Gust Flow Research Articles

Characteristics of gusts with different velocity profiles and control parameters

The characteristics of gust flow are essential for gust response and alleviation. To investigate the influence of control parameters on gusts with different velocity profiles, four vertical gust profiles were designed. Methods were proposed to generate them with two pitching airfoils in a low-speed water tunnel. The velocity field was measured via phase-locked particle image velocimetry. The coefficient of determination R2 was proposed to evaluate the generated gust profile quality, which referred to the quality of the vertical velocity profile. The influence of control parameters on different gust profiles was investigated, and the cause of the profile distortion was explored. For continuous sine gusts, the gust ratio GR increased approximately linearly with the pitching amplitude, while the gust ratio initially increased and then decreased with increasing frequency. As the two control parameters increased, the flow uniformity decreased because the airfoil wakes disturbed the measured flow field. In terms of continuous 1-cosine gusts, the gust ratio increased nonlinearly with pitching amplitude. Compared with those of the sine gusts, the GR values of the 1-cosine gusts were higher, whereas the R2 values were lower. In addition, the discrete and continuous gust profiles had similar distortion near the peaks. However, discrete gusts had lower R2 values than continuous gusts because the starting and stopping vortices of the pitching airfoils disturbed the gust flow. Based on these findings, a method to improve the profile quality and field uniformity by increasing the spacing of the pitching airfoils was proposed. This work can support further studies of gust response and alleviation during complex gust encounters.

Numerical study of the behaviors of two ventilated supercavities in periodic gust flow

In the past few years, scholars have shown great interest in supercavity behavior under unsteady gust flow. Based on the study of single cavity, the two ventilated cavities in the unsteady flow generated by the gust generator at different gust amplitude (Ag) and gust frequencies (fg) are numerically simulated. The shape and flow characteristics of two cavities in periodic gust flow are obtained. Generally, the behavior of two cavities under periodic gust flow is consistent with that of single cavity in some aspects. It is found that the reduction of total cavity length is related to the half wavelength and the vertical velocity, and the critical threshold decreases with higher total cavity length. Subsequently, the influence of the frequency and amplitude of the periodic gust flow on the flow field pressure was studied, which showed that the time corresponding to the minimum pressure under different amplitudes and frequencies is inconsistent. Finally, the influence of gust flow on the cavity pressure is illuminated from the perspective of wave motion on the cavity surface, emphasizes that the pressure change in the closure region directly affect the cavity pressure. Based on the theory, we explain the delay of cavity pressure under different length cavity caused by different amplitude, frequencies and interaction modes.

Gust-induced flow perturbations on circular cylinder: Investigating the effects and characteristics

This paper delves into the fascinating realm of gust flow past a circular cylinder, unveiling the dynamic interactions and key features that influence the system's behavior. In this study, ANSYS Fluent is used to perform numerical simulations based on finite volume method. The aim is to investigate the temporal evolution of the aerodynamic forces at different Reynolds numbers (Re) at different gust frequencies. In this study, gusts are generated by harmonically changing the direction of the inlet velocity at different frequencies. Unlike uniform flow, vortices are shed simultaneously from the upper and lower surface of cylinder, sometimes with equal strengths and sometimes with varying strengths depending upon the gust frequency. Cl, Cd and Strouhal number values increase with gust frequency corresponding to any Reynolds number. However, the values remain less than no-gust case under similar flow situations. Fast Fourier transform analysis has been used to examine the energy spectrum and kinetic energy contours of vorticity and turbulence to gain further insight into fluid physics. The results of this study provide valuable insight into the physics of gust flow and circular cylinders.

