# Ratio of velocity in CFD-post

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 October 15, 2013, 04:56 Ratio of velocity in CFD-post #1 New Member   Join Date: Aug 2012 Posts: 17 Rep Power: 4 Hi My question is about the post-process of a simulation using CFX. I already run a simulation of wind blowing on a opaque wall. Now I would like to visualise de velocity regions behind of the barrier. With the option of CFD-Post contour on a plane I can show contours of velocity. But the experiment that I am using as a base worked with a-dimensional expressions, where contours represent a ratio: U+ = U/Uref U= ameliorated speed (leeward side of the barrier) Uref= speed of reference before the barrier Is it possible to express this function U/Uref with contours? Regards

 October 15, 2013, 05:33 #2 Senior Member   Lance Join Date: Mar 2009 Posts: 522 Rep Power: 11 Im not sure I understand you correctly, but I'll give it a shot. Create an expression in cfd-post, e.g something like Velocity/Uref where Uref is an other expression you defined Then create a variable of your first expression and plot it as the contour.

October 15, 2013, 06:25
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Quote:
 Originally Posted by Lance Im not sure I understand you correctly, but I'll give it a shot. Create an expression in cfd-post, e.g something like Velocity/Uref where Uref is an other expression you defined Then create a variable of your first expression and plot it as the contour.
Thanks for your response.
What I mean: I want to replicate this isocurves:

this isocurves represent how much the wind speed changes around a building.
If the inlet wind is 5m/s and after the wall the speed decrease to 1.5m/s then, the region is represented as an area with the a-dimensional number of 0.3 (1.5/5).
If the area >1 that means the wind speed increases, if it is <1 the wind speed decreases.

Now, I don't know how to write the expression for the the wind speed after and before the barrier.

 October 15, 2013, 06:52 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,800 Rep Power: 85 Generate the CFD-Post variable as Lance suggests. This is quite simple to do, have a look at the tutorials if you need help on how to introduce post processing variables. Then put a plane through your domain and generate a contour plot on the plane of your variable and that will generate the equivalent plot.

 October 16, 2013, 06:39 #5 Senior Member   OJ Join Date: Apr 2012 Location: United Kindom Posts: 475 Rep Power: 11 On the same note, is it possible to show those values in different regions in the contour like the figure above? I tried once doing that but couldn't figure out how to do that. OJ

 October 16, 2013, 06:50 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,800 Rep Power: 85 I think you have to do that manually using text objects. I cannot think of a way of doing it automagically.

October 16, 2013, 07:18
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OJ
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Quote:
 ... I cannot think of a way of doing it automagically.
Freudian?

 October 16, 2013, 17:26 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,800 Rep Power: 85 Not at all. It is a very useful term: http://www.urbandictionary.com/defin...=automagically

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