Fluctuation Of Aerodynamic Force Research Articles

Base pressure fluctuations on levitated freestream-aligned circular cylinder

Base pressure fluctuations associated with the large-scale wake structures behind a freestream-aligned circular cylinder and aerodynamic force fluctuations related to them are experimentally investigated in the wind tunnel tests. Measurements at ReD=6.97×104 and 1.04×105 were conducted using pressure-sensitive paint (PSP) and a magnetic suspension and balance system (MSBS) for creating a supportless condition. The obtained pressure fields were mainly analyzed by a modal decomposition combining azimuthal Fourier decomposition and proper orthogonal decomposition (POD). The pressure fluctuations caused by large-scale vortex shedding were observed from the results of frequency analysis for mode coefficients. The states of the fluctuations were classified into three patterns, which are anticlockwise/clockwise circular and flapping patterns. These patterns have been observed in the previous studies for velocity fluctuations in the wake of a freestream-aligned circular cylinder. The conditional sampling analysis revealed that the trend in the amplitude of the pressure fluctuations is different by the state, and the flapping pattern causes a large pressure difference across the cylinder axis. Furthermore, the relationship between the antisymmetric pressure fluctuations and lift fluctuations, which act in the lateral direction of the cylinder, is confirmed.

Unsteady Aeroelastic Characterization and Scaling Relations of Flexible Membrane Wings

We present a numerical study to characterize nonlinear unsteady aeroelastic interactions of two-dimensional flexible wings at high angles of attack. The coupled fluid–flexible wing system is solved by a body-fitted variational aeroelastic solver based on the fully coupled Navier–Stokes and nonlinear structural equations. Using the coupled fluid–structure analysis, this study is aimed to provide physical insight and correlations for the aeroelastic behavior of flexible wings in the parameter space of the angle of attack and the aeroelastic number. The phase diagrams of the aerodynamic performance are established to obtain the envelope curves of the optimal performance and determine the transition line of the drag variation. The effects of the angle of attack and the aeroelastic number on the aeroelastic behaviors are systematically examined. The time-averaged membrane deformation is positively correlated with a nondimensional number, the so-called Weber number. A new scaling relation is proposed based on the dynamic equilibrium between the aerodynamic force fluctuation and the combined inertia–elastic fluctuation. The unsteady aerodynamic force can be adjusted by manipulating the membrane vibration, the mass ratio, the Strouhal number, and the aeroelastic number. The numerical investigations provide design guidelines and have the potential to enhance the maneuverability and flight agility of micro air vehicles with flexible wing structures.

Numerical study on compressible flow around a circular cylinder in proximity to the wall

In the present study, the compressible flow around a circular cylinder in proximity to the wall is investigated. By using direct numerical simulation approach, the flow behavior with the Reynolds number 100 and the Mach numbers 0.1–0.6 are examined. The numerical result shows that the compressibility effect would enhance the flow stability, whereas the strength of shedding vortices, the shedding frequency, and the fluctuation of aerodynamic force are reduced by increasing the Mach number. Through the examination of signed enstrophy and shear steepness in the cylinder boundary layer, it is found that the vortices shedding from the free-stream side and the wall side are concurrently inhibited by the wall when the cylinder is fully embedded in the wall boundary layer, and the strength of the wall-side vortices is relatively weaker. This imbalance would be intensified when the cylinder gets closer to the wall, and thus a reduction in the shedding frequency is produced. Based on this effect of wall proximity on frequency, the flow behavior is divided into three regimes, i.e., high-frequency, low-frequency, and completely suppression regimes. Finally, from the observation of mean forces, the trend of mean drag and lift force coefficients with regard to the Mach number and gap ratio is described as the scaling behavior in terms of two piecewise functions.

Multi-Parametric Investigations on Aerodynamic Force, Aeroacoustic, and Engine Energy Utilizations Based Development of Intercity Bus Associates with Various Drag Reduction Techniques through Advanced Engineering Approaches

