Turbulent Two-Dimensional Shear Flows

Abstract

Turbulent shear flows can be divided into three groups according to the number of fixed boundaries: 1. Free turbulent shear flows are said to be free, i.e. remote from walls, like jets, wakes, mixing layers and plumes. Jets and wakes differ from each other only by the sign of the momentum creating them. Free turbulent shear flows play a significant role in many kinds of engineering equipments, in rivers and in atmosphere: jets in propulsive devices for rockets and aircrafts and pneumatic control systems, wakes behind aeroplanes and submarines, dispersion in rivers, jet streams in the atmosphere are few examples. Most of 2D shear layers are regions in which there is one single predominant direction, the streamwise direction, while the shear stresses and diffusion fluxes are significant in the direction perpendicular to it. Most of shear layers, but not all, can be considered wiihin the framework of the boundary layer approximation, with only one small length scale, 8, in a direction perpendicular to the streamwise direction. Finally free shear layers include significant regions with both strong and weak strain making them rather sensitive to turbulence modeling which is more challenging there than in the vicinity of a wall where the flow is rather shear-dominated. 2. Turbulent shear flows bounded by one free and one fixed boundary, such as boundary layers or wall-jets, where the wall may be straight or curved, permeable or impermeable. The velocity of the ambiant flow may be in the same or in the opposite direction of the shear flow. 3. Turbulent shear flows bounded by two or more fixed boundaries, such as or channel flows, pipe flows (a pipe is a closed duct of circular cross section), duct flows (a duct has a non circular cross section).