Numerical and Mesh Resolution Requirements for Accurate Sonic Boom Prediction of Complete Aircraft Configurations

Abstract

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. INTRODUCTION SONIC boom phenomena is one of the main reasons preventing the acceptance of supersonic flight over populated areas. The importance of minimizing the environmental impact cannot be understated. In addition, the business case for low-boom supersonic aircraft is also quite compelling: a much larger market can be found should the aircraft be allowed to fly supersonically over land. For these reasons research efforts have been recently focused on various techniques for sonic boom mitigation.1-7 However, before sonic boom minimization design work can be credibly carried out, the accurate prediction of the fundamental sonic boom propagation problem has to be addressed in detail.By the time the pressure disturbance created by an airplane reaches the ground, most boom signatures develop into the well-known N-wave shape. This sig- * Graduate Student, AIAA Member nature is such that it contains two strong shocks at its beginning and end, causing a high perceived loudness (pldB). The propagation o...

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