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#1 |
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Daniel WEI (老魏)
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Hi developers,
Testing on OpenFOAM-2.1.0 ------------------- Here's the situation: ----------------------------------------------------------------- After running the case and writing out for a few times, eg. 1/, 2/, 3/, ... Then I stopped at time "3/" and then I change the deltaT value in "controlDict" file, from 0.0001 to 0.01, and then, I started the case again, (starting from the latestTime), HOWEVER, the next timestep is still 3.0001, the default deltaT is read from "3/uniform/time" file, not the "controlDict" file. So, is this considered as a feature? ![]()
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~ Daniel WEI ------------- Boeing Research & Technology - China Beijing, China |
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#2 |
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Alberto Passalacqua
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Is writeControl set to adjustableRunTime in controlDict?
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#4 |
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Alberto Passalacqua
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This just indicates OF will read dictionaries and settings during the run. You should have
writeControl adjustableRunTime; instead than writeControl runTime; Best,
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#5 |
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Daniel WEI (老魏)
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![]() Sorry, I didn't make it clear. My question is this, say I have a case with the following three folders: 163/ constant/ system/ when you start the solver, it tries to look for the deltaT value, NOT first from the "system/controlDict" file, but from "163/uniform/time" file. Which I would hope that the priority be, 1st, system/controlDict, and then 2nd, 163/uniform/time I understand it is necessary to read mesh with the priority as first time/polyMesh dir and then the constant/polyMesh dir. But I think deltaT is a different matter.
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~ Daniel WEI ------------- Boeing Research & Technology - China Beijing, China Last edited by lakeat; January 6, 2012 at 14:20. |
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#6 | |
Retired Super Moderator
Bruno Santos
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Greetings to all!
Daniel, what kind of solver are you using? If it is a transient solver, that probably is indeed a feature to avoid instantaneous crashes when restarting a case from a time snapshot >0. From past experiences, with transient solvers, the advised work-flow is to restart the simulation from the latest time snapshot, wait a few iterations for it to stabilize solving it and only then should you try to change said "deltaT" will it is running. If it is a stationary solver, then my guess is that it might be related to the need for N previous iterations of fields, which can be recreated simply by running a couple of steps with the same "deltaT" as before. Disclaimer: I don't enough about OpenFOAM's core code to give a sure answer, these are just speculations. Oh, and a question remains: if it is a transient solver, does it use an adaptive "deltaT"? edit: ah, this is what Alberto already asked - if you were using: Quote:
Bruno
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Last edited by wyldckat; January 6, 2012 at 12:42. Reason: see "edit:" |
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#7 |
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Daniel WEI (老魏)
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Hi Thanks Bruno,
I was/am using: writeControl runTime; It's nothing to do with the runTimeWriteOutControl feature, I can change this to another one, but the problem remains. solver is pisoFoam
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#8 | |
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Bruno Santos
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Hi Daniel,
After a bit of testing, it seems that one of my estimates has some validity: Quote:
Code:
deltaT deltaT0 Now, I suppose that the real issue is whether "deltaT" should be simply changeable in "controlDict" or continue to have to be changed in the latest time step. But after briefly looking at some code, my first conclusion is this: "deltaT" should be changed only if one knows what one is doing. Although, after looking at the code: http://foam.sourceforge.net/docs/cpp...ce.html#l00031 Code:
00031 void Foam::Time::readDict() 00032 { 00033 if (!deltaTchanged_) 00034 { 00035 deltaT_ = readScalar(controlDict_.lookup("deltaT")); 00036 } 00037 Code:
00903 void Foam::Time::setDeltaT(const scalar deltaT) 00904 { 00905 deltaT_ = deltaT; 00906 deltaTchanged_ = true; 00907 adjustDeltaT(); 00908 } Code:
