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Wall shear forces

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Old   April 29, 2007, 10:10
Default Hello, Hope everyone is hav
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Philippose Rajan
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Hope everyone is having a lovely weekend!

I am aware that there are quite a few other threads which deal with a similar topic, but I couldnt find the answer to my doubts... so... here I am!

I am trying to incorporate wall shear forces into a dynamic mesh solver with incompressible flow (turbForceFoam), and have the following questions after looking through various code snippets and discussions:

1. When a turbulent solver is used I read in the forum that, the wall shear force is calculated using:

shearForce = sum(- mesh.Sf().boundaryField()[pID]
& R.boundaryField()[pID];

On the other hand, when I was looking through the "liftDrag" utility, I found that the above calculation is used to calculate only that part of the shear force which is due to turbulence.

The viscous shear forces are then additionally calculated using the normal:

shearForce = sum(- mu * U.boundaryField()[pID].snGrad()
* mesh.magSf().boundaryField()[pID];

These two parts are then added together (along with the pressure force contribution) to get the total drag.

Do I always need to calculate both these contributing factors for general cases of calculating the shear forces on a wall due to flow? Or is this a special case only for airfoils?

2. In the equation above to calculate turbulent shear force, the final output is a vector, in the direction opposite to that of the surface normal (facing into the domain, and normal to the surface)... but, shouldnt a shear force be tangential to the surface? What is the exact interpretation of the equation?

3. Why does one require a negative sign for calculating the viscous force? Does the snGrad function return the gradient of velocity in a direction opposite to that of the flow?

Have a nice Sunday!

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