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June 16, 2011, 18:14 
OpenFOAM V&V

#1 
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Martin Hegedus
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The root of this question is a posting I made at Symscape, http://www.symscape.com/blog/newstr...fdwindtunnel I realize that OpenFOAM is used by industry, academia, and hobbyists. There is even a workshop for it. However, I am having difficulty finding systematic quantitative (i.e. not qualitative pretty colored pictures or top level slides) V&V information for OpenFOAM for simple geometries, i.e., flat plate, bump, forward/backward steps, lidded cavities, airfoils, wings, etc. This includes grid convergence studies. Examples of what I am referring to, in regards to a NASA code CFL3D, are, http://cfl3d.larc.nasa.gov/Cfl3dv6/c...testcases.html or http://turbmodels.larc.nasa.gov/, or even http://aaac.larc.nasa.gov/tsab/cfdlarc/aiaadpw/ Does such information, to the extent one can reproduce the results, exist for OpenFOAM? Or is it up to each individual/group to work through V&V cases from scratch on their own? 

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June 19, 2011, 16:24 

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Bruno Santos
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Greetings Martin,
I just saw this blog post and remembered about your thread: http://cfdtoy.blogspot.com/2011/05/m...ification.html Best regards, Bruno
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June 21, 2011, 16:53 

#3  
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Santiago Marquez Damian
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COMPARISON OF SINGLE PHASE LAMINAR AND LARGE EDDY SIMULATION (LES) SOLVERS USING THE OPENFOAM(R) SUITE VOLUME OF FLUID SIMULATION OF BORDA MOUTHPIECES Results are checked against experiments and Fluent. Hope this help
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Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC)  CONICET/UNL Tel: 543424511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe  Argentina. http://www.cimec.org.ar 

June 21, 2011, 17:33 

#4 
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Martin Hegedus
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Thanks, it's a start.
In general, I've seen others raise the issue about grid convergence. (LidDriven Cavity from first paper). Did you figure out why the lid driven cavity did not converge? The fact that the residuals do not converge to machine zero is a little scary. 

June 21, 2011, 17:42 

#5 
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Santiago Marquez Damian
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Hi, Do you refer to the problem of p residuals?
Regards.
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Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC)  CONICET/UNL Tel: 543424511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe  Argentina. http://www.cimec.org.ar 

June 21, 2011, 17:49 

#6 
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Martin Hegedus
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Yes, that is correct.


June 21, 2011, 18:00 

#7 
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Santiago Marquez Damian
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Martin, this problem was reported several times, but I couldn't find a cure at that time. This is related to PISO loop and tolerances and type in p solver (I never played enough time with GAMG solver for example), I'm working in that now. If you could find a set of parameters that perform better It would be nice to share it with the community, particularly avoiding the plateau at ~1E6.
Regards.
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Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC)  CONICET/UNL Tel: 543424511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe  Argentina. http://www.cimec.org.ar 

June 21, 2011, 19:49 

#8 
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Martin Hegedus
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I don't use OpenFOAM that much. I get very frustrated with it. The vast majority of my cases are steady state external aero, both compressible and incompressible. In general I haven't had much luck with OpenFOAM. I figure it is my own personal issue since so many others use the code. So, instead, I wrote my own solver from scratch (compressible with equations coupled). That's been a lot of work. Especially the V&V stuff. However, at some point it would be nice to have more confidence in OpenFOAM and use it more.
If I come up with something that works for the lidded cavity, I'll share it here on this forum. BTW, is there a better place to share cases and solutions for OpenFOAM? http://www.cfdonline.com/Wiki/Valid...and_test_cases seems sparse and www.openfoam.com doesn't seem to be very, well, open in the sense of supplying a place for the OpenFOAM community to go to. 

June 21, 2011, 22:44 

#9  
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Santiago Marquez Damian
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Quote:
Quote:
Regards.
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Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC)  CONICET/UNL Tel: 543424511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe  Argentina. http://www.cimec.org.ar 

June 22, 2011, 00:21 

#10  
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Martin Hegedus
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I haven't found one yet. A wiki (such as the V&V here at CFD Online), in my opinion, may not be appropriate. Wiki's are very formal and polished. 

