A comparison of lumped-parameter and diffusional models describing the effects of the outlet boundary conditions on the mixing in flow systems

A comparison is presented between two methods of modeling the dispersion and mixing processes in large (geophysical) turbulent flow systems, such as estuaries, coastal waters, and atmospheric systems. One method is based on the diffusional model and the other is a lumped-parameter model, referred to as “box-mode” or compartmental model. The models are compared on the basis of model free parameters defined here: the purging rate and remaining life distribution. It is found that there exists a good agreement between the results of both models for a one-dimensional system.The problem of the boundary condition at the outlet is discussed; it is concluded that the use of a mass transfer coefficient may be justified in some cases where inflows at the outlet (e.g. due to tidal motion) can be estimated. When these inflows are strong a sink at the outlet can be taken as a boundary condition.