Afterbody Effects on Axisymmetric Base Flows

Abstract

The influence of an afterbody on an axisymmetric wake flow is investigated with two-component particle image velocimetry at a Reynolds number of based on the model diameter. The reattachment distance is found to vary from two to four step heights as the afterbody diameter is increased. Measurements performed on equivalent planar backward-facing step geometries reveal that the reattachment length scales similarly to a planar backward-facing step when the afterbody to main-body diameter ratio approaches unity. The distribution of turbulent normal and shear stresses reveals a progressive inhibition of the radial fluctuations for increasing afterbody diameter, whereas a proper orthogonal decomposition analysis highlights a concurrent weakening of the shear-layer flapping mode. For , the frequency spectrum of the time coefficient of the first proper orthogonal decomposition mode appears associated with the wake flapping motion and peaks in the range of . This frequency is associated with the recirculating flow unsteadiness observed in the azimuthal plane of the wake. The configuration without afterbody features an additional contribution at , which is associated with vortex shedding. The current results reveal the dependence of low-frequency unsteadiness upon changes in the afterbody diameter.