Perturbed vortical layers and shear sheltering

New theoretical results and physical interpretations are presented concerni ng the interactions between different types of velocity fields that are sep arated by thin interfacial layers, where there are dynamically significant variations of vorticity across the layers and, in some cases within them. I t is shown how, in different types of complex engineering and environmental flow, the strengths of these interactions vary from the weakest kind of su perposition to those where they determine the flow structure, for example b y mutual exclusion of velocity fields from the other region across the inte rface, or by local resonance near the interface. We focus here on the exclu ding kinds of interactions between, on the one hand, elongated and compact regions containing vortical flows and large variations in velocity, and on the other hand various kinds of weak perturbation in the surrounding extern al flow region: rotational, irrotational; time-varying, steady; large, smal l; coplanar, non-coplanar; non-diffusive, diffusive. It is shown how all th ese kinds of external disturbances can be wholly, or partially,'blocked' at the interface with the vortical region, so that beyond a certain shelterin g distance into the interior of this region the fluctuations can be very sm all. For the special case of quasi-parallel co-planar external straining mo tions outside non-directional shear flows, weak sheltering occurs if the me an velocity of the shear flow increases otherwise the perturbations are amp lified. For non-parallel flows, the sheltering effect can be greater when t he vorticity is distributed in thin vortex sheets. The mechanism whereby th e vortical flow induces 'blocking' and 'shear-sheltering' effects can be qu antitatively explained in terms of the small adjustments of the vorticity i n the vortical layers, and in some cases by the change in impulse of these layers. If the vorticity in the outer part of the vortical region is weak, it can be 'stripped away' by the external disturbances until the remaining vorticity is strong enough to 'block' the disturbances and shelter the inne r flow of the vortical region. The mechanisms presented here appear to expl ain on the one hand some aspects of the observed robustness of vortical str uctures and jet or plume like shear flows in turbulent and geophysical flow s, and on the other hand the levels of external perturbation needed to erod e or breakdown turbulent shear flows. (C) 1999 Elsevier Science B.V. All ri ghts reserved.