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In RANS simulations for turbulent scalar transport, it is common that usingan eddy-viscosity (EV) model to close the Reynolds stress yields reasonablemean flow predictions but large errors in scalar transfer results regardless ofscalar flux model inadequacies. This failure mode of EV models is generallyrelated to the fact that the transport of momentum and scalar depends ondifferent Reynolds stress components. The present work addresses two commonissues relevant to such failures in turbulent scalar transport aroundobstacles. The first issue is the general overprediction of scalar transfernear the upwind surfaces, which is primarily attributed to the absence ofwall-blocking mechanism in conventional EV models. We accordingly propose aShear-Preserving-Wall-Blocking (SPWB) method to analytically correct theoverpredicted wall-normal stress under the realizability constraint. The secondissue is the general underprediction of scalar transfer in the downstream largeseparation regions, which is essentially attributed to the presence of vortexshedding invalidating the scaling ground in conventional EV models' dissipationclosures. We accordingly apply the recently proposed Double-ScaleDouble-Linear-EV (DSDL) model to scalar transport predictions. Consequently, ahybrid model SPWB-DSDL is developed. The model is then applied to two testcases, of which the first features a bluff obstacle with an upstreamimpingement flow and a downstream two-dimensional separation and the second astreamlined obstacle with an upstream concave surface flow and a downstreamthree-dimensional separation. In the two cases, the SPWB-DSDL model is capableof simultaneously yielding reasonable results of mean flow field, turbulenceenergy and stress, and scalar transfer in both upstream and downstream regions,thus demonstrating significant improvement upon a classical EV model.

On correcting the eddy-viscosity models in RANS simulations for turbulent flows and scalar transport around obstacles