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Old   July 30, 2012, 08:09
Default wall shear in immersed solid
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Hamed Abdul Majeed
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Hi,

In immersed solid module of cfx, I need to find the wall shear; basically use it to find the skin friction coefficient cf
It seems that the immersed solid does not recognize wall shear.
I calculate cf using the following expression:
Wall Shear/(0.5*massFlowAve(Density)@inlet *Uinf^2)

But Wall Shear seems unrecognized.
If I need to plot friction factor for an immersed solid how should I do it!!

Thank you

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Hamed
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Old   July 30, 2012, 18:26
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Immersed solid do not have wall shear as a variable as they are not modelled as walls, but as momentum source terms. You are going to have to calculate wall shear another way - maybe looking at the general flow stress tensor at the locations near the wall or some other calculation which evaluates to wall shear.
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Old   July 30, 2012, 22:23
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Hamed Abdul Majeed
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Thnx for the reply.

I figured out two ways, actually the problem is with wall shear.

Method 1:
As wall shear stress is given by

Then in cfx we can use the expression

Wall Shear = 0.0000198 [Pa s] *Velocity u.Gradient Y
where 0.0000198 is viscosity of air.

I tested this for a flat plate analysis done earlier

Using Wall Shear this was the plot


Then I used the expression 0.0000198 [Pa s] *Velocity u.Gradient Y, this was the plot


The results are fairly different,
What is the issue.

Method 2
I then found this link http://www.cfd-online.com/Wiki/Skin_...on_coefficient and decided to use and empirical value. My Max reynolds number is 10^6. I decided to use the first empirical expression. But the problem was that this error!
WARNING
No data exists for variable 'CF' specified in object 'Series 1'\'Chart Line 1'.



Please help!

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Hamed
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Old   July 30, 2012, 23:51
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For the first one: Immersed solids use a momentum sink to behave like a wall. They do this with a factor which has to be a balance between stopping the flow enough to act like a wall versus numerical stability. You need to do a sensitivity analysis on this parameter to check this parameter is set adequately before making any assessments about the accuracy of the approach.

For two: You have not defined the variable "CF" properly. The error message is pretty clear.
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Old   July 31, 2012, 00:51
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By the factor do you mean the momentum source scaling factor, well I have found out that momentum source scaling factor performs best for momentum source scaling factor of 100 or above. I did that for a flat plate

Flow over a flat plate as an immersed solid
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Old   July 31, 2012, 00:56
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About the method 1, I suggested earlier, it is for a general case not immersed solid. I compared the value of wall shear with the general accepted definition, but the results differ!!
I mean Wall Shear = 0.0000198 [Pa s] *Velocity u.Gradient Y is the definition of wall shear, so it must be justified, unless it is not the case.
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Old   July 31, 2012, 03:02
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I understand now. Are you reporting hybrid or conservative values?
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Old   July 31, 2012, 04:17
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I am using hybrid values.
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Old   July 31, 2012, 06:46
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What does it give you if you use conservative values?
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Old   July 31, 2012, 13:55
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Conservative or hybrid values makes no difference, also the difference between either using wall shear or 0.0000198 [Pa s] *Velocity u.Gradient Y is very large.
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Old   July 31, 2012, 19:07
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I suspect it is using conservative variables and hybrid variables are not available for this calculation.

I think the problem is that your equation is based on the value in the control volume next to the wall (ie an average of the wall to a distance out from the wall) and the wall shear stress is evaluated at the wall. To check this, if you do a mesh refinement you will find your equation getting closer as you refine the mesh.
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