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July 30, 2012, 08:09 
wall shear in immersed solid

#1 
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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 Regards Hamed 

July 30, 2012, 18:26 

#2 
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Glenn Horrocks
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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.


July 30, 2012, 22:23 

#3 
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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.cfdonline.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! Regards Hamed 

July 30, 2012, 23:51 

#4 
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Glenn Horrocks
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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. 

July 31, 2012, 00:51 

#5 
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Hamed Abdul Majeed
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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 

July 31, 2012, 00:56 

#6 
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Hamed Abdul Majeed
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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. 

July 31, 2012, 03:02 

#7 
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Glenn Horrocks
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I understand now. Are you reporting hybrid or conservative values?


July 31, 2012, 04:17 

#8 
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Hamed Abdul Majeed
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I am using hybrid values.


July 31, 2012, 06:46 

#9 
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Glenn Horrocks
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What does it give you if you use conservative values?


July 31, 2012, 13:55 

#10 
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Hamed Abdul Majeed
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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.


July 31, 2012, 19:07 

#11 
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Glenn Horrocks
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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. 

Tags 
immersed, immersed boundary, wall shear 
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