MEASUREMENTS AND IMAGE-ANALYSIS OF THE TURBULENT FIELD IN AN AXISYMMETRICAL JET SUBJECT TO A SUDDEN VELOCITY DECREASE

The unsteady turbulent far field of a transient axisymmetric turbulent jet is studied experimentally. The initial flow perturbation consists of a sudden and large decrease in the ejection velocity. Flow visuali zation provides a ualitative understanding of the response of the jet structure, and the temporal evolution of the mean and fluctuating velo city field is measured by using X-hot-wire probes. The longitudinal ad aptation of the perturbation time scale to the local jet time scale ca n be described by showing a local self-similarity property of the flow . Transient mean radial velocities are deduced, and a large radial inf low of external fluid is found to occur during the phase of velocity d ecrease. The anisotropic reaction of the turbulent second-order moment s reflects the particular energy-transfer mechanisms encountered in fr ee shear flows. A local negative energy production from the turbulent field to the mean field appears on the jet axis during the transient e volution. Moreover, we can expect that the kinetic energy budget measu red in steady jets is perturbed. (C) Elsevier Science Inc., 1997.