Flow Characteristics and Shear-Layer Vortex Shedding of Double Concentric Jets

Abstract

The characteristics of the flow structure and shear-layer vortex shedding of double concentric jets separated by a disk are studied experimentally. Average flow patterns in the prepenetration, transition, and penetration regimes are measured. In the prepenetration regime, five characteristic flow structures are identified in the shear layer evolving from the edge of the circular disk: laminar, subcritical, transitional, supercritical, and fully turbulent modes. Periodic vortices with small fluctuations shed intermittently along the subcritical shear layer. The transitional shear layer is characterized by chaotic motion. Periodic turbulent vortices shed intermittently in the supercritical shear layer. The Strouhal number of the subcritical vortex shedding increases with the increase of annulus Reynolds number. It is constant in the supercritical mode. With the increase of central jet Reynolds number, the Strouhal number increases to a maximum at a central/annular Reynolds number ratio of 0.11 and then decreases. The Taylor's integral length scale of the subcritical shear-layer vortex shedding decreases rapidly with the increase of annulus Reynolds number. It remains constant at about 0.4 disk diameters in the supercritical mode. The Reynolds stresses of the shear layer and the stagnation point are presented and discussed. At the maximum Strouhal number of shear-layer vortex shedding in the prepenetration regime, the turbulent kinetic energy attains a maximum in the shear layer and at the stagnation point. Nomenclature D = diameter of circular disk, 20 mm d = exit diameter of central jet, 3.4 mm / = frequency of shear-layer vortex shedding, Hz Lr = axial length of recirculation zone, from center of disk to aft stagnation point t - Taylor's integral length scale Rea = Reynolds number of annular jet (=ua D/v) Rec = Reynolds number of central jet (=ucd/v) r = radial coordinate, originating from center of circular disk rlvc = radial coordinate of inner vortex center rovc = radial coordinate of outer vortex center Sr = Strouhal number of vortex shedding (=fD/ua) u = instantaneous axial velocity ua = average exit velocity of annular jet uc = average exit velocity of central jet u_ = mean axial velocity u'u1 = axial normal stress of turbulence u'v' = turbulent shear stress tj_ = mean radial velocity v'v' = radial normal stress of turbulence x = axial coordinate, originating from center of circular disk ;tivc = axial coordinate of inner vortex center jcovc = axial coordinate of outer vortex center v = kinematic viscosity of air