Experimental Validation of an Analytical Prediction Model for Fan Buzz-Saw Noise

Despite the renewed interest observed in the early 2010’s towards the low-speed contra-rotating propulsion concepts (open rotors and ducted contra-fans), the conventional rotor-stator ducted fan stage has remained the technology of choice for the mid-to-long-range civil-aircraft market. Moreover, following the success of the gearbox technology, designs with a fan pressure ratio below 1.4 and engine bypass ratio beyond 12 are considered by manufacturers for the future generations of aircraft engines.

In that context, the topic addressed in the present paper is the importance of buzz-saw noise generated by fan rotors operating at transonic regimes, especially during take-off. Contrary to other acoustic sources such as jet noise or rotor-stator interaction noise, buzz-saw noise is not necessarily expected to decrease with lower fan pressure ratio, if the rotor blade tip still operates in supersonic or transonic regime with a significant inflow distortion typical of short nacelles under incidence. The purpose of the present contribution is to provide a number of application cases for a fully analytical prediction model for fan buzz-saw noise, which has been published in 2019 at the AIAA/CEAS Aeroacoustics Conference.  

The noise predictions provided by the model will be first validated against experimental data collected during measurements of four different fan stages: the high-speed NASA QHSF stage with a fan pressure ratio (FPR) around 1.8 at design point, the NASA SDT with FPR=1.5, and the low-speed DLR UHBR and AneCom ACAT1 stages with FPR=1.4. Also, the predictions of the new model will be compared with those obtained with the NASA ANOPP fan noise correlations developed by Heidmann and updated by Kontos.

The various comparisons mentioned above will be discussed for rotor speeds ranging from subsonic to supersonic operating conditions, on the design working line corresponding to the aerodynamic peak efficiency of each fan. Additionally the models will be tested on a low-loading and a high-loading working line in order to appraise their prediction capabilities. It will be shown that a fairly good agreement is obtained with the experimental data, both for the absolute noise levels as for the evolutions with rotor speed and working line.

Finally, an attempt will be made to assess the acoustic impact of a flow distortion ingested by the rotor, which is relevant for one of the experimental test cases. The distortion is characterized by a circumferentially non-uniform inflow into the rotor blades, with local variations in Mach number and blade incidence that affect the generation of shocks and their propagation. Especially for the low-pressure-ratio fans where the nacelle length must be kept short to avoid excessive weight and drag penalties, the presence of a marked inflow distortion may shift the onset of buzz-saw noise toward lower tip speeds, thus reducing the acoustic benefit of the ultra-high-bypass-ratio engine.