A numerical simulation of turbulent mixing layer by discrete vortex method

Two-dimensional turbulent high-Reynolds-number mixing layers with and without initially forced disturbances are simulated by a discrete vortex method. The direct summation method is used to follow the motion of up to 2600 vortices and 3000 marker particles. Flow visualization shows that entrainment occurs because clusters of vortices induce circumferential velocities on nonturbulent fluid out of the mixing layer. The vortex pairing process is seen to play a primary role in the development of an unforced mixing layer, though self-growth may also play an important role. The results also indicate that the mixing layer spreads more rapidly in the slower-speed region. With initially forced disturbances, mixing layers are divided into a Kelvin-Helmholtz instability region, a transition region, and a region where flow features are quite similar to those of an unforced mixing layer. In the transition region, contragradient diffusion occurs, there is little or no entrainment and the development of a mixing layer is suppressed.