Numerical and mesh resolution requirements for accurate sonic boom prediction of complete aircraft configurations

In this paper we conduct a careful study to assess the numerical mesh resolution requirements for the accurate computation of sonic boom ground signatures produced by complete aircraft configurations. The details of the ground signature can be highly dependent on the accurate prediction of the pressure distribution in the near-field of the aircraft. For this purpose it is necessary to describe the geometric detail of the configuration including the wing, fuselage, nacelles, diverters, etc. and to accurately capture the propagation of shock and expansion waves at large distances from the fuselage centerline. Unstructured, adaptive mesh technologies are ideally suited for this purpose since they use mesh points only in the appropriate locations within the flow field. In this work, we consider a supersonic business jet configuration (SBJ) which was tested at the NASA Langley Research Center and for which experimental near-field data was extracted at several locations underneath the flight track. The propagation of these near-field signatures from different altitudes can be shown to result in near N-wave ground booms. In order to examine the effect of both nacelles and empennage, results for three test cases are presented. These test cases represent the complete configuration with the large nacelles, the configuration without the nacelles, and the configuration without the nacelles and empennage. Inviscid solution adaptive unstructured meshes with up to 7.2 million nodes and 42.1 million tetrahedra are used to calculate the pressure distributions at several locations below each configuration where comparisons with experimental data are performed. All near-field pressure distributions are propagated to the ground (from and altitude of 50,000 ft) to predict the ground boom and the perceived noise level of the ground signature. For each case, the minimum number of mesh nodes and elements and the levels of refinement needed for accurate computations of near-field pressure distribution and ground boom signature are discussed.