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Old   March 4, 2008, 16:27
Default low-Re k-epsilon
  #1
Manu
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Hello, I'm using k-omega (lowâ€"re) model of turbulence on a Naca 0012 airfoil flying at a speed of 10 m/s, however I cannot get a fineness ratio smaller than 2 ! The lift coefficient is realistic, but the drag is 100 times much greater than it should be. Moreover when I plot the contours of omega it appears that the dissipation of turbulence only occurs within the boundary layer. Does anybody know how to solve this problem? Thanks, manu

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Old   March 4, 2008, 23:45
Default Re: low-Re k-epsilon
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NRD
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Hello,

RANS models in FLUENT are valid for flows which are fully turbulent. Its not clear when you say "low-re". Also if you are using standard wall functions, the value of y+ should be between 30 and 300. In my experience, results are sensitive to the value of y+, you can also try reducing the convergence criteria. Just wanted to share my views.
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Old   March 5, 2008, 10:39
Default Re: low-Re k-epsilon
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Dimitri
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Manu, this is an assignment we've given you; the use of Forums is strictly forbidden... Just joking, try to work it out using what NRD has said which is enough to find the solution to yr problem.

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Old   March 5, 2008, 10:49
Default Re: low-Re k-epsilon
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Manu
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Salut! En fait ça y est ça marche j'obtiens des coeff. de portance et trainee corrects. A plus!
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Old   March 5, 2008, 10:52
Default Re: low-Re k-epsilon
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Dimitri
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excellent, alors c'etait le wall-function le probleme ou bien les valeurs de reference?
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Old   March 5, 2008, 10:58
Default Re: low-Re k-epsilon
  #6
Manu
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un mix des deux en fait, ma vitesse de référence était fausse donc j'obtenai des mauvais coeff, et a cause du wall function le rapport Cl/Cd était faux. c cool en tt cas, merci et � plus!
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Old   March 10, 2008, 17:45
Default Re: low-Re k-epsilon
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Andrès
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Hi8 guys, I got the same problem; i can't get any closer but 127% (Cd); I've tried with several options, viscuos models etc, as u wrote in frech, i did't understand the full answer... can u translate plz.
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Old   March 11, 2008, 03:47
Default Re: low-Re k-epsilon
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Manu
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Ok no worries, sorry about that. Actually I'm student, and it was an assignment on NACA 0012 airfoil with flow velocity quite low (10m/s) and each student had a different Rans model of turbulence to test on it. I had to test the low-re number version (i.e. solved up to the wall) of the standard k-omega model. At the early begining I made stupid coarse mistakes, as for instance I forgot to set the velocity of the flow in the reference value, so obviously the dimensionnless coefficients were wrong. I had some problems as well with the Cd, not because of Y+ (it was almost equal or below 1 everywhere), but because of the free stream value of the dissipation of turbulence, which were wrong (you should take care in setting realistic values for this parameters) I hope it will help you, Manu
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Old   March 11, 2008, 10:16
Default Re: low-Re k-epsilon
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Andrès
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well, what I'm doing is also an assigment but it is at Re= 6xe06, it has been difficult to obtain the Cd (still working with a 126 % of error), the reference values are ok, the Y+ the same. can u tell me which are those free stream values you talked about?
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Old   March 11, 2008, 14:44
Default Re: low-Re k-epsilon
  #10
Andrès
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jeje , now I get what u mean, I'm using intensity (2,66 %)and hydraulic diameter (0.0248); and eventhought the results aren't as expected. any other tip? are those values right?
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