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July 26, 2007, 07:03 
Hi,
I tried to run the forw

#1 
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Lasse Boehling
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Hi,
I tried to run the forwardStep tutorial, but the Courant number is getting very big fast. So I tried to decrease the timestep. This resulted in bigger Courant numbers faster than before. How can that be? Co = (\delta(t)*U)/(\delta(x)) What can you do in order to run it properly? Initial conditions are those set by the tutorial. All hints and help will be much appreciated.  Lasse 

July 26, 2007, 08:12 
hi,
i am not in expert in t

#2 
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Stephan Gerber
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hi,
i am not in expert in this area had only some lessons on gasdynamics  but what exactly happend? if i run the case (foam 1.3) the max. co keeps constant and the mean co increases first fast and than slower and slower. what did you expect? i thought the shockwave runs in the area and therefore the mean co should become higher in the beginning. but if the flow becomes steady the mean co becomes steady too. greetings stephan 

July 26, 2007, 08:24 
hi,
i checked me setup agai

#3 
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Stephan Gerber
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hi,
i checked me setup again  and there are some changes to the tutcase: i use backward instead of euler for the time in system/fvScheme and changed the tolerances to 1e10 in system/fvSolution. with these changes it workes on my pc. greetings stephan 

July 26, 2007, 09:40 
Hi Stephan,
I expected a de

#4 
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Lasse Boehling
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Hi Stephan,
I expected a decrease in \delta(t) would result in a decrease in the Courant number (a slower divergence, (I found the opposite)). Backward is ofcourse more stable. But the Courant number still diverges after 0.5 sec. Both the mean and max increases, so after 0.57 sec they are ~e30. It still crashes. Any good ideas? Thanks, Lasse 

July 26, 2007, 10:08 
hi,
i run the calculation w

#5 
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Stephan Gerber
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hi,
i run the calculation with the settings i posted to you up to the endTime(10) and had no problems... btw i used 4 piso loops but even if i use euler with 2 piso loops (standard) i have no such problems. did you change any setting for the case? are you using foam 1.4 or 1.3? the problem with higher co together with lower dt is still strange. did you try recompiling the solver and building the mesh again? maybe you could post the bcfiles and the last timesteps which were showed up by the solver ... greetings stephan 

July 26, 2007, 10:54 
Hi,
I'm using foam 1.4. And

#6 
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Lasse Boehling
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Hi,
I'm using foam 1.4. And I ran it with the settings you told me. I'm pretty sure I didn't change any of the default settings, but I'll have to dobbelcheck that one. I'll let you know if I succed. Greetings Lasse 

July 26, 2007, 11:43 
hi,
actually why do you thi

#7 
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Stephan Gerber
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hi,
actually why do you think backward is more stable than implicit euler? greetings stephan 

July 27, 2007, 04:29 
Hi,
I just thought that Eul

#8 
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Lasse Boehling
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Hi,
I just thought that Euler automatically meant forward (explicit) Euler. And you made it backward Euler (implicit) by typing backward instead of euler. Therefore it would be more stable. Greetings Lasse 

July 27, 2007, 05:59 
hi,
if you look at the user

#9 
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Stephan Gerber
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hi,
if you look at the user/programmers guide fvm:ddt with "euler" in the fvScheme should be the implicit one  fvc:ddt should be the explicit one.(both first order in time). i dont know the word in english  but "backward" should be something like a multistepdiscr. which uses not only u_i to solve for u_(i+1) but u_(i1) too. (it is second order in time) in cases of solving ode both implicit euler and backward should be absolutly stable ( in sense of astable)  there are other stability criteria too. if you plot the stability region of both (implicit euler and backward) both should be in the upper right quadrant of the complex gauss diagramm (plots Re(z) over Im(z)) which says that they are both stable in case the timestep is positive. its is just my oppinion so please corect me if i am wrong!!! it is some time ago that i was working with stability... greetings stephan 

July 27, 2007, 08:16 
Hi,
You're right about fvm

#10 
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Lasse Boehling
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Hi,
You're right about fvm is implicit and fvc is explicit. But where do you see that Euler is fvm? I can only see the sonicFoam.C file where all the ddt are fvm. Maybe that's the place to see it. Implicit Euler and backward Euler is (as far as I know) the same. Forward Euler uses u_i and u'_i to calculate u_(i+1) where backward uses u_i and u'_(i+1). You're right about the stability wrt implicit euler and backward. Cheers Lasse 

July 27, 2007, 08:58 
hi,
in my opinion you can s

#11 
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Stephan Gerber
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hi,
in my opinion you can see it only in the solver equations  correct me if i am wrong. but still "backward" is not simply implicit euler  it has second order time errors since it uses more than one value from the past. just do a taylor expansion for u_(i1) and u_(i2) (in case you want to solve for u_i) and try to eliminate first order terms while solving for first deriv.. greetings stephan 

July 30, 2007, 04:02 
Hi,
I think you're right. I

#12 
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Lasse Boehling
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Hi,
I think you're right. I just don't know where the solver equations are. If I look in the fvSolution it says PBiCG and PBC under solvers for p, U, etc. Is PBiCG and PBC another way to see if it's implicit? Greetings Lasse Btw. Thanks for your answers so far. It helps me a lot 

July 30, 2007, 08:47 
hi,
as you mentioned, you c

#13 
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Stephan Gerber
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hi,
as you mentioned, you can see it in sonicFoam.C. in fvsolution you can find the name and parameters for the solvers of the linear equation system  in foam 1.3 sonicfoam uses versions of the cgsolver. i dont think you can see if its explicit or implicit in fvsolution although an explicit problem wont need a linear system at all.... regards stephan 

July 30, 2007, 09:49 
Hi,
Thanks. This may not be

#14 
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Lasse Boehling
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Hi,
Thanks. This may not be the correct place to pose this question, but I'm very curious: Are you just very helpful or do you work for openFoam? Either way, I appreciate your help. This question is also a little out of context. In the turbFoam.C file (and other turbulent source codes) they calculate U under the momentum predictor. In the equation there is a line: + turbulence>divR(U) What does that mean? Or where can I read about it? Thanks, Lasse 

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