Fundamental Entrainment Observations (VSL, etc.) for a SSSL

Fundamental observations of the entrainment process on the low speed side of a high $Re$ self-preserving single stream shear layer have been made using PIV realizations. The $Re$ value was: $U_0 \theta_{mid}/\nu$ = 6.75*$10^4$, where $\theta_{mid}$ = 13.7 cm is the momentum thickness at the mid-location ($x/\theta(0)$ = 390) of the observations. The VSL (Viscous Super Layer), 15-20 $\eta_K$ thick, is bounded by a well-defined border where the non-vortical/vortical transition occurs. The Kolmogorov microscale ($\eta_K$) was determined from the mean-square vorticity adjacent to the VSL. A threshold level to define the border ($\omega_z \theta_{mid}/U_0$ = 0.221) was selected by examination of the data. Quantitative measures of the entrainment process have been obtained, including: $i$) the convoluted length of the border ($L_b$) made non-dimensional with respect to the length ($L_m$) of the temporally averaged flow field ($L_b/L_m$ = 2.8) and $ii$) $\langle v_b^2 \rangle / v_e^2$ = 17, as a measure of the sink-effect at the border. $v_b$ is the measured velocity at the border; $v_e$ is the well-established entrainment velocity far from the active shear layer whose value: $v_e / U_0$ = 0.035, corresponds to the growth of the self-preserving SSSL (d$\theta /$d$x$).