Large‐eddy simulation of passive scalar diffusion in isotropic turbulence

A spectral large‐eddy simulation of the velocity and passive temperature fields in three‐dimensional isotropic turbulence is developed at high Reynolds numbers and a Prandtl number of the order of 1. The temperature spectrum in the large scales is shown to develop a range of the form E T (k)≈0.1η[〈u 2〉/ε]k − 1, where ε and η are the kinetic energy and temperature variance dissipation rates, respectively. This range, which is a result of shearing by large scale velocity gradients, is followed by a k − 5 / 3 inertial‐convective range at higher wavenumbers. The temperature variance decays much faster in time than the kinetic energy. Finally, the spectral eddy conductivity rises logarithmically with k toward small wavenumbers, contrary to the eddy viscosity that displays a plateau.