A physical flow model is devised based on the concepts of interaction between the dissipative shear flow and the adjacent free stream and the conservation of mass in the wake. Four flow components are integrated in the model; namely, the flow approaching the trailing edge, the expansion around the trailing edge, the mixing within the free-jet boundary, and the recompression at the end of the wake. A unique and stable solution results for the base pressure. Theoretical results obtained for thin approaching boundary layer do not require empirical information and are, therefore, best suited to evaluate the merits of the theory. Here emphasized is the case of isoenergetic constant-pressure mixing in the turbulent free-jet boundary and agreement is found between theory and experimental data.