  In this paper we present the results of numerical simulations of the
Kelvin-Helmholtz instability in a stratified shear layer. This shear
instability is believed to be responsible for extra mixing in differentially
rotating stellar interiors and is the prime candidate to explain the abundance
anomalies observed in many rotating stars. All mixing prescriptions currently
in use are based on phenomenological and heuristic estimates whose validity is
often unclear. Using three-dimensional numerical simulations, we study the
mixing efficiency as a function of the Richardson number and compare our
results with some semi-analytical formalisms of mixing.