Reynolds number effects on the bistable flows over a wavy circular cylinder

The wake of wavy cylinder has been shown to exhibit bistability. Depending on the initial condition, the final state of the wake can either develop into steady flow (state I), or periodic shedding (state II). In this paper, we perform numerical simulations to reveal the Reynolds number effects (Re=30 – 300) on these two wake states. With increasing Reynolds number, the steady vortical structures in state I wake sways back and forth in the spanwise direction, resulting in low-frequency fluctuations in drag forces, but not in lift. For state II, the increase in Reynolds number is associated with the emergence of another spectral peak in the lift coefficient. The secondary frequency is associated with highly three-dimensional vortical structures in the wake. For both states, the wakes transition to turbulent flows at higher Reynolds numbers, with the development of small-scale vortices. We further study the streamwise gust flows over the wavy cylinder. The time-varying inflow velocity results in a wide range of instantaneous Reynolds number spanning from the absolutely unstable flow regime to the bistable regime. Depending on the period of the inflow velocity variation, the wake perturbations grown at the absolutely unstable flow regime can be damped out in state I wake, or grow large enough to trigger the transition to state II, resulting in loss of flow control efficacy. The above analyses reveal novel flow physics of the bistable states at unexplored Reynolds numbers, and showcase the complex transition behavior between the two states in unsteady flows. The insights gained from this study improve the understanding of the wake dynamics of the wavy cylinder.

Pedestrian-level gust wind flow and comfort around a building array–Influencing assessment on the pocket park

• Wind flows around a building array and a row of trees are simulated and validated. • Mean and gust flow fields around building arrays with pocket parks are analyzed. • Pocket park amplifies PLW velocities under perpendicular inflow wind. • Pocket park promotes PLW comfort for sitting under oblique and parallel winds. Congested urban forms deteriorate urban ventilation, causing the problem of poor wind comfort at the pedestrian level. Pocket parks are small-scale open spaces for neighborhoods’ recreations, and they have the potential to alleviate this problem in cities, while this is rarely quantitatively investigated. This study investigates the pedestrian-level wind (PLW) flows around building arrays, and the influences of pocket park and vegetation on the mean and gust wind flows, and the wind comfort are further assessed. Steady-state Reynolds Averaged Navier-Stokes (RANS) simulations with the RNG k-ε model are used and validated with the benchmark experiments that give correlation coefficients above 0.80, including winds around a building array and trees. Results indicate that pocket park amplifies the mean and the gust flows in the streets under the perpendicular inflow wind, while attenuates the flows under the oblique and the parallel inflow winds. Inside the pocket park, an outdoor shelter space is created for pedestrian to relax and gather when winds in the streets are overly high. In this case, the vegetation promotes wind comfort for short-term sitting. These findings provide evidence for quantifying the effect of pocket parks with and without vegetation in achieving a healthy outdoor open space.

Physics of gust response mitigation in open-loop pitching manoeuvres

This paper experimentally investigates the flow field development and unsteady loading of three force-mitigating pitch manoeuvres during a transverse gust encounter. The manoeuvres are constructed using varying levels of theoretical and simulation fidelity and implemented as open-loop kinematics in a water towing tank. It is found that pitch actuation during a gust encounter results in two important changes in flow topology: (i) early detachment of the leading-edge vortex (LEV) and (ii) formation of an LEV on the pressure side of the wing upon gust exit. Each of the pitch manoeuvres is found to mitigate a significant portion of the circulatory contribution of the lift force while only manoeuvres with accurate modelling of the added-mass force are found to adequately mitigate the total lift force. The penalty of aerodynamic lift mitigation using pitch manoeuvres was a twofold increase in the pitching moment transients experienced by the wing for all cases. By quantifying changes in the vertical gust momentum before and after the encounter, lift-mitigating manoeuvres were found to reduce the disturbance to the gust's flow field, thereby reducing the momentum exchange between the gust and the wing.

Numerical Analysis of the Effects of Periodic Gust Flow on the Wake Structure of Ventilated Supercavities

A computational model is established to investigate the effects of a periodic gust flow on the wake structure of ventilated supercavities. The effectiveness of the computational model is validated by comparing with available experimental data. Benefited from this numerical model, the vertical velocity characteristics in the entire flow field can be easily monitored and analyzed under the action of a gust generator; further, the unsteady evolution of the flow parameters of the closed region of the supercavity can be captured in any location. To avoid the adverse effects of mounting struts in the experiments and to obtain more realistic results, the wake structure of a ventilated supercavity without mounting struts is investigated. Unsteady changes in the wake morphology and vorticity distribution pattern of the ventilated supercavity are determined. The results demonstrate that the periodic swing of the gust generator can generate a gust flow and, therefore, generate a periodic variation of the ventilated cavitation number σ. At the peak σ, a re-entrant jet closure appears in the wake of the ventilated supercavity. At the valley σ, a twin-vortex closure appears in the wake of the ventilated supercavity. For the forward facing model, the twin vortex appears as a pair of centrally rolled-up vortices, due to the closure of vortex is affected by the structure. For the backward facing model, however, the twin vortex appears alternately as a pair of centrally rolled-up vortices and a pair of centrally rolled-down vortices, against the periodic gust flow.

