Statistics of surface flow structures on decaying grid turbulence

Abstract

The surface flow pattern of decaying turbulence generated by a vertical bar grid in a towing tank has been studied. The velocities and positions of about 2×104 Al aluminum filing tracers following the surface motion have been measured and statistically analyzed. From ten photo series, each of 10 mesh width span and 40 mesh widths long, 2000 surface eddies were identified and studied. The flow field is decomposed into ‘‘closed’’ rotating flow structures (the surface eddies) and ‘‘open’’ flow structures (rivers) of predominantly translational motion. There are also relatively stagnant areas that grow in number and size with time. Immediately behind the grid the average energy densities of the surface eddies ee and the rivers er are about equal but ee decays at a greater rate than er. The eddy size distribution gradually shifts to larger eddies and an increasing fraction of the fluctuating kinetic energy is transferred into the rivers. In the collision-dominated equilibrium region, the eddy and the river energy distributions can be described by Boltzmann-type functions. These distributions may then be characterized by ‘‘temperature’’ θe and θr for eddies and rivers. These temperatures decay steadily throughout the observation period, but remain approximately equal to each other. The relationship between the surface flow dynamics and the subsurface and bulk turbulent grid-flow is examined.