Thermal and momentum diffusivity measurements in a turbulent stratified flow

Abstract

The effect of stratificatio n on mixing coefficients is examined using measurements of velocity and temperature across the interface between a layer of warm water flowing over a pool of colder water in a channel. Velocity profiles were obtained by generating local changes in pH to produce colored tracers that are neutrally buoyant at all points in the stratified flow. Temperature profiles were measured with small thermistor beads that traverse the water regularly at selected locations. The measurements of velocity and temperature and their gradients combined with the conservation equations of mass, momentum and energy yield the transfer coefficients for heat and momentum. Parameters characterizing the functional relationships between these eddy diffusivities and the Richardson number are determined and agree well with environmental data and analytical predictions. The water channel measurements confirm that the vertical thermal diffusivity profile can be expressed in terms of a Richardson number defined by surface conditions, depth and temperature gradient as assumed for the prediction of the thermal structure in a large temperate lake. A semiempirical parameter required in the expression relating thermal diffusivity to Richardson number has been determined from the measurements.