Reactive control of isolated unsteady streaks in a laminar boundary layer

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.

On the role of actuation for the control of streaky structures in boundary layers

“…Other applications may be found in the works of Hanson et al (2010) and Osmokrovic, Hanson & Lavoie (2014), who used plasma actuator arrays to reduce the energy of specifically targeted modes of streak disturbances generated from an array of roughness elements. Hanson et al (2014) and Bade et al (2016) provide experimental demonstrations of active control via feedback and feedforward controllers designed to target disturbances also introduced by an array of roughness elements. Papadakis, Lu & Ricco (2016) deal with a pair of Orr-Sommerfeld modes introduced in a numerical simulation and design an optimal controller to target them, obtaining significant attenuations in the resultant velocity field.…”

Opposition control of turbulent spots

“…The suction applied to the high-speed streaks, however, did not work, as it increased the disturbance level of the boundary layer. Motivated by controlling transient growth and subsequent bypass transition of the laminar boundary layer to turbulence, Bade et al (2016) developed a strategy to reactively control unsteady streaks. The streak disturbances in early stage of linear growth were detected by wall-shear-stress sensors, which provided a feedforward and feedback input to the plasma actuators.…”

Laminar Boundary-Layer Response to Step Input by an Array of Plasma Actuators

“…Due to the stochastic nature of the Bypass transition [71], a closed-loop control system will bring higher efficiency for controlling the streaks, since it enables to more accurately target the streaks [72]. Bade et al [71], used plasma actuators to control the transient growth of the streaks in a laminar boundary layer. They used a closed-loop system, by providing the boundary layer response to the generated vortices, using wall shear stress sensors.…”