The threedimensional turbulent flow past a backwardfacing step
Abstract
The incompressible turbulent flow over a backwardfacing step in a rectangular duct was investigated experimentally. The experiments were conducted in a singlesided sudden expansion duct having a fixed expansion ratio (downstreamtoupstream channel height) equal to two and a variable aspect ratio (step spantostep height) capability with a maximum AR = 30. The working fluid was air at atmospheric conditions and the Reynolds number, based on a freestream velocity of 16 m/s and a step height of 25.4 mm, was 26,500. Extensive surface flow visualizations were performed on the step, side, and flat walls for aspect ratios ranging from 1 to 28. These visualizations revealed a pair of counter rotating eddies, with axes normal to the step wall, immediately downstream of the step and near the side walls. Furthermore, flat wall visualizations revealed a three dimensional separation bubble downstream of the step. The spanwise extent and streamwise location of this bubble significantly depended on aspect ratio, and a critical aspect ratio existed below which no bubble appeared. Mean values of the pressure coefficient measured on the step and flat walls, and downstream of reattachment, were observed to decrease with smaller aspect ratios. Centerline fluctuating pressure levels near flow reattachment increased to a maximum at AR = 4, but decreased for smaller aspect ratios. The dependence on aspect ratio of the mean pressure coefficient leads to the introduction of a new scaling parameter, the downstreamtoupstream hydraulic diameter ratio, which was used to define a new normalized pressure coefficient that leads to the collapse of pressure distributions onto a single curve. Finally, thermal anemometry was used to perform twocomponent velocity measurements along the centerplane and in the nearwall region downstream of flow reattachment. The centerplane velocity field showed that the proximity of the side walls tends to inhibit the relaxation process past reattachment for smaller aspect ratios. Higher centerplane streamwise and transverse velocities obtained in the redevelopment region for AR = 2 and 4 indicated a threedimensional mean flow structure along the full span of the tunnel. The nearwall streamwise and spanwise velocities for larger aspect ratios were characterized by sharp spanwise gradients near the side walls.
 Publication:

Ph.D. Thesis
 Pub Date:
 1994
 Bibcode:
 1994PhDT........14P
 Keywords:

 Aspect Ratio;
 Backward Facing Steps;
 Ducted Flow;
 Flow Visualization;
 Incompressible Flow;
 Separated Flow;
 Three Dimensional Flow;
 Turbulent Flow;
 Wind Tunnel Tests;
 Counter Rotation;
 Pressure Distribution;
 Reattached Flow;
 Velocity Distribution;
 Vortices;
 Fluid Mechanics and Heat Transfer