Analytical and computational investigations of oblique blade-vortex interaction-generated noise

Abstract

This paper presents results from an analytical and computational study of oblique rotor blade-vortex interaction (BVI) acoustics. Our oblique BVI model utilized an isolated vortex interacting with a two-bladed rotor operating at zero thrust to minimize the influence of the rotor's own wake. One of the goals of this research is to examine the predictability of oblique BVI noise using three representative state-of-the-art computational schemes. These computational schemes included a CFD/Kirchhoff method, an Indicial Aerodynamics/Ffowcs-Williams Hawkings method and a simple BVI Impulse/Lowson method. Another goal of the paper is to illustrate the importance of BVI source phasing effects in blade-vortex interaction noise radiation. This is achieved by examining the BVI noise directivity plots for four oblique BVI configurations. Variation of BVI noise directivity for these oblique BVI cases were attributed to the formation of Mach cone noise wavefronts generated by supersonic trace Mach number sources. BVI sources with supersonic trace Mach numbers were found to radiate sound waves that arrived at some locations in-phase due to the characteristic positioning and timing of BVI events. This characteristic positioning and timing of the BVI events (i.e. BVI source phasing effect) is responsible for the highly directional propagation of BVI acoustics.