RE-EVALUATING MIXING LENGTH IN TURBULENT MIXING LAYER BY MEANS OF HIGH-ORDER STATISTICS OF VELOCITY FIELD

Abstract

A turbulent planar mixing layer is composed of two different flow types in its flow field, namely a shear layer in the central region and two free streams in each outer high- and low-speed side. Shear layer is formed after the trailing edge of the splitting plate and develops stream-wisely through successively distinct regions, namely the near field region and the self-preserving region. Two alternative definitions of the mixing lengths (lS and lF) are proposed in terms of the skewness and flatness factors, respectively, which are of third- and fourth-order of turbulence statistics. It is shown that the linear growth rate of the mixing length (either lS or lF) can be, then, used as one of the necessary and sufficient conditions to identify the achievement of the self-preserving state in turbulent mixing layer. Moreover, lF can be taken as the real length scale of the shear layer, which is of shear turbulence, bounded by the two outer high- and low-speed free streams in a given stream-wise station.