IMPLICIT {word1 [word2 word3 [zone_selector [tre]]]  \ JACOBI jiter jcnv [RELAX jrel] [zone_selector]  \ GAUSS giter gcnv [RELAX grel] [zone_selector]  \ MACCORMACK [MAFK k] [zone_selector]  \ OVERFLOW [zone_selector]} 
IMPLICIT {NONE [zone_selector]  \ UGAUSS [BLOCK  DIAGONAL] [LINE] [VISCOUS_JACOBIAN {FULL  SCALAR}] \ [SUBITERATIONS nsub] [APPROXIMATE_LHS  EXACT_LHS] [SECOND] \ [CONVERGE [ORDER order] [FREQUENCY freq]] [RELAX urel] [zone_selector]} 
This keyword allows user control of the implicit operator within each zone. There are five possible modes for structured grids, and two for unstructured grids.
IMPLICIT word1 [word2 word3 [zone_selector [tre]]] 
This form of the IMPLICIT keyword allows the user to turn off the implicit operator completely, resulting in an explicit calculation, or to use a scalar or full block implicit operator. With these options, a different implicit operator may be specified for each computational direction.
The userspecified parameters are defined as follows:
word1  A keyword controlling the ξdirection implicit operator  
word2  A keyword controlling the ηdirection implicit operator  
word3  A keyword controlling the ζdirection implicit operator  
tre  Specifies the trapezoidal time differencing factor.
The possible values are:

The keywords that may be used for word1, word2, and word3 are defined in the following table.
Keyword  Difference Scheme  

NONE  Explicit  
SCALAR  Scalar implicit operator (default for Euler solutions)  
FULL  Full block implicit operator (default for viscous solutions) 
Note that if only one of the above keywords is specified, WindUS assumes
that the specified operator is to be used in all directions in all zones.
IMPLICIT JACOBI jiter jcnv [RELAX jrel] [zone_selector] 
This form of the IMPLICIT keyword turns on the point Jacobi implicit operator in the selected zone. The userspecified parameters are defined as follows:
jiter  The number of subiterations allowed each time step. A typical value is 30.  
jcnv  The level of convergence to assume the subiterations are converged. A typical value is 0.0001.  
jrel  The relaxation factor. The default value is 1.0 (i.e., no relaxation). 
IMPLICIT GAUSS giter gcnv [RELAX grel] [zone_selector] 
This form of the IMPLICIT keyword turns on the GaussSeidel implicit operator in the selected zone. The userspecified parameters are defined as follows:
giter  The number of subiterations allowed each time step. A typical value is 10.  
gcnv  The level of convergence to assume the subiterations are converged. A typical value is 0.0001.  
grel  The relaxation factor. The default value is 1.0 (i.e., no relaxation). 
IMPLICIT MACCORMACK [MAFK k] [zone_selector] 
This form of the IMPLICIT keyword turns on MacCormack's firstorder modified approximate factorization (MAFk) procedure in the selected zone. The procedure attempts to remove the decomposition error by feeding the error term back into the matrix equations on the righthand side.
The solution process requires subiterations, with the number of subiterations specified with the MAFK option by the parameter k. The default for k is 2.
For stability, the IMPLICIT BOUNDARY keyword should be used to specify that implicit boundary conditions are to be used with the MAFk procedure. In addition, when large CFL numbers are to be used, it is recommended that the CFL number be increased gradually (over 200 iterations or so) to the desired value using the INCREMENT parameter in the CFL# keyword.
The MAFk procedure has been demonstrated in WindUS to be stable in two dimensions with very high CFL numbers (greater than 1000). In three dimensions, however, only limited testing has been done, and its efficiency has not yet been determined.
The MAFk procedure may not be used with
secondorder time marching.
IMPLICIT OVERFLOW [zone_selector] 
This form of the IMPLICIT keyword specifies that the "ARC3D 3factor diagonal scheme" as implemented in OVERFLOW 1.8q is to be used. Tests indicate that this scheme is faster than the other implicit schemes, and gives comparable answers.
This option is currently available only for 3D perfect gas flows, with explicit boundary conditions. It also uses more memory than the other implicit schemes, due to the interface coding used to implement it in WindUS.
See Also: IMPLICIT BOUNDARY, IMPLICIT ORDER
IMPLICIT NONE [zone_selector] 
This form of the IMPLICIT keyword allows the user to turn off
the implicit operator completely, resulting in an explicit calculation.
IMPLICIT UGAUSS [BLOCK  DIAGONAL] [LINE] [VISCOUS_JACOBIAN {FULL  SCALAR}] \ [SUBITERATIONS nsub] [APPROXIMATE_LHS  EXACT_LHS] [SECOND] \ [CONVERGE [ORDER order] [FREQUENCY freq]] [RELAX urel] [zone_selector] 
This form of the IMPLICIT keyword specifies that a GaussSeidel implicit operator is to be used in the selected zone. The various options are as follows:
BLOCK  DIAGONAL  Specifies that the equations are to be solved as either a full
block or diagonal matrix system.
The default is BLOCK.
 
LINE  A line is a group of cells that are solved tightly coupled.
They are typically used in the wallnormal direction of the
boundary layer to reduce the stiffness associated with the
grid anisotropy and are also used in the freestream along
streamlines. The idea is to couple, as tightly as possible,
the cells along dominant directions as indicated by physics
to accelerate convergence.
Lines are generated using a set of library functions which are called by the cfpart utility to insert boundary layer lines on viscous meshes. These functions are also called from the flow solver to generate inviscid lines along streamlines. The LINE option will solve the NavierStokes equations using a line GaussSeidel implicit solver. If this option is used, EXACT_LHS must also be specified. Note that even when LINE is specified, the turbulence equations will be solved using a point GaussSeidel solver. The default is a point GaussSeidel implicit solver.  
VISCOUS_JACOBIAN {FULL  SCALAR}  Use either a full or scalar (diagonal approximate) viscous
Jacobian.
The default is SCALAR.
 
SUBITERATIONS nsub  Specifies the number of subiterations to be performed on
each face, for each "iteration per cycle," for the point
GaussSeidel solver.
The default is 6.
 
APPROXIMATE_LHS  EXACT_LHS  Specifies how rigorously the implicit matrix is constructed.
The first option is a lowstorage algorithm with an
approximate Jacobian based on the linearization of the
analytical flux for the Rusanov scheme.
The second option uses the Jacobian of the
numerical flux, and requires storage of the entire
lefthandside in memory.
The default is EXACT_LHS.
 
SECOND  Apply a second order Jacobian correction to the implicit
matrix.
 
CONVERGE [ORDER order] [FREQUENCY freq]  
Specifies the convergence criteria for the subiterations for the
point GaussSeidel solver (i.e., the LINE option is not
being used).
order is the desired order of convergence, and
freq is the frequency for checking convergence, in terms
of GaussSeidel subiterations (i.e., the value specified using
SUBITERATIONS).
This only applies when EXACT_LHS is specified, and
CFL# AUTOMATIC is being
used.
The defaults for order and freq are 2 and 100,
respectively.
 
RELAX urel  Specifies the relaxation factor. The default value is 1.0 (i.e., no relaxation). 
The following additional options specify actions that are now always done in the current version of WindUS. They have been retained for backward compatibility, so that existing input data (.dat) files may be used, but otherwise have no effect.
JACOBIAN [PER] FACE  Specifies that the flux Jacobian is to be computed on each face.
 
SAVE_JACOBIAN  Specifies that the Jacobians are to be saved between iterations. 