Experimental study of the near field and transition region of a free jet issuing from a sharp-edged elliptic orifice plate

Abstract

The mixing characteristics in the near field and transition region, the region of interest in most jet mixing applications, of an isothermal, incompressible, free jet of air issuing from a sharp-edged elliptic orifice plate of aspect ratio 2 into still air surroundings have been studied experimentally using hot-wire anemometry and a pitot-static tube. For comparison, some experiments were performed in round jets issuing from a sharp-edged orifice plate and from a contoured nozzle. All the test jets were unforced and the Reynolds number, based on the equivalent diameter of the sharp-edged elliptic orifice plate which was the same as the diameter of the sharp-edged round orifice plate or the contoured round nozzle, was 1.88 × 10 5 . The three components of the mean velocity vector, time-averaged and phase-averaged fluctuating velocity data, autocorrelation coefficients and one-dimensional spectra of the fluctuating velocities and the mean static pressure were measured. The mean streamwise vorticity, mass entrainment rates and the jet half-velocity widths have been obtained from the measured data. The results show that mixing in an elliptic jet issuing from a sharp-edged orifice plate is higher than in round jets and in elliptic jets issuing from contoured nozzles. The near field and transition region of the elliptic jet were deduced to be dominated by the axisymmetric instability mode which triggers the generation of symmetric shear layer structures. The results also show the presence of coherent structures in the near field and transition region of the jet and the effect of these coherent structures on momentum transport in the jet.