Experimental Investigation of Swirling Vortex Motion in Jets

Abstract

Experiments have been carried out in a series of axisymmetric free turbulent jets with degrees of swirl covering the weak, moderate, and strong ranges, including the case of the onset of reversed flow in the central region of the jet. Measurements are reported of mean axial and swirl velocities, static pressure, and jet width at axial stations up to 15 orifice diameters. Mean velocity and pressure profiles are shown to be effectively similar from an axial distance of four diameters for weak and moderate swirl. For the case of strong swirl, a vortex is generated in the region close to the orifice resulting in a displacement of the axial velocity maximum from the jet axis. After a distance of 10 diameters, the influence of the vortex motion becomes small, and similarity of the profiles is obtained farther downstream. Experimentally determined profiles are described in terms of Gaussian error curves and third-order polynomials. Jet width and mass flow rates of entrained fluid are shown to increase according to the degree of swirl so that, for strong swirl, jet width and rate of entrainment are almost twice those for the nonswirling jet. Results of the decay of velocity and pressure along the axis are compared with values predicted by an approximate theory based on the integration of the Reynolds’ equations of motion. Good agreement is found between results and predictions, and a set of semi-empirical equations is provided from which a complete description of the mean velocity and pressure fields can be obtained for swirling jets.