A benchmark of Reynolds-Averaged Navier-Stokes (RANS)-informed analytical methods, which are attractive for predicting fan broadband noise, was conducted within the framework of the European project TurboNoiseBB. This paper discusses the first part of the benchmark, which investigates the influence of the RANS inputs. Its companion paper focuses on the influence of the applied acoustic models on predicted fan broadband noise levels. While similar benchmarking activities were conducted in the past, this benchmark is unique due to its large and diverse data set involving members from more than ten institutions. In this work, the authors analyze RANS solutions performed at approach conditions for the ACAT1 fan. The RANS solutions were obtained using different CFD codes, mesh resolutions, and computational settings. The flow, turbulence, and resulting fan broadband noise predictions are analyzed to pinpoint critical influencing parameters related to the RANS inputs. Experimental data are used for comparison. It is shown that when turbomachinery experts perform RANS simulations using the same geometry and the same operating conditions, the most crucial choices in terms of predicted fan broadband noise are the type of turbulence model and applied turbulence model extensions. Chosen mesh resolutions, CFD solvers, and other computational settings are less critical.

A benchmark dedicated to RANS-informed analytical methods for the prediction of turbofan rotor–stator interaction broadband noise was organised within the framework of the European project TurboNoiseBB. The second part of this benchmark focuses on the impact of the acoustic models. Twelve different approaches implemented in seven different acoustic solvers are compared. Some of the methods resort to the acoustic analogy, while some use a direct approach bypassing the calculation of a source term. Due to differing application objectives, the studied methods vary in terms of complexity to represent the turbulence, to calculate the acoustic response of the stator and to model the boundary and flow conditions for the generation and propagation of the acoustic waves. This diversity of approaches constitutes the unique quality of this work. The overall agreement of the predicted sound power spectra is satisfactory. While the comparison between the models show significant deviations at low frequency, the power levels vary within an interval of ±3 dB at mid and high frequencies. The trends predicted by increasing the rotor speed are similar for almost all models. However, most predicted levels are some decibels lower than the experimental results. This comparison is not completely fair—particularly at low frequency—because of the presence of noise sources in the experimental results, which were not considered in the simulations.

This work assesses the risks of increased fan noise for high bypass ratio aeroengines with short intakes. The close proximity between the fan and inlet contributes to the increase in radiation of the rotor-alone tones and reinforces the interaction of the inflow distortion with the fan. Thus, the closer the fan is to the inlet, the higher the risk for noise generation. This article discusses the results of Harmonic Balance simulations performed on a conceptual turbofan operated at the approach condition. The inflow distortion created by the nacelle incidence is dominated by the circumferential component [ ] = 1. Its presence is visible throughout the nacelle. A thorough analysis of the unsteady pressure and velocity fields shows that the new acoustic source created by the periodic unsteady loading of the rotor cutting the inflow distortion is negligible compared to the rotor–stator interaction. But the amplitude of the rotor–stator interaction tones is affected by the unsteadiness of the rotor wake shape, particularly in the tip region where a pronounced flow separation on the rotor blade is created at a certain range of azimuthal position. The variations of the flow incidence at the rotor leading edge, due to the axial and tangential components of the mean velocity, cannot explain that flow separation. Instead, the origin is attributed to the azimuthal variations of the radial component of the mean flow velocity near the casing which slightly points inward to the spinner, i.e. in the opposite direction of the casing contour line. The flow separation induces a pronounced scattering of the wake azimuthal components m_w = hB into [ ] in the tip region, whereas the same effect is rather limited on the rest of the blade height. This leads to a moderate increase of the tonal sound power level compared to the case with clean inflow. The azimuthal scattering due to the propagation of the sound waves through the distortion is found to be weak in the bypass duct. However, this effect is very important in the inlet lip vicinity, where the strong asymmetry of the flow modifies the path of the sound waves up to the far field.

The objective of this work is to better understand the acoustic interferences created by distributed propulsion engines based on an approach that combines RANS simulation results for the aerodynamic prediction and analytical models to calculate acoustics. A multi-propeller configuration without wing is considered for this investigation. The propeller geometry and the operating conditions are realistic for a regional transport airplane. In the first part of the paper, the results obtained by two different and independent prediction methods are compared. One method is well-established and serves as validation for the second, low-order method, which is better suited for design-to-noise applications since it requires less details as input and is computationally faster by several orders of magnitude. The good agreement between both methods, obtained for a single propeller as well as for the distributed propeller configuration, is exploited in the second part of the paper to investigate the role of acoustic interferences. Taking acoustic interferences into account drastically affects the directivity of the tonal emission. Compared to the results obtained by considering the propellers as if they were uncorrelated, the far-field sound pressure levels can be significantly lower at the radiation nodes or amplified up to the theoretical limit of 9 dB calculated for eight propellers. The directivity patterns depend on the relative initial angular positions of the propellers. When these positions are randomly varied according to the uniform probability density distribution model, the mean result (expectation) is the same as if the propellers were considered as uncorrelated. Finally, the results show that the probability that the acoustic level is lower than the mean value is higher than 50% because of the positive skewness of the probability distribution of the resulting pressure amplitude. Even though the propeller–propeller and propeller–wing interactions were not considered, the essence of the findings is expected to remain valid for more complex configurations because those interactions are rotor phase-locked.