XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX''"> 3.8 Compatibility Requirements for Model and VFAC Templates
The model of the problem and its VFAC boundary condition data must satisfy certain compatibility requirements or else an error will occur in Viewfactor while attempting to combine the viewfactor data and the VFAC template data to make resistors. Should this occur, you may need to change the VFAC boundary condition and rerun the viewfactor analysis. The best way to avoid this is careful planning.
Origin of the Problem
The compatibility requirements primarily originate from the fact that thermal radiation interchange is between pairs of surfaces and the requirement in Patran Thermal that the radiation resistors be symmetric. Thus, the resistor that would be made from surface one to surface two must be the same as would be made from surface two to surface one. Which resistors are made is determined by the VFAC template data and VFAC boundary condition data for each surface. Incompatibility results, for example, when one surface is gray (nbands = 0) and the other surface has spectral wavebands (nbands > 0).
Compatibility problems can also arise just from a single surface and its associated VFAC data. An example of this is a VFAC boundary condition with an AMBNOD specified and a VFAC template with KFLAG set to one. The problem here is that the KFLAG setting will require a distance for calculating the transmissivity from Beer’s Law and the distance from the surface to the ambient environment (AMBNOD) is not well defined or known.
The specific compatibility requirements are:
• All UIDs referenced in the VFAC boundary condition, input data form must be available TIDs in the VFAC templates.
• All VFAC templates referenced by the VFAC boundary condition UIDs in an enclosure must have the same number of wavebands. This means that all surfaces in an enclosure must either be gray (nbands = 0) or have the same spectral wavebands (nbands > 0).
• Surfaces with an AMBNOD in their VFAC LBC are incompatible with VFAC templates that have their KFLAG set to one. If the user wishes to model the problem with KFLAG set to one, then the ambient environment must be modeled as surfaces at the ambient temperature.
• If the surface has not been given a participating media node, MEDNOD, in its VFAC boundary condition and the constant transmissivity, CONSTANT_TAU, is not set to 1.0 in the VFAC template identified by the UID for that surface, then an error will occur. This is because no participating media has been defined, yet the VFAC template says that some of the radiant energy from the surface is absorbed by the medium (since the transmissivity is not 1.0).
• For every pair of surfaces in an enclosure that can see each other, the following quantities for each must be equal:
MEDNOD, CONSTANT_TAU, TMPID, LAMBDA1, LAMBDA2, and KFLAG.
• If there are multiple wavebands, then this condition must hold for each waveband. Note also that by the previous requirement on wavebands in the enclosure that the number of wavebands for every pair of surfaces which can see each other is equal.
Suggested Procedures to Avoid Compatibility Problems
Careful planning is the key to avoiding compatibility problems between the model’s VFAC boundary condition data and the VFAC templates in the TEMPLATEDAT file. Plan out the modeling strategy, paying particular attention to the most complex model of the problem. If for example, a simple model does not include a participating media node even though it is not used, the viewfactor calculations will have to be redone if in the future a participating media needs to be modeled. If, on the other hand, the participating media node had been included in the model, even though it will not be used in the simple model when it is needed in the more complex model with the participating media, it will be there and the viewfactors will not have to be recalculated. Only the VFAC templates will need to be changed and new Patran Thermal radiation resistors made.
If you plan or want to be able to use the KFLAG set equal to one option, then do not use ambient environment nodes, AMBNOD, in the model. Instead, model the ambient environment as surfaces at the ambient temperature.
If you plan or want to be able to model participating media, include the participating media node, MEDNOD, in the model. There is no harm if it is not used, but it will be there when it is needed.
Take care when planning the enclosures and participating media nodes, MEDNOD, to ensure that all surface pairs in an enclosure which can see each other have the same MEDNOD.
The above suggestions are the most important, as failure to follow them may require that the viewfactors be recalculated and this is the most computationally expensive part of the Viewfactor analysis.
The requirements for the equality of the CONSTANT_TAU, TMPID, LAMBDA1, LAMBDA2, and KFLAG between surface pairs which can see each other in an enclosure is not so troublesome if violated. It can be corrected by changing the VFAC templates but does not in general require recalculating the viewfactors. Still, careful planning is the best prevention. If there are two or more enclosures, the user may wish to use different VFAC templates for the same material in the different enclosures. This will give the user greater latitude in modeling different, but simultaneous, phenomena in the different enclosures.
If a constant value for transmissivity, CONSTANT_TAU, is used, it must be the same for all surfaces in an enclosure which can see each other.
The TMPID must be the same for all surfaces in an enclosure which can see each other.
The KFLAG must be the same for all surfaces in an enclosure which can see each other.
If the KFLAG is zero, then the CONSTANT_TAU or TMPID data must be for transmissivity data directly. If the KFLAG is one, then the CONSTANT_TAU or TMPID data must be for extinction coefficient data to be used in Beer’s Law.
The beginning and ending wavelengths for each waveband in the enclosure must be in the same order and must be equal for all surfaces in an enclosure which can see each other.
Given these restrictions on the data in an enclosure, the user may wish to model the same material in each different enclosure with different UIDs so that for example it may be represented with different VFAC templates in each different enclosure. This involves some additional work, but greatly extends the flexibility and capabilities of the model.