The impacts of conflicting aerodynamic forces and side drifting forces are the primary unstable elements in automobiles. The action of an unstable environment in automobile vehicles increases the chance of an accident occurring. As a result, much study is required to determine how opposing aerodynamic forces and side drifting force affects function, as well as how to deal with them for safe and smooth navigation. In this work, an intercity bus is chosen as a main object, and computational fluid dynamics (CFD) analysis is used to estimate aerodynamic forces on the bus in all major directions. Experimentation is also carried out for validation reasons. CFD findings for a scaled base model and a dimple-loaded model based on experimental results from a subsonic wind tunnel are demonstrated to be correct. The drag forces generated by CFD simulations on test models are carefully compared to the experimental drag findings of same-dimensioned models. The error percentages between the results of these two methods are acquired and the percentages are determined to be within an acceptable range of significant limitations. Following these validations, CATIA is used to create a total of nine distinct models, the first of which is a standard intercity bus, whereas the other eight models are fitted with drag reduction techniques such as dimples, riblets, and fins on the surface of their upper cumulus side. A sophisticated computational tool, ANSYS Fluent 17.2, is used to estimate the comparative assessments of the predictions of aerodynamic force fluctuations on bus models. Finally, dimples on the top and side surfaces of the bus model (DESIGN–I) are proposed as a more efficient model than other models because dimples are a vital component that may lower pressure drag on the bus by 18% in the main flow direction and up to 43% in the sideslip direction. Furthermore, by minimizing the different aerodynamic force sources without impacting the preparatory needs, the proposed model may provide comfortable travel. The real-time bus is created, and the finalized drag reduction is applied to the optimized places over the whole bus model. In addition, five distinct size-based bus models are developed and studied in terms of aerodynamic forces, necessary energy to resist aerodynamic drag, required forward force for successful movement, instantaneous demand for particular power, and fuel consumption rate. Finally, the formation of aeroacoustic noise owing to turbulence is estimated using sophisticated computer simulation. Last, for real-time applications, multi-parametric studies based on appropriate intercity buses are established.

Three-dimensional characteristics and axial flow pattern in the wake flow of an oblique circular cylinder

Many circular cylindrical engineering structures submerged in fluid are not always orthogonal to flow direction. Oblique circular cylinder is a more general model to study the flow around circular cylindrical engineering structures. Comparing with a normal circular cylinder, there are more three-dimensional effects reflected in the wake flow and aerodynamic forces of an oblique circular cylinder. Moreover, axial flow is a unique and important flow phenomenon in the oblique circular cylinder wake flow. This study introduces a numerical study based on an oblique circular cylinder model and unsteady Reynolds-Averaged Navier-Stokes equations (URANS) to reveal the three-dimensional external flow and axial flow. Effects of the yaw angles take the most attentions. The analyzed flow features include aerodynamic force coefficients, wake flow velocities, correlations between aerodynamic forces and wake flow structures, three-dimensional vortex structures and three-dimensional axial flow. Spectrum signatures of wake flow fluctuation and time-average axial flow pattern are verified by a visualized wind tunnel experiment. Numerical results prove that there are obvious three-dimensional effects in wake flow velocities and aerodynamic forces. Yaw angle, as a crucial parameter, determines the configuration of vortices. Furthermore, yaw angle affects the spectrum signatures of wake flow fluctuation and aerodynamic force fluctuation, and change the spanwise correlation of wake flow. A novel time-average axial flow velocity-distribution pattern is observed in the numerical results and experiment results. Its mechanism is analyzed based on the wake vortex structures.

LES study of unsteady cavitation characteristics of a 3-D hydrofoil with wavy leading edge

The present study seeks to conduct numerical investigations of the cavitating flow characteristics around a sinusoidal wavy leading edge (WLE) 3-D hydrofoil underlying a NACA 634–021 profile with an aspect ratio of 4.3. Cavitational and non-cavitational characteristics of hydrofoils are numerically examined at a chord-based Reynolds number of 7.2 × 105. The sinusoidal leading edge geometries include two WLE amplitudes of 5% and 25% and two WLE wavelengths of 25% and 50% of the mean chord length. We examined the cavitating flow around the hydrofoils in different cavitation numbers, namely σ = 0.8 and σ = 1.2. The flow over the protuberances of the WLE hydrofoil is considered at varying chord lengths and a constant angle of attack α = 6°, where significant spanwise variations in all flow properties, in contrast to the straight leading edge (SLE) hydrofoil, were observed. Large eddy simulation (LES) and Kunz mass transfer models are employed to simulate the dynamic and unsteady behavior of the cavitating flow. Besides, the compressive volume of fluid (VOF) method is used to track the cavity interface. Simulation is performed under the two-phase flow solver —interPhaseChangeFoam— of the OpenFOAM package. Compared to the SLE hydrofoil, we provided an exhaustive report of the time-averaged and instantaneous fluid dynamic characteristics of the cavitating flow around the sinusoidal leading edge hydrofoil, i.e., pressure, velocity, and vorticity fields, as well as lift and drag coefficients, and turbulent kinetic energy are reported. Furthermore, detailed analyses of the instantaneous cavity leading edge and flow separation treatment, vortical structure of the flow, vorticity stretching and dilatation, details of the spanwise flow, the formation of a low-pressure zone behind the WLE hydrofoil, streamwise velocity fluctuation, and evolution of the cavity dynamics through a complete cycle are reported. Results show that early development of the laminar separation bubble (LSBs) on the suction side of WLE hydrofoil prevents significant flow separation. Furthermore, the WLE cases exhibit a significantly reduced level of unsteady fluctuations in aerodynamic forces at the frequency of periodic vortex shedding.

