Laplacian viscous stress term in compressible solver

jelmer

Hello,

for compressible viscous computations I intend to use/modify the sonicFoam solver, but a few things I don't understand:

- the viscous term is given as a laplacian (the case forwardStep is discussed in sec. 3.3 of the P-guide) and I can not see why. Only for incompressible flows the stress reduces to laplacian(mu, U). Is there a reason for it?

I tried to incorporate the full stress tensor, but this is only possible in explicit form because the function grad(U) is only available in the fvc class and now I model it by:

volTensorField gradU = fvc::grad(U);

mu*(( gradU + gradU.T() ) - 2.0/3.0*fvc::div(U)*tensor::I);

but the solver gets less stable, especially for larger mu.
Is there a reason why I can't use grad(U) in implicit form (why it is not available in the fvm class)? Is there another way to do this implicitly?

-Furthermore I see from the results that the choice of outflow boundary conditions (zerogradient on p, T, U) is not ideal since it generates an artificial boundary layer on the outflow, see also figure 3.8 in the P-guide). For my work a correct boundary treatment is essential.
Is there a specific reason for this choice of boundary conditions? It seems to mee that it is not the usual case for Navier-Stokes outflow conditions.

regards, Jelmer