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pressure eq. "converges" after few time steps 

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February 7, 2011, 09:08 

#41  
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Felix L.
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these were simple aerodynamic cases (2D, incompressible, RANS) and mostly hexa meshes. The case I was looking deeper into the performance of the linear solvers was a ground effect study of two interacting airfoils. The speedup I obtained with PCG was impressive, I was able to reduce the simulation time from 3 days (finest grid) with GAMG to less than 12 hours with PCG and otherwise same settings. Why do you use such low relative tolerances  any particular reason behind that? Greetings, Felix. 

February 7, 2011, 09:08 

#42  
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Franco Marra
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It should be possible by adding, in fvSolution, the following keyword: Code:
maxIter 300; Code:
solvers { p { solver PCG; preconditioner DIC; tolerance 1e06; relTol 0.1; maxIter 1; } ... Regards, Franco 

February 7, 2011, 09:19 

#43 
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Travis Carrigan
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Just curious, have you tried using leastSquares for the gradScheme?
I did some 2D calculations for a NACA airfoil using both structured and unstructured grids. I too suffered convergence issues when running the calculation for the unstructured case. However, switching the gradScheme to a cellLimited leastSquares happened to solve the problem. Let me know if this works. 

February 7, 2011, 09:58 

#44  
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maddalena
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Ok, first results using all you suggested above. I have not get to steady state, but at least I have a sort of solution now.
All goes fine until some boundary epsilon and k appears. They prevent my pressure residual to get under 0.1. How can I solve it? using lower relax factor on them? Quote:
mad 

February 7, 2011, 10:11 

#45  
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Vesselin Krastev
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Best Regards V. 

February 8, 2011, 00:02 

#46  
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Arjun
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There are chances that you do not observe efficiency changes. I will try to explain the reason. First matrix solver is sensetive to direct solve and number of equations. Why is it so? It is related to the main reason why multigrid algorithms work for the first place. simple rule of thumb is that larger the equations in direct solver faster the convergence multigrid would show. (it might work against it though some times, but it rarely does in properly implemented multigrid code). to show this I will show you an example. The mesh is 60 x 60 x 60 and Poisson equation. We will use smoothed aggregation multigrid (of Vanek et al). (I haven't used my additive corrective multigrid code for loong time so i will not waste time searching it). Here is how multgrid levels are generated in this case. ncells = 216000 Size [ 0 ] = 216000 Size [ 1 ] = 25939 Size [ 2 ] = 611 Size [ 3 ] = 10 Size [ 4 ] = 1 Max AMG levels = 4 For this problem initial residual is 1000. Here is how convergence went for this: Res start = 1000 [1 ] Res = 1652.59 ratio 0.605112 [2 ] Res = 633.144 ratio 1.57942 [3 ] Res = 254.718 ratio 3.9259 [4 ] Res = 110.734 ratio 9.03068 [5 ] Res = 51.5134 ratio 19.4124 [6 ] Res = 25.3378 ratio 39.4668 [7 ] Res = 13.0345 ratio 76.7195 [8 ] Res = 6.96472 ratio 143.581 [9 ] Res = 3.84651 ratio 259.976 [10 ] Res = 2.18456 ratio 457.759 [11 ] Res = 1.26968 ratio 787.598 [12 ] Res = 0.751132 ratio 1331.32 [13 ] Res = 0.449974 ratio 2222.35 [14 ] Res = 0.271864 ratio 3678.31 [15 ] Res = 0.165201 ratio 6053.23 [16 ] Res = 0.100779 ratio 9922.68 [17 ] Res = 0.0616454 ratio 16221.8 [18 ] Res = 0.03778 ratio 26469 [19 ] Res = 0.0231871 ratio 43127.5 [20 ] Res = 0.0142468 ratio 70191.2 [21 ] Res = 0.00876183 ratio 114131 [22 ] Res = 0.00539288 ratio 185430 [23 ] Res = 0.00332168 ratio 301052 [24 ] Res = 0.00204727 ratio 488455 [25 ] Res = 0.0012626 ratio 792017 [26 ] Res = 0.00077915 ratio 1.28345e+006 It took 26 iterations to drop error by factor of 1.28345e+006 Now lets fix level 2 as direct solve. That is the level with 611 will be solved directly. This is how convergence went for this case: Res start = 1000 [1 ] Res = 1625.02 ratio 0.615378 [2 ] Res = 512.776 ratio 1.95017 [3 ] Res = 167.994 ratio 5.95259 [4 ] Res = 57.2169 ratio 17.4774 [5 ] Res = 20.0184 ratio 49.9541 [6 ] Res = 7.11943 ratio 140.461 [7 ] Res = 2.56062 ratio 390.53 [8 ] Res = 0.928676 ratio 1076.8 [9 ] Res = 0.338887 ratio 2950.83 [10 ] Res = 0.124259 ratio 8047.74 [11 ] Res = 0.0457355 ratio 21864.9 [12 ] Res = 0.0168867 ratio 59218.1 [13 ] Res = 0.00625179 ratio 159954 [14 ] Res = 0.00232032 ratio 430975 [15 ] Res = 0.00086324 ratio 1.15843e+006 You see by increasing direct solve size i could do the same thing in 15 iterations. So there are two things: (a) Direct solve takes time (b) By increasing direct solve size you can speed up convergence. A good choice of direct solver size would be when the time saved in convergence is more than the time lost in direct solve. So sometimes they can cancel each other out. This is the reason you might not have noticed the efficiency change. If you really want to observe the change then try putting the direct solve size to very large , say 100000 or so. 

