Investigation of the Wall Scalar Fluctuations Effect on Passive Scalar Turbulent Fields at Several Prandtl Numbers by Means of Direct Numerical Simulations

摘要

We investigate the effect of the wall-scalar fluctuations on passive scalar turbulent fields for a moderate Reynolds number R-s = 395 and for several Prandtl numbers ranging from the very low value P-r = 0.01 to the high value P-r = 10 by means of direct numerical simulation (DNS) simulations. Several cases of plane channel flows are considered. Case I is a channel flow heated on both walls with a constant imposed heat flux q(w). We consider for this case two different types of boundary conditions. For the first one, the isoscalar boundary condition theta(w) = 0 is imposed at the wall implying that its fluctuation and therefore its rms scalar fluctuations theta(rms) = root is not affected by the scalar fluctuations at the wall. But owing to the different boundary conditions applied at the wall, significant differences in the evolution of the rms scalar fluctuations hrms are observed in the immediate vicinity of the wall. Surprisingly, the maximum theta(rms) intensity remains almost unchanged in the near wall region whatever the type of boundary condition is applied at the wall. In addition, the turbulent heat fluxes that play a major role in heat transfer are found to be independent of the wall scalar fluctuations. This study demonstrates that the impact of the wall scalar fluctuations is appreciable mainly in the near wall region. This outcome must be taken into account when simulating industrial flows with thermal boundary conditions involving different fluid/solid combinations.