Open Rotor Tone Acoustics: Near-field Prediction & Projection

Abstract

Recent interest in countra-rotating open rotor engines by the aeronautical industry has prompted a renewal in research methods for predicting open rotor engine noise. The following paper outlines frequency domain methods to predict open rotor tone noise. The methods are exact and are valid in the nearfield, and are used to analyse and understand cruise test data from rig scale open rotor experiments conducted by Rolls-Royce plc. Since the experimental data is only collected in the nearfield and at various observer distances, analytical methods for projecting open rotor tone noise to different observer locations are also proposed. The nearfield acoustic methods are used to visualise and provide insight into the sound field in regions near to the open rotor. The exact expressions are also compared with faster more approximate methods, to show their suitability in predicting the typical level of sound. The projection methods are based on the nearfield frequency domain acoustic expressions, where both exact and approximate methods are described. Both methods are assessed for their robustness and the approximate projection method is then used on open rotor cruise data from experiments. Approximating loading and thickness noise with rotating point forces and rotating point acoustic doublets respectively is shown to be a suitable method. The projection methods are shown to work extremely well on theoretical input data, and the approximate projection method is also used on experimental data with promising results. Nearfield acoustic data, whether theoretical or experimental, can be effectively projected from one observer distance to another, using the analytical projection methods outlined in this paper, for a range of operating conditions and open rotor geometries.