Effect of Single-Row and Double-Row Passive Vortex Generators on the Deep Dynamic Stall of a Wind Turbine Airfoil

Passive vortex generators (VGs) have been widely applied on wind turbines to boost the aerodynamic performance. Although VGs can delay the onset of static stall, the effect of VGs on dynamic stall is still incompletely understood. Therefore, this paper aims at investigating the deep dynamic stall of NREL S809 airfoil controlled by single-row and double-row VGs. The URANS method with VGs fully resolved is used to simulate the unsteady airfoil flow. Firstly, both single-row and double-row VGs effectively suppress the flow separation and reduce the fluctuations in aerodynamic forces when the airfoil pitches up. The maximum lift coefficient is therefore increased beyond 40%, and the onset of deep dynamic stall is also delayed. This suggests that deep dynamic-stall behaviors can be properly controlled by VGs. Secondly, there is a great difference in aerodynamic performance between single-row and double-row VGs when the airfoil pitches down. Single-row VGs severely reduce the aerodynamic pitch damping by 64%, thereby undermining the torsional aeroelastic stability of airfoil. Double-row VGs quickly restore the decreased aerodynamic efficiency near the maximum angle of attack, and also significantly accelerate the flow reattachment. The second-row VGs can help the near-wall flow to withstand the adverse pressure gradient and then suppress the trailing-edge flow separation, particularly during the downstroke process. Generally, double-row VGs are better than single-row VGs concerning controlling deep dynamic stall. This work also gives a performance assessment of VGs in controlling the highly unsteady aerodynamic forces of a wind turbine airfoil.

Numerical Investigation on Heat Transfer and Flow Characteristics of a Confined Circular Cylinder With Slit

Abstract Heat transfer and flow characteristics of channel-bounded circular cylinder with a slit vent parallel to the flow direction are numerically investigated using openfoam. The interesting feature of this configuration is the formation of the separation bubble behind the cylinder, which significantly alters the near wake characteristics. In this study, the emphasis is given to understand the effect of the slit on forced convection from the cylinder. Simulations were performed by varying the slit width from 0 to 0.25 (in steps of 0.05) for the range of Reynolds number (Re) 60–240. Re is defined based on the diameter of the cylinder (d) and centerline velocity (Uc) at the inlet of the channel. The influence of s/d and Re on the separation bubble, aerodynamic forces, and heat transfer characteristics are studied in detail. Results demonstrate that the slit can manipulate the flow to mitigate adverse effects of vortex shedding and thus can be used as a passive flow control technique. It was observed that the inclusion of the slit in the cylinder delays the onset of vortex shedding, and it also reduces the fluctuations in aerodynamic forces up to 99%. Compared with the solid cylinder, around 38% increase in vortex shedding frequency, a 16% reduction in drag, and a 10% increase in average Nusselt number is observed when the slit width is 0.25d. It was found that the introduction of slit vent in the cylinder not only enhances the heat transfer along with the reduction in expenditure of pressure loss across the cylinder but also suppresses the fluctuations in aerodynamic forces, which causes vortex-induced vibrations and thus improves structural stability and integrity.

Analysis on low-frequency fluctuation of aerodynamic force acting on an automobile and flow using proper orthogonal decomposition

Analysis on low-frequency fluctuation of aerodynamic force acting on an automobile and flow using proper orthogonal decomposition

Streamline Computation Study on Rotation Aerodynamic Noise Prediction of Cross-flow Fan

The approach of resolving analysis the cross-flow relative motion streamline on the cross-flow straight blade is presented to predict the rotation aerodynamic noise performance of cross-flow fan by solving the dipole source term of Ffowcs Williams and Hawkings equation, while the cross-flow fan is applied in the indoor unit of split-type air-conditioner and operated in the rated condition. The calculating results of the method are respectively drawn by programming in Matlab computational language, and compared with the results of CAA numerical simulation and noise experiment. There are some differences in the distribution condition of aerodynamic force fluctuation amplitude on the blade surface, and sound pressure in the related frequency on the indoor unit casing, or on the sphere far field, between the streamline computation approach and numerical simulation. The orders of magnitudes of these calculating results solved by the analysing streamline method are similar with that attained by the numerical simulation. The resolving analysis approach has the characteristics of decreasing the computing cost and not constructing the acoustics grid model of fan, compared with the numerical simulation. The error between the result of numerical simulation and noise test is larger than that between the result of theoretical calculation and noise test, so the approach could be used for the rotation aerodynamic noise analysis of the cross-flow fan.