June 22, 2011, 08:45 

#11  
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Patricio Bohorquez
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At low Reynolds numbers (i.e. incompressible and laminar flow) we have reported successful comparison between OpenFOAM and wind tunnel experiments for external flows both for steady and unsteady regime. You could find more information in the following paper:
Bohorquez, P., SanmiguelRojas, E., Sevilla, A., JiménezGonzález, J., MartínezBazán, C. Stability and dynamics of the laminar wake past a slender bluntbased axisymmetric body. Journal of Fluid Mechanics, 676: 110144 (2011) http://dx.doi.org/10.1017/s0022112011000358 Quote:


June 22, 2011, 09:52 

#12 
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Santiago Marquez Damian
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Another ref. for my thesis!! Thanks for it and the downloading link.
Regards.
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Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC)  CONICET/UNL Tel: 543424511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe  Argentina. http://www.cimec.org.ar 

June 22, 2011, 12:55 

#13  
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Martin Hegedus
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June 22, 2011, 13:16 

#14 
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Martin Hegedus
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June 22, 2011, 14:25 

#15 
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Patricio Bohorquez
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Yes, you are right. On the numerical side there are lots of parameters that affect the solution of any problem because they introduce errors: the topology of the mesh, cell elements, the implementation (segregated/coupled), the order of consistency, etc. And they affect the results because the mesh is always coarser than we want in the absence of exceptional numerical facilities.
But this happens with any numerical solver, not just with OpenFOAM. There are suitable problems that can be solved if you know how to drive the tool, otherwise the numericist wont succeed. Numerical algorithms are designed for specific purpose and, consequently, they continue growing. OpenFOAM implements a classical FVM formulations, it is not the Panacea. 

June 22, 2011, 14:57 

#16  
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Martin Hegedus
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A question then. Were you able to converge the residuals for your steady results to machine zero, or at least to the point where you were very confident the residuals were heading there? I assume you did, but I'm looking for data points not based on my assumptions. 

June 22, 2011, 15:27 

#17 
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Patricio Bohorquez
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In the case described in the paper, if the boundary condition of the body is set to slip, then the pressure and velocity residual drop to zero. When using 'no slip' the pressure residual may reach an asymptotic value (usually between 10^{6} and 10^{4}). However, I cannot ensure that it is due to the boundary condition. Why can you use a different interpolation and discretization scheme for each differential operator in each equation? Is there an optimal choice to guarantee the "well balanced" property and drop the residuals to machine accuracy?
Anyway, we are very happy with OpenFOAM results for incompressible flows. They converge as the mesh is refined and it is able to reproduce many nonlinear transitions for a wide range of physical problems, even in the presence of the pressure plateau. 

June 22, 2011, 16:11 

#18  
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Martin Hegedus
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One of the reasons this is interesting is that eddy viscosity in the RANS equation basically lowers the local Reynolds number, i.e. viscosity goes up. Thus there is somewhat of a connection between the flows I usually deal with, and your low Reynolds number shapes. This seems to match with santiagomarquezd. Has anyone done a flat plate analysis with OpenFOAM and converged it to machine zero? (Edit: Oh, at low to high reynolds numbers) 

June 22, 2011, 17:03 

#19 
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Patricio Bohorquez
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Nice thoughts. The flat plane analysis is a good suggestion. If someone knows the answer please share it.
With respect to the shear stress in the cell, figure 16 in Alves, Oliveira & Pinho (2003), www.fe.up.pt/~fpinho/pdfs/ijnmf1.pdf, came to my mind. I don't know if it is a crazy idea but iterations in SIMPLE are analogous to "pseudotime", so maybe there is some analogy between the asymptotic values for the pressure and for \tau_{xx}. 

June 22, 2011, 17:24 

#20 
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Martin Hegedus
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I did I quick internet search and found OpenFOAM results for flat plates, both laminar and turbulent. I have not seen any residual plots and the results I saw are for x stations that have much higher Rex than the reynolds number of interest here. So nothing conclusive.


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