February 8, 2011, 00:20 

#47  
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Arjun
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The main issue is that multigrid is single word BUT it represents a whole world of matrix solvers. Some multigrids have issues and thats why a lot of research is going on in this area. But some of modern multigrid solvers are really very impressive. Good read would be this http://neumann.math.tufts.edu/~scott/research/aSA2.pdf just to see which direction we are heading. 

February 8, 2011, 01:32 

#48 
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maddalena
one reason for you residuals not going very low can be, that your solution is of transient nature. BTW, what the reason to set different nonorthogongonal correction (limited 0.5 an limited 0.33) for k and epsilon? My point of view, I would try to converge all with the first order, and then switch to the "second" order linearUpwind.
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Best regards, Dr. Alexander VAKHRUSHEV Christian Doppler Laboratory for "Metallurgical Applications of Magnetohydrodynamics" Simulation and Modelling of Metallurgical Processes Department of Metallurgy University of Leoben http://smmp.unileoben.ac.at 

February 8, 2011, 02:38 

#49  
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maddalena
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Good morning!
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Quote:
My task of today is trying to reduce the pressure residual, and the first step can be to use first order everywhere on laplacian. And maybe trying least square on grad U, as suggested by Travis. Thanks to your support, I will keep you informed! mad 

February 8, 2011, 03:44 

#50 
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maddalena
did you try potentialFoam on your setup? Is it converging till required accuracy?
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Best regards, Dr. Alexander VAKHRUSHEV Christian Doppler Laboratory for "Metallurgical Applications of Magnetohydrodynamics" Simulation and Modelling of Metallurgical Processes Department of Metallurgy University of Leoben http://smmp.unileoben.ac.at 

February 8, 2011, 03:54 

#51  
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maddalena
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Quote:
mad 

February 8, 2011, 04:05 

#52 
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I have mentioned potentialFoam, because it gives some tips about your required nonorthogonal corrections and gives a good first approximation to start with.
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Best regards, Dr. Alexander VAKHRUSHEV Christian Doppler Laboratory for "Metallurgical Applications of Magnetohydrodynamics" Simulation and Modelling of Metallurgical Processes Department of Metallurgy University of Leoben http://smmp.unileoben.ac.at 

February 8, 2011, 05:00 

#53  
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Vesselin Krastev
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Best Regards V. 

February 8, 2011, 05:22 

#54  
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Felix L.
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Hello, Arjun, thanks for the text recommendation, I will have a look into it. Yeah, I know MG methods are a complex topic and there are many different directions evolving at the moment. I was working with the DLR TAU code (a code of the german aerospace center used both in research and industry) and the multigrid method used there was really, really helpful to save resources. I'm pretty sure, GAMG won't be the only option of MG approaches in OF, so I'm very much looking forward to the upcoming updates. Greetings, Felix. 

February 8, 2011, 09:19 

#55 
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maddalena
hope I found the reason for your poor pressure residuals. Look at the residual level for my pressure equation test with 1. laplacian corrected 2. laplacian limited 0.5 1200cells_corrected_linear_100iter.png 1200cells_limited_0.5_linear_100iter.png
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Best regards, Dr. Alexander VAKHRUSHEV Christian Doppler Laboratory for "Metallurgical Applications of Magnetohydrodynamics" Simulation and Modelling of Metallurgical Processes Department of Metallurgy University of Leoben http://smmp.unileoben.ac.at 

February 8, 2011, 09:30 

#56  
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Vesselin Krastev
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February 8, 2011, 10:00 

#57  
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maddalena
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February 8, 2011, 11:53 

#58 
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Felix L.
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Hello, all,
I can reproduce this behaviour using one of my aerodynamic cases with different laplacian schemes and otherwise same settings (see the attachements). It has to be noted that the aerodynamic coefficients differ only by max. 0.1%, so this incomplete convergence of pressure  though not really goodlooking  maybe has a minor influence on the result (at least for my simple 2D case). But I was able to get the resiudal of p below 1e3 for the limited laplacian case, so maybe this is already accurate enough, I can't tell. A deeper investigation would be interesting. Greetings, Felix. 

February 9, 2011, 02:01 

#59 
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maddalena
the same behavior of the pressure residuals I got when changing gradSchemes from linear (or leastSquares) gradient scheme to its limited version (cell/face(MD)Limited) another my observation was, that leastSquares (not limited) scheme for gradients gave more smooth and physical solution, while one from the linear scheme was distorted by skewed cells of the unstructured grid part. So if you are using limited version of pressure laplacian and gradient discretization, the order of 10^2  10^3 might be normal for your pressure residuals. On the one hand, introducing limiting, you provide more physically correct solution due to its boundness. On the other hand, that would result in convergence issues. You can read more about accuracy/convergence in so called "Gamma paper" http://powerlab.fsb.hr/ped/kturbo/Op...GammaPaper.pdf PS. By the way, Maddalena, thank you for rising this topic, it helped me to discover some important points for my self))) PS1. Hope you understood, that I suggested to try non limited scheme versions to achieve desired convergence criterion.
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Best regards, Dr. Alexander VAKHRUSHEV Christian Doppler Laboratory for "Metallurgical Applications of Magnetohydrodynamics" Simulation and Modelling of Metallurgical Processes Department of Metallurgy University of Leoben http://smmp.unileoben.ac.at 

February 9, 2011, 03:19 
Thank you!

#60 
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maddalena
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From my point of view, this is one of the most interesting thread about schemes and convergence of the OF forum! All the suggestions have been demonstrated widely. Indeed, they have been not "limited" to the only: Do this because I know it works, but people showed that what they say is true for specific reasons, with specific test cases.
Of course, the thread is open for similar contribution in the future, hoping that the discussion level remains the same. Thank you, FOAMers! maddalena 

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convergence issues, pipe flow, simplefoam 
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