2-D Airfoil Simulation

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 October 26, 2012, 17:18 2-D Airfoil Simulation #1 New Member   J Join Date: Oct 2012 Posts: 5 Rep Power: 4 Hello, I am working on a senior capstone project and need to analyze several airfoil sections in Fluent. I am using ANSA to generate the meshes. The solution methods and solution controls are as follows: -Methods- Scheme>SIMPLE Gradient>Gree-Gauss Node Based Pressure>Standard Density>Second Order Upwind Momentum>Second Order Upwind Energy>Second Order Upwind -Controls- Pressure>0.3 Density>1 Body Forces>1 Momentum>0.7 Energy>1 The far-field boundary is defined as a pressure far-field and the airfoil geometry is defined as a wall. Currently the solution is not converging, the residuals drop initially but then they slowly start to either level off or rise. Does anyone have any tips on setting Fluent parameters? Thanks! Last edited by OSUStudent; October 26, 2012 at 17:44.

 October 26, 2012, 19:54 #2 Member   Ryne Join Date: Jan 2010 Posts: 32 Rep Power: 7 Please post some good pictures of your mesh before we go any further.

 October 27, 2012, 09:25 #3 Senior Member   Stuart Buckingham Join Date: May 2010 Location: United Kingdom Posts: 267 Rep Power: 16 You can increase your URFs if its converging and then not doing much more. Also, plot your drag and lift coefficients, they will help you decide if your solution is well converged. Convergence is a subjective thing. Just because the default setting says something is converged when the residuals get below 10^-4, doesn't mean a sloution is converged. Stu __________________ http://bc247.wordpress.com

October 27, 2012, 11:34
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Sijal Ahmed Memon (turboenginner@gmail.com)
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Quote:
 Originally Posted by stuart23 You can increase your URFs if its converging and then not doing much more. Also, plot your drag and lift coefficients, they will help you decide if your solution is well converged. Convergence is a subjective thing. Just because the default setting says something is converged when the residuals get below 10^-4, doesn't mean a sloution is converged. Stu
Very true. May be for some one 1e-4 is converged solution and for other case you may require 1e-6. Check your main parameters with both convergence criteria and see if there is any change in solution.

 October 28, 2012, 16:25 #5 Member     Pedram Mojtabavi Join Date: Apr 2011 Location: Iran Posts: 66 Rep Power: 6 What is your Mach number? If you determine the both inlet and outlet boundaries as pressure far field, then you can calculate the inlet and outlet static pressure based on the total pressure of your case and enter them in the panel. There will be no problem if you do so since I have done the similar case before. P.S : Try first order schemes for initial calculations and capture Y+ of your grid. You may need too refine your mesh as well. Hope it will help with your problem, Best regards.

November 1, 2012, 17:50
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J
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Thank you for the help!

Here are some pictures of the mesh I am using, I don't have that much experience using ANSA and am not really sure how to determine the quality of the mesh.
Attached Images
 mesh1.jpg (37.2 KB, 25 views) mesh2.jpg (78.8 KB, 32 views) mesh3.jpg (42.8 KB, 28 views)

 November 1, 2012, 18:04 #7 Member     Pedram Mojtabavi Join Date: Apr 2011 Location: Iran Posts: 66 Rep Power: 6 You better consider structured quadrilateral grid close to the wall to resolve the boundary layer more accurately. outside the boundary layer You can use either unstructured triangular or tetrahedral grid.

November 1, 2012, 18:22
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J
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I know how to check the mesh quality in FLUENT where it gives you a value between 0 and 1 but is there a way to do it in ANSA?
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 mesh1.2.jpg (47.5 KB, 9 views) mesh2.2.jpg (73.6 KB, 10 views) mesh3.2.jpg (45.4 KB, 15 views)

 November 1, 2012, 18:32 #9 Member     Pedram Mojtabavi Join Date: Apr 2011 Location: Iran Posts: 66 Rep Power: 6 I'm afraid I don't have information about ANSA. But since your quality does not exceed more than 0.9 the mesh is fine. If the skewness gets too high, FLUENT will notify you in command window. Your recent mesh is fine. You just need to keep Y+ low.

 November 1, 2012, 18:45 #10 New Member   J Join Date: Oct 2012 Posts: 5 Rep Power: 4 Ok, thank you. In reference to you earlier question my velocity is M=0.02035, I have read elsewhere that if the velocity gets too low then the calculations won't work out.

 November 6, 2012, 08:57 #11 Senior Member   Vangelis Skaperdas Join Date: Mar 2009 Location: Thessaloniki, Greece Posts: 163 Rep Power: 9 Just some tips for the use of ANSA When you mesh with CFD meshing algorithm make sure that you specify a Maximum Length at the Options window at the bottom right of the ANSA GUI. In this way the mesh will not grow in size inside the macro above the value that you set. You can find bad elements if you activate HIDDEN button (near SHADOW at the bottom) If you start ANSA in CFD mode then the default Fluent skewness values are already used. ANSA will report elements failing this limit as OFF and will display them in green or red color if the fail Fluent equiarea or equiangle skewness respecitively. Finally in order to have a nice spacing near the wall you could make some cuts in the inner macro, and then use the functions PERIMETERs>NUMBER and SPACING to aling the nodes and use MAP QUAD mesh. Vangelis

November 8, 2012, 20:23
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J
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Thank you for the reply.

I was able to create a quad map mesh in the boundary layer and checked the mesh using the HIDDEN button like you described but I am still having convergence issues within fluent. My current mesh has about 650K points and fluent reports its quality as 0.60. At this point I think the problem might be something I'm doing (or not doing) in fluent. Do you have any suggestions for this problem?
Attached Images
 mesh1.3.jpg (63.1 KB, 14 views) mesh2.3.jpg (96.7 KB, 18 views) mesh3.3.jpg (62.7 KB, 13 views) resid.3.jpg (37.4 KB, 19 views)

 November 9, 2012, 00:03 #13 Super Moderator     Sijal Ahmed Memon (turboenginner@gmail.com) Join Date: Mar 2009 Location: Islamabad Pakistan Posts: 3,909 Blog Entries: 6 Rep Power: 38 did you check the scale of problem in Fluent? What about the boundary conditions?

November 9, 2012, 02:20
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Quote:
 Originally Posted by OSUStudent Thank you for the reply. I was able to create a quad map mesh in the boundary layer and checked the mesh using the HIDDEN button like you described but I am still having convergence issues within fluent. My current mesh has about 650K points and fluent reports its quality as 0.60. At this point I think the problem might be something I'm doing (or not doing) in fluent. Do you have any suggestions for this problem?
Strange behavior of residuals,there is something wrong in the setup of problem. From which zone you have initialized the problem?

 Tags aerodynamics, airfoils, ansa, fluent, fluid

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