Abstract
Eddy-diffusion-type interfacial mixing in a two-layered density current was investigated experimentally and theoretically. In this type of flow analyzed in the present paper, because both upper and lower layers are in motion, they cause a change in both the average salinity and the volume flow rate through this interfacial mixing. The mass flux, passing through the interface in both an upward and downward direction, can be separated into a turbulent diffusion component and a convection component, investigated in previous studies on the entrainment-type transport. This separation was achieved by integrating the two-dimensional convection-diffusion equation in each layer.
The mixing rate, defined on the basis of the one-dimensional mass conservation equations of both layers, was roughly inversely proportional to the Richardson Number in each layer, because of non-breaking of internal gravity waves. However, in a strict sense, the separated nondimensional turbulent diffusion flux of salt was inversely proportional to the overall Richardson Number, Ri, in agreement with the results obtained by Moore & Long. Also, the direction and magnitude of the non-dimensional convective flux which causes the change in the volume flow rate of both layers seemed to be governed by the parameter (α) expressing the degree of stability and turbulence of each layer, or in other words, the potentiality of one layer to entrain the other fluid particles. The functional relationship between a and C which is the constant in the power law of non-dimensional convective velocity, C=func (α), was obtained experimentally. The critical value of C when α=0 coincided with that of entrainment-type mixing which had been investigated previously, and when α=1.0 this coincided with the Moore & Long flow where Wm=0.
