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Wall Shear Stress: Ansys Fluent

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Old   November 13, 2015, 13:58
Default Wall Shear Stress: Ansys Fluent
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Md. Nasimul Hoque Chowdhury
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Hello Everyone,

I am trying to verify the reported wall shear stress of a model in Ansys Fluent. This two dimensional model uses standard k-e turbulent closure scheme with standard wall functions. The manual says that, it uses Launder and Spalding method. I tried to verify it with my result. However, the results do not match. Fluent results gives wall shear stress around 20.62 N/m2.
If I follow the process explained in the Fluent manual, it gives me around 8.95 N/m2.
Note that, If I use Log(base 10) instead of Log(base e) in the procedure, I can reproduce the same value around 20.62 N/m2. Now, I am thinking what can be the source of discrepancy. I would appreciate if anyone can suggest me regarding this issue.

FYI: The velocity in the first cell is 1.111 m/s, distance from the wall is 0.0008 m and kinetic energy at the point is 0.059828 m2/s2.
Note: I would like to mention that my first cell from the wall is in the log-law region. I have around 2-3 cells in the log-law region.

Regards,
Md Nasimul
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Old   November 14, 2015, 12:12
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Lucky Tran
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What's your y* for the same cell?

I don't think you're in the log region.

Or maybe you can show the arithmetic to arrive at those numbers.
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Old   November 16, 2015, 19:51
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Md. Nasimul Hoque Chowdhury
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Thank You for the response. I checked my Y* value. It is 107.14. So, it is in the log-law region. One more thing, I should mention that it is an open channel flow (multi-phase) model.

Here is the actual velocity profile data.

y data:
0.00E+00, 0.0008 ,0.0016 ,0.0024 ,0.0032, 0.004, 0.0048, 0.0056, 0.0064, 0.0072, 0.008, 0.0088, 0.0096, 0.0104, 0.0112, 0.012.
x-velocity:
0, 1.11141, 1.30493, 1.43661, 1.53633, 1.61681, 1.68387, 1.7406, 1.78878, 1.82945, 1.86322, 1.89045, 1.91138, 1.92617, 1.93549, 1.94082.
Turbulent K-E:
0.0637314, 0.0598278, 0.0543767, 0.0509705, 0.0471932, 0.0433579, 0.0395679, 0.0358986, 0.0324181, 0.0291987, 0.02632, 0.0238651, 0.0219147, 0.0205486, 0.0197962, 0.0197106.

If I take yp as 0.0008 and corresponding x-velocity (up) as 1.11141 m/s, density= 998.2 kg/m3, dynamic viscosity= 0.001002, and C_mu= 0.09.
y*=yp*c_mu^(0.25)*k^(0.5)*density/dynamic viscosity =107.14.



taw_w becomes 8.95 N/m2. whereas Fluent gives about 20.62 N/m2. Is there something I am missing? Thank you again for looking in to the issue.
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Old   November 17, 2015, 01:09
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The value of k is consistent with the wall shear stress reported by Fluent.
But I also don't get agreement between velocity & y with the shear stress.

How about y* reported by Fluent? Does it agree with y*~100?

Also did you list the node values or cell centered values? You should be using cell centered values but it's probably not enough of a difference.

You might also want to read through here as an additional reference:
http://www.cfd-online.com/Wiki/Two_e...bulence_models
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