View Issue Details

IDProjectCategoryView StatusLast Update
0002558OpenFOAMBugpublic2017-05-19 18:47
ReporterSahas Assigned Tohenry  
PrioritynormalSeveritytweakReproducibilityalways
Status closedResolutionno change required 
Summary0002558: wallShearStress is calculated using nu instead of mu also for multiphase inciompressible solvers (like interFoam)
DescriptionMay be it is not a bug but current behavior of wallShearStress utility for multiphase solvers can lead to confusion and further mistakes due to using nu instead of mu in wall shear stress evaluation.
Steps To ReproduceRun tutorials/multiphase/interFoam/ras/DTCHull. Evaluate wallShearStress and visualize it on the patch named "hull". One can see that wallShearStress is greatest in region of air. Correct values can be obtained using next formula:
wallShearStress_correct = wallShearStress*(rho_water*alpha.water+rho_air*(1-alpha.water))
TagsinterFoam, multiphase

Activities

henry

2017-05-19 17:29

manager   ~0008166

The wallShearStress calculates

        \f[
            Stress = R \dot n
        \f]

    where
    \vartable
        R | stress tensor
        n | patch normal vector (into the domain)
    \endvartable

If you want rho*R rather than R you will need to multiply the result by rho.

Sahas

2017-05-19 17:36

reporter   ~0008168

Yes, and for compressible fluid R is evaluated as model.devRhoReff() whereas for incompressible -- as model.devReff() (function Foam::functionObjects::wallShearStress::execute()). In multiphase solver rho is not constant, so this leads to confusion with values of wallShearStress. I think, it is needed to include some description of such behavior in user manual or fix the issue or may be output warning when using wallShearStress with interFoam.

henry

2017-05-19 17:58

manager   ~0008169

It is clear from the description and dimensions of the wall shear stress that it is kinematic rather than dynamic for incompressible solvers and interFoam is an incompressible solver. If you need the wall shear stress to be multiplied by rho why not do that?

Sahas

2017-05-19 18:23

reporter   ~0008170

Ok, it is not a problem. It is just a matter of habit: usually (and physically) wall shear stress is measured in Pa, in case of constant density we can divide it by rho for consistency with incompressible pressure in OpenFOAM. But in interFoam pressure is measured in Pa (due to non-constant density), so for consistency wall shear stress should be also measured in Pa.

And again, this is not a problem but a point to pay attention.

Issue History

Date Modified Username Field Change
2017-05-19 17:09 Sahas New Issue
2017-05-19 17:09 Sahas Tag Attached: interFoam
2017-05-19 17:09 Sahas Tag Attached: multiphase
2017-05-19 17:29 henry Note Added: 0008166
2017-05-19 17:29 henry Assigned To => henry
2017-05-19 17:29 henry Status new => closed
2017-05-19 17:29 henry Resolution open => no change required
2017-05-19 17:32 Sahas Status closed => feedback
2017-05-19 17:32 Sahas Resolution no change required => reopened
2017-05-19 17:36 Sahas Note Added: 0008168
2017-05-19 17:36 Sahas Status feedback => assigned
2017-05-19 17:58 henry Note Added: 0008169
2017-05-19 18:23 Sahas Note Added: 0008170
2017-05-19 18:47 henry Status assigned => closed
2017-05-19 18:47 henry Resolution reopened => no change required