Reactive control of isolated unsteady streaks in a laminar boundary layer. (21st April 2016)

• Record Type: Journal Article
• Authors: Bade, Kyle M.; Hanson, Ronald E.; Belson, Brandt A.; Naguib, Ahmed M.; Lavoie, Philippe; Rowley, Clarence W.
• Is Part Of: Journal of fluid mechanics. Volume 795(2016:May)
• Page Start: 808
• Page End: 846
• Publication Date: 2016-04-21
• Subjects: boundary layer control, -- flow control, -- instability control; Fluid mechanics -- Periodicals; 532.005
• DOI: 10.1017/jfm.2016.200
• Languages: English
• ISSNs: 0022-1120

Abstract

Abstract : This study is motivated by controlling transient growth and subsequent bypass transition of the laminar boundary layer to turbulence. In experiments employing a model problem, an active roughness element is used to introduce steady/unsteady streak disturbances in a Blasius boundary layer. This tractable arrangement enables a systematic investigation of the evolution of the disturbances and of potential methods to control them in real time. The control strategy utilizes wall-shear-stress sensors, upstream and downstream of a plasma actuator, as inputs to a model-based controller. The controller is designed using empirical input/output data to determine the parameters of simple models, approximating the boundary layer dynamics. The models are used to tune feedforward and feedback controllers. The control effect is examined over a range of roughness-element heights, free stream velocities, feedback sensor positions, unsteady disturbance frequencies and control strategies; and is found to nearly completely cancel the steady-state disturbance at the downstream sensor location. The control of unsteady disturbances exhibits a limited bandwidth of less than 1.3 Hz. However, concurrent modelling demonstrates that substantially higher bandwidth is achievable by improving the feedforward controller and/or optimizing the feedback sensor location. Moreover, the model analysis shows that the difference in the convective time delay of the roughness- and actuator-induced disturbances over the control domain must be known with high accuracy for effective feedforward control. This poses a limitation for control effectiveness in a stochastic environment, such as in bypass transition beneath a turbulent free stream; nonetheless, feedback can remedy some of this limitation.