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-   -   High drag coeff. estimated by Gambit+fluent (http://www.cfd-online.com/Forums/fluent/42603-high-drag-coeff-estimated-gambit-fluent.html)

M. suuri October 18, 2006 03:01

High drag coeff. estimated by Gambit+fluent
 
I am using gambit grid generator and fluent as solver to simulate flow over a low Re. number airfoil (150,000 to 700,000). I have used BL. while mesh generation. (I donot bay attantion to y+!!).

I used k_omega and k_epsilon modles with diffrent combinations of options.

results were compared to both (1)old german wind tunnel data and also to (2) 2004 wind tunnel data.

Lift coeff. results were between the two exp. results. Drag and pithcing momnet coeff.s show high errors(difrences)

Drag coeff. for example has about 50% error with both exp. results.

Can I get explantion or advise to modify my solver or grid generator configurations.

M. Surri

Ralf Schmidt October 18, 2006 04:54

Re: High drag coeff. estimated by Gambit+fluent
 
Hi!

check the reference valus (report->reference values) the area, that the drag coefficient is related to, is given there.

Ralf

M. ssuri October 18, 2006 06:45

Re: High drag coeff. estimated by Gambit+fluent
 
Even when I take the drag force in Newtons from fluent and I make the calculation of dynamic pressure (0.5*density*Vinf^2) times the refrence area(1 unit length span * my airfil chord in meters)

I obtain same results as by fluent which means that I used correct ref. values (dayn. preesure ,ref. area and ref length).

thakns


Jason October 18, 2006 08:25

Re: High drag coeff. estimated by Gambit+fluent
 
Why aren't you paying attention to y+???

Drag force will especially be sensitive. If you're worried about it, you should be working with y+ of about 1, k-omega sst... and compare that to k-epsilon with near wall treatment. If you're not paying attention to y+, you may be getting spurious results in the BL...

y+ << 1 is bad

y+ = 1 use near wall treatment (k-epsilon)

12 < y+ < 25 transitioning from the laminar sublayer to the turbulent BL and results are a bit questionable in this region

y+ of 30-300 don't use near wall treatment (k-epsilon)

y+ >> 300 is bad

If you're not paying attention to your y+ values, then you can't really predict what your turbulence model is going to do.

Good luck,<br. Jason

amir October 28, 2006 07:11

Re: High drag coeff. estimated by Gambit+fluent
 
Hi

If you want to consider the drag over an airfoil you must use the spalart allmaras turbulent model. also you must get the Y+ between (30-60)

Good luck

D. Dixon November 5, 2006 15:47

Re: High drag coeff. estimated by Gambit+fluent
 
Amir, why is it important to use the Spalart-Allmaras turbulence model for drag on an airfoil? Shouldn't all of the drag modeling approaches yield approximately correct drag?

amir November 6, 2006 09:02

Re: High drag coeff. estimated by Gambit+fluent
 
no. because each model will be proper for different useage. for example is low Reinolds number we must use the k-w sst. for airfil you can use the both k-w sst and spallart.


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