Broadband noise generation of a contra-rotating open rotor blade

Requirements to improve the propulsive efficiency of aircraft engines have revived interest in contra-rotating open rotor (CROR) engines. However, CRORs exhibit tonal and broadband noise emissions which are a barrier of entry into commercial service. Until recently, studies have concentrated on tonal noise emissions because they are considered to be louder than broadband emissions. However, recent work has shown that CROR broadband noise can also be significant. This project uses computational aeroacoustic methods to study the mechanisms underlying broadband rotor-wake interaction (BRWI) noise, which is a dominant CROR broadband noise source. Predictions of BRWI noise have thus far assumed the rotor blades to be flat plates, and have not accounted for the effects of the blade geometry on the noise. In this project, the effects of blade geometry on turbulence interaction noise are comprehensively studied, including the effects on the noise due to airfoil thickness, leading edge radius, angle-of-attack and camber. Airfoil thickness and leading edge radius are shown to reduce the noise at high reduced frequencies, with the noise being more sensitive to thickness than to leading edge radius. The effects of angle-of-attack and camber are found to be small for interactions between an airfoil and isotropic turbulence. The mechanisms which cause the changes to the noise are also investigated. The project concludes with a study of the turbulence interaction noise for realistic CROR blades and conditions to evaluate the error incurred when rotor blades are assumed to be flat plates. It is found that while CROR blade geometry does affect BRWI noise, the effect is sufficiently small that flat plate theory can be confidently used in most circumstances. In addition to examining the effects of airfoil geometry on the noise, this project investigates efficient methods to computationally simulate turbulence interaction noise. It is shown that the noise is sensitive to transverse turbulent disturbances, but is not sensitive to spanwise or streamwise ones. Therefore, accurate noise predictions are obtained by representing the turbulence as only transverse disturbances, which leads to significant savings in computational cost