Turbulent time scales and the dissipation rate of temperature variance in the thermal mixing layer

In a turbulent flow where the mechanisms and/or the importance of the generation processes of turbulent kinetic energy k and mean square scalar variance ∼(c2) are dissimilar, no simple connection exists between the turnover time scale of the fluctuating velocity and fluctuating scalar fields. To allow the computation of the turbulent scalar field in these situations a means of calculating the scalar time is required. Here, in an extension of work by Newman, Launder, and Lumley [J. Fluid Mech. 111, 217 (1981)], the time scale is obtained via a proposed transport equation for the dissipation rate of mean square scalar variance. The modeled equation has been applied successfully to the calculation of the spread of a thermal mixing layer in grid‐generated turbulence and the decay of temperature variance in a homogeneous field.