Turbulence Production in a Jet Assessed by the Time-Repetitive Measurement of Instantaneous Velocity Distributions.

Abstract

Cross-correlation PIV is repetitively applied to obtain the time-change of instantaneous velocity vectors, which enables us to analyze turbulence kinetics. Each term in the balance equation of turbulence energy is evaluated, based on the ensembles of fluctuating velocities and instantaneous velocity-gradients, thereby finding primary factors of production and dissipation of turbulence energy. It is shown that turbulence production rate is high in the shear layer because of a large Reynolds stress, and viscous dissipation of turbulence energy occurs mainly near the jet axis. Furthermore, typical turbulence eddies in the shear layer are sampled to discuss the effect of instantaneous vortex-motions and fluctuating flows on turbulence production. It is observed that the pair of counter-rotating vortices appears and accelerates the flow in between, resulting in an increase in quantities of instantaneous Reynolds stress at a particular direction.