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ACCELERATE - Convergence acceleration (block)

Structured Grids

ACCELERATE
   ZONE range1[,range2[, ... rangen]]
   [CFLRAMP START iter1 END iter2]
   [SECOND START iter1 END iter2]
   [FOURTH START iter1 END iter2]
ENDACCELERATE

Unstructured Grids

ACCELERATE
   ZONE range1[,range2[, ... rangen]]
   [CFLRAMP START iter1 END iter2]
ENDACCELERATE

The objective of Wind-US's convergence acceleration scheme is to approach the steady state solution more quickly by allowing the use of a large CFL number early in the calculation.

Structured Grids

The ZONE keyword identifies the zone(s) within which convergence acceleration is to be used. The range parameter(s) must be one of the following forms:

    zonenum   Selects zone zonenum
begzone:endzone Selects all zones from begzone to endzone
ALL Selects all zones

The CFL number will be ramped down linearly from its starting value, specified using the CFL# keyword, to 1.0. The ACCELERATE keyword does not work when CFL# MODE 3 (i.e., the time step calculation procedure from OVERFLOW) is used.

With structured grids, first- and second-order smoothing are normally applied to maintain numerical stability. The amount of smoothing, when it is applied, and how it varies with iteration is controlled by the SMOOTHING and/or BOUNDARY-DAMP keywords, the iter1 and iter2 values specified with ACCELERATE, and the mode used with TEST option 49.

For the second-order smoothing,

    mode   Second-Order Smoothing

< 2 Smoothing varies with iteration i, as follows:

iiter1   Smoothing applied as determined by the SMOOTHING keyword
iter1 < i < iter2 Smoothing coefficient decreased linearly to 0
iiter2 No smoothing applied

2 Smoothing varies with iteration i, as follows:

iiter1   Smoothing applied as determined by the SMOOTHING and/or BOUNDARY-DAMP keywords
iter1 < i < iter2 Smoothing coefficient decreased linearly to 0
iiter2 No smoothing applied

3 No smoothing applied

And for the fourth-order smoothing,

    mode   Fourth-Order Smoothing

< 2 Smoothing applied as determined by the SMOOTHING keyword

2 Smoothing varies with iteration i, as follows:

iiter1   No smoothing applied
iter1 < i < (iter1 + iter2) / 2 Smoothing coefficient increased linearly from 0 to value determined by the SMOOTHING and/or BOUNDARY-DAMP keywords
(iter1 + iter2) / 2 < i < iter2 Smoothing coefficient decreased linearly to 0
iiter2 No smoothing applied

3 Smoothing applied as determined by the SMOOTHING and/or BOUNDARY-DAMP keywords

If iter1 and iter2 aren't specified, the following default values are used.

    Option   iter1   iter2
SECOND1150
FOURTH50500
CFLRAMP50500

Recommended values for the second- and fourth-order smoothing parameters val1 and val2 specified with the SMOOTHING keyword, and for the starting CFL number, are:

    Dimensions   val1   val2   CFL #
2D0.10.0310-15
3D0.060.012.5

Example

In the following example, convergence acceleration is applied in zones 3 and 4. Second-order smoothing is applied between iterations 1 and 200, and fourth-order smoothing is applied between the default values of iterations 50 and 500. The CFL number specified with the CFL# keyword will be used for the first 250 iterations, then ramped down to a value of 1.0 between iterations 250 and 500.

   SMOOTHING SECOND 0.06 FOURTH 0.01 SMLIMT 0.
   ACCELERATE
      ZONE 3,4
      SECOND START 1 END 200
      CFLRAMP START 250 END 500
   ENDACCELERATE
   TEST 49 2

See Also: SMOOTHING, BOUNDARY-DAMP, CFL#, TEST 49

Unstructured Grids

The ZONE keyword identifies the zone(s) within which convergence acceleration is to be used. The range parameter(s) must be one of the following forms:

    zonenum   Selects zone zonenum
begzone:endzone Selects all zones from begzone to endzone
ALL Selects all zones

iter1 and iter2 specify starting and ending iteration values for varying the CFL number. Just as for structured grids, for unstructured grids the CFL number will be ramped down linearly from its starting value, specified using the CFL# keyword, to 1.0. If the CFLRAMP keyword isn't specified, default values of 50 and 500 are used, for iter1 and iter2.

For unstructured grids, however, no variation in smoothing is done.

See Also: CFL#


Last updated 17 Feb 2009