LATERAL STRAINING OF TURBULENT BOUNDARY-LAYERS .2. STREAMLINE CONVERGENCE

Experimental measurements are presented showing the effects of streaml ine convergence on developing turbulent boundary layers. The longitudi nal pressure-gradient in these experiments is nominally zero so the on ly extra rate-of-strain is the lateral convergence. Measurements have been made of mean flow and turbulence quantities at two different Reyn olds numbers. The results show that convergence leads to a significant reduction in the skin-friction and an increase in the boundary layer thickness. There are also large changes in the Reynolds stresses with reductions occurring in the inner region and some increase in the oute r flow. This is in contrast to the results of Saddoughi & Joubert (199 1) for a diverging flow of the same included angle and zero pressure-g radient which show much smaller changes in the stresses and an approac h to equilibrium. A new non-dimensional parameter, beta(D), is propose d to characterize the local effect of the convergence and it is shown how this parameter is related to Clauser's pressure-gradient parameter , beta(x). It is suggested that this is an equilibrium parameter for t urbulent boundary layers with lateral straining. In the present flow c ase beta(D) increases rapidly with streamwise distance leading to a si gnificant departure from equilibrium. Measurement of terms in the tran sport equations suggest that streamline convergence leads to a reducti on in production and generation and large increases in mean advection. The recovery of the flow after the removal of convergence has been sh own to be characterized by a significant increase in the turbulent tra nsport of shear-stress and turbulent kinetic energy from the very near -wall region to the flow further out where the stresses have been depl eted by convergence.