Experimental study of turbulence in plane separated flows

The improvement of the pulsed hot wire anemometer and the laser Doppler anemometer have made possible such studies and this paper describes an investigation using the laser Doppler anemometer in a separated flow situation. Results are given of an experimental study of the plane turbulent separated flow of water over a double backward facing step of duct height ratio 3:1. Time mean streamwise, transverse and cross-stream components of velocity are reported, together with the turbulent velocity fluctuations. The measurements are made using a laser Doppler anemometer operating in the differential Doppler mode with forward scattering. The problem of high turbulence intensity and instantaneous flow reversals was overcome by frequency shifting of the incident laser light beams using a pair of Bragg cells. Substantial changes in flow patterns are detected together with turbulence intensities based on local streamwise velocity of up to 100% were recorded in the recirculating zones.