SHEAR-FLOW TURBULENCE STRUCTURE AND ITS LAGRANGIAN STATISTICS

Rapid distortion theory and kinematic simulations are used to investig ate the effects of a high shear rate on the structure of a homogeneous turbulence. The results show that an important effect of the shear ac ting on an initially isotropic turbulence is the selective amplificati on of structures having a large length scale in the mean flow directio n, which leads to velocity fields that contain many large-scale cohere nt structures. Using kinematic simulation, we show how the streamwise, spanwise and vertical integral length scales change at different shea r rates. Particular attention is given to the Lagrangian statistics. T he computations of the kinematic simulation indicate that for a unifor m shear, the Lagrangian time scales for all three components T-ii(L) ( i = 1, 2 and 3) are proportional to L(ii)(x)/sigma(u), in contrast to the generally assumed estimates T-11(L) similar to L(11)(x)/sigma(u), T-22(L) similar to L(22)(y)/sigma(nu) and T-33(L) similar to L(33)(z)/ sigma(w). This is consistent with the results of Turfus and Hunt (1986 ) obtained for a rigid surface which blocks the vertical velocity fluc tuations at z = 0. Also we show that T-33(L) increases, while T-11(L) and T-22(L) decrease, as the shear rate increases.