Abstract

Sensing and cancellation of streaks early within their growth extent is key to the delay of bypass transition and eventual turbulence. Hanson et al., [1], [2], utilised feedforward and/or feedback control to weaken artificially induced streaks in a Blasius boundary layer; Lundell [3] demonstrated bypass transition delay, utilising wall-shear sensing, but manually-tuned feedforward control. In contrast, the current work aims to delay bypass transition utilising feedforward and feedback controllers driving plasma actuators. The experimental setup, Figure 1, comprises a turbulence-generating grid ahead of a flat plate with a sharp leading edge and a small trailing edge flap. Naturally-occurring high and low-speed streaks exhibiting linear transient growth were observed within the first 300 mm, eventually followed by turbulent spot formations first detected at a streamwise location x ≈ 350 mm from the leading edge. Two wall-shear-stress sensors (SU and SD) and two plasma actuators (capable of producing positive and negative wall-normal forcing to oppose high and low-speed streaks respectively) were placed in the linear growth region. SU detects streaks in their early stages of growth. A Feedforward (FF) control system makes use of the output from SU and combined with single-point Linear Stochastic Estimation (LSE), an estimate of the streaks’ streamwise shear disturbance τ at the x location of SD is obtained. The estimate is used to modulate the plasma actuator voltage to produce a counter-disturbance which, at SD , is equal in magnitude and opposite in sign to the estimate. A Feedback (FB) loop uses any output from SD in a PI controller to correct for remaining, uncancelled disturbances resulting from, for example, inaccuracies in the LSE model of the dynamics of streak growth. Notable changes in the mean and rms wall normal velocity profiles, turbulent spot intermittency and energy spectra, for FF, and combined FF + FB control, demonstrate the viability of the control scheme to weaken boundary layer streaks and delay bypass transition.