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TEMPORAL - Time-marching input (block)

TEMPORAL
   [TIME LEVELS ntlvls
      SUBITERATIONS [MODE] {NEWTON|DUALDT} ]
   [[PHYSICAL] TIMESTEP PhysDt | [STEADY] CFL cfl [zone_selector]]
   [CONVERGENCE {LEVEL|ORDER} newcvg]]
   [IMPLICIT TIME {FIRST|SECOND} [ORDER] [zone_selector]]
   [CFTVARS {LOCAL|LINK}]
   [NEWTVARS {LOCAL|LINK}]
   [SORDTVARS {LOCAL|LINK}]
   [SAVE {NEWTON|NO_NEWTON}]
   [SAVE {TIME|NO_TIME}]
   [SAVE_CFT]
ENDTEMPORAL

The TEMPORAL keyword block is used for various input options related to the time-marching procedure to be used. The various elements of the TEMPORAL keyword block are defined as follows:

TEMPORAL

Defines the beginning of the TEMPORAL block.

TIME LEVELS ntlvls
   SUBITERATIONS [MODE] {NEWTON|DUALDT}

The TIME LEVELS keyword may be used to specify the number of global time levels (ntlvls) to advance.

The SUBITERATIONS [MODE] {NEWTON|DUALDT} keyword must appear on a subsequent line and allows the user to specify either Newton subiterations or dual time stepping. The two modes yield similar formulations, but with slightly different coefficients.

Newton fixed-point methods are known to converge extremely quickly when they converge, but may not converge at all if the starting point is too far from the root. In limited testing it has been shown that the NEWTON mode converges faster than the DUALDT mode. However, the DUALDT mode may be more stable in cases where the NEWTON mode fails to converge or requires extremely small time steps. This is because the dual time approach allows the use of local time stepping in the pseudo-time iterations while still advancing the solution by a time step that is driven by the physics of the problem.

In older versions of the code, the less descriptive MODE {NPARC|BCFD} keyword combination was used to select between the Newton and dual time stepping algorithms. Prior to the MODE keyword, the only way to control this option was by adjusting the newton_mode variable in modules/global_variables_module.f90 and recompiling.

[PHYSICAL] TIMESTEP PhysDt | [STEADY] CFL cfl [zone_selector]

There are two methods for specifying the outer (global) time step. The first method specifies a physical time step, PhysDt, in seconds. The second method specifies a constant CFL. If neither option is specified, the minimum zonal value from the CFL keyword is used.

The CFL keyword is used to control the inner (pseudo-)time step.

CONVERGENCE {LEVEL|ORDER} newcvg

There are two possible procedures for determining the overall global convergence with subiteration.

    CONVERGENCE LEVEL   Convergence is assumed when

Qn − Qn−1 | < newcvg

where Q represents the vector of dependent variables, and n is the subiteration time level.

CONVERGENCE ORDER Convergence is assumed when

Qn − Qn−1 | / max ( | Qn − Qn−1 | ) < 10newcvg

In this case, newcvg is an integer value.

The default, if the CONVERGENCE keyword isn't used, is CONVERGENCE ORDER 3.

Within a subiteration time level, the number of cycles and iterations to be run is specified using the CYCLES and ITERATIONS keywords.

The convergence criteria within a subiteration time level (i.e., the inner loop level) may be specified using the CONVERGE keyword. Note that for structured grids the default for the CONVERGE keyword is a four order of magnitude decrease in the maximum residual. If Newton iteration is being used for a steady flow problem with structured grids, with the default of three orders of magnitude for the global convergence criteria, it would make sense to also use three orders of magnitude for the convergence criteria within a Newton time step.

See Also: CFL, CYCLES, ITERATIONS, CONVERGE, NEWTON

IMPLICIT TIME {FIRST|SECOND} [ORDER] [zone_selector]

This keyword sets the order of the implicit time marching scheme. The default, if the IMPLICIT TIME keyword isn't used, is first-order.

Second-order time marching may be used with both structured and unstructured grids, and with or without the subiteration methods. With structured grids, it may be used with any implicit operator except MacCormack's modified approximate factorization procedure (IMPLICIT MACCORMACK).

See Also: IMPLICIT, IMPLICIT ORDER

CFTVARS {LOCAL|LINK}
NEWTVARS {LOCAL|LINK}
SORDTVARS {LOCAL|LINK}
SAVE {NEWTON|NO_NEWTON}
SAVE {TIME|NO_TIME}

Using the subiteration techniques and/or second-order time differencing requires the storage of flow field data at additional time levels. When CFTVARS LOCAL is specified, the extra time levels required for subiteration and second-order time differencing are stored in the common flow (.cfl) file. Specifying CFTVARS LINK causes the extra time levels to be stored in a separate time data (.cft) file, and linked to the .cfl file. The NEWTVARS and SORDTVARS keywords may be used instead of CFTVARS, and apply independently to Global Newton iteration (or dual time stepping) and second-order time differencing, respectively.

The default is to store the extra time levels for Newton iteration or dual time stepping in the .cft file, and for second-order time differencing in the .cfl file.

The SAVE keyword may be used to specify whether or not the data at these extra time levels is retained in the .cfl or .cft file at the end of a run. The default is to save the data for second-order time differencing, but not for Global Newton iteration or dual time stepping.

The CFTVARS, NEWTVARS, SORDTVARS, and SAVE keywords may also be specified outside the TEMPORAL keyword block.

SAVE_CFT

Retains the (linked) time data (.cft) file for seamless restarts of unsteady simulations. Otherwise the file is deleted at the end of the run.

ENDTEMPORAL

Defines the end of the TEMPORAL block.

Example

In the following example, dual time stepping is used to advance the solution by 1000 global time levels, with each time level representing 1.0E-06 seconds. Second-order implicit time marching will be used, and the run will terminate early if the global solution has converged 4 orders of magnitude. Within each global time step, 10 cycles of subiterations will be used with a CFL number of 1.0. The subiterations will terminate early if the solution has converged 4 orders of magnitude.

   / These values control the global solution advancement.
   TEMPORAL
      TIME LEVELS 1000
        SUBITERATIONS MODE DUALDT
      PHYSICAL TIMESTEP 1.0E-06
      CONVERGENCE ORDER 4
      IMPLICIT TIME SECOND ORDER
   ENDTEMPORAL
   / These values control the subiteration advancement.
   CYCLES 10
   ITERATIONS PER CYCLE 1
   CONVERGE ORDER 4
   CFL 1.0


Last updated 1 Apr 2016