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-   -   Calculating average wall shear? (http://www.cfd-online.com/Forums/cfx/23722-calculating-average-wall-shear.html)

 Joe March 8, 2007 07:12

Calculating average wall shear?

I need to calculate the time averaged wall shear stress for a transient flow application. Wall shear stress magnitude is not available in the Transient Statistics tab.

What I normally do in this situation is to write the value of a CEL expression (which calculates the variable I'm interested in) into an algebraic additional variable e.g.

algebraic additional variable: avMyVar, [units], unspecified domain additional variables: avMyVar: algebraic equation, set value to expMyVar define CEL expression: expMyVar = ((Wall Shear x Component)^2+(Wall Shear y Component)^2+(Wall Shear z Component)^2)^0.5

Once this is done, avMyVar appears in the Transient Statistics tab and the appropriate average can be selected.

While this approach works fine when using volumetric variables in expMyVar, it generates an error in CFX solver for wall shear magnitude:

+--------------------------------------------------------------------+ | ERROR #001100279 has occurred in subroutine ErrAction. | | Message: | | A transient statistic has been requested but could not be initial- | | ised. A likely reason for this is that the specified variable on- | | ly exists on boundaries. The following statistic will not be gene- | | rated - TRNAVG: avMyVar | | | | | +--------------------------------------------------------------------+

Clearly it doesnt like the fact that wall shear magnitude is a non-volumetric quantity. Does anyone know how to get around this problem?

 Stumpy March 12, 2007 17:12

Re: Calculating average wall shear?

I *think* you can do this in version 11.0. No need to create the AV and CEL expression, Wall Shear should just be there in the list of variables on the Trn Stats tab in Pre. There's also a new Trnavg CEL operator, e.g you can refer to variable.Trnavg in CEL expressions.

 Joe March 12, 2007 18:55

Re: Calculating average wall shear?

I see.

I'm still on 10 and was planning to write a fortran routine to access wall shear tomorrow.

If anyone could confirm Stumpy's findings for CFX 11 I could just bite the bullet and do the upgrade now.

 Joe March 17, 2007 10:42

Re: Calculating average wall shear?

I can now confirm that wall shear is available in Transient Statistics for CFX 11.

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