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akultane January 12, 2010 04:40

Modifying pressure field in CFX
 
I am developing a method to calculate hydro turbine blade torque in the direction of blades rotational axis. So far I have used CFX function "torque" for that. At some operation points my results correspond very well with measurements. However, at other operation points the difference between measured and calculated values becomes significant. In those OPs I often find strong low pressure peaks at the leading edge of the blade. In reality peaks do not exist or at least those are not so strong.

I assume that peaks might generate some error because those are located at the leading edge of blade. The displacement from blade rotational axis is large and thus the effect on blade torque becomes large also. If I could filter the pressure field on blade to remove the low pressure peaks my results could be better.

My question is that is it possible to filter the pressure field in CFX in such way that I could remove the low pressure peaks, for example by setting some treshold value. Another alternative that I have though is to export the pressure field, modify it, import it back to CFX and then use function "torque". However I am not sure if it is possible to do that.

ghorrocks January 12, 2010 18:02

In CFD-Post create a new variable which is a copy of the pressure field but clipped to the value you wish. The calculate the torque on this variable.

But this is not a good approach - you really should look into the low pressure regions and determine whether they are real or not, and if not you should improve your simulation so they do not occur. Better to fix the root problem than do some dodgy work to make the result fit the answer you like.

akultane January 13, 2010 15:42

Quote:

Originally Posted by ghorrocks (Post 242349)
In CFD-Post create a new variable which is a copy of the pressure field but clipped to the value you wish. The calculate the torque on this variable.

But this is not a good approach - you really should look into the low pressure regions and determine whether they are real or not, and if not you should improve your simulation so they do not occur. Better to fix the root problem than do some dodgy work to make the result fit the answer you like.



Thanks for your feedback. It is definitely true what you are saying about fixing the root problem. I have been woking on that quite a while. Refining the mesh reduces the size of the peak a little bit. However I think that the problem lies in somewhere else, perhaps in near wall modelling or in turbulence model.

To get back to the issue. I can create a new variable of pressure where the low values are clipped but I don't know how to tell CFX function "torque" to use that variable, instead of pressure in calculation of hydraulic torque on blade surface. If I can't do that, one option would be to export blade surface coordinates and pressure values in each coordinate axis direction and do the torque calculation outside CFX.


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