The bifurcation of circular jets in crossflow

Abstract

An experimental study of an incompressible circular jet in a crossflow and theoretical analysis based on inviscid flow models are described. The jet exits from a rigidly mounted pipe projecting distant from the floor of a tunnel carrying a steady stream of water; density of the jet and the stream are the same. The results of scalar and velocity measurements and visualizations showed that the jet bifurcated into two separated, counterrotating arms for values of ε=U∞/UJET, the ratio of the mean crossflow velocity U∞ to the mean jet discharge velocity UJET, less than or equal to 0.25. The angle of separation between the two arms of the bifurcated jet was found to vary inversely with ε. For higher values of ε the jet does not bifurcate but is dominated by a different mode of instability. The structure of the flow field, which is different for bifurcated and nonbifurcated jets, comprised a variety of vortical structures which survived for very long distances x beyond x/2a≳400, where a is the radius of the jet exit and x is distance downstream from the jet axis. The location of the point of bifurcation is predicted from consideration of potential flow models and the characteristics of bifurcating elliptical jets. The location of the point of bifurcation is more distant from the jet exit for smaller values of ε, and experimental results were in good agreement with the theoretical predictions. The initial jet trajectory is shown to be associated with the presence in the wake of vorticity shed from the pipe. The near-field geometry and centerline trajectory of the jet are also found to be in accord with predictions in that it is observed that z∼x1/2 and z∼ε−1. Dilutions of bifurcated jets are found to be greater than for nonbifurcated jets.