CAVERN: a prediction code for cavity electromagnetic analysis

The authors describe an application of the fast Fourier Bessel transform to speed up their jet-engine scattering-analysis code, CAVERN. Because of the engine termination, it is typically necessary to break up the computational domain into that of the inlet and engine regions. In the inlet region, the fields are modeled using ray techniques, whereas the complex engine must be modeled using a different and more-rigorous formulation. A need then arises for interfacing the two computational domains, in a manner that maintains field continuity. Since the engine is always housed in a cylindrical or near-cylindrical inlet section, one way for interfacing the two computational regions is to represent the ray and engine fields using a cylindrical-modal expansion. The engine can then be characterized, via the modal-scattering matrix, in a manner independent of the excitation. Ray fields can be readily turned into modes, and vice versa, and continuity of the fields at the interface of the two regions is maintained, since both use the same modal representation. However, for inlets whose diameter spans 50λ or so, the task of transforming rays into modes and vice versa can become a bottleneck, because of the large number of propagating modes present in the cavity. Use of the fast Fourier Bessel transform can alleviate this difficulty, as described