Observations of turbulent reattachment behind an axisymmetric downstream-facing step in supersonic flow

Abstract

Supersonic flow over a downstream-facing step on the circumference of a large, ducted, axisymmetric body was used to study flow reattachment. Step heights h were 0.25, 1.00, and 1.68 in., compared to a body radius of 6 in. Freestream Mach numbers were in the range 2 to 4.5. Theturbulent boundary-layer thickness just ahead of the step varied from 0.14 to 0.19 in. (momentum thicknesses of about 0.01 in.). Surface pressure distributions throughout the region of separation and reattachment were measured, and points of reattachment were determined. Comparison of the shapes of the pressure distributions for various step heights shows that the initial (steepest) parts of the reattachment pressure rise, up to the point of reattachment, tend to become superimposed when plotted against x/h. Downstream reattachment the curves branch out, exhibiting a dependence on geometry and probably on initial shear layer profile. In the region of the initial pressure rise (near the end of the "dead air" region) dynamic pressures are low; the pressure rise there apparently is balanced by turbulent shear stress.