Abstract
The internal steady and unsteady flows with a frequency and amplitude are examined through a backward facing step (expansion ratio 2), for low Reynolds numbers (Re=400, Re=800), using the immersed boundary method. A lower part of the backward facing step is oscillating with the same frequency as the unsteady flow. The effect of the frequency, the amplitude, and the length of this oscillation is investigated. By suitable active control regulation, the recirculation lengths are reduced, and, for a percentage of the time period, no upper wall, negative velocity, region occurs. Moreover, substituting the prescriptively moving surface by a pressure responsive homogeneous membrane, the fluid–structure interaction is examined. We show that, by selecting proper values for the membrane parameters, such as membrane tension and applied external pressure, the upper wall flow separation bubble vanishes, while the lower one diminishes significantly in both the steady and the unsteady cases. Furthermore, for the time varying case, the length fluctuation of the lower wall reversed flow region is fairly contracted. The findings of the study have applications at the control of confined and external flows where separation occurs.
Highlights
IntroductionThe suppression of the effects of the detached flow, such as pressure drop or drag, is a common goal of fluid dynamics analysis and design
Flow separation arises innately in internal and external flows, incurring losses
Backward facing step flow applications abound in everyday life
Summary
The suppression of the effects of the detached flow, such as pressure drop or drag, is a common goal of fluid dynamics analysis and design. The inner flow past a backward facing step (BFS). Constitutes a benchmark problem for the analysis of recirculation owing to its simplicity, along with the fixation of the lower wall detachment position at the step location [1,2]. Backward facing step flow applications abound in everyday life. Bubble zones at the wake of vehicles, separated flow over airfoils at large attack angles, spoiler flows, detachment of inflow in an engine, and bubble downstream flows around constructions/ships are common cases of backward facing step flow applications occuring in modern practices [3]. For the control of the flow, various methods have been applied, such as plasma actuation, electromagnetic actuation, synthetic jet, oscillating flap, inlet pulsation, periodic perturbation, vortex generators, local forcing, and visual feedback [3,4,5,6,7,8,9,10,11] etc
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