One of the dominant noise sources of modern Ultra High Bypass Ratio (UHBR) engines is the interaction of the rotor wakes with the leading edges of the stator vanes in the fan stage. While the tonal components of this noise generation mechanism are fairly well understood by now, the broadband components are not. This calls to further the understanding of the broadband noise generation in the fan stage. This article introduces the cyclostationary stochastic hybrid (CSH) method, which accommodates in-depth studies of the impact of cyclostationary wake characteristics on the broadband noise in the fan stage. The Random Particle Mesh (RPM) method is used to synthesize a turbulence field in the stator domain using a URANS simulation characterized by time-periodic turbulence and mean flow. The rotor-stator interaction noise is predicted by a two-dimensional CAA computation of the stator cascade. The impact of cyclostationarity is decomposed into various effects investigated separately. This leads to the finding that the periodic turbulent kinetic energy (TKE) and periodic flow have only a negligible effect on the radiated sound power. The impact of the periodic integral length scale (TLS) is, however, substantial. The limits of a stationary representation of the TLS are demonstrated making the CSH method indispensable when background and wake TKE are of comparable level. Good agreement of the CSH method with measurements obtained from the 2015 AIAA Fan Broadband Noise Prediction Workshop are also shown.

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.

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The prediction of fan broadband noise presents a challenge. Fully scale-resolving approaches are exceedingly demanding in computational resources, whereas analytical techniques rely on simplifying assumptions. RANS-informed synthetic turbulence methods are a compromise between accuracy and cost. In particular, a two-dimensional approach based on a simulation at midspan is attractive because it can easily be computed on a conventional PC. However, this approach raises some concerns: It is uncertain, if the mean flow and turbulence statistics at midspan are truly representative for the entire fan stage and if the neglect of three-dimensional flow effects is permissible. In this paper, the authors improve the current method to account for the three-dimensional nature of the flow by applying corrections based on turbulence spectra and by weighting results obtained at three spanwise positions at 20%, 50%, and 80% of the stator height. Cyclostationarity needs to be considered for all simulations as the resulting turbulence spectra cannot be described by conventional von Kármán spectra. Instead, the differing shape of the spectra can be attributed to turbulence in the wake and in the background flow. For this configuration, the deviation between the overall sound power at midspan and the overall sound power taking into account the three radial positions is less than 0.5 dB. Thus, the results at midspan are, in fact, representative for the entire fan as long as contributions close to the duct walls are ignored. Lastly, it is shown that non-linearities are negligible for the investigated, full-scale fan at approach.

Next generation fan designs featuring large bypass ratios have the potential to reduce rotor-stator-interaction noise of fans. However, these designs pose new challenges and long-held beliefs regarding the acoustic benet of such concepts need to be evaluated. At o-design operating points, the increase in nozzle area serves to maintain the stall margin by reducing the fan loading. To an extent, this may lower the contribution of wake turbulence to fan broadband noise. Thus, investigating the relative contribution of other noise sources becomes relevant. In this paper, the authors examine the contribution of wake and ingested turbulence to fan broadband noise levels at approach for the ASPIRE fan, which is a next generation fan concept. To enable the investigation of background turbulence, a Reynolds stress turbulence model is used. The CFD solution then provides inputs for a 2D cyclostationary synthetic turbulence method. The noise predictions are improved by performing simulations at three spanwise positions and by introducing a correction technique to account for the three-dimensional interstage ow. For the investigated case, it is shown that broadband noise generated at the stator is as high as for conventional concepts. Furthermore, both wake and background turbulence contribute signicantly to the overall fan broadband noise.