Thermal phenomena tend to be complex physical processes and the analysis of these processes by digital computer is equally complex. The tools provided by MSC.Software Corporation in the form of Patran and P⁄THERMAL provide advanced capabilities to model some very complex thermal problems. While great effort has been taken to make the tools simple and easy to use, the very complexity of the thermal analysis problem necessitates some degree of complexity in the software provided to perform the analysis. This section provides an overview of the generic thermal analysis cycle using Patran and Patran Thermal. Since this Guide deals with Viewfactor analysis, this summary emphasizes parts of the cycle peculiar to the analysis of a problem containing thermal radiation.

The first step is to define the problem. This includes identifying the geometry, boundary conditions, materials, material properties and approximations to be used in the analysis. You may find it useful and efficient to outline the entire analysis procedure as it pertains to the problem at hand. This should help avoid unpleasant surprises later on. It also ensures that all the steps in a complex process are followed.

This step involves creating or inputting the geometric model, creating a finite element mesh on the model, and assigning boundary conditions and material identifications, in Patran. This step requires close coordination with the next step, General Patran Thermal Preparation, so that the boundary conditions and material properties identified in Patran correspond to material definitions and boundary conditions in the supporting Patran Thermal files. These activities and entities are described more fully in the MSC Patran Thermal User’s Guide. Planning at this stage of the analysis is important if you wish to be able to easily change boundary conditions and/or material property definitions in the future.

The model of the thermal analysis problem built with the Patran preprocessor generally has only identification numbers attached to boundary conditions and material properties. These identification numbers are used by the Patran Thermal module to point into various databases and files for the actual data describing the boundary condition or material property. If these files do not already exist, they must be created. Details concerning these files are contained in the MSC Patran Thermal User’s Guide.

Thermal radiation problems analyzed with the Patran System of products require some specific preprocessing in Patran. You must identify the material surfaces which are participating in the radiation interchange and identify the radiative properties of these material surfaces. The VFAC LBC form has been introduced into Patran specifically to facilitate the modeling of thermal radiation problems. The VFAC form provides support for basic thermal radiation boundary conditions, for participating absorbing and emitting media between surfaces, for identifying convex surfaces which cannot radiate to themselves directly, for identifying surfaces that are not obstructions, and for radiation to an ambient node. This Patran form is described in detail in Advanced Features of the VFAC Boundary Condition, 42.

In support of these modeling capabilities, you will also need to enter data into the Patran Thermal files for the surface emissivity properties and the participating media (if any) extinction or transmissivity properties, including in both cases waveband data if applicable. This data is typically entered into the Patran Thermal TEMPLATEDAT, MATDAT, and MICRODAT files. These files and data relevant to a thermal radiation problem will be briefly described in Patran Thermal TEMPLATEDAT Files for Surface Property Description, 52. For full details on these files, refer to the MSC Patran Thermal User’s Guide.

Facilities have also been programmed into Patran Thermal and Viewfactor to accept and process information about symmetry occurring in the thermal analysis problem. Problem symmetry is also input in Patran at this stage of the analysis cycle. The use of symmetry in thermal radiation problems is described in more detail in Symmetry as Applied to the Model and Viewfactor Radiation Exchange, 67.

After the model of the problem to be analyzed has been prepared in Patran and the required supporting Patran Thermal files have been created, there are two steps to prepare the problem for processing by Viewfactor. These are:

The translation of the Patran neutral file is done using a menu pick from Patran Thermal’s PATQ menu and is described in detail in Preparation for Analysis (Ch. 4). The VFCTL file is typically created using your editor. It is about 20 lines of identifying keywords and associated parameter values. This file is described in Viewfactor Execution From Patran Thermal, 79.

Viewfactor will usually be executed as a noninteractive batch process. Merely invoke the command procedure to submit Viewfactor and its control file, VFCTL, for execution, or select “Execute Viewfactor Analysis” in the Analysis / Submit Options form.

Since Viewfactor analysis tends to be computationally expensive, review all aspects of the model carefully before beginning the viewfactor analysis. This will help to minimize the number of Viewfactor analyses submitted with incorrect or incomplete data. Viewfactor has some data checking and error detection capabilities, but it cannot detect all user errors. The procedure for submitting a Viewfactor job is described in detail in Submitting a Viewfactor Job for Analysis, 98.

Viewfactor will create a number of output files. The files created depend on some parameters in the VFCTL file. The various files created as Viewfactor output are described in Viewfactor Data and Program Flow, 16 and Output Created by a Viewfactor Execution, 101. When the viewfactor analysis is completed, the Viewfactor diagnostic files, VFDIAG and VFMSG, should be reviewed for acceptable diagnostic data values and possible error messages, as described in Reviewing the Viewfactor Output, 103.

After the Viewfactor analysis is complete and the rest of the Patran Thermal input files are complete, the user is ready to perform the thermal network analysis using Patran Thermal. Viewfactor will have created two files to which Patran Thermal must be given access. These are the radiation resistor file, VFRESDAT, and the radiosity node file, VFNODEDAT. This access is usually provided by giving Patran Thermal the names of these files through the Patran Thermal QINDAT file. The QINDAT file is described in the MSC Patran Thermal User’s Guide. The particular aspects of the QINDAT file relevant to the viewfactor analysis and the Viewfactor files VFRESDAT and VFNODEDAT are described in Interface From Viewfactor to Patran Thermal, 111.

At this point in the analysis cycle you may also translate the binary radiation resistor data file, VFRESDAT, into a text file which you may examine. This capability is provided by a menu pick in Patran Thermal’s PATQ and is described in Interface From Viewfactor to Patran Thermal, 111. This text file has no other purpose in the analysis. It is provided merely for your convenience.

The viewfactor portion of the analysis is now complete. The remaining steps in the analysis cycle all concern general thermal analysis.

The procedure for submitting a Patran Thermal analysis is described in the MSC Patran Thermal User’s Guide. Briefly, the process involves generating some FORTRAN source code for the particular problem, compiling the new source code, linking with the Patran Thermal QTRAN run-time library, and submitting the job for execution. The results of the thermal analysis will be contained in the Patran Thermal QOUTDAT file and in nodal results files for use with the Patran postprocessing tools.

The capabilities of Patran and the Patran Thermal interface permit analysis results to be displayed and examined quickly and efficiently. For more information about thermal results postprocessing, refer to the Patran Thermal User’s Guide. Refer to the Patran Reference Manual for general postprocessing information.

After examining the analysis results, you may be satisfied with the analysis, in which case this analysis cycle terminates. You may wish to refine or modify the computer model of the problem and perform the analysis again, in which case the analysis cycle starts over and repeats itself as applicable.