Simplified Models for Propeller Potential Interaction Noise

Nowadays, a major engineering concern involved in the design of future aircraft and drone configurations is related to aerodynamic and acoustic rotor-airframe interactions. As low-noise emissions are highly required for the development of such technologies, a thorough understanding of the noise-generating phenomena is a necessary step to investigate aerodynamic performances and acoustic control strategies. However, accurate flow computations and sound predictions are challenging tasks when complex physical mechanisms are involved. Hence, at the early design stage, where the detailed geometry of the propulsion systems is unknown, analytical approaches are considered reliable tools to exploit. The present work aims at modeling and predicting the aerodynamic sound generated by a propeller placed upstream of a rod. Simplified analytical formulations are considered, involving the distorted potential flow field generated by the propeller-rod interaction. The latter is modeled by the unsteady flow generated by a circle and a moving upstream airfoil. The aerodynamic forces are employed as inputs to an acoustic model, based on Ffowcs-Williams and Hawkings’ analogy, to model the tonal peaks at the blade passing frequency and harmonics. To compare with the modeled results, sound directivity patterns emitted by a drone propeller have been measured in the anechoic laboratory at von Karman Institute for Fluid Dynamics, Belgium, by means of an array of 7 microphones. The parameters selected for the experimental characterization are the rod diameter and the relative distance between the propeller and the rod. The experimental results are compared with the analytical model. It is observed that higher level of noise is measured when increasing the spacing among the propeller-rod, as well as when a larger rod diameter is selected. An acceptable agreement has been observed among experimental and analytical results, given the simplifications. The outcome of the work consists of a fast-running analytical tool that takes into account the well-known potential flow formulations able to obtain a preliminary estimation of the propeller noise with little computational effort. Overall, this work brings more knowledge to the framework of potential-interaction noise, whilst getting more insights into the physical noise-generating mechanism.