DIRECT NUMERICAL-SIMULATION OF A PASSIVE SCALAR WITH IMPOSED MEAN GRADIENT IN ISOTROPIC TURBULENCE

Mixing of a passive scalar in statistically homogeneous: isotropic, an d stationary turbulence with a mean scalar gradient is investigated vi a direct numerical simulation, for Taylor-scale Reynolds numbers, R(la mbda), from 25 to 185. Multiple independent simulations are performed to get confidence intervals, and local; regression smoothing is used t o further reduce statistical fluctuations, The scalar fluctuation fiel d, phi(x,t), is initially zero, and develops to a statistically statio nary state after about four eddy turnover times. Quantities investigat ed include the dissipation of scalar flux, which is found to be signif icant; probability density functions (pdfs) and joint-pdfs of the scal ar, its derivatives, scalar dissipation, and mechanical dissipation; a nd conditional expectations of scalar mixing, del(2) phi. A linear mod el for scalar mixing jointly conditioned on the scalar and nu-velocity is developed, and reproduces the data quite well. Also considered is scalar mixing jointly conditioned on the scalar and scalar dissipation . Terms appearing in the balance equation for the pdf of phi are exami ned. From a solution of the scalar pdf equation two sufficient conditi ons arise for the scalar pdf to be Gaussian, These are shown to be wel l satisfied for moderate values of the scalar, and approximately so fo r large fluctuations, Many correlations are also presented, including rho(nu,phi), which changes during the evolution of the scalar from a v alue of unity when initialized to the stationary value of 0.5-0.6. (C) 1996 American Institute of Physics.