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What does “Mean Wall Shear Stress” mean?

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Old   September 16, 2009, 08:15
Question What does “Mean Wall Shear Stress” mean?
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I am running an unsteady simulation and I have a question about unsteady wall statistics.

Having already activated the data sampling from the iteration menu I use the Unsteady wall statistics and more specificly the Mean wall shear stress option from Display/Contours../ .
And now the question is: How does fluent calculate this “Mean” values??
Is it the mean from the magnitudes of the shear vectors or
the magnitude of the mean shear vector?? (Please see the following equations)

Mean WSS=\frac{\sum|\vec{WSS}|_{t}}{number of time steps} or

Mean WSS=\frac{|\sum\vec{WSS}_{t}|}{number of time steps}

In some cases it is the same but in my case I have a variable velocity inlet (with positive and negative values) and thus the Shear Vector change direction.

Thank you for your answers,
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