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

Abstract

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.

Citation

Hu, R., Wang, L. and Fu, S. (2016), "Investigation of the coherent structures in flow behind a backward-facing step", International Journal of Numerical Methods for Heat & Fluid Flow, Vol. 26 No. 3/4, pp. 1050-1068. https://doi.org/10.1108/HFF-09-2015-0403