THE INFLUENCE OF LINEAR MECHANISMS DURING THE ADJUSTMENT OF SHEARED TURBULENCE TO FLOW CURVATURE

Recent experimental studies on homogeneous curved shear flow have shown that the imposition of strong mean flow curvature can cause a reversal of the turbulent shear stress, giving it the same sign as the gradient of mean velocity. Measurements of the coherence spectrum for these flows has revealed that this reversal is not uniform across all scales, and that eddies of different sizes can have opposite orientations and transport momentum in opposite directions. To evaluate the influence of linear mechanisms in the shear stress reversal a ''rapid distortion'' type of model was applied to those flows which demonstrated this phenomenon. The model predicts that flow curvature causes a periodic modulation of the structure of sheared turbulence, and that the sign of the shear stress reverses because of these oscillations. The period of the modulation, in terms of the total strain, was found to decrease as the turning rate increases relative to the shearing rate. For those flows which showed a reversal of the shear stress, the range of experimental observation was only a fraction of the predicted period, but interpreting the observed development as a portion of an oscillation the measurements were found to be qualitatively similar to the predictions of the linear theory. In cases of stronger turbulence a self-preserving structure developed, before the shear stress could reverse, and the measurements deviated significantly from the predictions.