Active control of streamwise vortices and streaks in boundary layers

Abstract

Coherent structures play an important role in the dynamics of turbulent shear flows. The ability to control coherent structures could have significant technological benefits with respect to flow phenomena such as skin friction drag, transition, mixing, and separation. This paper describes the development of an actuator concept that could be used in large arrays for actively controlling transitional and turbulent boundary layers. The actuator consists of a piezoelectrically driven cantilever mounted flush with the flow wall. When driven, the resulting flow disturbance over the actuator is a quasi-steady pair of counter-rotating streamwise vortices with common-flow away from the wall. The vortices decay rapidly downstream of the actuator; however, they produce a set of high-and low-speed streaks that persist far downstream (well over 40 displacement thicknesses). The amplitude of the actuator drive signal controls the strength of the generated vortices. The actuator is fast, compact, and generates a substantial disturbance in the flow. Its performance has been demonstrated using a small array of sensors and actuators in low-speed water laminar boundary layers with imposed steady and unsteady disturbances. Experiments are reported in which transition from a large disturbance was delayed by 40 displacement thicknesses, and in which the mean and spanwise variation of wall shear under an array of high-and low-speed streaks was substantially reduced downstream of a single transverse array of actuators. † Present address:

Active control of turbulent boundary layer using an array of piezo-ceramic actuators

“…Therefore, the averaged spanwise ( z ) and longitudinal ( x ) spacing between near-wall steaks, and the bursting frequency (f B ) may be estimated to be 10 mm, 100 mm, and 7 Hz, respectively, provided that the present flow is a canonical turbulent boundary layer (Kline et al 1967, Blackwelder & Eckelmann 1977. In order to disturb the coherent structures in a turbulent boundary layer, the physical size of an actuator is required to be in the order of 20 wall units and 200 wall units in the spanwise and streamwise directions, respectively, and its excitation frequency has to be at least 5 times of the bursting frequency (f B ) (Jacobson & Reynolds 1998). The actuators used currently satisfy these requirements.…”

Effect of Heat Transfer on Materials Selection for Bimaterial Electrothermal Actuators

“…15(a) and (b) shows the contours of equal actuation frequency for the two cases; L = 60 µm and L = 100 µm, with the operating temperature range of ∼423 K−573 K, respectively. Hitherto, piezoelectric actuators [27], [28] and electrostatic actuators [29] have been preferred for applications such as boundary-layer flow-control devices, which operate at high frequencies on the order ∼10 kHz. The results presented herein suggest the point at which electrothermal actuators might be considered as viable alternatives.…”

“…By normalizing (26), (27), and (28) with respect to geometry, indices for effectiveness based on the displacement, force, and work per cycle can be obtained…”

Experimental control of natural perturbations in channel flow

“…Nonetheless, it exceeds previous values from experimental studies by a considerable margin: as mentioned above, Rathnasingham & Breuer (2003) apply related techniques to control natural flow disturbances in a turbulent boundary layer at Re = 1960 (based on the momentum thickness) and report a maximum localized reduction of 30 % in the streamwise velocity fluctuations; more recently, Lundell (2007) used a threshold-and-delay control algorithm and achieved a maximum disturbance reduction of 18 %. A related study by Jacobson & Reynolds (1998) attempted to control disturbances in an Re = 600 boundary layer, but introduced the upstream perturbations artificially and deterministically.…”