00080 void Foam::Time::adjustDeltaT() 00081 { 00082 if (writeControl_ == wcAdjustableRunTime) 00083 { 00084 scalar interval = writeInterval_; 00085 if (secondaryWriteControl_ == wcAdjustableRunTime) 00086 { 00087 interval = min(interval, secondaryWriteInterval_); 00088 } 00089 00090 00091 scalar timeToNextWrite = max 00092 ( 00093 0.0, 00094 (outputTimeIndex_ + 1)*interval - (value() - startTime_) 00095 ); 00096 00097 scalar nSteps = timeToNextWrite/deltaT_ - SMALL; 00098 00099 // For tiny deltaT the label can overflow! 00100 if (nSteps < labelMax) 00101 { 00102 label nStepsToNextWrite = label(nSteps) + 1; 00103 00104 scalar newDeltaT = timeToNextWrite/nStepsToNextWrite; 00105 00106 // Control the increase of the time step to within a factor of 2 00107 // and the decrease within a factor of 5. 00108 if (newDeltaT >= deltaT_) 00109 { 00110 deltaT_ = min(newDeltaT, 2.0*deltaT_); 00111 } 00112 else 00113 { 00114 deltaT_ = max(newDeltaT, 0.2*deltaT_); 00115 } 00116 } 00117 } 00118 } Looking deeper, it seems that all of this mess happens because of the events in "Foam::Time::readModifiedObjects()": http://foam.sourceforge.net/docs/cpp...ce.html#l00251 - specifically this part of the code Code:
00268 // Time handling is special since controlDict_ is the one dictionary 00269 // that is not registered to any database. 00270 00271 if (controlDict_.readIfModified()) 00272 { 00273 readDict(); 00274 functionObjects_.read(); 00275 } 00276 00277 bool registryModified = objectRegistry::modified(); 00278 00279 if (registryModified) 00280 { 00281 objectRegistry::readModifiedObjects(); 00282 } My final conclusion is this: making OpenFOAM sane to update "deltaT" simply via "controlDict", can drive the programmer insane... therefore, why bother if one can change "deltaT" in at least one of two ways:
![]() Best regards, Bruno
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#9 |
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Alberto Passalacqua
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Honestly I have some problem understanding what the issue is. First of all, pisoFoam uses a fixed time step, second if you change the time step in controlDict after stopping the simulation and then you restart it, the new time-step is used, as expected.
As a guideline, changing the time step by hand is a poor practice. If you need to change your time-step, do it automatically by using a solver with adaption (pimpleFoam), and eventually implement a CFL condition that reflects your adaption criterion. For what concerns the precedence given to what time-stepping information has precedence, giving priority to the uniform/time information seems correct to me, since your run terminated with that information. This however does not mean OpenFOAM will ignore the new time-step set in controlDict. Finally, since the problems seemed to be that OpenFOAM saved a different time than the desired one. If you change your time-step during the simulation, you must set writeControl to adjustableTimeStep, and not to timeStep. However, this is explained in the User's Guide ;-) Best,
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#10 | ||||
Retired Super Moderator
Bruno Santos
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Hi Alberto,
Quote:
Quote:
Quote:
Quote:
![]() My guess is that Daniel needs a fast running solver and that a higher or lower "deltaT" was needed only for the first few iterations. Best regards, Bruno
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#11 | |
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Daniel WEI (老魏)
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Quote:
I thought controlDict has higher privilege in "controlling" the deltaT value. Anyway, its no big deal. And I feel it's too quick to judge it good or bad practice, we all have freedom to do something. I just came across to find this issue (or it is not an issue at all), when I was trying to write and test some fancy stuffs. Thank you guys
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#12 | ||
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Alberto Passalacqua
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Quote:
Quote:
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#13 | |
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Alberto Passalacqua
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Quote:
- To decide the time-step you have to manually compute the maximum Courant number, which is time-consuming. A code can do it for you, exactly and reducing the chance you make mistakes. On the long run, a few more lines of code will save you a lot of time. - You have to manually account for physical time scales you want to resolve. This can be done easily by implementing the relevant criteria. - Manual adaption usually is less smooth than what a well-defined adaption can do for you. This reflects on accuracy and stability. Best,
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#14 |
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Alberto Passalacqua
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As a second thought, you can change the time-step by hand directly while the simulation is running, by editing controlDict, and saving it, if runTimeModifiable is set to true in the same dictionary.
