Separation bubbles at high Reynolds number: Measurement and computation
Abstract
Two flows through a sudden expansion in a pipe (an axisymmetric backward facing step) of area ratio 3.56, were studied experimentally. Both flows separated at the step, where the boundary layer was turbulent, and reattached to the pipe wall downstream enclosing a separation bubble. For one of the flows, a centerbody was mounted downstream of the step to provide an annular flow contraction and to impose a strong negative streamwise pressure gradient on the flow in the reattachment region. Both flows were effectively incompressible and of Reynolds number 1.5 x 10(exp 5). Pulsedwire techniques were used to measure velocity and skin friction. A new pulsedwire probe was developed to make detailed measurements of velocity in turbulent reversing flow between 0.15 mm and 0.55 mm from a wall. Without the centerbody, the bubble length was 10.7 step heights. For some distance downstream of the step the flow was in some ways similar to a free jet, though the shear layer grew at a slower rate and turbulence intensities were higher than in a jet. With the centerbody, the bubble length was 3.1 step heights. The separated shear layer was strongly curved towards its lowspeed side by the centerbody. This had a stabilizing effect on turbulence in the layer. The shear layer approached the wall at a larger angle and was thinner in the reattachment region than it was without the centerbody. In both cases, the nearwall flow upstream of reattachment and in the attachment region was very different to that in a normal attached turbulent boundary layer. An inviscid empirically based computation method for flows with separation bubbles was developed. The flow is computed only on the highspeed side of the timemean dividing streamline, the position of which is computed by specifying a staticpressure distribution along it. The effects of turbulence in the shear layer are modelled by specifying a distribution of stagnation pressure here, similar to that found in a fullydeveloped plane mixing layer. Several axisymmetric flows with separation bubbles were computed. The level of agreement with measurements is encouraging.
 Publication:

Ph.D. Thesis
 Pub Date:
 June 1985
 Bibcode:
 1985PhDT........27D
 Keywords:

 Axisymmetric Flow;
 Backward Facing Steps;
 Boundary Layer Separation;
 Flow Distribution;
 High Reynolds Number;
 Pipe Flow;
 Reattached Flow;
 Shear Layers;
 Turbulence Effects;
 Turbulent Flow;
 Wind Tunnel Tests;
 Computational Fluid Dynamics;
 Incompressible Flow;
 Reynolds Equation;
 Fluid Mechanics and Heat Transfer