Re-examining the thermal mixing layer with numerical simulations

The question of whether a temperature mixing layer evolves in a self-simila r manner is of importance in developing and validating theories about scala r mixing. The simplicity of the flow encourages the thought that it is self -similar, but several laboratory experiments at moderate Peclet numbers hav e found inconsistencies with self-similar behavior. The experimentalists ar e limited, however, by the length of the wind tunnels and by difficulties i n aligning the virtual origins of the scalar and velocity fields. Direct nu merical simulations virtually eliminate both these problems, and large-eddy simulations add the ability to study an approximation to the case of an in finite Peclet number. These two simulation techniques are used in this pape r to show that the mixing layer at a moderate Peclet number very nearly evo lves 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 a ddition, 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 stud ying the large-scale characteristics of mixing. (C) 2000 American Institute of Physics. [S1070-6631(00)00801-1].