An investigation of rotor noise generatoin by aerodynamic disturbance

An open rotor has been considered as a process for converting an unsteady velocity inflow into sound radiation. With the aid of crude assumptions 'aero-acoustic transfer functions' have been defined theoretically for both discrete frequency and broad band noise. An experimental study of the validity of these transfer functions has yielded results which show good agreement at discrete frequencies though slightly less good for broad band noise. Agreement in both cases holds over three or more decades of the relevant parameters. The experimental work involved has necessitated the development of a rotating hot wire anemometry system. A single hot wire probe has been mounted in the nose-cone of the rotor and used to quantify fluctuations in the airflow onto a single rotor blade for the transfer function results. Further theoretical analysis has revealed that the sound fieid can be expressed in terms of blade-to-blade correlations in the airflow, and results from two probes rotating simultaneously have been modelled mathematically and inserted in the theory. Preliminary results show.encouraging agreement with experimental data.