Study on Double Coaxial Pipe Jets. 2nd Report. Vortex Formation in Nozzle.

Abstract

Flow of double coaxial pipe jets which consist of a central round air jet (an inner jet) and an annular air jet (an outer jet) has been experimentally studied. Several nozzles with outer pipe lengths of L/D0=0 to 2 and different widths B of the outer annular jet were employed to examine the effects of self-excitation by the outer pipe. The strong intensity of fluctuating velocity appears in the nozzle with the pipe lengths of L/D0=0. 5 to 1. The characteristics of the jet have great resemblance to those of a self-excited axisymmetric jet driven by a whistler nozzle (i. e., pipe-collar). It is found that the values of the Strouhal number StL based on the pipe length L are constant, and that these values increase in a stepwise manner to the values equal to integral multiples, with further increase in the L/Bratio. The results of the measured distributions of vorticity and the flow visualization show the coherent structures in the inner and outer mixing regions of the jets. It is confirmed that the strong intensity of fluctuating velocity is related to the subharmonic component by vortex pairing.