4 Replies Latest reply on Apr 30, 2014 2:46 AM by Jared Conway

Not getting downforce readings on bluff body diffuser. help

Terter Ertyery Apr 28, 2014 9:05 PM

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?

Re: Not getting downforce readings on bluff body diffuser. help

Terter Ertyery Apr 28, 2014 9:08 PM (in response to Terter Ertyery)

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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Re: Not getting downforce readings on bluff body diffuser. help

Jared Conway Apr 28, 2014 10:23 PM (in response to Terter Ertyery)

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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- Re: Not getting downforce readings on bluff body diffuser. help
  
  Terter Ertyery Apr 29, 2014 2:01 PM (in response to Jared Conway)
  
  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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  - Re: Not getting downforce readings on bluff body diffuser. help
    
    Jared Conway Apr 30, 2014 2:46 AM (in response to Terter Ertyery)
    
    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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