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
A phase-averaging technique was employed to study the evolution of flow behind an oscillating bluff plate immersed in a fiat-plate turbulent boundary layer. The experiments were performed for a reduced frequency of 0.0044. The large-scale disturbance generated by the plate developed to an organized form over 20 maximum plate height and then diffused rapidly, as quantified by the ratio of Reynolds stress of the phase-averaged fluctuation to that of the total fluctuation. The small-scale fluctuations embedded in the large-scale disturbance were almost removed by phase averaging. However, their contributions in Reynolds stress and kinetic energy were pronounced along the path of the core of the large-scale structure.
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