As a predictive model for the uniflow scavenging process, a phenomenological model has been suggested. Based on jet mixing and propagation, this model can give an approximate description of velocity and concentration fields, thus obviating the uncertainty of the thermodynamic models. The unsteady jet model is soundly founded on the conservation laws of mass, momentum and energy for compressible flow. A comparison between the computational and experimental results of an opposed-piston two-stroke engine demonstrates good agreement. The experimental work on a simple water rig has been done. It also validates that the unsteady jet model is satisfactory.