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What variable should I output to get wall force?

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Old   August 13, 2016, 11:27
Default What variable should I output to get wall force?
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Pauli
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Hello,

I am running a CFX v16.2 large & long transient multi-reference frame simulation. I want to minimize my transient result file storage space so I only output the variables I really want/need.

I need to have transient results for the wall force components in both the moving and the stationary domains. I am outputting the static and total pressure, density and velocity, and wall shear. Yet when I open a .trn file, I do not get force as a variable.

Here is my current output command snippet:
FLOW: Flow Analysis 1
OUTPUT CONTROL:
RESULTS:
File Compression Level = Default
Option = Standard
END
TRANSIENT RESULTS: Transient Results 1
File Compression Level = Default
Include Mesh = On
Option = Selected Variables
Output Variables List = Absolute Pressure,Density,Total Pressure in Stn \
Frame,Velocity,Wall Shear
OUTPUT FREQUENCY:
Option = Timestep Interval
Timestep Interval = 1
END
END
END
END

Can someone please point out what I am missing. Do I need additional output parameters or is this some idiosyncrasy of how CFX processes wall data?

I ran a test where I used standard output for a few transient time steps. When I loaded one of those .trn files I was able to get wall force. So I am thinking I need an additional variable or two.

Thanks,
Pauli

Last edited by Pauli; August 13, 2016 at 11:28. Reason: change notifications
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Old   August 14, 2016, 07:43
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I suspect you will need to include the pressure variable to get forces acting on surfaces.
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Old   August 15, 2016, 11:02
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I tried outputting the static and total pressure in stn frame, density, velocity, and wall shear. I expected that was sufficient to compute wall forces. However, when I opened a .trn file, force was not available.
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Old   August 16, 2016, 02:29
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Well, you can calculate the force on your own:

Fx = areaInt_x(Pressure)@surface + areaInt_x(Wall Shear)@surface

and Fy, Fz similarly.

The expression above is similar to force_x()@surface, however not exactly the same.
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Old   August 16, 2016, 02:39
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Quote:
Originally Posted by Jiricbeng View Post
Well, you can calculate the force on your own:
Fx = areaInt_x(Pressure)@surface + areaInt_x(Wall Shear)@surface
Wouldnt it be
Fx = areaInt_x(Pressure)@surface + areaInt(Wall Shear X)@surface instead?
Because I think that integration in X of the whole wall shear vector is not the same as integration of the x-component of the wall shear vector?
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Old   August 16, 2016, 05:19
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Yes, I think you are right.
However if you compare

Fx = areaInt_x(Pressure)@surface + areaInt(Wall Shear X)@surface

and

Fx = force_x()@surface

there is still very small difference, but I think it could be neglected
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Old   May 15, 2018, 07:58
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You just need to turn on the Output Boundary Flows option.

Code:
...
    RESULTS:
      File Compression Level = Default
      Include Mesh = On
      Option = Selected Variables
      Output Boundary Flows = All
      Output Variables List = Pressure,Velocity,Wall Shear
    END
...
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