Feedback tracking control for dynamic morphing of piezocomposite actuated flexible wings

Aerodynamic properties of flexible wings can be improved via shape morphing using piezocomposite materials. Dynamic shape control of flexible wings is investigated in this study by considering the interactions between structural dynamics, unsteady aerodynamics and piezo-actuations. A novel antisymmetric angle-ply bimorph configuration of piezocomposite actuators is presented to realize coupled bending-torsional shape control. The active aeroelastic model is derived using finite element method and Theodorsen unsteady aerodynamic loads. A time-varying linear quadratic Gaussian (LQG) tracking control system is designed to enhance aerodynamic lift with pre-defined trajectories. Proof-of-concept simulations of static and dynamic shape control are presented for a scaled high-aspect-ratio wing model. Vibrations of the wing and fluctuations in aerodynamic forces are caused by using the static voltages directly in dynamic shape control. The lift response has tracked the trajectories well with favorable dynamic morphing performance via feedback tracking control.

A large-eddy simulation on a deep-stalled aerofoil with a wavy leading edge

A numerical investigation on the stalled flow characteristics of a NACA0021 aerofoil with a sinusoidal wavy leading edge (WLE) at chord-based Reynolds number$Re_{\infty }=1.2\times 10^{5}$and angle of attack$\unicode[STIX]{x1D6FC}=20^{\circ }$is presented in this paper. It is observed that laminar separation bubbles (LSBs) form at the trough areas of the WLE in a collocated fashion rather than uniformly/periodically distributed over the span. It is found that the distribution of LSBs and their influence on the aerodynamic forces is strongly dependent on the spanwise domain size of the simulation, i.e. the wavenumber of the WLE used. The creation of a pair of counter-rotating streamwise vortices from the WLE and their evolution as an interface/buffer between the LSBs and the adjacent fully separated shear layers are discussed in detail. The current simulation results confirm that an increased lift and a decreased drag are achieved by using the WLEs compared to the straight leading edge (SLE) case, as observed in previous experiments. Additionally, the WLE cases exhibit a significantly reduced level of unsteady fluctuations in aerodynamic forces at the frequency of periodic vortex shedding. The beneficial aerodynamic characteristics of the WLE cases are attributed to the following three major events observed in the current simulations: (i) the appearance of a large low-pressure zone near the leading edge created by the LSBs; (ii) the reattachment of flow behind the LSBs resulting in a decreased volume of the rear wake; and, (iii) the deterioration of von-Kármán (periodic) vortex shedding due to the breakdown of spanwise coherent structures.

Two-dimensional and three-dimensional numerical simulations of cycloidal propellers in hover

The cycloidal propellers for micro aerial vehicle scale cyclocopter in hovering status were studied in this paper based on the URANs solver using 2D, 2.5D, 3D half blade and 3D full blade model. The results from all numerical models were validated with the experimental results. It was found that 2.5D model cannot produce more accurate results than 2D model, hence results from 2D model were employed to discuss cycloidal rotor with infinite blade span. It was also indicated that the 3D half blade model produced the same results as 3D full blade model, but was more efficient than 3D model. The numerical simulation results of cycloidal rotor with finite (3D model) and infinite span (2D model) were compared. The results indicated that for the 2D cycloidal rotor with large blade pitching amplitude, there were leading edge and trailing edge vortices due to dynamic stall, which resulted in parallel blade vortex interactions. The parallel blade vortex interactions will also induce the fluctuation of aerodynamic forces. For the 3D blade with small aspect ratio, the flow was dominated by 3D dynamic stall and blade vortex interactions. The 3D flow due to finite blade span resulted in smooth dynamic stall and can weaken the parallel blade vortex interactions induced by dynamic stall vortices, hence no strong aerodynamic force fluctuation was observed. The perpendicular blade vortex interactions caused by blade tip vortices can induce cross flow when the azimuth angle of the rotor is between 270° and 360°, which reduces the strength of downwash in the region where the rotor azimuth angle is between 180° and 360°. This results in much smaller side force. Although sometimes the time-averaged aerodynamic forces obtained by 2D and 3D model were quite close to each other, the physics lying behind is quite different. Hence, it was not correct to use the 2D models to discuss the principles of cycloidal rotors with finite blade span.

