In the past, the only way for manufacturers to evaluate a proposed automotive HVAC system design was to build an expensive prototype and test it in the laboratory. Several years ago, engineers at Visteon began experimenting with CFD with the goal of evaluating automotive air handling system designs without the need to build a prototype. The complex geometries used in air handling systems present major obstacles in computer simulation. They typically include ducts that expand and contract, change from round to square cross-sections, go through complex curves throughout their length, and have many branches and internal walls.

The CFD codes used in the past at Visteon involved a structured mesh approach that required that the block structure be defined by hand before a volume mesh was generated. It took weeks or months to produce a grid for analysis of complex air handling system components using this approach - as long as building a prototype. Recently, Visteon began using TGrid 3 to generate an unstructured tetrahedral volume mesh from a triangular surface mesh. While tetrahedral meshes are useful for complex geometries, prism elements are more suitable for resolving boundary layers. The prism elements, extruded from surface triangles, allow a more accurate solution with fewer elements. TGrid creates a hybrid mesh consisting of prism layers in near-wall regions and tetrahedral cells in the remainder of the domain. It also layers prism elements at the walls. The result is better accuracy without the time-consuming task of building an all-hex mesh.

By switching to FLUENT and taking advantage of parallel processing, Visteon made dramatic improvements in solver technology. The latest FLUENT release partitions the flow domain for multiple processors and runs an instance of the solver on each processor. Visteon uses both a 16-node, shared-memory Silicon Graphics compute server and clusters of workstations for parallel processing of CFD models. On the SGI compute server, the FLUENT solver achieves about 90% processing efficiency – 14.4 times the speed of a single process. This makes it possible to solve many typical models overnight.

As an example of the design improvements that can be achieved by applying these new capabilities, consider the design specification that performance of the HVAC system be linear with respect to the temperature dial on the instrument panel. In other words, moving the dial from position one to position two should have the same impact on temperature as moving from position two to position three. In the past, the linearity of the temperature dial couldn’t even be estimated until full-vehicle prototypes were constructed. At that point, changes were costly and the testing data provided little or no input on what types of changes were required. Using FLUENT, Visteon engineers can determine the linearity of a proposed design as soon as the solid model has been created – in a matter of days. The flow patterns revealed in the analysis help them to quickly iterate to an optimized design.

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