An empirical approach is used to correlate the mixing of isothermal incompressible coaxial air flows in straight pip es of constant circular cross-section. An assumed decay function for kinetic momentum is correlated in terms of initial geometry and initial velocities of the mixing streams. The method allows the rapid estimation of quantities of engineering interest including the axial distribution of both static pressure and total momentum. The method has been tested against published data on thrust augmenting ejectors of the parietal type, and found to provide satisfactory reproduction of measured performance as affected by mixing pipe length and area ratio.