Code:
Time = 0.05161 Courant Number mean: 0.000432668 max: 0.00169887 DILUPBiCG: Solving for Ux, Initial residual = 8.17931e-06, Final residual = 8.17931e-06, No Iterations 0 DILUPBiCG: Solving for Uy, Initial residual = 1.05228e-05, Final residual = 2.49306e-11, No Iterations 1 DICPCG: Solving for p, Initial residual = 2.25639e-05, Final residual = 6.67366e-07, No Iterations 17 time step continuity errors : sum local = 3.62613e-13, global = 2.84931e-21, cumulative = 6.5081e-20 DICPCG: Solving for p, Initial residual = 2.2504e-05, Final residual = 5.38693e-07, No Iterations 17 time step continuity errors : sum local = 2.92495e-13, global = 9.48445e-22, cumulative = 6.60294e-20 ExecutionTime = 9.39 s ClockTime = 9 s Time = 0.05162 Courant Number mean: 0.000432674 max: 0.00169887 DILUPBiCG: Solving for Ux, Initial residual = 8.17484e-06, Final residual = 8.17484e-06, No Iterations 0 DILUPBiCG: Solving for Uy, Initial residual = 1.05171e-05, Final residual = 2.49175e-11, No Iterations 1 DICPCG: Solving for p, Initial residual = 2.25517e-05, Final residual = 6.6703e-07, No Iterations 17 time step continuity errors : sum local = 3.62432e-13, global = 1.69903e-22, cumulative = 6.61993e-20 DICPCG: Solving for p, Initial residual = 2.24918e-05, Final residual = 5.38372e-07, No Iterations 17 time step continuity errors : sum local = 2.92321e-13, global = 1.62789e-21, cumulative = 6.78272e-20 ExecutionTime = 9.39 s ClockTime = 9 s regIOobject::readIfModified() : Re-reading object controlDict from file "/home/alberto/OpenFOAM/alberto-2.1.x/tutorials/incompressible/icoFoam/cavity/system/controlDict" Time = 0.05262 Courant Number mean: 0.043268 max: 0.169888 DILUPBiCG: Solving for Ux, Initial residual = 0.000633579, Final residual = 2.75111e-07, No Iterations 2 DILUPBiCG: Solving for Uy, Initial residual = 0.000954272, Final residual = 1.37441e-06, No Iterations 2 DICPCG: Solving for p, Initial residual = 0.325047, Final residual = 6.71319e-07, No Iterations 30 time step continuity errors : sum local = 9.50543e-10, global = -9.0295e-20, cumulative = -2.24678e-20 DICPCG: Solving for p, Initial residual = 0.0408667, Final residual = 7.39858e-07, No Iterations 29 time step continuity errors : sum local = 8.20548e-10, global = 1.77083e-19, cumulative = 1.54615e-19 ExecutionTime = 9.39 s ClockTime = 9 s Time = 0.05362 Courant Number mean: 0.0433123 max: 0.169856 DILUPBiCG: Solving for Ux, Initial residual = 0.00295721, Final residual = 6.88127e-06, No Iterations 2 DILUPBiCG: Solving for Uy, Initial residual = 0.00340695, Final residual = 4.3901e-07, No Iterations 3 DICPCG: Solving for p, Initial residual = 0.047135, Final residual = 7.22096e-07, No Iterations 29 time step continuity errors : sum local = 8.5687e-10, global = 6.6009e-21, cumulative = 1.61216e-19 DICPCG: Solving for p, Initial residual = 0.0108227, Final residual = 6.78693e-07, No Iterations 28 time step continuity errors : sum local = 8.32457e-10, global = 8.08983e-20, cumulative = 2.42114e-19 ExecutionTime = 9.39 s ClockTime = 9 s Best,
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#15 |
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Alberto Passalacqua
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Reported here: http://www.openfoam.com/mantisbt/view.php?id=382
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#17 |
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Alberto Passalacqua
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You are welcome. The behavior is indeed not what a user would expect, so I guess it's a bug (and one I haven't noticed in a loooong time!).
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#18 |
Retired Super Moderator
Bruno Santos
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Hi,
Just saw now Henry's answer on the bug report... ROFL... So another of my conclusions was correct. ![]() Basically it's one of those features were one should look at the code when no documentation is available ![]() ![]() I didn't test changing it during runtime because it looked that it wasn't humanly possible to do so with such a small case. But I stand corrected ![]() Best regards, Bruno
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#19 | |||
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Alberto Passalacqua
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Quote:
Quote:
Quote:
Best,
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#20 |
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Daniel WEI (老魏)
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Dear Henry,
How about this, let the code give an inconsistency warning for the users, something like: "deltaT value inconsistency found between snapshot/uniform/time and system/controlDict, blahblahblah". so to be more human/user oriented. ![]() PS: Sorry to speak here, because I found the bug report had been closed...
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