PHASE AVERAGED VELOCITY-FIELD IN AN AXISYMMETRICAL JET SUBJECT TO A SUDDEN VELOCITY DECREASE

An unsteady transient axisymmetric turbulent jet was studied experimen tally. The initial how perturbation consisted of a sudden and large de crease in the ejection velocity. The temporal evolution of the mean an d fluctuating unsteady velocity held was measured by using X hot-wire probes. In the jet far field, adaptation of the externally imposed uns teadiness to the local jet time scale is confirmed quantitatively. The main features of the phase averaged velocity field are presented and comments are made about the instantaneous state of the turbulence ener getics. Transient mean radial velocities are deduced and an important increase of the instantaneous rate of entraining external fluid into t he jet is found. Finally, we show that the pressure effect due to radi al impusle terms plays an important role in the propagation of the mea n perturbation. The longitudinal adaptation of the perturbation time s cale driven by the local jet time scale provides a turbulent how that is intermediate to quasi-static flows and rapidly distorted flows.