The Initiation, Development, and Decay of the Secondary Flow in a Bounded Jet

Abstract

The secondary flow in a low aspect ratio incompressible turbulent bounded jet is described in terms of a near, middle, and far field in which the secondary motion is initiated, developed, and decayed, respectively. The initiation of the secondary flow is explained by the distortion of the planar vortex loops which bound the jet at the exit plane. In the region away from the bounding plates, the vortex loop distortion is similar to that found in rectangular free jets; however, the bounding plates cause an additional production of streamwise vorticity near the plates which has no counterpart in the free jet flow. Downstream of the jet core region, a large-scale secondary flow develops from this vorticity. Farther downstream the secondary flow decays; the resultant flow may be characterized as a combination of a plane jet and boundary layer flows. This explanation is supported by the vorticity and velocity data of this investigation. Velocity measurements of this study are sufficiently comprehensive to allow formulation and evaluation of several quantitative measures of the secondary flow strength. The average (over a transverse plane) momentum flux thickness, and the far field behavior show the secondary flow to be dynamically passive. Properly nondimensionalized transverse velocity profiles exhibit characteristic distortions from a basically “self-similar” shape from which the center of the secondary flow rotation can be determined. Integrals of the velocity data allow the inference of mass transport across planes parallel to the bounding plates.