Card 6a - View Factor Request

This opional card specifies the parameters and options for view factor calculations.

Parameters: L, N1, N2, FLAG, REVN1, REVN2, SPACEFLAG

L

L is the type of view factor calculation request.

  • L = VFSALL (or 1) calculates all the view factors with shadowing, starting with the view factor from N1 to N2, and following the order in which the elements are defined on Card 5a. To calculate all the view factors leave all fields except the first one blank.
  • L = VFNALL (or 2) is the same as VFSALL with no shadowing.
  • L = VFS12 (or 3) calculates a single view factor from N1 to N2 with shadowing. If N1 and N2 are both group names, all the view factors from the elements of N1 to the elements of N2 are calculated. For this option, the REVN1, REVN2, and SPACEFLAG fields may also be active.
  • L = VFN12 (or 4) is the same as VFS12 with no shadowing.
  • L = VFS1ALL (or 5) calculates all the view factors for N1 with shadowing, starting with the view factor from N1 to N2 and following the order in which the elements are defined on Card 5a. N2 = blank calculates all view factors for N1. If N1 is a group name, all view factors for the elements of N1 will be calculated. For this option, the REVN1, REVN2, and SPACEFLAG fields may also be active.
  • L = VFN1ALL (or 6) is the same as VFS1ALL with no shadowing.

N1

N1 may be an element number or a group name.

N2

N2 may be an element number, group name, or blank.

FLAG

FLAG activates ray-tracing to calculate view factor.

  • FLAG may be blank or 0 does not activate any special options.
  • FLAG = VFTRACE (or –3.4E5) uses ray-tracing to calculate view factor if specular and/or transparent surfaces are present. For more information, see VFTRACE option in the Notes.

REVN1

REVN1 may be 0, or an element number, or a group name, and it specifies that the reverse sides of the REVN1 element(s) are to be added to the N1 elements. It is applicable for L = VFN12, VFS12, VFS1ALL, or VFN1ALL.

REVN2

REVN2 may be 0, or an element number, or a group name, and it specifies that the reverse sides of the REVN2 element(s) are to be added to the N2 elements. It is applicable for L = VFN12, VFS12, otherwise it must be blank or 0.

SPACEFLAG

  • SPACEFLAG may be blank or 0
  • SPACEFLAG = SPACE is applicable only for L = VFN12, VFS12, VFS1ALL, VFN1ALL.
  • SPACEFLAG = NOSPACE is applicable only for L = VFNALL, VFSALL.
  • If L = VFN12 or VFS12 and SPACE is present, then the space elements of Card 5d are added to the N2 group.
  • If L = VFN1ALL or VFS1ALL and SPACE is present, then the space elements of Card 5d are added to the N1 group.
  • If L = VFNALL or VFSALL and NOSPACE is present, then the space elements of Card 5d are removed from the N2 group.

Code example


VFSALL
$ ALL VIEW FACTORS ARE CALCULATED WITH SHADOWING

VFS12 ENC ENC
$ ONLY THE VIEW FACTORS WITHIN THE ENCLOSURE
$ WHOSE ELEMENTS ARE ASSOCIATED WITH THE NAME
$ ENC ARE CALCULATED

MESH 1
$ ALL THE VIEW FACTORS ARE CALCULATED WITH MESH = 1

VFSALL
MESH 2

VFS1ALL SHIELD
$ THEN ALL THE VIEW FACTORS TO THE GROUP OF
$ ELEMENTS CALLED SHIELD ARE RECALCULATED FOR
$ GREATER ACCURACY WITH MESH = 2

VFS12 4 5
$ VIEW FACTOR FROM ELEMENT 4 TO 5 IS CALCULATED

Notes

Restart option:

  • If VFSALL or VFNALL are used, and the VUFAC module stops prematurely, e.g. at the view factor of elements J1 to J2, the rest may be calculated in a restart run by using the same L, N1 = J1 and N2 = J2. The previously created file VUFF must be present, the new view factors will be automatically appended to it.

Accuracy check:

  • The GRAYB module performs a view factor calculation accuracy check by summing the view factors for each element.
  • The view factor sum should be close to 1. Compensation for view factor calculation inaccuracy is provided by the Card 2a KSP parameter. View factor calculation accuracy may be controlled with a Card 6f MESH ERROR Card.
  • If the N =512 option is not specified on Card 2a, the view factor module will write a detailed view factor calculati ons to the report log file. You may use this to debug your model.

No shadowing and no radiation flags:

  • The Card 9 PARAM NOSHADOW option can be used to specify non-shadowing elements.
  • The Card 9 PARAM NORAD option can be used to specify SURFACE elements that take part only in convective conductance calculations and not radiative calculations.
  • The Card 9 PARAM NOSOLAR option can be used to specify elements that will not take part in solar spectrum calculations.

VFTRACE option

  • If the VFTRACEoption is specified view factors are calculated with ray–tracing whenever specular and/or transparent surfaces are encountered.
  • If specular and/or transparent surfaces are present the effect of VFTRACE is increased accuracy in two areas: radiative couplings between elements, and the diffusely reflected component of incident solar radiation or radiation originating from Card 6n heat flux view factors. Ray-tracing is always performed for directly incident component of solar radiation and Card 6n heat flux view factor requests.
  • If the VFTRACE option is not specified, view factors are calculated with the assumption that all surfaces reflect incident radiation diffusely. It is important to note the ray-tracing is Cpu-intensive and its benefits in increased accuracy are often negligible.
  • If transparency/specularity values are specified only in the solar spectrum of an element referenced from Card 9 - MAT Material Property Definition , they are considered by default to be valid for the IR spectrum as well. If the IR spectrum transparency or specularity is also specified, a separate set of view factors will be calculated for the IR spectrum as well.

The view factors calculated with ray-tracing are written on file VUFF with the mnemonic BVF. They are distinguished from view factors calculated without ray-tracing by the following:

  1. For view factors calculated without ray-tracing the flag in the TIME field is -1.E36, for view factors calculated with ray-tracing in the solar spectrum the flag is -1.1E36, and for view factors calculated with ray-tracing in the IR spectrum the flag is -1.2E36.
  2. The area specified in the AR field is not the area of the element, but its area multiplied by the factor (1-specularity/transmissivity). The emissivity and absorptivity values in the E and A values divided by (1-specularity/transmissivity).

Calculating view factors in an enclosure.

  • For calculating all the view factors in an enclosure with VfxENC Cards, see Card 6r - View Factor Request in an Enclosure – Optional.