An APU is a gas turbine used to provide electrical and pneumatic power during ground operations. The unit also acts as a backup supply of electrical power when airborne. The design of the APU for the Embraer jet was more complicated than usual.

"It was a particularly challenging component to design because there were constraints regarding size, drag, and acoustics," says Jose Goldstein, senior engineer at Hamilton Sundstrand Power Systems.

Hamilton Sundstrand turned to CFD as a less expensive method of designing the APU inlet. By starting with a CAD model of the aircraft, and a CFD solution performed on the aircraft by Embraer, the engineers were able to perform a detailed CFD analysis of the APU inlet. Hamilton Sundstrand engineers used Fluent's GAMBIT mesh generation program to simplify grid generation. Through the use of GAMBIT's "virtual geometry" feature, the engineers were able to quickly remove duplicate lines and gaps between surfaces in the CAD model.

"This is a typical problem with CAD to mesh package translation," says Gao Yang, Master engineer at Hamilton Sundstrand Power Systems. "But GAMBIT makes it a little bit easier with a feature called virtual geometry. It stitches bad geometry and makes it workable. Without the virtual geometry feature, we would have needed much more time to repair the geometry manually"

Hamilton Sundstrand analyzed a number of inlet designs which included varying the number of splitter plates, altering the de-icing fence, and even the orientation and number of inlets. In the end, a single inlet without splitter plates proved to be the best.

"This inlet was a big design challenge for us, with so many parameters that could vary and the performance requirements being so critical," says Jose. "It would have been very difficult to do without a powerful CFD tool. Even if we had come up with a design that worked, we would not have been able to optimize it the way we did. Tools such as virtual geometry and unstructured meshing were key in this application because they made the use of CFD possible."