Experimental Validation of an Analytical Sound Transmission Model for Fan Stages

Sound propagation effects play an important role for the tonal radiation from fan stages. Acoustic modes originate predominantly from the stator leading edge as a result of aerodynamic rotor-stator interaction. Modes propagate through the rotor and stator before they radiate into the far-field. During propagation through the blade rows, reflection and scattering redirects the acoustic modes or shifts sound power to different mode orders and frequencies. Analytical models enable the evaluation of mode propagation effects in turbomachinery stages, but they rely on simplifications of blade geometry and flow field and therefore, require experimental validation. To this end, an experimental investigation was undertaken that provides specific, highly resolved data for the validation of analytical and numerical prediction tools of propagation effects in turbomachinery stages. The mode propagation is modeled using an cascade model in combination with a model to account for the impact of flow deflection at the blade rows. By comparing predicted and experimental results for mode transmission, reflection and scattering the model is validated over a wide range of mode orders, frequencies and operating conditions. Overall, good agreement of the model prediction is observed. Shortcomings of the modeling are identified and possible model improvements to increase accuracy are discussed.