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Not getting downforce readings on bluff body diffuser. help

Question asked by Terter Ertyery on Apr 28, 2014
Latest reply on Apr 30, 2014 by Jared Conway

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Hi, I am using flow simulation 2012 and I am running tests on bluff body diffusers however i am not getting enough downforce as compared to other studies, The drag force is double what the downforce is.

I have a real wall moving at the same speed as the airflow to act as a moving floor,

I attached images of the pressure and velocity profiles, on the left are my results and on the right from someone elses study in different software. As you can see i dont get increased speed under the body and therefore no pressure difference.

Is it something to do with the meshing? or anything else i am missing?

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Terter Ertyery Apr 28, 2014 9:08 PM
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Edit: my global goals are Force x and force y.

x is in direction of the flow.

reading from 40 m/s were -36N for force y and 75N for force x
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Jared Conway Apr 28, 2014 10:23 PM
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what does the mesh look like?

is this a 2d study or 3d study?

what are you comparing the results to?

i would also recommend that you do surface goals selecting all surfaces other than the moving wall. global would include them.

i'd also recommend starting very simple, ie no moving wall and 2d if you aren't there yet. if you look at other discussion about drag/lift in flow or review the solidworks kb, you will find it is highly reliant on having a good mesh and a large enough size comp domain.
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- Terter Ertyery @ Jared Conway on Apr 29, 2014 2:01 PM
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  I set it on number 5 on the wizard settings.
  
  I changed it from a 3d to 2d study and got correct results ( the lift force decreases by a factor of 10, drag stays similar value), so why wont the 3d results match up? Is it because of the mesh density then i assume? so is it possible to do a good enough analysis in 3d on a not so good computer?
  
  Other studies, I am trying to set the tests up well so that i can do more complex testing later with slightly modified bodies.
  
  I did the surface goals now.
  
  And when calculating Cl and Cd, what area would you use for the dynamic pressure?
  
  thanks.
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  - Jared Conway @ Terter Ertyery on Apr 30, 2014 2:46 AM
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    3d issues, could be your expectations, could be mesh
    is it possible to do on a normal computer? possibly, you'll have to iterate a bit on mesh until the solve time takes too long, but you should be able to learn a lot from 2d or half symmetry
    dynamic pressure, use an average, there is no right answer to this even if you do a hand calc
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Not getting downforce readings on bluff body diffuser. help

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