Hybrid finite element modal analysis for jet engine inlet scattering.

Abstract

Motivated by the need to predict radar signatures of aircraft, a numerical simulation of the radar scattering from jet engine cavities is developed. Finite element techniques are used for modeling the engine face which has been an elusive target in computational electromagnetics for many years. The finite element method is used to compute the cylindrical mode scattering solution of the fields within the circular inlet directly in front of the engine face. This modal solution is represented by a generalized modal scattering matrix and can be coupled to a high frequency solution for the remainder of the aircraft. Important advancements are made by examining the radar return of the angularly periodic fan blades and uncovering the phenomenon of limited mode coupling. Limited mode coupling is then used to predict the non-zero entries in the modal scattering matrix representing the engine face. Since there can be a large number of traveling modes in the engine inlet, the sparsity of the modal scattering matrix is exploited to allow storage and computational scaling. Additionally, the entire finite element problem can be scaled down to a single slice of the fan. This scaling reduces the size of the domain so that electrically large fans could be modeled on available computing platforms. Finally, a novel technique is introduced where the jet engine modulation can be computed as a post-processing step, based on a single solution for stationary blades.