Measurements in plane separated flow

Abstract

This paper reports an experimental study of turbulent separated flow in water over a double backward facing step. The dimension of the step was 2.5 mm in depth expanding symmetrically to 7.5 mm. The duct was 76 mm wide and 300 mm long and time mean streamwise, transverse and cross-stream components of velocity and turbulent fluctuation were measured using a laser Doppler anemometer incorporating frequency tracking and frequency shifting of the incident beams. For all measurements water flowed upstream of the step at an average velocity of 1.8 m/s corresponding to a Reynolds number, based on upstream duct width of 6 × 10 3. Substantial changes of flow patterns, and increases in kinetic energy of turbulence and velocity fluctuations were detected as a consequence of the double step expansion. Turbulent intensities of up to the order of 100% have been measured in the recirculation zones. Transverse turbulent intensities based on the mean duct exit velocity were in general about 50% of the stream wise components. The maximum streamwise turbulent velocity measured was 150% more than the corresponding value at the step. A contour plot of non-dimensionalised constant turbulent kinetic energy compares favourably with similar results obtained for other duct configurations. The effect on heat transfer in asymmetrical flows is discussed.