A cyclic perspective on transient gust encounters through the lens of persistent homology

Large amplitude gust encounters exhibit a range of separated flow phenomena, making them difficult to characterize using the traditional tools of aerodynamics. In this work, we propose a dynamical systems approach to gust encounters, viewing the flow as a cycle (or a closed trajectory) in state space. We posit that the topology of this cycle, or its shape and structure, provides a compact description of the flow, and can be used to identify coordinates in which the dynamics evolve in a simple, intuitive way. To demonstrate this idea, we consider flowfield measurements of a transverse gust encounter. For each case in the dataset, we characterize the full-state dynamics of the flow using persistent homology, a tool that identifies holes in point cloud data, and transform the dynamics to a reduced-order space using a nonlinear autoencoder. Critically, we constrain the autoencoder such that it preserves topologically relevant features of the original dynamics, or those features identified by persistent homology. Using this approach, we are able to transform six separate gust encounters to a three-dimensional latent space, in which each gust encounter reduces to a simple circle, and from which the original flow can be reconstructed. This result shows that topology can guide the creation of low-dimensional state representations for strong transverse gust encounters, a crucial step toward the modeling and control of airfoil-gust interactions

Characterization of Aerodynamic Forces on Wings in Plunge Maneuvers

We present experiments and simulations of plunging maneuvers of large amplitude, for velocity ratios of G=1 and 2, defined as the ratio between the peak plunge velocity and the freestream velocity. We explore the effect of the airfoil shape by considering a NACA 0012 wing and a flat plate. The experiments are performed with wings with aspect ratios of 4 and 4.86, whereas the simulations are performed using a model of an infinite-aspect-ratio wing. We report the time evolution of the force coefficients and flow visualizations. A good qualitative agreement is found between experiments and simulations, with small discrepancies in the maximum and minimum lift coefficients observed during the maneuvers and somewhat larger discrepancies during the postmaneuver phase. It is found that the airfoil shape has a small effect on the lift coefficient but a somewhat larger effect on the drag coefficient. We also perform a force decomposition analysis to relate vortical structures to the force on the wings, providing a quantitative measurement of the effect of the leading-edge vortex and trailing-edge vortex on the peak aerodynamic forces.This work was partially supported by the State Research Agency of Spain (AEI) under grant DPI2016-76151-C2-2-R, including funding from the European Regional Development Fund and the U.S. Air Force Office of Scientific Research under grant FA9550-16-1-0508. The computations were partially performed at the supercomputer Picasso from the Red Española de Supercomputación in activity FI-2019-1-0030.Publicad

Mitigation of transverse gusts via open- and closed-loop pitching maneuvers

Unsteady flow conditions present significant challenges to stable flight, and gust rejectionremains a concern for flight control in many modern flight environments. Examples of gustdominated flight conditions include flight in stormy conditions, aircraft takeoff and landing in strong crosswinds or ship air wakes, and micro air vehicles in strong shear flow engendered by urban settings and complex terrain. Improving flight stability during gust encounters relies on an improved understanding of the flow physics and the development of effective mitigation control strategies. To this end, the present work seeks to (1) improve our understanding of the unsteady flow physics of a pitching wing encountering a transverse gust and (2) develop and characterize successful open- and closed-loop control strategies to mitigate aerodynamic lift transients induced by the gust using wing pitching input. Classic unsteady aerodynamic theory was used to construct the open-loop pitch maneuvers and tune the closed-loop controller for closed-loop control. The dynamical systems treatment of the problem during control design revealed several important physical features important to vehicle control. Two sets of wing-gust encounter experiments were conducted using a flat-plate wing model in a water towing tank. The transverse gust was generated in the center of the towing tank using a recirculating water jet. Data was acquired using a combination of Particle Image Velocimetry (PIV), force, and torque measurements. In the first set of experiments, the constructed openloop pitch maneuvers were implemented as open-loop kinematics in the water towing tank. This study revealed several findings regarding the change in the flow topology due to pitch actuation, the necessity of modeling added mass for open-loop pitch maneuver construction, and the increase in the pitching moment transients due pitch control. This study also demonstrated how lift-mitigating pitching maneuvers minimized the disturbance to the gust’s flow field, thereby reducing the momentum exchange between the gust and the wing. The second set of experiments implemented a proportional control strategy based on classic unsteady aerodynamic theory using a pitch acceleration input and real-time force measurements. The closed-loop control experiments spanned upwards and downwards gusts of various strengths and lift tracking at pre- and post-stall angles of attack. The controller yielded an average rejection performance of 80% without a priori knowledge of gust strength or onset time and for various aerodynamic conditions. Reasons for the controller’s success include using lift measurements directly in control feedback, aerodynamic models that capture the salient physics in the control design process, and wing pitching as input. Simultaneous time-resolved PIV and force measurements were used to discover and understand the flow physics underlying the lift transients and how applying closed-loop control mitigated those transients

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.https://doi.org/10.1017/jfm.2022.50