# low-Re k-epsilon

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 March 4, 2008, 16:27 low-Re k-epsilon #1 Manu Guest   Posts: n/a 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

 March 4, 2008, 23:45 Re: low-Re k-epsilon #2 NRD Guest   Posts: n/a 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.

 March 5, 2008, 10:39 Re: low-Re k-epsilon #3 Dimitri Guest   Posts: n/a 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.

 March 5, 2008, 10:49 Re: low-Re k-epsilon #4 Manu Guest   Posts: n/a Salut! En fait ça y est ça marche j'obtiens des coeff. de portance et trainee corrects. A plus!

 March 5, 2008, 10:52 Re: low-Re k-epsilon #5 Dimitri Guest   Posts: n/a excellent, alors c'etait le wall-function le probleme ou bien les valeurs de reference?

 March 5, 2008, 10:58 Re: low-Re k-epsilon #6 Manu Guest   Posts: n/a 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 ï¿½ plus!

 March 10, 2008, 17:45 Re: low-Re k-epsilon #7 AndrÃ¨s Guest   Posts: n/a 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.

 March 11, 2008, 03:47 Re: low-Re k-epsilon #8 Manu Guest   Posts: n/a 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

 March 11, 2008, 10:16 Re: low-Re k-epsilon #9 AndrÃ¨s Guest   Posts: n/a 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?

 March 11, 2008, 14:44 Re: low-Re k-epsilon #10 AndrÃ¨s Guest   Posts: n/a 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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