Effects of the initial condition on the axisymmetric free shear layer: Effects of the initial momentum thickness

Abstract

Spurred by large discrepancies among previous data on the free shear layer, effects of the initial condition on the characteristic measures of an axisymmetric air free shear layer were investigated experimentally. The initial, boundary layer state (i.e., laminar or turbulent), momentum thickness Reynolds number Rϑe, and fluctuation intensity u″pe/Ue have been taken as three characteristic identifiers of the initial condition. The discrepancies among published data are reviewed, and data showing the effects of variations in Rϑe (at constant u″pe/Ue) for both initially laminar and tripped (turbulent) boundary layers are reported. It is found that the spread rate, similarity parameter, and peak turbulent intensity in the self-preserving region are essentially independent of Rϑe, but dependent on whether the initial boundary layer is laminar or tripped (turbulent). Initially, tripped shear layers manifest two stages of linear growth. The virtual origin as well as the distance required for attainment of self-preservation depend noticeably and systematically on Rϑe. The mean velocity and turbulence intensity profiles appear to reach self-similarity together when the initial boundary layer is laminar, but not when the initial boundary layer is turbulent.