In the second model the salt water is stagnant and fresh water flows at a constant velocity u. Between the fluids a resistance layer is assumed that controls the movement of salt across the interface by the following condition:

=> (3)

The concentration in the salt water is assumed to remain constant. The parameter h is a measure for the resistance to the flux of salt. The full solution is given again as an integral expression. For large x it reduces to:

α (4)

The influence of the velocity u is clearly seen. A large value of u decreases the concentration, which confirms the flushing effect. However, a comment on the resistance parameter h should be made. Verruijt refers for this concept to the theory of heat conduction. In Carslaw and Jaeger [2] (page 19-20) a similar resistance layer is applied for the heat flux to a cooling liquid flowing along a warm body. If the analogy with heat flow is correct, then the coefficient h is a function of u. Carslaw and Jaeger report for turbulent flow over a heated pipe that h is proportional to u-8. This is of particular interest. With h proportional to u-1, the velocity cancels out, while for any negative power other than –1 flushing still occurs, but significantly less than (4) suggests. So, the question remains: is there a flushing effect and if so, how strong is it.