DEVELOPMENT OF A 2-STREAM MIXING LAYER FROM TRIPPED AND UNTRIPPED BOUNDARY-LAYERS

A two-stream mixing layer with a velocity ratio of 0.6 was generated with two different initial conditions; one with both initial boundary layers laminar and one where both boundary layers were tripped. Some recent measurements have shown that relatively large spanwise variations can occur in the mean flow and turbulence properties of plane mixing layers, especially in the untripped case. Therefore, for the first time, all the data presented here were averaged over several spanwise locations. The results indicate that both the near- and far-field growth rates for the untripped case are significantly higher than for the tripped case. The maximum Reynolds stresses and higher order products for the two cases behave very differently in the near-field, but asymptote to approximately the same constant levels far downstream. The mean velocity and turbulence profiles in this region also collapse adequately for the two cases when plotted in similarity coordinates. The distance required to achieve self-similarity was found to be distinctly shorter for the tripped case, in contrast to previous observations. The higher growth rate for the untripped case is attributed to the presence of streamwise vortices, which result in additional entrainment by the mixing layer.