TimeDiscrete

model.sol(sname).create(fname,"TimeDiscrete")

model.sol(sname).feature(fname).set(pname,value)

The time interval and possible intermediate time values are given in the property tlist. The output times are controlled by the property tout.

The feature TimeDiscrete accepts the following property/values:

Table 6-69: Valid Properties for the time-Discrete Solver
Property	Values	Default	Description
atol	String	empty	Absolute tolerance per field. See below,
atolmethod	String	empty	How to interpret the atolfields value. See below.
atolglobal	positive scalar	1e-3	Global absolute tolerance, if atolglobalvaluemethod is manual.
atolglobalfactor	positive scalar	0.1	Global absolute tolerance as a factor of the relative tolerance, if atolglobalvaluemethod is factor.
atolglobalmethod	scaled \| unscaled	scaled	How to interpret the atolglobal value.
atolglobalvaluemethod	factor \| manual	factor	Use a factor of the relative tolerance or a user-defined value for the absolute tolerance.
clist	String array		Provide values for constants as input parameters using a string array; for the corresponding constant names, use cname.
cname	String array		Provide names of constants as input parameters using a string array; for the corresponding constant values, use clist.
control	String	user	Name of the controlling study step or "user" if the feature is controlled manually.
keeplog	on \| off	off	Keep warnings in stored log.
message	String		The log message from the last solution process.
plot	on \| off	off	Plot while solving.
plotfreq	tsteps \| tout	tout	Times to update plot.
plotgroup	String		Name of plot group for plot while solving.
prevlevels	positive integer	2	Number of previous time levels to store.
probefreq	tsteps \| tout	tsteps	Times to update probe.
probes	vector of strings		Probed to use if probesel=manual.
probesel	all \| none \| manual	all	The probes to compute.
rtol	numeric	0.01	Relative tolerance.
timestepdiscrete	numeric scalar \| numeric vector \| string with expression	0.01	Time step when manual time stepping.
tlist	numeric vector		Time list.
tout	tlist \| tsteps	tlist	Output times.

The TimeDiscrete solver is used for solving time-dependent PDEs that have already been discretized in time using, for example, the prev operator or the bdf operator. Such discretization requires the solution at previous time steps. Different discretizations require different number of previous time steps. For example, the first order accurate bdf method requires the solution at the previous time step, while the second-order accurate bdf-method requires the solution at the two preceding time steps. How many previous time steps should be accessible to the solver is controlled through the property prevlevels.

You can control the process of solving the linear or nonlinear system of equations in each time step manually. For a coupled problem, this is done through the properties Damp, Dtech, Hnlin, Initstep, Jtech, Maxiter, Minstep, and Rstep listed under femnlin. For a segregated problem, the properties listed under femstatic that are related to the segregated solver are available.

Because only manual time stepping is available, there is no estimation of the error made in a time step. However, the tolerances, specified through the properties rtol, atol, atolmethod, atolglobal, and atolglobalmethod are still important as tolerances when solving the nonlinear system of equations in each time step. For a description of these properties, see Time. They should in general be set to the desired accuracy in the final solution.

The property tlist must be a strictly monotone vector of real numbers. Commonly, the vector consists of a start time and a stop time. If more than two numbers are given, the intermediate times can be used as output times, or to control the size of the time-steps (see below). If just a single number is given, it represents the stop time, and the start time is 0.

The property tout determines the times that occur in the output. If tout=tsteps, then the output contains the time steps actually taken by the solver. If tout=tlist, then the output contains interpolated solutions for the times in the tlist property. The default is tout=tlist.

The size of the time step is controlled through the property timestepdiscrete. If timestepdiscrete is a scalar value, this time step is taken in the entire simulation. When timestepdiscrete is a (strictly monotone) numeric vector, the solver computes the solution at the times in the vector. The start time and stop time is still obtained from tlist; the vector given in timestepdiscrete is truncated and/or expanded using the first and/or last time step in the vector so that the start time and stop time agrees with the values in tlist. Finally, an expression using variables with global scope and which results in a scalar can be used as timestepdiscrete.


	For more information about the time discrete solver; see Time Discrete Solver in the COMSOL Multiphysics Reference Manual.