Abstract
Direct numerical simulations of the incompressible turbulent flow over the backward-facing step have been performed. To control the separation flow, the traveling wave-like body force control is applied to the top surface of the step. We choose four different cases to discuss the control effect on the reattachment length: stationary wave cases with the opposite phase and the downstream and upstream traveling wave cases. The reattachment length decreased in all the cases; however, the decrease patterns differed. The stationary wave affects the velocity of the flow above the step, which reduces the reattachment length. In traveling wave cases, the strong separation bubble is generated upstream and released periodically (i.e., oscillation). The phase-average analysis shows that the period of the traveling wave dominates the oscillation of the separation bubble. The process of separation bubble oscillation can be divided into two parts. The negative body force instantly induces the flow downward after flow separation, which generates new separation bubbles and pushes out the original one. The positive body force induces the flow upward, which increases the size and rotation of the separation bubble. Therefore, the release of the separation bubble is attributed to this switch. The three-component decomposition clarifies contributions of traveling waves and turbulence.
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