Active control of tollmien-schlichting waves in a blasius boundary layer

Abstract

Tollmien-Schlichting (TS) waves occur as random wave packets naturally due to the receptivity to typically any two of three disturbances, surface roughness, free-stream vorticity or acoustic waves. It is well established that in a low-turbulence environment, growth of TS waves is the first phase of the transition process in boundary-layer flows. Control of TS waves is important because attenuating their growth leads to a delay in transition and consequently, reduction in skin-friction drag, which clearly has a strong application for aircraft. Although deterministic approaches to attenuate TS waves such as anti-wave superposition with open-loop control have been shown to provide good attenuation, they lack the essential robustness required to adapt to the inherent uncertainty present in real-life scenario. Therefore, only a stable, real-time, closed-loop system can provide a realistic and practical control scheme. In this work, attenuation of the growth of TS waves using a single-input and single-output (SISO) active control system is investigated experimentally. Both feedback and feedforward configurations are tested, where the TS wave/wavepacket is generated by either a surface point source or through surface receptivity to freestream vorticity in a flat-plate Blasius boundary layer. In the first instance, the experimental set-up to generate TS waves is very similar to that used by \cite{Li2006b}. In the second, an oscillating ribbon placed above the boundary layer, as previously performed by \cite{Dietz1999} is used. The aim is to investigate the performance of the SISO control system in attenuating single-frequency, two-dimensional disturbances, as well as wave packets generated by these configuations. The necessary plant models are obtained using system identification. Controllers are then designed based on these models and implemented in real-time to test their performance. The cancellation of the RMS streamwise velocity fluctuation of TS waves is evident over a significant domain on implementing the controllers.