To build the model, specify the jobname and a title for your analysis. Then, use the Mechanical APDL preprocessor (PREP7) to define the element types, element real constants, material properties, and the model geometry. (These tasks are common to most analyses. The Modeling and Meshing Guide explains them in detail.)
For a thermal analysis, you also need to keep these points in mind:
To specify element types, use the ET command.
To define constant material properties, use the MP command.
The material properties can be input as numerical values or as table inputs for some elements. Tabular material properties are calculated before the first iteration (that is, using initial values (IC)). See the MP command for more information on which elements can use tabular material properties.
To define temperature-dependent properties, first use either the MPTEMP command or the MPTGEN command to define a table of temperatures. Then, use the MPDATA command to define the corresponding material property values.
As an alternative to the MP command, you can define thermal properties for some thermal elements via the command TB,THERM. See Material Model Support for Elements for more information.
Use the same commands to define temperature-dependent film coefficients (HF) for convection.
Caution: If you specify temperature-dependent film coefficients (HF) in polynomial form, you should specify a temperature table before you define other materials having constant properties.
You can use the surface-effect elements (SURF151, SURF152) to apply heat transfer for convection/radiation effects on a finite element mesh. The surface-effect elements also allow you to generate film coefficients and bulk temperatures from FLUID116 elements and to model radiation to a point. You can also transfer external loads (such as from CFX) to Mechanical APDL using these elements.
The guidelines for using surface-effect elements follow:
Create and mesh the thermal region as described above.
Use ESURF to generate the SURF151 or SURF152 elements on the surfaces of the finite element mesh.
For SHELL131 and SHELL132 models, you must use SURF152. Set KEYOPT(11) = 1 for the top layer and KEYOPT(11) = 2 for the bottom layer.
For FLUID116 models, the SURF151 and SURF152 elements can use the single extra node option (KEYOPT(5) = 1, KEYOPT(6) = 0) to get the bulk temperature from a FLUID116 element (KEYOPT(2) = 1).
SURF151 and SURF152 elements can also be used to define film effectiveness on a convection surface. For more information on film effectiveness, see Conduction, Convection, and Mass Transport (Advection) in the Theory Reference.
For greater accuracy, the SURF151 and SURF152 elements can use the option of two extra nodes (KEYOPT(5) =2, KEYOPT(6) = 0) to get bulk temperatures from FLUID116 elements (KEYOPT(2) = 1). For two extra nodes, you must set KEYOPT(5) to 0 before issuing the ESURF command. After issuing ESURF, you set KEYOPT(5) to 2 and issue the MSTOLE command to add the two extra nodes to the SURF151 or SURF152 elements.
The following methods are available for mapping the FLUID116 nodes to the SURF151 or SURF152 elements with MSTOLE.
Minimum centroid distance method: The centroids of the FLUID116 and SURF151 or SURF152 elements are determined and the nodes of each FLUID116 element are mapped to the SURF151 or SURF152 element that has the minimum centroid distance. The minimum centroid distance method will always work, but it might not give the most accurate results.
Projection method: The nodes of a FLUID116 element are mapped to a SURF151 or SURF152 element if the projection from the centroid of the SURF151 or SURF152 element to the FLUID116 element intersects the FLUID116 element perpendicularly. An error message is issued If a projection from a SURF151 or SURF152 element does not intersect any FLUID116 element perpendicularly.
Hybrid method: The hybrid method is a combination of the projection and minimum centroid distance methods. In this method, the projection method is tried first. If the projection method fails to map correctly, a switch is made to the minimum centroid distance method. Any necessary switching is done on a per-element basis.
If the FLUID116 element lengths vary significantly as shown in the following two figures, the projection method is better than the minimum centroid distance method. The minimum centroid distance method would map the nodes of the shorter FLUID116 element to the SURF151 or SURF152 element, but the projection method would map the nodes of the longer FLUID116 element to the SURF151 or SURF152 element.
The projection method will not map any FLUID116 nodes to the SURF151 or SURF152 elements that are circled in the following figure. However, the hybrid method will work because a switch will be made to the minimum centroid distance method on the second pass.
Solve the analysis.
For example problems using SURF152 with a FLUID116 model, see VM271 and Thermal-Stress Analysis of a Cooled Turbine Blade in the Technology Showcase: Example Problems.
For information in using surface-effect elements to model radiation to a point, see Modeling Radiation Between a Surface and a Point.
For information on transferring external loads from CFX to Mechanical APDL, see the CFD-Post User's Guide, or the Coupled-Field Analysis Guide.
There is no single procedure for building model geometry. The tasks you must perform to create it vary greatly, depending on the size and shape of the structure you wish to model. Therefore, the next few paragraphs provide only a generic overview of the tasks typically required to build model geometry. For more detailed information about modeling and meshing procedures and techniques, see the Modeling and Meshing Guide.
The first step in creating geometry is to build a solid model of the item you are analyzing. You can use either predefined geometric shapes such as circles and rectangles (known as primitives), or you can manually define nodes and elements for your model. The 2D primitives are called areas, and 3D primitives are called volumes.
Model dimensions are based on a global coordinate system. By default, the global coordinate system is Cartesian, with X, Y, and Z axes. However, you can choose a different coordinate system if you wish. Modeling also uses a working plane -- a movable reference plane used to locate and orient modeling entities. You can turn on the working plane grid to serve as a "drawing tablet" for your model.
You can tie together, or sculpt, the modeling entities you create via Boolean operations, For example, you can add two areas together to create a new, single area that includes all parts of the original areas. Similarly, you can overlay an area with a second area, then subtract the second area from the first. Doing so creates a new, single area with the overlapping portion of area 2 removed from area 1.
Once you finish building your solid model, you use meshing to "fill" the model with nodes and elements. For more information about meshing, see the Modeling and Meshing Guide.