Experimental Study on Unsteady Stuctures in a Reattaching Separated Flow Field

Separated and reattaching flow fields observed in engineering equipments are extremely complex because of its unsteadiness and three-dimensional structure. It is well known that of organized large-scale vortical structures appear in the separated shear layer. The purpose of this study is to explore experimentally the large-scale vortices in a separated and reattaching shear layer evolving from a backward facing step edge and to find the width of the free stream and the periodicity of motion of large-scale vortices. In this investigation, the reattachment length of the shear layer was determined based on the measurements with a thermal-tuft probe. The wall pressure and the velocity fluctuations were measured with a pressure sensor and an I type hot wire probe respectively. Those data were analyzed by a FFT analyzer. The obtained results are described as follows. (1) The reattachment length increases with the expansion ratio, W_2/W_1. When the distance X in the flow direction is normalized by the width of reattachment region ΔX_R, the forward flow fraction distributions at different expansion ratio, W_2/W_1, fall on one curve near the reattachment region. (2) According to analysis of the frequency of pressure fluctuations, the frequency of large-scale vortices is f・H/U_0=0.7, where H is height of the step. f is frequency and U_0 is velocity of the main flow. (3) the peak of power spectrum of the velocity fluctuations is f・H/U_0=0.3～0.5