# Limiting an expression

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 August 30, 2013, 08:25 Limiting an expression #1 New Member   Join Date: Aug 2013 Posts: 5 Rep Power: 3 Hi everyone, my question is about limiting an expression in CFX post. I know how to get the total force on a certain surface, just define "force_x()@surface". But is it also possible to just evaluate the force of a certain part of a surface without splitting the surface in different parts? My geometry for example extends from x=0 to x=1000 and i now want to calculate the force at this surface between x=0 and x=200. Thanks for your input!

 August 30, 2013, 12:57 #2 New Member   Join Date: Aug 2013 Posts: 5 Rep Power: 3 In the meantime, i found a way to calculate what i asked before. 1. Open the .res file in CFX post 2. Search for the "export" command 3. In the new window, select the surface you want to work on and "Force X" 4. Save the .csv file (contains four columns X, Y, Z and ForceX ) 5. Import the .csv file into Excel or OpenOffice 6. Sort all four columns according to X 7. Sum up the ForceX column for a certain X range and voila!

 August 30, 2013, 16:10 #3 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,097 Rep Power: 19 This method will surely do the trick, but wouldnt it be easier to create a new clipped plane from the actual plane in CFD post?

 August 30, 2013, 21:54 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,665 Rep Power: 84 Also - the method described by Phil will be approximate. When evaluated in CFX it uses the integration points to calculate to forces, whereas the exported data is simplified back to the nodes. So you will get a more accurate force measurement with Alex's suggestion.

 August 31, 2013, 02:15 #5 New Member   Join Date: Aug 2013 Posts: 5 Rep Power: 3 Thanks for your hints! But will the clip plane method really work? I can't access CFX at the weekends, so i'll have to wait until Monday to try it. From the cfx_post.pdf, i get the following: "10.11 clip command ...A clip plane will act on all objects in the Viewer, including the Wireframe, but will not affect other functions such as calculations (that is, a calculation will still use the entire location, whether visible or not)." Another method to use could be a contour plot of X, as described here: Clipping Plane

August 31, 2013, 03:36
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Alex
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Quote:
 Originally Posted by philblitz ...A clip plane will act on all objects in the Viewer, including the Wireframe, but will not affect other functions such as calculations (that is, a calculation will still use the entire location, whether visible or not)
"force_x()@surface" "force_x()@clipped_surface"

 August 31, 2013, 08:12 #7 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,665 Rep Power: 84 CFD-Post has a lot of options for generating surfaces. You can also generate them by planes, between contour lines and from isosurfaces. So if the clip plane option does not work there are plenty of others.

 September 2, 2013, 13:18 #8 New Member   Join Date: Aug 2013 Posts: 5 Rep Power: 3 Today it compared the different methods with the expression "force_x()@surface", which has a value of 1,09183 N. My "export to Excel" method resulted in 1,09183 N. The second method (Iso Clip from the original surface) gave a result of 1,08742 N. The third method (user surface from Contour plot level) showed a value of 0,29218 N. The following picture shows where how much drag is produced across my surface (from ~-2,15m to 0,9m). The upper two lines (Excel and Iso Clip method) match each other, but the third (Contour) shows huge differences. The overall shape is the same although.

 September 2, 2013, 13:48 #9 Senior Member   Bruno Join Date: Mar 2009 Location: Brazil Posts: 236 Rep Power: 12 Use the method which calculates the force based on the iso-clip. That should be the correct one. For future simulations, split the surface on the place you'll want to measure. That way you'll be able to measure forces directly from the solver, eliminating any possible sources of error from interpolation the mesh/results. By the way, how are you measuring force with a contour?

 September 3, 2013, 03:37 #10 New Member   Join Date: Aug 2013 Posts: 5 Rep Power: 3 Measuring a force via a contour plot works the following. But i would not recommend it as the differences are to big compared to the other methods for my geometry. 1. Create a contour plot on a surface, in my case contour of X 2. Create an user surface from this contour and select the contour level 3. Create an expression: force_x()@user surface

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