The user function ID, UID, identifies the VFAC template in P⁄THERMAL’s TEMPLATEDAT file which will be used to identify the material properties associated with this surface. These properties include surface emissivity and participating media transmissivity data. This parameter is required and is entered as an integer. In Patran Thermal and Viewfactor only positive UIDs are valid. Patran does not check for nonpositive UIDs and so it is up to you to observe this restriction. A nonpositive UID causes an error in Patran Thermal and Viewfactor.

This parameter identifies the participating media (if any) node by its node ID number. The default value is 0 (zero), indicating that no media node is present. If this referenced node ID number changes as a result of optimization, equivalencing, or node renumbering in Patran, the corresponding reference in the VFAC record will not be automatically updated to match this change. So it is up to you to make sure that the node ID for the media node has not changed.

This parameter identifies the ambient or space node (if any) by its node ID number. The default value is 0 (zero), indicating that no ambient node is present. This is the node which radiation escaping the enclosure will reach and which will represent the ambient radiation from space for this surface. If this referenced node ID number changes as a result of optimization, equivalencing, or node renumbering in Patran, then the corresponding reference in the VFAC record will not be automatically updated to match this change.

The convex surface ID, CNVSID, is used to identify convex surfaces in an enclosure. This is used to reduce computer time for the Viewfactor raw viewfactor calculations. A convex surface is one for which no point on the surface has a direct line of sight view of any other point on the surface. For our purposes, plane surfaces may be considered convex. Note that the scope of the CNVSID is confined to the present enclosure and thus the user may reuse convex surface ID numbers in different enclosures without adverse effects. The default value is 0 (zero), indicating that this is not a convex surface. Special care must be taken in axisymmetric and 3-D models to make sure that saddle-like surfaces are not mistakenly thought to be convex.

The value of 1, causes the nonobstruction flag to be set. This means to Viewfactor that this surface is not capable of obstructing the view between any other pair of surfaces in this enclosure, including the view between this surface and other surfaces. This facility provides the option to reduce Viewfactor calculation time by identifying the nonobstructing surfaces in an enclosure.

The bottom surface flag, “1”, is used when applying VFAC boundary conditions to the bottom surface (not edges) of quadrilateral, triangular, or bar elements. In these cases, VFAC DFEGs can be applied either to the top or bottom, the default being the top. The bottom is selected when the character “1”is present in the TOP/BOTTOM FLAG data box. The bottom surface flag has no meaning for solid elements or for the edges of quadrilateral and triangular elements, and is ignored. The top of a quadrilateral or triangular element is defined by the right-hand rule. For bar elements, the beam orientation is used. The top of bars points towards the beam orientation, the bottom points away. See the Patran Reference Manual for more information on beam orientation.

Entering the correct enclosure ID is critical to the proper performance of Viewfactor. There is no way for Patran, Patran Thermal or Viewfactor to check the correctness of the enclosure ID. Exercise extreme care in this regard.

Note: This is required even if there is only one enclosure.