Re-examining the thermal mixing layer with numerical simulations

The question of whether a temperature mixing layer evolves in a self-similar manner is of importance in developing and validating theories about scalar mixing. The simplicity of the flow encourages the thought that it is self-similar, but several laboratory experiments at moderate Peclet numbers have found inconsistencies with self-similar behavior. The experimentalists are limited, however, by the length of the wind tunnels and by difficulties in aligning the virtual origins of the scalar and velocity fields. Direct numerical simulations virtually eliminate both these problems, and large-eddy simulations add the ability to study an approximation to the case of an infinite Peclet number. These two simulation techniques are used in this paper to show that the mixing layer at a moderate Peclet number very nearly evolves with a single length and time scale, and that behavior consistent with self-similarity is observed in the case of an infinite Peclet number. In addition, the results show that direct numerical simulations can accurately reproduce the data from wind tunnel experiments downstream of a turbulence grid, and that large-eddy simulations are a valuable research tool for studying the large-scale characteristics of mixing.