break-up of the jet, that in combination with the destabilizing effect of the aerodynamic forces lead to the bursting of the jet.

Determining the aerodynamic performance of a high-speed unmanned marine wig craft

The study object of this paper is the aerodynamic processes during the movement of an unmanned aerial vehicle flying low over the underlying surface. The ground effect is known to enhance the aerodynamic performance of low-flying aircraft, particularly larger ones. However, this effect is most noticeable for large objects. Unmanned vehicles are typically characterized by their relatively compact geometry. This study explores the aerodynamic processes involved in the flight of a small-sized vehicle using the principle of dynamic support over a surface. The particular prototype of the unmanned aerial vehicle suggested by the authors has been examined herein. The aim of the study is to evaluate the aerodynamic forces affecting a small-sized unmanned high-speed vehicle that employs the dynamic principle of support over the surface (WIG craft) by using CFD modeling. In contrast to most available studies on the ground effect, a 3D problem statement was used in this work. Computational experiments have visualized the physical fields surrounding an aircraft in flight over the ground. This study determines how the distance from the surface affects the aerodynamic properties of a small-sized aircraft, as well as the height of the effective zone where ground effect influences the small-sized WIG craft within . It has been shown that approaching the surface leads to a shift in the center of pressure of the vehicle, which leads to a change in aerodynamic momentum. This phenomenon must be taken into account when designing a control system to provide a stable flight. The study's findings are directly relevant for the development of a type of unmanned vehicles that use the dynamic principle of support over the surface.