Effect of vortex shedding in unsteady aerodynamic forces for a low Reynolds number stationary wing at low angle of attack

This study elucidates the relation between wake vortex shedding and aerodynamic force fluctuations for a low Reynolds number wing from time resolved particle image velocimetry (TR-PIV) experimental measurements. The results reveal a periodic lift and drag variation within the shedding cycle and resolve the frequencies of those fluctuations from a proper orthogonal decomposition (POD) and power spectral density (PSD) analysis. To show the effect of vortex shedding on the body force fluctuations, the evolution of instantaneous aerodynamic forces is compared to the pressure field of the fluid flow and to the vortical structures in the wake of the airfoil. A six step model describing the vortex-force relation is proposed. It shows that changes in lift such as maximum lift and minimum lift are associated with the detachment of a vortex. It also shows that the minimum or local minimum drag value is obtained at the onset formation of a vortex on the airfoil wake. Similarly, the maximum or local maximum drag is obtained at the onset formation of the saddle on the airfoil wake. The model further explains the asymmetry observed in the unsteady drag force evolution. The model can be used to optimize flow control and fluid-structure interaction applications.

Application of porous material to reduce aerodynamic sound from bluff bodies

Aerodynamic sound derived from bluff bodies can be considerably reduced by flow control. In this paper, the authors propose a new method in which porous material covers a body surface as one of the flow control methods. From wind tunnel tests on flows around a bare cylinder and a cylinder with porous material, it has been clarified that the application of porous materials is effective in reducing aerodynamic sound. Correlation between aerodynamic sound and aerodynamic force fluctuation, and a surface pressure distribution of cylinders are measured to investigate a mechanism of aerodynamic sound reduction. As a result, the correlation between aerodynamic sound and aerodynamic force fluctuation exists in the flow around the bare cylinder and disappears in the flow around the cylinder with porous material. Moreover, the aerodynamic force fluctuation of the cylinder with porous material is less than that of the bare cylinder. The surface pressure distribution of the cylinder with porous material is quite different from that of the bare cylinder. These facts indicate that aerodynamic sound is reduced by suppressing the motion of vortices because aerodynamic sound is induced by the unstable motion of vortices. In addition, an instantaneous flow field in the wake of the cylinder is measured by application of the PIV technique. Vortices that are shed alternately from the bare cylinder disappear by application of porous material, and the region of zero velocity spreads widely behind the cylinder with porous material. Shear layers between the stationary region and the uniform flow become thin and stable. These results suggest that porous material mainly affects the flow field adjacent to bluff bodies and reduces aerodynamic sound by depriving momentum of the wake and suppressing the unsteady motion of vortices.

Sting-free measurements on a magnetically supported right circular cylinder aligned with the free stream

The flow over cylinders of varying fineness ratio (length to diameter) aligned with the free stream was examined using a magnetic suspension and balance system in order to avoid model support interference. The drag coefficient variation of a right circular cylinder was obtained for a wide range of fineness ratios. Particle image velocimetry (PIV) was used to examine the flow field, particularly the behaviour of the leading-edge separation shear layer and its effect on the wake. Reynolds numbers based on the cylinder diameter ranged from 5×104to 1.1×105, while the major portion of the experiment was conducted atReD=1.0×105. For moderately large fineness ratio, the shear layer reattaches with subsequent growth of the boundary layer, whereas over shorter cylinders, the shear layer remains detached. Differences in the wake recirculation region and the immediate wake patterns are clarified in terms of both the mean velocity and turbulent flow fields, including longitudinal vortical structures in the cross-flow plane of the wake. The minimum drag corresponded to the fineness ratio for which the separated shear layer reattached at the trailing edge of the cylinder. The base pressure was obtained with a telemetry technique. Pressure fields and aerodynamic force fluctuations are also discussed.

Fundamentals Regarding Reduction of Noise Arising from Jet Interaction with Deployed Lift-Augmenting Flaps

Proposed lift-augmented STOL aircraft are expected to require noise reductions on the order of 20 dB to meet community noise goals, according to a number of recent model studies. A predominant source of noise associated with the concepts and configurations currently under consideration is the interaction of the exhaust stream with deployed flaps. This flow-surface interaction produces both large-scale aerodynamic force fluctuations, which radiate like point dipoles at low frequencies, and smaller-scale surface pressure fluctuations, which radiate only from the leading and trailing edges of these surfaces. The fundamentals of the relevant noise-producing processes are reviewed and basic noise reduction concepts developed using parametric source models. Experimentally observed noise reductions on a 1/15 scale externally blown flap (EBF) system are presented for two of the noise reduction concepts—modification of the acoustic transduction process using variable impedance surfaces, and modification of the incident flow field through introduction of high porosity mesh grids upstream of the flaps.