Mr. Overholt et Sb. Pope, DIRECT NUMERICAL-SIMULATION OF A PASSIVE SCALAR WITH IMPOSED MEAN GRADIENT IN ISOTROPIC TURBULENCE, Physics of fluids, 8(11), 1996, pp. 3128-3148
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.