This keyword allows the user to specify the CFL number or time step for all zones, or on a zone-by-zone basis. All keywords and parameters must be on one line in the data file. If this keyword is not used, a constant CFL number is used, with a value of 1.3 for structured grids and 10.0 for unstructured grids. For unstructured grids, a larger CFL value can usually be used than for structured grids. Suggested values for unstructured grids are 10 at the start of a calculation, and as high as 50 for later runs.

For structured grids there are three different options, as specified by the MODE value. For unstructured grids there are two options; mode 3 is not available. Note that the MODE keyword is optional for modes 1 and 2, but required for mode 3. (Modes 1 and 2 are distinguished in the code by whether or not the word INCREMENT is present.)

{CFL# | TIMESTEP} [MODE 1] [CFL | SECONDS] cfl1 [zone_selector [ityp]]

In this mode, the CFL number or time step is specified directly, by the input value cfl1.

	CFL		A CFL number is being specified. This is the default, and should be used for steady flow problems.
	SECONDS		The time step is being specified in seconds. This option is generally used for unsteady flow problems.

When a CFL number is being specified, the value of ityp may be set to 1 to indicate that a global time step (i.e., constant in space) should be used, equal to the minimum value in the zone. This allows the time step to be determined through a CFL number for unsteady flow problems. Currently, this option is only available for single-zone grids. When a CFL number is specified, and ityp = 0 or is omitted, a local time step (i.e., varying in space) will be used.

For unstructured grids with local time stepping, the size of the physical time step will automatically be limited to the largest time step in the grid that corresponds to a CFL number of one. Of course, due to variations in grid spacing and flow conditions, the CFL number at other locations may be much higher.

{CFL# | TIMESTEP} [MODE 2] [CFL | SECONDS] INCREMENT \    cfl1 cflmax cflfac inccfl istart [zone_selector [ityp]]

In this mode, the CFL number or time step will be gradually increased as the calculation proceeds. The CFL and SECONDS options have the same meaning as for mode 1, again with CFL as the default. The increase in the CFL number or time step is controlled by the following input values:

	cfl1		CFL number or time step for iteration istart
	cflmax		Maximum CFL number or time step allowed
	cflfac		Factor which multiplies the CFL number or time step every inccfl iterations
	inccfl		Iteration increment at which the CFL number or time step is multiplied by cflfac. The multiplication is done whenever mod(iteration,inccfl) = 1.
	istart		Starting iteration in the increment calculation. Note that the iteration values are cumulative, starting from the initial run. I.e., to gradually increase the CFL number at the beginning of a restart case, istart should be set to the value of the iteration in the existing .cfl file, from the previous run.

As in mode 1, when a CFL number is being specified for a single-zone grid, the value of ityp may be set to 1 to indicate that a global time step (i.e., constant in space) should be used, equal to the minimum value in the zone. Also as in mode 1, for unstructured grids with local time stepping, the size of the physical time step will automatically be limited to the largest time step in the grid that corresponds to a CFL number of one.

If mode 2 is used in a staged solution, it should only be used at the start of the run, and SET CFL shouldn't be specified in the SOLVER-STAGES block for the first stage.

{CFL# | TIMESTEP} MODE 3 [DT cfl1] [CFLMIN cflmin] [CFLMAX cflmax] \    [ITIME itime] [zone_selector]

This mode uses the time step calculation procedure originally used in the OVERFLOW code, and is only available for structured grids.

