Model-Based Control of Tollmien-Schlichting Waves using DBD Plasma Actuators

Abstract

One of the main goals of laminar flow control is to reduce skin-friction drag by delaying the onset of laminar to turbulent transition. Over unswept wings, the leading cause of this is the growth of flow instabilities called Tollmien-Schlichting(TS) waves. This thesis aims to test the feasibility and performance of a designed Linear Quadratic Gaussian (LQG) controller in attenuating TS waves, in an experimental setting. A modified flat-plate test setup with an adverse pressure gradient was used to generate and characterize these waves, while control was performed using a Dielectric-Barrier Discharge (DBD) plasma actuator. Embedded microphones were used to measure the pressure fluctuations within the boundary layer, while Particle Image Velocimetery (PIV) was used to quantify the flow. The performance of the controller was investigated at its nominal and off-design conditions (robustness), as well as its comparison to open-loop continuous forcing. For the nominal design conditions, a reduction in the spectral energy of the peak frequencies of four orders of magnitude are observable for the closed-loop case, two orders more than the open-loop case. The root-mean-square (RMS) of the downstream signals showed an overall maximum additional reduction of 55% of the pressure fluctuations, compared to the open-loop case. The controller was more robust at lower peak-to-peak voltages, with an overall 30-60% reduction in RMS of the pressure fluctuations, compared to open-loop forcing .This study demonstrates the feasibility of using LQG model-based techniques as a viable flow-control strategy in damping flow-instabilities, and is an option worth further investigation.