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NACA 0012 Airfoil with Spalart-Allmaras turbulence model

Posted 17.10.2012, 06:11 GMT-4 Fluid & Heat, Computational Fluid Dynamics (CFD) Version 4.3a 4 Replies

Mark Dekker
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Hello,

I am new to COMSOL and I thought I would start with a relatively simple model: a NACA 0012 airfoil at 0 angle of attack.

The domain is small and the mesh resolution is relatively coarse to get faster solutions. (Once it works I will try a larger domain and finer mesh)

I chose the Spalart-Allmaras Turbulence model given its robustness and accuracy in these conditions.

Unfortunately, the simulation always exits prematurely with an error:
Segregated group 2
Expression evaluates to NaN.
- Expression: (((mod1.spf.cb1*mod1.spf.Stilde)*mod1.nutilde)-((mod1.spf.fw*mod1.spf.cw1)*((mod1.nutilde/mod1.spf.l_w)^2)))

I have played with different boundary conditions, mesh densities, solvers. Nothing seems to work.

Any help on this would be greatly appreciated!

Attachments:

4 Replies Last Post 19.04.2013, 04:27 GMT-4
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Hello Mark Dekker

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Shawn.F
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Posted: 1 decade ago 02.01.2013, 22:56 GMT-5
Hi Mark,

I noticed that you are using incompressible turbulent flow. That does not seem to be meaningful. Attached is a flow model around NACA0012, as I am working on it at the moment. I decided to leave it for you in case could help you. As you can see, I am using high Mach number flow module. There is a tutorial on the Comsol manual as well, which you may want to take a look. Note that this module is the most fit for Mach# > 0.3.

My model is not 100% accurate at this time, and I am trying to figure out how exactly I should configure the compressible flow in Comsol.

I also appreciate it if you come up with any suggestion-modifications if you found anything appropriate for boundary conditions or other setups.

Good luck

Shawn
Hi Mark, I noticed that you are using incompressible turbulent flow. That does not seem to be meaningful. Attached is a flow model around NACA0012, as I am working on it at the moment. I decided to leave it for you in case could help you. As you can see, I am using high Mach number flow module. There is a tutorial on the Comsol manual as well, which you may want to take a look. Note that this module is the most fit for Mach# > 0.3. My model is not 100% accurate at this time, and I am trying to figure out how exactly I should configure the compressible flow in Comsol. I also appreciate it if you come up with any suggestion-modifications if you found anything appropriate for boundary conditions or other setups. Good luck Shawn

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Mark Dekker
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Posted: 1 decade ago 11.04.2013, 08:44 GMT-4
Hi Shawn,

Sorry its taken a while for me to get back to you, I have been busy on another project of late.

I chose Incompressible flow because this is the regime I am interested in. I am specifically interested in validating COMSOL wit the data presented here:

turbmodels.larc.nasa.gov/naca0012_val.html

I am still not getting very good results and I think it may have to do with the mesh and I will need o see how fix that.

I had a quick look at your model and my first impression was that your domain is far too small. The Domain extents should be hundreds of chord lengths away from the airfoil. This may feel excessive, but it makes a big difference.

If you are trying to simulate a wind tunnel you may need to provide more domain ahead of the airfoil. I would also resolve the wake more finely, up to the outlet.

Have you considered using boundary layer cells? They would increase your near wall resolution a lot!

Maybe it helps. Thanks for your comments.

Hi Shawn, Sorry its taken a while for me to get back to you, I have been busy on another project of late. I chose Incompressible flow because this is the regime I am interested in. I am specifically interested in validating COMSOL wit the data presented here: http://turbmodels.larc.nasa.gov/naca0012_val.html I am still not getting very good results and I think it may have to do with the mesh and I will need o see how fix that. I had a quick look at your model and my first impression was that your domain is far too small. The Domain extents should be hundreds of chord lengths away from the airfoil. This may feel excessive, but it makes a big difference. If you are trying to simulate a wind tunnel you may need to provide more domain ahead of the airfoil. I would also resolve the wake more finely, up to the outlet. Have you considered using boundary layer cells? They would increase your near wall resolution a lot! Maybe it helps. Thanks for your comments.
Shawn.F
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Posted: 1 decade ago 11.04.2013, 12:51 GMT-4
Mark,

I managed to solve the problem, but by using the compressible flow (high Mach number). You are right, the domain was so small, and mesh was horrible at the model that I had posted earlier. But eventually, I got good results. I made the domain 25 times of the chord in each direction, and I also used boundary layer mesh. In case if you are using compressible flow, make sure you gradually increase the Re number the way that is mentioned in the COMSOL example for the high Mach number module.

Good luck

Shawn
Mark, I managed to solve the problem, but by using the compressible flow (high Mach number). You are right, the domain was so small, and mesh was horrible at the model that I had posted earlier. But eventually, I got good results. I made the domain 25 times of the chord in each direction, and I also used boundary layer mesh. In case if you are using compressible flow, make sure you gradually increase the Re number the way that is mentioned in the COMSOL example for the high Mach number module. Good luck Shawn
Mark Dekker
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Posted: 1 decade ago 19.04.2013, 04:27 GMT-4
Hey Shawn,

Glad you could sort it out.

I am using the k-omega turbulence model now and it works reasonably well, but nowhere near as well as other dedicated commercial CFD solvers. Not sure where m errors are coming in, but at least I am within the ballpark now.

Mark

Hey Shawn, Glad you could sort it out. I am using the k-omega turbulence model now and it works reasonably well, but nowhere near as well as other dedicated commercial CFD solvers. Not sure where m errors are coming in, but at least I am within the ballpark now. Mark

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