	itime		A flag indicating the type of time step to be used     0    Time-accurate run (i.e., time step is constant in space) 1 Time step scaled by local metric Jacobian, with fudge factor of 0.005 to keep the time step from getting too small for really tiny cells 2 Time step scaled by local metric Jacobian, without a fudge factor 3 Constant CFL number equal to cflmax The default value is 1.
	cfl1		Time step parameter. For itime = 0 (i.e., time-accurate cases), this is the time step in seconds nondimensionalized by Lr / ar, where Lr is the grid reference length and ar is the freestream speed of sound. For itime = 1 and 2, this is a CFL number, before scaling by the local metric Jacobian. For itime = 3, cfl1 is not used. The default value is 0.5.
	cflmin, cflmax		For itime = 3, the CFL number is set to cflmax, and cflmin is not used. The CFL number is defined using the sum of the maximum eigenvalues in each coordinate direction. For other itime values, cflmin and cflmax are used to limit the CFL number to values within the specified range. A value of 0.0 indicates no limiting. If either cflmin or cflmax is negative, the absolute values are used, and the CFL number is defined using the method of Gnoffo, with a viscous correction by Tannehill. If both cflmin and cflmax are non-negative, the usual one-dimensional (inviscid) CFL number definition using the maximum eigenvalue is used. The default values are both 0.0.

If mode 3 is used, it must be used in all zones, but the values of cfl1, cflmin, etc., may vary from zone to zone.

Examples

Set the CFL number to 1.5 in all zones.

   CFL# CFL 1.5

Set the CFL number to 1.5 in zones 1 and 5, and 0.7 in zones 2, 3, and 4.

   CFL# CFL 1.5 ZONE 1,5
   CFL# CFL 0.7 ZONE 2:4

The following example sets the CFL number to 1.0, computes the corresponding time step at every point in the grid, finds the minimum of those time steps, and resets the time step at every point to that minimum value. This allows the time step to be set for an unsteady flow problem by specifying a CFL number. Note that even though this may only be done for single-zone grids, the syntax requires that the zone be explicitly specified.

   CFL# CFL 1.0 ZONE 1 1

Set the time step equal to 1×10⁻⁶ seconds at every point.

   CFL# SECONDS 0.000001

The next example sets the CFL number to 0.5 for the first 500 iterations, then increases it by a factor of 1.1 every 100 iterations. The maximum value allowed will be 2.0, so the actual final CFL number will be some value just below 2.0. (For this example, it's 1.899.)

   CFL# CFL INCREMENT 0.5 2.0 1.1 100 501

The remaining examples use the mode 3 time step calculation procedure, only available for structured grids. The first example sets the time step parameter to 0.1, and defaults the remaining input parameters, causing the actual time step to be scaled by the local metric Jacobian with the 0.005 fudge factor, with no minimum or maximum value.

   CFL# MODE 3 DT 0.1

In this example the mode 3 time step calculation procedure is used to set the time step parameter to 0.5. The actual time step will be scaled by the local metric Jacobian with the 0.005 fudge factor, with minimum and maximum CFL numbers of 1.0 and 5.0. Since the minimum and maximum values are specified as positive numbers, the standard one-dimensional (inviscid) CFL number definition using the maximum eigenvalue is used.

   CFL# MODE 3 DT 0.5 CFLMIN 1.0 CFLMAX 5.0

The next example uses the mode 3 time step calculation procedure to set the time step parameter to 0.05. The actual time step will be scaled by the local metric Jacobian without the 0.005 fudge factor, with minimum and maximum CFL numbers of 1.0 and 5.0. Since the minimum and maximum values are specified as negative numbers, the CFL definition of Gnoffo is used, with a viscous correction by Tannehill.

   CFL# MODE 3 DT 0.05 ITIME 2 CFLMIN -1.0 CFLMAX -5.0

Use the mode 3 time step calculation procedure to set the CFL number to a constant value of 1.3.

   CFL# MODE 3 ITIME 3 CFLMAX 1.3

Use the mode 3 time step calculation procedure for an unsteady case, with the nondimensional time step set to a constant value of 0.05.

   CFL# MODE 3 ITIME 0 DT 0.05

The following example uses the mode 3 time step calculation procedure for an unsteady case, again with a specified nondimensional time step of 0.05. In this case, however, if the specified time step results in a CFL number at some point in the flow field less than 0.25 or greater than 5.0, the actual time step throughout the flow field will be reset to the corresponding value.

   CFL# MODE 3 ITIME 0 DT 0.05 CFLMIN 0.25 CFLMAX 5.0