ILUT

Options for ILUT preconditioner implemented by Xiaodong Wang. ilut_opts is used by Hom and SolverOpts.

**Description of ILUT:**
The ILU(p) fill methodology usually employed is based on stencil only and does not account for element size, though for highly-diagonally dominant systems, deep fill elements tend to be small and so this method is fast and effective for most CFD applications. ILUT stands for ILU(tau), where tau is a drop tolerance. This method is much slower than ILU(p) but according to Yousef Saad (1993), is far more effective than ILU(p) for indefinite systems or systems that are less diagonally dominant. Turning on pivoting is also generally recommended for performance, in which case the algorithm is called ILUTp. However, our implementation does not seem to work very well.

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ilut% droptol
This parameter greatly affects performance, smaller drop tolerance will lead to higher fill levels (better conditioning, but more expensive and more data storage).
 * //Description://** Drop tolerance - values smaller than this will be dropped.
 * //Parameter Type://** real
 * //Range of Acceptable values://** (0, inf)
 * //User Notes://**

ilut% adpfator

 * //Description://** Adaptive tolerance for drop tolerance.
 * //Parameter Type://** real
 * //Range of Acceptable values://** (?)

ilut% lev_fil
This acts as an upper bound on the fill level.It is not the same as the fill level in iluk, it actually takes values 4-5 times larger.
 * //Description://** Fill level for ilut.
 * //Parameter Type://** Integer
 * //Range of Acceptable values://** (0, inf)
 * //User Notes://**

ilut% bnorm

 * //Description://** Measurement for block dropping.
 * //Parameter Type://** Integer
 * //Range of Acceptable values://** (?)

ilut% pivoting

 * //Description://** Use pivoting?
 * //Parameter Type://** logical

ilut% diagcomp

 * //Description://** ?
 * //Parameter Type://** logical

ilut% rowave

 * //Description://** Use row-averaged value as a reference for ILUT?
 * //Parameter Type://** Logical