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-   -   Drag coefficient do not match of the experimental result (https://www.cfd-online.com/Forums/fluent/212650-drag-coefficient-do-not-match-experimental-result.html)

recep911234 December 5, 2018 15:13

Drag coefficient do not match of the experimental result
 
1 Attachment(s)
Hi i have a problem to my naca0012 airfoil (2d) analysis. I set the reference values, boundary conditions and i solve this with k-omega method but i get confused because of drag coefficient. im trying to match my values to experimental values(nasa's thesis) and i didnt. lift coefficient is excatly match but drag is not. what is the solution of this error(or whatever)?

mesh element number:40000

https://www.cfd-online.com/Forums/at...1&d=1544040397

Thats my values

https://www.cfd-online.com/Forums/at...1&d=1544040237

Thats Experimental Values

https://www.cfd-online.com/Forums/at...1&d=1544040280

Model

https://www.cfd-online.com/Forums/at...1&d=1544040549

Method

https://www.cfd-online.com/Forums/at...1&d=1544040579

Reference Values

https://www.cfd-online.com/Forums/at...1&d=1544040602

Pressure Farfield b.c

https://www.cfd-online.com/Forums/at...1&d=1544040631

Also initial values (i computed from farfield)

https://www.cfd-online.com/Forums/at...1&d=1544040696

LuckyTran December 5, 2018 16:01

Drop the coefficients and try comparing the forces in units of N.

recep911234 December 6, 2018 06:53

Quote:

Originally Posted by LuckyTran (Post 718115)
Drop the coefficients and try comparing the forces in units of N.

drop the coefficients? i didnt get it and forces dont matter on my problem i interested in coefficient also i have not force values to compare forces? Am i misunderstand about forces? Thx for comment btw.

recep911234 December 12, 2018 18:02

any help???

LuckyTran December 12, 2018 18:07

If one coefficient matches and not the other, usually it's because one of the coefficients is using different reference values than the other. That's why it's a good idea to forget about the coefficients altogether and compare raw forces (N) to forces (N) even if it means you have to back-calculate some parameters.


Also you might find that you're unable to convert from the coefficients back to the force because something is missing in the publication or maybe you just don't know what you are doing. It is always a good idea to do some basic calculations.

ASCT December 12, 2018 23:54

Please cross check the values of turbulence and pressure
 
note: cross check the values of turbulence, try using k epsilon and at boundary conditions check the turbulence intensity (usually 5 % as turbulence intensity in default.) use analytical formula and find the corresponding values before using them.
cross check the pressure at the boundary condition, it should be in corresponding with the gauge pressure. (else you can put the gauge pressure as 0 for less confusion)


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