Gust encounters of rigid wings: Taming the parameter space

Recent efforts to study discrete large-amplitude gust encounters have often focused on one of three canonical problems: transverse, vortex, or streamwise gust encounters. A selection of this work is highlighted, concentrating on two-dimensional problems where the length scale of the gust flow is similar to the wing chord, and the amplitude of the gust flow is similar to the freestream. Current and outstanding questions are outlined for future work, including the role of low-order models and the limitations of using canonical problems to represent real-world gust encounters.

Numerical Study of the Behaviors of Ventilated Supercavities in a Periodic Gust Flow

Abstract We conducted a numerical simulation of ventilated supercavitation from a forward-facing cavitator in unsteady flows generated by a gust generator under different gust angles of attack and gust frequencies. The numerical method is validated through the experimental results under specific steady and unsteady conditions. It is shown that the simulation can capture the degree of cavity shape fluctuation and internal pressure variation in a gust cycle. Specifically, the cavity centerline shows periodic wavelike undulation with a maximum amplitude matching that of the incoming flow perturbation. The cavity internal pressure also fluctuates periodically, causing the corresponding change of difference between internal and external pressure across the closure that leads to the closure mode change in a gust cycle. In addition, the simulation captures the variation of cavity internal flow, particularly the development internal flow boundary layer along the cavitator mounting strut, upon the incoming flow perturbation, correlating with cavity deformation and closure mode variation. With increasing angle of attack, the cavity exhibits augmented wavelike undulation and pressure fluctuation. As the wavelength of the flow perturbation approaches the cavity length with increasing gust frequency, the cavity experiences stronger wavelike undulation and internal pressure fluctuation but reduced cavitation number variation.

Morphological changes of ventilated supercavities in continuous and discrete incident gust flows

The behavior of ventilated supercavities under continuous and discrete gust flows was tested in a medium-sized high-speed cavitation tunnel at the Korea Research Institute of Ships and Ocean Engineering. To investigate the influences of experimental facilities on supercavitation observations, the experimental results were compared with experiments at the Saint Anthony Falls Laboratory under similar flow conditions. We suggested a new threshold criterion at which the total cavity length is shortened, which predicts that the total cavity length shrinks when the vertical velocity of an incident flow reaches approximately 4.5% of the horizontal flow velocity. Additionally, we found that discrete gust flows resulted in changes in the cavity shape that were smaller than those in continuous gust flows.

Development of Efficient and Accurate Parallel Computation Algorithm Using Moving Overset Grids on Background Multi-Domains for Complex Two-Phase Flows

The goal of this study involves developing an efficient and accurate parallel computation method for two-phase flow problems including complex moving foreign bodies. The proposed parallel computing techniques are based on the moving body-fitted grids’ overset on background multidomains with grid-overlapping at their interface. First, the cavitation flow over the hemispherical head form is investigated using the two-phase flow solver, which is validated by comparing the numerical and experimental results. Subsequently, the parallel computing technique based on the multidomain method that divides the computational domain into several smaller subdomains is proposed to facilitate more efficient numerical simulations. At the interface of the subdomains, the grid-overlapping method is proposed for more accurate simulations. The illustrative computations indicate that the accuracy of the parallel computation combined with the grid-overlapping method on multidomains is identical to that of the serial computation based on a single block, albeit with a significant reduction in the computation time. Finally, the moving overset grid technique is combined with the background multidomain method and applied to simulate the gust flow that is generated by the pitching motions of the twin hydrofoils. The overset grid technique includes the following three sequential steps: hole-cutting, finding donor cells, and bilinear interpolation. The prediction results for the inflow gust generated by oscillating hydrofoils closely follow the measured results.

