Thermal analyses are the second most frequently performed simulations.
(Structural analyses top the list.) And although most thermal simulations are done with finite elements, there are advantages to methods that work with networks of thermal bodies connected by links that simulate conduction, convection, and radiation.
"For example, complex systems, such as the cooling system for a car engine takes a few nodes and a working knowledge of how much heat an engine generates and how it's transferred to air," says say Ron Behee, president of Network Analysis Inc., Chandler, Ariz. (www.sindag.com). The company develops the thermal network-analysis program Sinda/G. "The software is said to solve models faster than FEA, handle radiation more efficiently, and it's more stable solving nonlinear thermal problems. But using network analyses and thermal FEA gives engineers a better look at thermal characteristics than either soft ware alone," he says.
Recently, his company linked Sinda/G to MSC.Patran from MSC. Software, Santa Ana, Calif. (www.msc-software.com), the widely used pre and postprocessor for Nastran. "The combination lets users solve problems with network analyses within the finite-element model builder," he says. Behee suggests using network analysis to get a handle on a thermal problem and then turn to a finite-element modeler to compute de tailed temperatures for each part of an assembly.
What's more, the network analyzer includes a programming language based on Fortran that lets engineers customize a thermal model and add features not available in Patran. "For example, if a 15-W heat source on node 40 is to come on 45 sec into a simulation, the user could add a statement that reads, ‘If t = 45, then Q40 = 15'," he says.
Another plus to linking the two programs, adds Behee, is that engineers can generate models directly on imported CAD geometry instead of manually rebuilding them. This reduces errors and modeling time. Engineers can also postprocess results in Patran to get color-coded temperature contours. And the thermal software works with less-frequently encountered thermal radiation and orbital-heating codes such as Trasys, Nevada, Thermica, and TSS.
The network analyzer integrated into Patran solves other engineering problems as well. For instance, finite-element models of ten have too many elements for radiation codes. "To get around this, users can create coarse radiation meshes that overlay finer conduction meshes. The radiation mesh uses superelements that group many small finite-element faces into a few larger elements. Geometric primitive surfaces created in Patran can model such things as cylinders, disks, or spheres. So an external fuel tank could be modeled with three superelements, two hemispheres, and a cylinder. Patran passes this curved geometry directly to the thermal-radiation and orbital-heating codes. Conduction models might consist of 20,000 nodes, but radiation models often work well with only 1,000 geometric surfaces," he says.
In addition designers can run both thermal and structural problems from the same Patran model. Thermal analysis finds temperatures that can be applied to Nastran models that solve for thermal distortion or stresses. "Meshes from the two simulations can be the same or different, but typically, structural meshes are finer and thermal meshes more coarse," says Behee.