Encyclopaedia Index

EARTH-GENERATED WALL FUNCTIONS

1) Nature

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). The EGWF facility provides all of the wall functions which appear in GXWALL, namely:

Full account is taken of the reuirements of the various Low-Reynolds Number turbulence models available in PHOENICS.

2) Activation

The EGWFs are activated by making the Q1-file setting;

EGWF=T

This causes any CONPOR commands or BLOCKAGE objects 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. 

The default wall-function option is specified by means of the variable WALLCO, for which the default value is GRND2, eg.

WALLCO=GRND1

Default wall-roughness may be specified by setting the variable

WALLA > 0

where the value set should be the sand-grain roughness height, excapt for the GRND5 fully-rough option when it is the actual 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 and HTCO

into which will into which will be written, for wall-cells, the skin-friction factor, the Stanton number, the shear-stress,  y+, the dimensionless distance from the wall and the heat transfer coefficient. These will then be printed along with the more usual field values.

3) Settings for individual CONPORs or BLOCKAGEs

The settings of WALLCO and WALLA are default values, used everywhere unless specifically overwritten. This can be done by setting SPEDAT commands as follows:

To change the wall-function option for a CONPOR named name:

SPEDAT(SET, name, WALLCO, R, GRNDn)

where n is 1,2,3 or 5

To change the roughness height for a CONPOR named name:

SPEDAT(SET, name, ROUGH, R, roughness_height)

To set a heat-transfer coefficient (in W/m2/K) for a CONPOR named name:

SPEDAT(SET, name, HTCO, R, heat-transfer-coeff).

If the surface of the solid is moving, the slide velocities are set by:

SPEDAT(SET, name, VELX, R, X-direction velocity)
SPEDAT(SET, name, VELY, R, Y-direction velocity)
SPEDAT(SET, name, VELZ, R, Z-direction velocity)

where the surface velocities are all in m/s, with a default value of 0.0 (in which case the settings are not needed).

In polar co-ordinates, the meanings of the surface velocities are set as follows:

SPEDAT(SET, name, IURVAL, I, iflag)

where iflag = 0 - all suface velocities are grid-aligned in m/s
                       1  - X velocity is angular momentum in m2/s
                      -1  - U velocity is angular velocity in radians/s
                      -2 - all surface velocities are cartesian components

In cartesian co-ordinates, the solid may be rotating about the X, Y or Z axes. The settings for this are:

SPEDAT(SET, name, VELX, R, omega) where omega is the rotation speed in radians/s.

For rotation about the X axis:

SPEDAT(SET, name, IURVAL, I, 4)
SPEDAT(SET, name, Y-CEN, R, y-coord)
SPEDAT(SET, name, Z-CEN, R, z-coord)

For rotation about the Y axis:

SPEDAT(SET, name, IURVAL, I, 3)
SPEDAT(SET, name, X-CEN, R, x-coord)
SPEDAT(SET, name, Z-CEN, R, z-coord)

For rotation about the Z axis:

SPEDAT(SET, name, IURVAL, I, 2)
SPEDAT(SET, name, X-CEN, R, x-coord)
SPEDAT(SET, name, Y-CEN, R, y-coord)

The same settings are used for BLOCKAGE objects, but do not have to be entered by the user. They are generated internally from the object attributes, as described here and here.

4) Internal-to-EARTH transactions

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: e default),

no wall functions are required (because the harmonic-mean option does all that is needed). They are therefore neither created nor printed.