Axisymmetric Mixing Layer: Influence of the Initial and Boundary Conditions

Abstract

The mixing layer in the near field of a 12.7-cm-diam circular air jet has been experimentally investigated at a jet speed of 30 m/s. Data have been obtained with a computer-trave rsed hot wire for laminar as well as equilibrium turbulent exit boundary layers, both without and with a large end plate. The integral measures of the mixing layer and their streamwise evolutions show essentially no dependence on the end plane geometry (i.e., exit boundary condition) except for a small A range (x/0e — 500) when initially laminar, but show strong dependence on the initial condition (i.e., laminar vs turbulent). The mean velocity and turbulence intensity profiles and streamwise spectral evolutions indicate that the shear layers have achieved self-preservation. Initially turbulent layers show two stages of linear growth; compared to the initially laminar layers, the growth rate is lower in the first stage but higher in the second stage. The turbulent field achieves self-similarity much later than the mean field when the initial state is turbulent, but essentially together when the initial state is laminar. The virtual origin is upstream of the lip for the initially laminar layers, but downstream for initially turbulent layers. Suprisingly, the present data are in both qualitative and quantitative agreement with plane mixing layer results. The Rex range (up to 10 6) in this study is apparently insufficient to resolve the question of the asymptotic effect of the initial condition of a mixing layer.