Detached eddy simulation of pedestrian-level wind and gust around an elevated building

Wind flow turbulence is known to have a major influence on the pedestrian-level wind (PLW) environments, particularly around a building. The elevated design of a building, as a special feature, proved to improve pedestrian-level weak wind conditions in high-density cities. The present study aims to assess three turbulence models, the detached eddy simulation (DES), the steady-state RANS (SRANS), and the unsteady-state RANS (URANS), in their simulation of the PLW flow turbulence concerning wind gust. The simulated mean wind velocities around isolated buildings with and without an elevated design were compared with those obtained from a wind tunnel experiment. The effects of mesh resolution and inflow fluctuating algorithm on the performance of the DES model were thoroughly evaluated. The DES model can better reproduce the mean flow fields than the other two models. Finally, the unsteady fluctuations of wind flow around the buildings with and without the elevated design are analyzed in terms of instantaneous wind velocity, lift coefficient, energy spectral density, and turbulence intensity. The predicted lift coefficient and Strouhal number are approximately 0.01 and 0.09, respectively, which is consistent with what are reported in the literature. Modifications of the frequency of vortex shedding, periodical wind flow pattern, and the normalized wind gust flow fields around the two types of buildings are compared in detail. The work reveals that transient turbulent flow pattern can be reasonably obtained with the DES model, indicating the potential of using the DES for PLW gust assessments in urban planning.

A comparative study of behaviors of ventilated supercavities between experimental models with different mounting configurations

Small-scale water tunnel experiments of the phenomenon of supercavitation can be carried out broadly using two different kinds of experimental models–in the first model (forward facing model, or FFM), the incoming flow first interacts with the cavitator at front, which is connected to the strut through a ventilation pipe. The second model could have the strut and the ventilation pipe preceding the cavitator (backward facing model, or BFM). This is the continuation of a water tunnel study of the effects of unsteady flows on axisymmetric supercavities. In this study, the unwanted effect of test model configuration on supercavity shape in periodic flows was explored through a comparison of FFM and BFM models. In our experiments, it was found that periodic gust flows have only a minimal effect on the maximum diameter and the cavity length can be shortened above a certain vertical velocity of periodic flows. These findings appear to be robust regardless of the model configuration.

Design approach for cavitation tolerant hydrofoils and blades

Cavitation inception and growth on conventional shape hydrofoils and blades leads initially to a jump of their flow-induced noise, further to an amplification of flow-induced vibration with frequently assisted erosion and finally, to a lift/thrust decrease combined with the drag increase. These undesirable cavitation effects can be mitigated or even suppressed for stable partial cavities experiencing no tail pulsations. A design approach enhancing performance of cavitating hydrofoils/blades by maintaining stable partial cavities is described. Experimental data manifesting an increase of hydrofoil lift with reduction of its drag and of force pulsations by such design are provided. Application of this design approach to propeller/turbine blades and advantages of its employment for blades operating in non-uniform incoming flows are analyzed. The possibility of an increase of the lift to drag ratio and of a reduction of the cavity volume oscillation in gust flows for blade sections is numerically manifested.

Investigation of the Behavior of Ventilated Supercavities in a Periodic Gust Flow

A ventilated supercavity consists of a large, gas-filled bubble enveloped around an underwater vehicle that allows for significant drag reduction and increases in vehicle speed. Previous studies at the Saint Anthony Falls Laboratory (SAFL) focused on the behavior of ventilated supercavities in steady horizontal flows. In open waters, vehicles can encounter unsteady flows, especially when traveling near the surface, under waves. In supercavitation technology, it is critical that the vehicle remains within the cavity while traveling through water to avoid unwanted planing forces. A study has been carried out in the high-speed water tunnel to investigate the effects of unsteady flow on axisymmetric supercavities. An attempt is made to duplicate sea states seen in open waters. In an effort to track cavity dimensions throughout a wave cycle, an automated cavity-tracking script has been developed. Using a high-speed camera and the proper software, it is possible to synchronize cavity dimensions with pressure measurements taken inside the cavity. Results regarding supercavity appearance, cavitation parameters, and their relation to sea state conditions are presented. It was found that flow unsteadiness caused a decrease in the overall length of the supercavity while having only a minimal effect on the maximum diameter. The supercavity volume varied with cavitation number, and a possible relationship between the two was explored.

