Afterbody Effects on Axisymmetric Base Flows

摘要

The influence of an afterbody on an axisymmetric wake flow is investigated with two-component particle image velocimetry at a Reynolds number of Re-D = 6.7 . 10(4) 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 d/D <= 0.4, 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 St(D) = 10(-3) -10(-2). 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 St(D) = 0.2, which is associated with vortex shedding. The current results reveal the dependence of low-frequency unsteadiness upon changes in the afterbody diameter.