Reasonable error in lift and drag coefficients?

xTamx420 · May 17, 2011, 07:54

Hi, I am a new user to Fluent and CFD in general. I have been running simulations on NACA 0012 foils and I was wondering what a reasonable Cd and Cl are? I get around 0.009 for Cd at 0 angle of attack with a reynolds number of 3000000. The data for NACA 0012 in Theory of Wing Sections has Cd around 0.006. Is this a reasonable error or am I doing something wrong? Cl us usually between 0.55 and 0.65 depending on the viscosity model for an angle of attack of 6 degrees which matches the data in Theory of Wing Sections pretty well.

k_k · May 18, 2011, 08:12

hi,

did ur solution converge?

have u set proper reference values?

Cheers

xTamx420 · May 18, 2011, 08:32

The solution did converge. I got around 0.009 for Spalart-Allmaras model at a couple different Reynolds Numbers. In Theory of Wing Sections the Cd at 0 angle of attack also is the same number for different reynolds numbers its just that mine is off by 50%. I was just looking at the Fluent manual and I think my mesh might be too coarse. I have the same mesh shape as the one in a tutorial I used with 40000 cells. Is that too coarse?

This is the tutorial I used:

https://confluence.cornell.edu/displ...+Specification

k_k · May 18, 2011, 12:44

Hi,

1. Check the y+ of the grid. Try to refine the grid to reasonably low y+.

2. Try running with a different turbulence model.

Come back if the results still dont improve.

Cheers

xTamx420 · May 18, 2011, 13:40

How do you check y+?

k_k · May 19, 2011, 05:00

Read this http://my.fit.edu/itresources/manual...e517.htm#76840

May 17, 2011, 07:54	Reasonable error in lift and drag coefficients?	#1
xTamx420 New Member Tom Join Date: May 2011 Posts: 14 Rep Power: 14	Hi, I am a new user to Fluent and CFD in general. I have been running simulations on NACA 0012 foils and I was wondering what a reasonable Cd and Cl are? I get around 0.009 for Cd at 0 angle of attack with a reynolds number of 3000000. The data for NACA 0012 in Theory of Wing Sections has Cd around 0.006. Is this a reasonable error or am I doing something wrong? Cl us usually between 0.55 and 0.65 depending on the viscosity model for an angle of attack of 6 degrees which matches the data in Theory of Wing Sections pretty well.

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May 18, 2011, 08:12		#2
k_k Member Kailash Join Date: May 2011 Location: London, UK Posts: 45 Rep Power: 14	hi, did ur solution converge? have u set proper reference values? Cheers

May 18, 2011, 08:32		#3
xTamx420 New Member Tom Join Date: May 2011 Posts: 14 Rep Power: 14	The solution did converge. I got around 0.009 for Spalart-Allmaras model at a couple different Reynolds Numbers. In Theory of Wing Sections the Cd at 0 angle of attack also is the same number for different reynolds numbers its just that mine is off by 50%. I was just looking at the Fluent manual and I think my mesh might be too coarse. I have the same mesh shape as the one in a tutorial I used with 40000 cells. Is that too coarse? This is the tutorial I used: https://confluence.cornell.edu/displ...+Specification

May 18, 2011, 12:44		#4
k_k Member Kailash Join Date: May 2011 Location: London, UK Posts: 45 Rep Power: 14	Hi, 1. Check the y+ of the grid. Try to refine the grid to reasonably low y+. 2. Try running with a different turbulence model. Come back if the results still dont improve. Cheers

May 18, 2011, 13:40		#5
xTamx420 New Member Tom Join Date: May 2011 Posts: 14 Rep Power: 14	How do you check y+?

May 19, 2011, 05:00		#6
k_k Member Kailash Join Date: May 2011 Location: London, UK Posts: 45 Rep Power: 14	Read this http://my.fit.edu/itresources/manual...e517.htm#76840