Measurements of the growth rate and structure in plane turbulent mixing layers

Abstract

Mean velocity and turbulent intensity measurements are reported for five different plane turbulent mixing layers, each with a different velocity ratio. These experiments confirm that increasing free-stream turbulence causes increases in the growth rate and in the Reynolds stresses. Cross-correlation measurements with time delay made in the mixing layer with the lowest free-stream turbulence level show that the large-eddy structure had length scales in the two cross-stream directions which were roughly equal, unlike the results reported by Brown & Roshko (1974) and others. Further measurements showed that a vortex-street wake existed immediately downstream of the splitter plate and that transition occurred in the wake flow rather than in a normal laminar mixing layer. This is thought to have prevented the Brown–Roshko structures from forming. Comparison of the growth rate observed in this case with other measured results suggests that the essential or effective turbulent structure in mixing layers is independent both of velocity ratio and of the degree of two-dimensionality which exists in the largest scales of turbulence.