Turbulence measurement in a separated and reattaching flow over a backward-facing step with the aid of three-dimensional particle tracking velocimetry

Abstract

With a three-dimensional particle tracking velocimeter, detailed turbulent flow measurement was made in a plane channel with a one-sided 50% abrupt expansion, which acted as a backward-facing step. The plane channel flow reached a fully developed state well upstream of the step. The Reynolds number based on the center-line velocity, U c, and the step height, H, was about 5540. The measurement region ranged from −2H upstream to 12H downstream of the step, and it was divided into about 300 × 120 sub-regions when the turbulence statistics were calculated. The mean reattachment position, where the forward flow fraction near the wall equaled to 0.5, was found to be located at 6.51H downstream of the step. All the distributions of mean and fluctuating velocities well demonstrated characteristics of the separated flow. As in the previous experimental investigations, the Reynolds normal and shear stresses had the maximum values upstream of the reattachment. The anisotropy tensor, which was calculated from the Reynolds stress tensor, revealed a peculiar phenomenon near the reattachment point that, even without any direct production mechanism, the spanwise normal stress was the largest among the three normal stresses. The ratio between the Reynolds shear stress and the turbulent kinetic energy showed considerably large values in the free shear layer downstream of the step shoulder.