A simple 3-D airframe noise interaction problem is utilized to investigate volume source term effects arising in the context of an acoustic analogy based on a uniform now. Simulations of the model problem were conducted to assess the effect of volume source terms and to determine whether they are due to true physical quadrupole noise. In these simulations, a cylinder is placed in the vicinity of the trailing edge of a NACA 0012 airfoil at incidence. LES is performed to compute the noise source field as well as to directly capture the quasi-far field noise. The acoustic field is additionally computed using both a fast Ffowcs Williams-Hawkings (FW-H) solver with the ability to directly include volume source terms, and a standard permeable surface FW-H solver. In addition to studying the origin and effect of volume source terms, the possible generation of spurious noise with commonly used permeable surface predictions is investigated. The Reynolds number for these simulations was chosen to be high enough to ensure turbulent breakdown at the trailing edge of the airfoil and in the near wake of the cylinder. Several cases were run to parameterize the problem, including variations in Mach number as well as variations in gap spacing between the cylinder and airfoil. Results show that the modification to far-field noise due to volume sources is quite pronounced and excluding their contribution results in a significant mis-prediction. This held for all cases studied, even at relatively low Mach numbers where volume sources are commonly excluded. By including only subsets of the volume source field in the FW-H predictions, it was observed that the majority of the acoustic modification stems from sources in the near wake of the cylinder. In all cases, additional spurious noise was also generated downstream when using various permeable FW-H surfaces.
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