A note on the stabilization of bluff-body wakes by low density base bleed

Abstract

Base blowing is a simple and well-known means of stabilizing both two-dimensional and axisymmetric bluff-body wakes at supercritical Reynolds numbers. In the present Brief Communication we consider a generalization of previous works by studying the effect of the bleed-to-freestream density ratio, S=ρb∕ρ∞, on the stability properties of the wake behind an axisymmetric, slender body with a blunt trailing edge. Since a lower density in the slow stream is known to inhibit absolute instability, here we restrict our attention to the case of light bleed fluid, S<1. This mechanism is shown to inhibit the wake for values of the bleed coefficient, defined as the bleed-to-freestream velocity ratio Cb=ub∕u∞, smaller than those obtained in the homogeneous case of S=1. Our approach consists of obtaining the basic, axisymmetric flow by integration of the full set of Navier-Stokes and species conservation equations in cylindrical coordinates, and calculating its linear, local, spatio-temporal stability downstream of the body base. The analysis predicts a critical bleed coefficient Cb*, which decreases with the density ratio according to the linear law Cb*≃0.01+0.05S for 0.1⩽S⩽1.