Application of Active Flow Control Technique for Gust Load Alleviation

A new gust load alleviation technique is presented in this paper based on active flow control. Numerical studies are conducted to investigate the beneficial effects on the aerodynamic characteristics of the quasi “Global Hawk” airfoil using arrays of jets during the gust process. Based on unsteady Navier-Stokes equations, the grid-velocity method is introduced to simulate the gust influence, and dynamic response in vertical gust flow perturbation is investigated for the airfoil as well. An unsteady surface transpiration boundary condition is enforced over a user specified portion of the airfoil's surface to emulate the time dependent velocity boundary conditions. Firstly, after applying this method to simulate typical NACA0006 airfoil gust response to a step change in the angle of attack, it shows that the indicial responses of the airfoil make good agreement with the exact theoretical values and the calculated values in references. Furthermore, gust response characteristic for the quasi “Global Hawk” airfoil is analyzed. Five kinds of flow control techniques are introduced as steady blowing, steady suction, unsteady blowing, unsteady suction and synthetic jets. The physical analysis of the influence on the effects of gust load alleviation is proposed to provide some guidelines for practice. Numerical results have indicated that active flow control technique, as a new technology of gust load alleviation, can affect and suppress the fluid disturbances caused by gust so as to achieve the purpose of gust load alleviation.

Creation and Maintenance of Cavities Under Horizontal Surfaces in Steady and Gust Flows

An experimental study of air supply to bottom cavities stabilized within a recess under a horizontal surface has been carried out in a specially designed water tunnel. The air supply necessary for creating and maintaining an air cavity in steady and gust flows has been determined over a wide range of speed. Flux-free ventilated cavitation at low flow speeds has been observed. Stable multiwave cavity forms at subcritical values of Froude number were also observed. It was found that the cross-sectional area of the air supply ducting has a substantial effect on the air demand. Air supply scaling laws were deduced and verified with the experimental data obtained.

Improvement of Hydrofoil Performance by Partial Ventilated Cavitation in Steady Flow and Periodic Gusts

This paper describes a study of the response of a recently developed low-drag partially cavitating hydrofoil (denoted as OK-2003) to periodical perturbations of incoming flow. A two-flap assembly specially designed to simulate sea wave impact on the cavitating hydrofoil generates the perturbations. The design range of cavitation number was maintained by ventilation. Unsteady flow can be simulated over a range of ratios of gust flow wavelength to cavity length. The measurement of time-average lift and drag coefficients and their fluctuating values over a range of inflow characteristics allows a determination of hydrofoil performance over a range of conditions that could be expected for a prototype hydrofoil. Both regular interaction with practically linear perturbations and resonancelike singular interaction with substantial nonlinear effects were noted. The observations are accompanied by a numerical analysis that identifies resonance phenomena as a function of excitation frequency.

Numerical Analyses of Discrete Gust Response for an Aircraft

Based on Navier-Stokes equations and structural and flight dynamic equations of motion, dynamic responses in vertical discrete gust flow perturbation are investigated for a supersonic transport model. A tightly coupled method was developed by subiterations between aerodynamic equations and dynamic equations of motion. First, under the assumption of rigid-body and single freedom of motion in the vertical plunging, the results of a direct-coupling method are compared with the results of quasi-steady model method. Then, gust responses for the one-minus-cosine gust profile arc analyzed with two freedoms of motion in plunging and pitching for the airplane configurations with and without the consideration of structural deformation.

A Theoretical Analysis of Unsteady Fluid Forces and Pressure Distributions on a Blade of Cavitating Cascades. 2nd Report, Sinusoidal Gust Flow.

An analysis of cavitation in flat plate cascades under the interaction with sinusoidal gust flow was carried out by a singularity method based on the closed cavity model allowing the cavity length to change freely. The fluid forces and pressure distributions on the blade were calculated. The effects of cavity length and the frequency of the gust flow on the unsteady forces and pressure distributions are discussed for lower and higher solidity cascades. The results are compared with experiment on 4-bladed inducer under the interaction with inlet distortion.