Vortex Quadrupoles and Propagation of Grid Turbulence

Abstract

The purpose of this communication is to present the results of experiments dealing with the propagation of turbulent fronts induced by oscillating grids in homogeneous fluids and to explane the experimental results theoretically. In most of the previous experiments grids made with large square bars were used to study a variety of problems ranging from velocity decay law to mixing across density interfaces (e.g., see Fernando, 1991). Barenblatt (1977) modeled the grid forcing by a source of turbulent kinetic energy distributed homogeneously in the grid’s plane and analyzed the propagation of a turbulent front. Voropayev et al. (1980) demonstrated experimentally that this modeling leads to the conclusion that the turbulent kinetic energy flux from the grid, oscillating with constant frequency and amplitude, rapidly decreases with time, which seems unrealistic (also see Barenblatt and Voropayev, 1983; Benilov et al. ,1983). Dickinson and Long (1978) studied the propagation of a turbulent layer by employing a fine grid with small mesh and bar diameter. Considering such a grid, it is possible to simplify the problem and develop an idealized model to describe the grid forcing on the fluid by using some singularities distributed in the grid’s plane. Such an attempt was made by Long (1978), who modeled the flow near the grid by a system of point source sink doublets. An essential shortcoming of this model is the absence of vorticity in the flow, and in a recent paper Long (1995) attempted to modify this model because the source-sink doublets produce no vorticity — “the sine qua non of turbulence”.