Phase-averaged velocity in a fluidic precessing jet nozzle and in its near external field

Abstract

Phase averaged laser-Doppler measurements of the axial velocity components made within and in the near exit field of a precessing-jet nozzle have verified a number of flow features reported in the research literature. The nozzle is a short cylindrical tube with an axisymmetric inlet at one end, and with a centrebody and a small exit lip at the other end. The diameter ratio of the abrupt expansion at the inlet is 1:5. The measurements of the internal flow field reveal a radially deflected internal jet which reattaches asymmetrically and precesses around the wall of the chamber. The phase-averaged flow inside the chamber can be divided into regions of forward flow and regions of reverse flow. The distribution of these regions inside the chamber implies the presence of large-scale recirculation. Representative reverse mean flow speeds of recirculation are about 30% of the forward flow speed. Measurements inside the chamber suggest that the effect of reversed flow on the velocity decay of the inlet-jet flow is similar to that of an ambient counter flow. Measurements in the external jet suggest that the initial entrainment rate of the external precessing jet is between six and seven times that of an equivalent free turbulent jet. The phase-averaged deflection angle of the present emerging jet is 50° but this decreases to about 30° within 0.4 chamber diameters of the exit plane.