Investigation of Volumetric Sources in Airframe Noise Simulations

Hybrid methods for the prediction of airframe noise involve a simulation of the near field flow that is used as input to an acoustic propagati on formula. The acoustic formulations discussed herein are those based on the Ffowcs Will iams and Hawkings equation. Some questions have arisen in the published literature i n regard to an apparently significant dependence of radiated noise predictions on the loc ation of the integration surface used in the solution of the Ffowcs Williams and Hawkings equation. These differences in radiated noise levels are most pronounced between solid-body surface integrals and off-body, permeable surface integrals. Such differences sugg est that either a non-negligible volumetric source is contributing to the total radi ation or the input flow simulation is suspect. The focus of the current work is the issu e of internal consistency of the flow calculations that are currently used as input to ai rframe noise predictions. The case study for this research is a computer simulation for a th ree-element, high-lift wing profile during landing conditions. The noise radiated from this f low is predicted by a two-dimensional, frequency-domain formulation of the Ffowcs Williams and Hawkings equation. Radiated sound from volumetric sources is assessed by comparison of a permeable surface integration with the sum of a solid-body surface integral and a volume integral. The separate noise predictions are found in good agreement.