Measurements and image analysis of the turbulent field in an axisymmetric 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 visualization provides a qualitative understanding of the response of the jet structure, and the temporal evolution of the mean and fluctuating velocity field is measured by using X-hot-wire probes. The longitudinal adaptation of the perturbation time scale to the local jet time scale can be described by showing a local self-similarity property of the flow. Transient mean radial velocities are deduced, and a large radial inflow of external fluid is found to occur during the phase of velocity decrease. The anisotropic reaction of the turbulent second-order moments reflects the particular energy-transfer mechanisms encountered in free shear flows. A local negative energy production from the turbulent field to the mean field appears on the jet axis during the transient evolution. Moreover, we can expect that the kinetic energy budget measured in steady jets is perturbed.