Identifying Dominant Flow Structures and their Frequencies in a DNS of a Backward Facing Step for Flow Separation Control

This computational study complements a joint experimental effort in identifying dominant flow structures, their frequencies, and the correlations with the wall pressure signal, in a turbulent flow over a backward facing step. A long-term goal of this study is to identify potential strategies for a design of metamaterials to control flow separation in turbulent flows. To this end, we perform Direct Numerical Simulations (DNS) of flow over a backward-facing step at ReH = 5100 with a spectral-element method. To provide a turbulent inflow to the step, we perform simultaneous auxiliary simulations of a turbulent half-channel flow that intersects with the step domain. We apply spectral analysis and reduced-order modeling techniques to extract dominant spatial and temporal patterns from the flow. We focus on key regions: upstream, near-wake, far-wake, and the recirculation bubble to understand how different flow features, such as shear layer flapping and vortex shedding, are reflected in wall signals. Two main questions guide our work: 1) What are the dominant frequencies driving the separated flow? 2) Where should metamaterial strips be placed to interact with these dynamics? Our results suggest that upstream and beneath the recirculation bubble are promising regions for control, as they show strong, coherent signatures in both pressure and shear stresses.