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IDProjectCategoryView StatusDate SubmittedLast Update
0000064OpenFOAMBugpublic2010-10-25 18:002010-10-25 19:15
Reporteruser43Assigned Tohenry  
PrioritynormalSeverityminorReproducibilityalways
Status resolvedResolutionfixed 
PlatformLinuxOSUbuntuOS Version9.10
Summary0000064: Semi-implicit Bullirsch-Stoer ODE solver is not reducing the timestep correctly
Descriptionsrc/ODE/ODESolvers/SIBS/SIBS.C

To solve the problem, change line 41 of this file:

Original:
       SIBS::redMax = 1.0e-5, SIBS::redMin = 0.0, SIBS::scaleMX = 0.1;

New:
       SIBS::redMax = 1.0e-5, SIBS::redMin = 0.7, SIBS::scaleMX = 0.1;

Background:
ODE solver "semi-implicit Bullirsch-Stoer" uses adaptive timestepping roughly according to:

1- Take the suggested timestep, and calculate the error
2- Is the error acceptable?
Yes - Calculate a timestep increase factor.
    - Apply timestep increase and move to the next time step.
No - Calculate a timestep reduction factor.
   - Apply timestep reduction, and retry the current timestep.

The timestep reduction (red) is tested against maximum (redMax) and minimum (redMin) limits.

Problem:
These timestep reduction limits are set incorrectly, and red always evaluates to the greatest possible timestep reduction: 1.0e-5. redMin is set to zero in the constructor (line 41). Should be set to 0.7.

This may have been cause by confusion over variable "red". It is a reduction factor... so a maximum reduction factor corresponds to a minimum next timestep. redMax and redMin are named for their effect on the timestep, not on their absolute value. redMax should therefore be smaller than redMin.

Solution:
The code is directly adapted from Numerical Recipes. The values used in this case are:

redMin = 0.7
redMax = 1.0e-5
Steps To ReproducePerform any ODE solution, set the timestep to an unreasonably large value. Observe value of red as it goes through lines 207 and 208.
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henry

2010-10-25 19:15

manager   ~0000090

Thanks for the bug-report.
Resolved in OpenFOAM-1.7.x by commit e5e8175dbfaf1d5a6d77332ac934b37de47ce7a4

Issue History

Date Modified Username Field Change
2010-10-25 18:00 user43 New Issue
2010-10-25 19:13 henry Assigned To => henry
2010-10-25 19:13 henry Status new => assigned
2010-10-25 19:15 henry Note Added: 0000090
2010-10-25 19:15 henry Status assigned => resolved
2010-10-25 19:15 henry Resolution open => fixed