Control of separated flow by a two-dimensional oscillating fence

Abstract

Control of separated flow behind a backward-facing step using a two-dimensional oscillating fence installed upstream has been investigated in this work. Parameters of the flow considered included the reduced frequency of the oscillating fence, the distance from the oscillating fence to the backward-facing step, the ratio of the maximum height of the oscillating fence to the step height, and the Reynolds number. It was found that with the experimental parameters properly selected the time-mean reattachment length of the separation region could be reduced over 40%, compared to the case without the presence of an oscillating fence. The evolution of unsteady flow behind a backward-facing step was further studied in detail by a phase-averaging measurement technique. The results obtained indicate that suppression of the separated flow behind the step is mainly due to the downwash motion induced by the vortical structure released upstream from the oscillating fence, when it convects over the step. Nomenclature / = frequency of the oscillating fence h = instantaneous height of the fence h f = maximum height of the fence h s = height of backward-facing step, =1.5 cm I r = inter mitt ency function defined in Eq.(2) K = reduced frequency, =fhf/U 0 K cr = critical reduced frequency, above which organized vortical structure develops behind the oscillating fence as it extends into the flow L s = distance from the oscillating fence to the step R eQ = Reynolds number, = U 0 /0v t = time T = time period of the oscillating motion of the fence U, u = time-mean stream wise velocity and streamwise velocity fluctuation U 0 = freestream velocity measured at the inlet of the test section U a = streamwise growth rate of the vortical flow structure behind the oscillating fence X = streamwise coordinate X r = time-mean reattachment length of the separated flowbehind the backward-facing step, without the presence of the oscillating fence, but with a squarewave trip upstream X' r = time-mean reattachment length of the separated flow behind the backward-facing step measured Y = vertical coordinate Z = spanwise coordinate 6 = boundary-layer momentum thickness measured at the step, without the presence of the oscillating fence v = kinematic viscosity of air (/> = phase angle of the oscillating motion of the fence Q z -time-mean vorticity in the Z direction -= time-mean quantity < > = ensemble-averaged quantity AX r = the reduction of the time-mean reattachment length measured, -X' r -X r

Control of the separated flow downstream of a two-dimensional fence by low-frequency forcing

“…Miau et al [7] report experiments with a plate oscillating perpendicular to the wall. With this device, they successfully reduced the length of the recirculation zone behind a backward-facing step in a turbulent flow [8].…”

Flow structures behind a vertically oscillating fence immersed in a flat-plate turbulent boundary layer

“…This configuration can be regarded as a basic flow model of the experiments of Francis et al (1979) and is relevant to other separated flows (Miau et al 1991). Lu and Huang (1989) visualized a similar flow with emphasis on the development of the large-scale vortical structure formed behind the oscillating plate.…”

Active Control of Pressure Fluctuations Due to Flow Over Helmholtz Resonators

“…The organized #ow structure convected behind oscillated spoilers and fences immersed in a turbulent boundary layer was studied for vehicle drag reduction and airfoil lift enhancement by Nelson et al [13], and Miau et al [14]. Oscillated spoilers and #aps have been used previously for the control of vortex shedding behind cylinders or the control of airfoil #utter.…”