Light attenuation experiments on double diffusive plumes and fountains

Abstract

By adapting the method of light attenuation to the study of axisymmetric disturbances, we examine the structure of forced plumes and fountains whose buoyancy is set by salinity and/or temperature differences between the turbulent flow and the otherwise stationary ambient. The attenuation measurements are used to infer the statistically steady-state density as a function of radius and height. These compare well with in situ measurements of density taken from a conductivity-temperature probe that repeatedly traversed the centreline of the plume. The theory for forced plumes also agrees well with the centreline density profile. The radial structure of plumes and fountains near the source is found to correspond well to a Gaussian profile, although the standard deviation is found to increase moderately faster with height than spreading rates reported in the existing literature. This is attributed in part to averaging biases resulting from light attenuating through transient eddies. In most experiments, hot and salty fountains reached a steady state height consistent with existing semi-empirical theories for fountains with just one diffusive component. However, if the density of the hot and salty fluid near the source is close to, but moderately larger than, that of the ambient, over time, the fountain is observed to increase progressively in height. This occurs presumably as a consequence of heat diffusion at the top of the fountain making the ambient above it increasingly buoyant relative to the negative buoyancy associated with less diffusive salt.