interFoam Ueqn
the LHS of UEqn is as follows,
fvVectorMatrix UEqn ( fvm::ddt(rho, U) + fvm::div(rhoPhi, U)  fvm::laplacian(muEff, U)  (fvc::grad(U) & fvc::grad(muEff)) ); I want to know the physical meaning of the last term, (fvc::grad(U) & fvc::grad(muEff)) and wher it come form, 
Hi Alvin, this term is due spatial variation of muEff. From the Cauchy equation you have for the viscous term div & [muEff*grad(U)], since muEff is not a constant, using the chain rule: muEff*laplacian(U)+grad(muEff)&U.
Regards. 
Thanks,very helpful

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