Table of Contents
Managing Time Steps
Archiving manager
General Points
Archiving is done with metafor.getTimeStepManager()
. An initial time and time step is set using setInitialTime()
, then a series of subsequent steps are set using setNextTime()
. Then number of archives between two time steps is given as argument to setNextTime()
.
tsm = metafor.getTimeStepManager() tsm.setInitialTime(t0, muli) tsm.setNextTime(t1, npas, mulm) ...
where
t0  initial time 
t1  next time 
muli  initial time step 
mulm  maximal time step between t0 and t1 
npas  number of archives between t0 and t1 
Use of Metafac
Introduction
This section describes how to deal with Metafor output, which are called Fac files, and created by the object called Metafac
(.bfac
, .tfac
).
Each time step is saved in a different file. This way, useless steps can be easily deleted, and a restart
can be done.
By default, archiving is done in binary (.bfac
 in opposition to .tfac
, text version or ASCII). If zlib is available, files are compressed immediately (into .bfac.gz
).
Note that a binary file is usually not portable between various OS.
By default, archiving files are sent to a folder called workspace/test_name/
and are called step_*.bfac.gz
.
Useful scripts
Some scripts, found in oo_meta/toolbox/utilities.py
, were written to easily see the result of a simulation, to convert into a different format, … These files can be modified by the user to suit a particular application.
View a given step with VizWin
To load the 10th time step,
loadFac('rep1.rep2.mytest', 10)
(assuming that the file mytest.py
is located in /rep1/rep2/
). If no time step is specified, Metafor loads the one having the highest number.
To view it :
vizu('rep1.rep2.mytest')
Movie
Automatic conversion from bfac/tfac to bmp:
 Type
makeAnimation('rep1.rep2.mytest')
 A
VizWin
window is opened with the first time step and the user is put on hold.  Activate options (scales, etc)
 Check that the
VizWin
window is not hidden by an object on the screen (otherwise it might appear in the screenshots).  Type ENTER once ready
Automatic creation of the movie:
 Run a program to create movies from the files (for example AVImaker).
Time step manager
The time step is monitored using the commands:
tsm = metafor.getTimeStepManager() tsm.setTimeStepComputationMethod(meth) tsm.setTolOnFac(tolOnFac) tsm.setIntegrationErrorTolerance(prec) tsm.setExplicitTimeStepSecurity(secu) tsm.setNbOfStepsForCriticalTimeStepUpdate(nrit) tsm.setMaxNbOfItInAdaptOfTimeStep(nbIter) (option: used in implicit or quasistatic) tsm.setMinimumTimeStep(minTimeStep)
^ Parameter ^ Default value ^ Description ^
meth  = TSM_NBOFITERATIONSMETHOD : in implicit, the current time step depends on the number of iterations in the previous time step, when in explicit it only depends on the critical one and the parameter secu 

= TSM_INTEGRATIONERRORMETHOD : the current time step is computed based on an integration error (and based on the critical one and the parameter secu in explicit) 

prec  1.E4  accuracy on the integration error 
secu  2.0/3.0  safety coefficient which multiplies the critical time step in explicit 
nrit  1  number of time steps before updating the critical time step 
nbIter  5  Minimal number of iterations in the algorithm in charge of reducing the time step. If the required number is greater than nbIter , the subsequent time step will be lower than the current one. Otherwise, it will be equal or greater. 
minTimeStep  1.0E10  minimum time step below which metafor stops running. 
tolOnFac  0.1  tolerance on time step for the discrete times corresponding to archiving (FAC). 
Quasistatic and dynamic implicit schemes
Two possibilities are used to compute the time step:
 A method based on the number of iterations before convergence. If lower than 3, the time step is increased. If greater than 4, it is decreased. This method does not provide any guaranty on accuracy.
 A method based on an integration error. It computes an integration error based on the difference of the norm of the acceleration (dynamic case) or speed (quasistatic case) to adjust the time step, depending on the accuracy required.
Dynamic explicit schemes
Since these schemes are conditionally stable, the time step $\Delta t$ is given by :
$ \Delta t = \gamma_s \Delta t_{crit} $
Where $\Delta t_{crit}$ depends on the maximal eigenfrequency of the system (obtained using the method called “power iteration”, and updated regularly) and $\gamma_s$, a safety coefficient which can be computed two ways:
 Constant safety, coefficient, no guaranty on the accuracy.
 Safety coefficient based on an integration error. It computes an integration error based on the difference of the norm of the acceleration. A maximal value is set by the user. Practically, the safety coefficient will only be lower than this user parameter for highly dynamic situations, such as impact.