On the physical accuracy of scalar transport modeling in inhomogeneous turbulence

Direct numerical simulations of scalar fields produced by uniform and line sources in channel flow are used as the basis for examining the accuracy of random flight and closure models in predicting turbulent scalar transport rates. Closure models of gradient form with an anisotropic eddy diffusivity tensor perform well for the uniform source flow and the far field of plumes. In the near field, the plumes are seriously distorted due to the inappropriateness of gradient transport in modeling the streamwise flux rate. Random flight models are most successful in producing a qualitative rendering of the near field of plumes, but are subject to significant quantitative inaccuracies for the low Reynolds and Schmidt number flows considered here. Ensembles of particle paths having common end points are used to explore the physics of the scalar transport correlation. For plume flows, transport in the near field is found to be primarily due to the average effect of the meandering of the turbulent fluid over the source, in which the amount of scalar dispersed by a fluid particle correlates with the local velocity fluctuation. Farther downstream, displacement transport---which may be reasonably modeled via gradient physics---emerges as the principal mechanism behind the scalar flux