Investigation of the coherent structures in flow behind a backward-facing step

Purpose - The purpose of this paper is to investigate the characteristic flow structures behind a backward-facing step. With better understanding of unsteady features, effective control practice with harmonic actuation is illustrated. Design/methodology/approach - The present study employs Improved Delayed Detached Eddy Simulation to resolve flow turbulence with a finite-volume approach on structured grid mesh. The coherent structure is displayed through temporal- and spatial-evolution of pressure fluctuations. Characteristic frequencies in different flow regions are extracted using fast Fourier transform. Dynamic mode decomposition method is applied to uncover the critical dynamic modes. Findings - The time- and spanwise-averaged quantities agree well with experimental data. It is observed that two distinct modes exist: shear layer mode and shedding mode. The former is related to Kelvin-Helmholtz instability mechanism, vortex pairing and step mode with non-dimensional frequency, St(h,st) at around 0.2. The latter is of multi-scale, with a typical coherent structure shedding frequency, St(h,st) at 0.074. Step mode interacts with shedding mode in the reattachment region, resulting in the low-frequency characteristics. Originality/value - An optimal excitation frequency to reduce recirculation bubble length is obtained at about St(h,st) = 0.2 with an explanation.