Mixing of dense fluid in a turbulent pipe flow Part 1. Overall description of the flow

This paper concerns an investigation into the behaviour of a layer of dense salt solution on the floor of a sloping rectangular pipe in which there is a turbulent flow. The various phenomena which are observed are described qualitatively and by the presentation of typical concentration profiles. Numerical values are obtained experimentally for the rates of spread of the edge of the layer in the case where the salt is moving entirely in the direction of the main stream. The rate of spread is found to depend mainly on the slope α and on the pipe Richardson number, defined by Ri p = D Δ d cos α/ V 2 , where D is the depth of the pipe, Δ d = g (ρ d −ρ a )/ρ a , ρ d is the density of the fully mixed discharge and ρ a is the density of the ambient flow. In the range of Ri ρ from 0 to 0·005 the rate of spread of the layer decreases by a factor of about 3 at small slopes. Some discussion is given of the factors determining the initial rate of spread just after the layer leaves the slit. Finally, it is shown how the depth measurements can be related to the determination of the concentration at the floor.