Control of flow separation from an axisymmetric body using tangentially steady bowing jets

Abstract

This work investigates flow separation control and drag reduction (DR) of an axisymmetric body using six tangentially steady blowing jets placed around the periphery of the semi-spherical after-body. The Reynolds number (Re_D) examined is from 1.2 × 10⁴ to 5.4 × 10⁵. Comprehensive measurements using hot-wire, force balance, pressure scanner, particle image velocimetry and flow visualization have been conducted with and without control. The unforced flow exhibits the characteristics of a sphere wake and may be divided into subcritical and supercritical regimes based on whether the separating boundary layer from the after-body is laminar or turbulent. The measured after-body pressure drag coefficient $C_{D,p}^a$, which is linearly correlated to DR, depends on the volume flow rate ratio (C_m) of the jets and Re_D. It is found that flow separation from the after-body can be completely suppressed, resulting in a maximum DR of 24.1 %. Furthermore, $C_{D,p}^a=g_1\left(C_m,{\mathrm{Re}}_D\right)$ may be reduced to $C_{D,p}^a/C_{D,p}^{a,0}=g_2\left(C_m\right)$, where g₁ and g₂ are two different functions and $C_{D,p}^{a,0}$ is the after-body pressure drag coefficient in the absence of control. This scaling law may be divided into three distinct regions. The flow physics associated with the three regions is discussed in detail, along with its impact upon the DR and the control efficiency. A conceptual model is proposed for the control mechanisms.