Acoustic Resonances in a 2D High Lift Configuration and 3D Open Cavity

Abstract

Acoustic resonances are computed numerically for a generic two-dimensional three-element high lift configuration and a three-dimensional open cavity. Both are major sources of tonal airframe noise. All computations are made for zero mean flow. The high-order finite element code NGSolve of Joachim Schöberl is used in conjunction with Perfectly Matched Layer (PML) absorbing boundary conditions to avoid unphysical reflections at the necessarily finite grid boundaries. Whatever the actual source of the observed discrete tones is, these tones may be enhanced by or lock onto resonant acoustic frequencies. For the high lift configuration our computed resonant frequencies are indeed fairly close to experimentally observed low-frequency slat tones and may be interpreted as resonances of leaky surface waves. These low-frequency resonances are practically unaffected by changes in slat cove geometry contrary to high-frequency resonances (gap tones). The radiation loss of resonances in a three-dimensional open cavity was found to be lower than in a two-dimensional cavity and a confining duct can change the damping of cavity resonances.