wallShearStress, wallGradU are inconsistent
I just tried to calculate the wallShearStress entries of my case from wallGradU (dU/dy), rho (all found using OF postprocessors and sample utility), and nu (from transportProperties). Since tau = mu * (dU/dy) = rho * nu * (dU/dy), I figure that tau / (rho * dU/dy) should be constant, however this is not the case.
Can anyone explain why there is a discrepancy? I feel like I should be able to calculate wallShearStress somehow using its constituent parameters. This is important to me since the wallShearStress utility does not work for laminar flow and I want to compare the forces on an airfoil in laminar and turbulent flow for the same case.
Thanks in advance,
Issue resolved - rather, issues:
1) for compressible cases, you must run "wallShearSress -compressible" with that flag in place. The only way to know to do this is to read the code for the wallShearStress utility. Suggestion: the utility should be able to distinguish between compressible and incompressible cases by itself, or force the user to use a flag for compressible and incompressible, to make this utility more user friendly.
2) by looking at the turbulence model (kwSST) code and the wallShearStress code, I found that OF calculates wallShearStress as follows:
tau_w = -(mu + mut) * (wallGradU)
When I made that change, the data matched.
Therefore, to calculate laminar, tau_w = -mu * (wallGradU) since mut = 0.
May others learn from my mistakes!
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