Periodic Vortical Gust Encounter and Mitigation Using Closed Loop Control

Date of Award

2023

Degree Name

M.S. in Aerospace Engineering

Department

Department of Mechanical and Aerospace Engineering

Advisor/Chair

Sidaard Gunasekaran

Abstract

Contending with gusts has been a problem people have been working on for a very long time, in fact the Wright Brothers' first day of flights was cut short after a gust of wind overturned and damaged their first flyer. Long before airplanes, people contended with gusts when they built sailboats to be resilient and even harness sudden bursts of wind without tipping over. Gusts are ever present in the atmosphere whether that be from weather, terrain, buildings, or other objects. To navigate through the air effectively, gusts must be contended with. While people have been working on this problem for a long time, the issue of understanding gust encounters, and in particular mitigating them has become even more important today. Wind turbines can be made more efficient if their interactions with gusts are better understood, and with the rise of more susceptible aircraft like smaller Uncrewed Aerial Vehicles (UAVs) and even Micro Air Vehicles (MAVs), gust mitigation has become even more important to ensure these new technologies can operate effectively. Due to these vehicles' smaller size, a gust can have a large effect on their stability and the loads they experience. Especially if these vehicles are to operate in complex environments like cities gust mitigation strategies will be essential to their success. Gust mitigation strategies in the past have mainly focused on using unsteady aerodynamic models to predict the forces a wing or flat plate will experience and building an open loop controller from that model to mitigate the gusts. While this method has had success, the use of an open loop controller limits applications to a known gust. This work instead makes use of a closed loop controller that does not necessarily need to know the structure of a gust a priori to mitigate it. Unsteady aerodynamic models are also examined and evaluated as a method of combining the two strategies for possible future controller improvements. A gust generator was designed to produce several periodic vortical gusts for a range of amplitudes and reduced frequencies. Time Resolved PIV (TR-PIV) measurements were captured along with force data while the controller mitigated periodic vortical gusts to examine the flow physics present in a closed loop gust mitigation encounter. When compared to the unmitigated open loop encounters, the closed loop controller was able to mitigate up to 95.7% of the peak open loop forces and up to 62.31% of the periodic disturbance. PIV flow analysis revealed that the controller limited flow separation and LEV formation that was present in open loop at the peak forces. For most of the gust encounter the controller was able to align the plate more closely to the flow to reduce lift deviations. The controller was also able to offset the angle to maintain a positive lift throughout a gust encounter. PIV also revealed that the controller performed best while the vortical gusts approached the plate. During this phase there was a gradual increase in induced Angle of Attack (AoA) it could compensate for. The controller struggled as the vortices crossed the leading edge (LE) of the flat plate rapidly changing the induced AoA as the vertical component of velocity due to the gust changed from one side of the vortex to the other. Three unsteady aerodynamic models, Theodorsen, Wagner, and Küssner, are explained and evaluated to see how well they can predict the experimental forces measured in both open and closed loop gust encounters. Using the upstream induced AoA and the angle of the flat plate, the models were all able to predict the general trends in the coefficient of lift CL histories for the range of gusts tested. Wagner, in particular, was also successful in predicting the magnitudes of the forces experienced during the gust encounters with an average error as low as 0.07. The success of the models indicate that they could be leveraged to further improve periodic vortical gust mitigation with a closed loop controller.

Keywords

Aerospace Engineering, Engineering, Fluid Dynamics, Mechanical Engineering, Vortical gusts, Gust mitigation, Unsteady aerodynamics, Closed loop control, Gust encounters, Urban air mobility

Rights Statement

Copyright © 2023, author

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