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1. Scaling the vorticity dynamics in the leading-edge vortices of revolving wings with two directional length scales

   https://doi.org/doi/10.1063/5.0024213  

   Nathaniel H Werner, Junshi Wang (December 2021, Physics of fluids)

   null (Ed.)

   In revolving or flapping wings, radial planetary vorticity tilting (PVTr) is a mechanism that contributes to the removal of radial (spanwise) vorticity within the leading-edge vortex (LEV), while vorticity advection increases its strength. Dimensional analysis predicts that the PVTr and advection should scale with the wing aspect-ratio (AR) in identical fashion, assuming a uniform characteristic length is used. However, the authors’ previous work suggests that the vorticity advection decreases more rapidly than the PVTr as AR increases, indicating that separate normalizations should be applied. Here, we aim to develop a comprehensive scaling for the PVTr and vorticity advection based on simulation results using computational fluid dynamics. Two sets of simulations of revolving rectangular wings at an angle of attack of 45° were performed, the first set with the wing-tip velocity maintained constant, so that the Reynolds number (Re) defined at the radius of gyration equals 110, and the second set with the wing angular velocity maintained constant, so that Re defined at one chord length equals 63.5. We proposed two independent length scales based on LEV geometry, i.e., wing-span for the radial and tangential directions and wing chord for the vertical direction. The LEV size in the radial and tangential directions was limited by the wing-span, while the vertical depth remained invariant. The use of two length scales successfully predicted not only the scaling for the PVTr and the vorticity advection but also the relative magnitude of advection in three directions, i.e., tangential advection was strongest, followed by the vertical (downwash) and then the radial that was negligible. 

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2. Evaluating Dynamic Stall-Onset Criteria for Mixed and Trailing-Edge Stall

   https://doi.org/10.2514/1.J062011  

   Sudharsan, Sarasija; Narsipur, Shreyas; Sharma, Anupam (March 2023, AIAA Journal)

   We evaluate two leading-edge-based dynamic stall-onset criteria (namely, the maximum magnitudes of the leading-edge suction parameter and the boundary enstrophy flux) for mixed and trailing-edge stall. These criteria have been shown to successfully predict the onset of leading-edge stall at Reynolds numbers of O(10^5), where the leading-edge suction drops abruptly. However, for mixed/trailing-edge stall, leading-edge suction tends to persist even when there is significant trailing-edge reversed flow and stall is underway, necessitating further investigation into the effectiveness of these criteria. Using wall-resolved large-eddy simulations and the unsteady Reynolds-averaged Navier–Stokes method, we simulate one leading-edge stall and three mixed/trailing-edge stall cases at Reynolds numbers of 200,000 and 300,000. We contrast the progression of flow features such as trailing-edge separation and vortex formation across different stall types and evaluate the stall-onset criteria relative to critical points in the flow. We find that the criteria nearly coincide with the instance of leading-edge suction collapse and are reached in advance of dynamic stall vortex formation and lift stall for all four cases. We conclude that the two criteria effectively signal dynamic stall onset in cases where the dynamic stall vortex plays a prominent role. 

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3. Evaluating Dynamic Stall Onset Criteria for Mixed and Trailing-Edge Stall

   https://doi.org/10.2514/6.2022-1983  

   Sudharsan, Sarasija; Narsipur, Shreyas; Sharma, Anupam (January 2022, 2022 AIAA SciTech Meeting)

   We evaluate two leading-edge-based dynamic stall onset criteria, namely, the maximum magnitudes of the Leading Edge Suction Parameter and the Boundary Enstrophy Flux, for mixed and trailing-edge stall. These criteria have been shown to successfully predict the onset of leading-edge stall at Reynolds numbers >= O(10^5), where the leading-edge suction drops abruptly. However, for mixed/trailing-edge stall, leading-edge suction tends to persist even when there is significant trailing-edge reversed flow and stall is underway, necessitating further investigation of the effectiveness of these criteria. Using wall-resolved, large-eddy simulations and unsteady Reynolds-Averaged Navier-Stokes method, we simulate one leading-edge stall and three mixed/trailing-edge stall cases at Reynolds numbers 2x10^5 and 3x10^6. We contrast the progression of flow features such as trailing-edge separation and vortex formation across different stall types and evaluate the stall onset criteria relative to critical points in the flow. We find that the criteria nearly coincide with the instance of leading-edge suction collapse and are reached in advance of dynamic stall vortex formation and lift stall for all four cases. We conclude that the two criteria effectively signal dynamic stall onset in cases where the dynamic stall vortex plays a prominent role. 

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4. Unsteady Aerodynamics and Wake Structures of Butterfly in Forward Flight

   https://doi.org/10.2514/6.2023-4241  

   Lou, Zhipeng; Li, Chengyu (June 2023, AIAA paper)

   The unsteady aerodynamics mechanisms, such as coupled wing-body aerodynamics, are believed to benefit the flapping flight of the insects. The butterfly takes more advantage of it than other insects because of its unique wing-body morphology and periodical body rotational motion. Our study conducted 3D reconstruction of a monarch butterfly and we adopted an in-house three-dimensional immersed-boundary-method Navier-Stokes equation solver to simulate the natural forward flight of the butterfly. By comparing the simulation with and without the influence of the body, we present a parametric study that proves the coupled wing-body interaction can improve the thrust-to-power ratio. During the upstroke the thrust is improved by 10%. During the upstroke, a posterior body vortex (PBV) that is attached beneath the body is induced by wing motion, which forms a jet flow as upstroke goes on. We visualized wake structures by Q-criterion and observed that the LEV has the strongest circulation at 68% wingspan. The circulation along the leading-edge shows similar trend as the instantaneous lift. 

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5. Physics of gust response mitigation in open-loop pitching manoeuvres

   https://doi.org/10.1017/jfm.2022.509  

   Sedky, Girguis; Gementzopoulos, Antonios; Andreu-Angulo, Ignacio; Lagor, Francis D.; Jones, Anya R. (August 2022, Journal of Fluid Mechanics)

   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. 

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