SOME EFFECTS OF SWIRL ON TURBULENT MIXING LAYER DEVELOPMENT

An experimental study has been conducted to investigate the effects of swirl on the streamwise development of an axisymmetric turbulent mixing layer. The initial boundary layer was tripped well upstream of the lip. All three mean velocities and the six independent components of the Reynolds stress tensor were measured using a rotatable cross-wire probe for three swirl numbers (0, 0.2, and 0.4). The zero swirl case was found to develop as expected, with a linear growth rate and the peak Reynolds stresses approaching constant asymptotic levels. The results show that the swirling mixing layer is affected by the angular momentum instability over almost the whole layer. The effect of the instability is to increase all the peak Reynolds stresses within the mixing layer, with the peaks increasing monotonically with swirl number. The largest increases occurred in the spanwise normal stress and the two secondary shear stresses. A part of the increase in the turbulence stresses is attributed to "extra" production through terms activated by the angular momentum instability. The thickness and growth rate for the lower swirl rate were found to be comparable to the zero swirl case, while those for the higher swirl rate were increased significantly, in line with the increased Reynolds stresses. This nonmonotonic behavior of the mixing layer growth rate is attributed to the effects of a favorable pressure gradient in the core flow.