The facility exists for the automatic provision of turbulent wall- functions on the interfaces between solids, (defined using the PRPS variable), and fluids. These wall functions are set in EARTH itself, and hence are known as EARTH-Generated Wall Functions (EGWFs). At present the EGWF facility provides only the simplest and most- frequently-used of the wall functions which appear in GXWALL, namely:
The EGWFs are activated by making the Q1-file setting;
EGWF=T
This causes any CONPOR commands in the user's Q1 file to refrain from the creation of wall PATCHes on solids specified using the PRPS variable. Wall PATCHes will still be set, where appropriate, on faces for which the porosity is zero. (See the entries on CONPOR for further information).
The wall-function option is specified by means of the variable WALLCO, for which the default value is GRND2, eg.
WALLCO=GRND1
Wall-roughness may be specified by setting the variable
WALLA > 0
where the value set should be the sand-grain roughness height.
Point-by-point print-out of skin-friction factors, Stanton numbers etc may be generated by setting:
WALPRN=T
This print-out is controlled by the IXPRF, IXPRL, IYPRF and IYPRL settings, in the usual way.
Alternatively, the user may STORE one or more dependent variables with names;
SKIN, STAN, STRS & YPLS
into which will be written, for wall-cells, the skin-friction factor, the Stanton number, the shear-stress, and y+, the dimensionless distance from the wall. These will then be printed along with the more usual field values.
The EGWFs are created internally, cell-by-cell, without the creation of patches. Their action is to modify the momentum-diffusion links which cross solid-fluid interfaces, which links have been calculated by first activating the harmonic-mean diffusion option for each variable.
However, if there is no turbulence, which is signalled by the Q1 setting:
ENUT=0.0 (which is the default),
no wall functions are required (because the harmonic-mean option does all that is needed). They are therefore neither created nor printed.