When a wing passes through a strong gust, large amounts of lift are generated very quickly due to the high effective angle of attack of the wing, large-scale flow separation and vortex formation, and the presence of vorticity in the gust flow. The resulting lift transient, and the speed at which it builds, presents a challenge to maintaining vehicle control and can result in structural deformation or failure. It is therefore of interest to equip wings with ways to mitigate a strong gust response, i.e., regulate lift. Results of this work apply to a broad range of applications including wind and water turbines operating in tides, waves, and wakes; and air and water vehicles of all scales, from small aerial vehicles operating in urban environments to manned vehicles operating in complex terrain, air wakes, and extreme weather. 

This project applied tools from fluid dynamics, reduced-order modeling, and optimal and robust control theory to (1) elucidate and model the underlying flow physics of an unsteady and uncertain large-amplitude transverse gust encounter, and (2) apply this knowledge to design control laws for regulation of lift production through kinematic actuation during and after the disturbance event.

The technical approach was to (1) synthesize a physics-based low-order model that includes sufficient consideration of leading- and trailing-edge vortex dynamics in a wing-gust encounter; (2) construct a robust control framework for lift regulation in the gust that properly accounts for uncertainties in the gust flow and the wing's response thereto; and (3) implement a closed-loop control framework in real-time gust encounter experiments in the laboratory.

Analytical models are useful for real-time control as they are computationally inexpensive and do not require large training data sets. Küssner’s model, for example, has been shown to perform well in predicting force histories of wing-gust encounters in low to moderate gust ratios, with its performance deteriorating in high gust ratios and when nearing the gust exit. This is likely due to its omission of non-linear physics that become important in those scenarios. In the current work, time-resolved PIV data was used to evaluate model assumptions and propose improved models based on knowledge of the flowfield. Ultimately, classic unsteady aerodynamic theory was used to construct open-loop pitch maneuvers and tune a closed-loop controller for closed-loop control. A dynamical systems treatment of the problem during control design revealed several important physical features important to vehicle control.

Open-loop pitch maneuvers were constructed based on the aforementioned models and implemented in a gust-enabled water-filled 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 to pitch control. It 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.

Further experiments implemented a proportional control strategy based on classic unsteady aerodynamic theory using a pitch acceleration input and real-time force measurements. These closed-loop control experiments included both 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% for various aerodynamic conditions without a priori knowledge of gust strength or onset time. 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 the choice of controller input. Simultaneous time-resolved velocity field and force measurements were used to observe and understand the flow physics underlying the lift transients in the disturbance and the physics of how applying closed-loop control mitigated those transients. One result from this work was the realization that non-circulatory forces can play a large role in gust mitigation for a pitching wing in a large flow disturbance.

Over the course of this project, several canonical pitch maneuvers with varying levels of fidelity were designed and studied using concurrent force and flow field measurements. While low order, potential flow models were found to be successful at predicting (and therefore mitigating) forces during the early parts of the gust encounter, they are not sufficient during gust exit. Large lift transients were observed during gust exit due to unmodelled effects of the gust-pitch maneuver coupling. Future work will likely need to consider gust exit explicitly.

Last Modified: 10/25/2024
Modified by: Anya Jones