Advanced open rotor far-field tone noise

An extensive analysis of the far-field tonal noise produced by an advanced open rotor is presented. The basis of the work is analytical methods which have been developed to predict the tonal noise generated by several different mechanisms. Also to provide further insight and validation, an in-depth analysis of rig-scale wind tunnel measurements and Computational Fluid Dynamics (CFD) data has been carried out.The loading noise produced by a propeller operating at angle of attack is formulated modeling the blades as a cascade of a two-dimensional flat plates. The simplified geometry enables the prediction of rotor-alone tones with a reduced computer processing time. The method shows a good ability to predict the relative change of the sound pressure level with changes of the angle of attack.The radiated sound field caused by aerodynamic interactions between the two blade rows is investigated. A hybrid analytical/computational method is developed to predict the tonal noise produced by the interaction between the front rotor viscous wakes and the rear rotor. The method assumes a Gaussian wake profile whose characteristics are determined numerically in the inter-rotor region and propagated analytically to the downstream blade row. A cross validation of the prediction method against wind tunnel measurements and CFD data shows reasonable agreement between the three techniques for a wide number of interaction tones.Additionally a theoretical formulation is derived for expressions for the tonal noise generated by the interaction between the bound potential field of one rotor with the adjacent rotor. The bound potential field produced by the thickness and bound circulation of each blade is determined using a distributed source model. The analytical model is compared with an existing model for a point source and is validated using advanced CFD computations. Results show that the potential field mostly affects the first interaction tone. The addition of the potential field component provides an improvement in the agreement with wind tunnel measurements.Overall this work provides new tools for the rapid assessment of the tonal noise produced by an advance open rotor at various operating conditions, and the analytical result can provide further physical insight into the sources of noise.