Large and small scales in a turbulent orifice round jet: Reynolds number effects and departures from isotropy

Abstract

An experimental investigation of the near field of a turbulent orifice jet is performed using high resolution Particle Image Velocimetry, aiming to highlight effects on the flow field due to changes in Reynolds number. The attention is focused onto departures from isotropy for large and small scales, by considering statistics of mean square velocity and velocity derivatives and specifically the non-dimensional ratios of such quantities. The results compare well with available literature data and pointed out that the effects of Reynolds number on large scales are usually small and limited to a region ranging less than seven-ten diameters from the jet outlet. For small scales, such Reynolds number dependence is extended up to ten-fifteen diameters. Farther from the jet exit, Reynolds number dependence almost disappears and all data approach similar asymptotic behaviors. On the other hand, velocity and some velocity derivative statistics clearly show that neither large nor small scale statistics strictly follow the isotropy condition; nonetheless, differences from that condition are limited to a factor which is almost constant in the whole measured field. In order to provide a link between such large and small scale departures from isotropy, a relation among mean square velocity ratios and mean square derivative ratios is proposed and proved to be well verified in the measured region and interval of Reynolds numbers. This relation allows deriving small scale derivative ratios, which are difficult to measure experimentally or to obtain numerically, due to high resolution requirements, from large scale velocity ratios, which are achieved much easier.