The Kelvin-Helmholtz Instability and Turbulent Mixing.

Detailed two-dimensional simulations of the Kelvin-Helmholtz instability in a compressible fluid have been carried out to study several microscopic aspects of turbulent entrainment and mixing occurring in low altitude nuclear explosions. The numerical simulations, performed by the SHAS2D computer code. treat accurately the three important regimes 1 Linear growth of the unstable modes, 2 Quasi-linear saturation of these unstable modes by shear-layer broadening, and 3 Shear-flow mixing of the large amplitude perturbations. A dispersion-relation code has been developed to study the unstable spectrum in the linear regime for arbitrary sheared-flow profiles and sound-speed profiles. This involves solving an inhomogeneous eigenvalue problem and has shown qualitative differences between the low Mach number problem and the incompressible problem. Using the results of the linear analysis and the numerical simulation for nonlinear effects, a phenomenological mixing model has been developed for the cascade of wave energy to long wavelengths via mode coupling and quasi-linear stabilization of the shorter wavelengths. Author