Fan noise is comprised of tonal and broadband noise. Tonal noise has been exten-sively studied resulting in effective measures for reducing dominant tones at discrete frequencies. Noise abatement techniques include using liners, choosing an appro-priate blade-vane count combination, increasing the distance between the rotor and stator blades, operating the fan in the subsonic range, and improving the blade geometry. Gliebe et al. [43] predicted the maximum expected reduction in engine system noise of an ultra-high bypass ratio (UHBR) engine due to the elimination of fan tones to range between 0.5 and 1.5 EPNdB. Once fan tones are mostly elimi-nated, another 3 to 4 EPNdB in noise reduction can be achieved by decreasing the contribution of fan broadband noise. This highlights the need to further the un-derstanding of fan broadband noise mechanisms in order to develop suitable noise
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Figure 1.2: Schematic of fan broadband noise sources and of the respective turbu-lence components (indicated by circular markers) [Source of the fan picture: Rolls-Royce].
mitigation measures.
Fan broadband noise can be categorized into two groups: self-noise and interac-tion noise. Self-noise mainly consists of the noise produced by turbulent eddies in the boundary layer of a blade interacting with the trailing edge. This noise mechanism is relevant for rotor blades and outlet guide vanes (OGV’s) of a fan stage. Inter-action noise occurs when turbulence interacts with blade leading edges. Relevant turbulence components for the rotor are ingested or background turbulence (rotor ingestion noise) and boundary layer turbulence (rotor boundary layer noise). For the OGV’s, rotor wake turbulence, background turbulence, and boundary layer tur-bulence are relevant. In this work, self-noise and rotor interaction noise sources are neglected. The focus is to study the interaction noise sources at the stator vanes. In the context of this thesis, the terms fan broadband noise and rotor-stator-interaction (RSI) noise are used interchangeably. This is technically imprecise: Fan broadband noise typically describes all broadband noise components of a fan. The use of this collective term therefore implies that self-noise and rotor interaction noise sources are negligible. In addition, RSI noise is oftentimes used in literature to describe the interaction noise related to only the wake turbulence component. In publications II and III, the contribution of different turbulence components to stator interac-tion noise are discussed. Ingesinterac-tion noise refers to the contribuinterac-tion of background
turbulence and boundary layer noise refers to the contribution of boundary layer tur-bulence. All mentioned fan broadband noise sources are depicted in Fig. 1.2. Note that the respective turbulence components are schematically indicated by circular markers: rotor wake turbulence (red), ingested or background turbulence (blue), boundary layer turbulence on the casing walls (green), and boundary layer tur-bulence on the blade surfaces (orange). The relevance of the different broadband noise sources to overall fan broadband noise levels is discussed in the subsequent paragraphs.
In the absence of interaction noise, self-noise can be understood as the minimum achievable level of fan broadband noise. In fact, Moreau and Roger [69] showed that self-noise is significantly lower than interaction noise for several applications. Yet, it is thought that this noise source can be relevant at low Mach number.
One of the main sources of interaction noise is RSI noise caused by the interaction of the rotor wake turbulence with the stator leading edges. Ganz et al. [37] studied an 18 inch model-scale fan in the Boeing Low-Speed Aeroacoustic Facility. They showed that the RSI noise is the loudest noise source for this fan configuration. For the NASA Source Diagnostic Test (SDT) fan, which was investigated in the NASA 9’ x 15’ Low-Speed Wind Tunnel, the RSI noise was also found to be the domi-nant fan broadband noise source [72]. This finding was confirmed in an analytical study performed by Envia et al. [30]. The authors used a RANS-informed ana-lytical technique, which is based on the work by Ventres et al. [89] and described in more detail by Nallasamy and Envia [72]. The code is restricted to the consid-eration of RSI noise but only slightly underpredicted the measured sound power levels. As measured data also contained some rotor noise, the slight discrepancy is reasonable and still shows that RSI noise is dominant. However, the authors also investigated two other fans: the NASA/PW Advanced Ducted Propulsor Fan 1 and the NASA/Honeywell Quiet High Speed Fan 2. For these two fans, the discrepancies between analytically predicted and experimental sound power level spectra were sig-nificantly larger at all operating points. This could indicate that other broadband noise sources significantly contribute to the overall fan broadband noise for these configurations.
Other potentially relevant interaction noise sources are ingestion and boundary layer noise. For a low-speed fan stage, which was studied in the low-speed fan rig at DLR in the framework of the PROBAND project, Jurdic et al. [54] reported
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that the stator ingestion noise due to background turbulence was several decibels louder than the RSI noise due to the wake turbulence. The significance of ingestion turbulence was also highlighted in a study by Moreau and Oertwig [68]. They analyzed the contributions of different broadband noise sources using the analytical tool PropNoise [70] for the DLR UHBR fan. This high-speed, low-pressure ratio fan was tested in the M2VP test facility in Cologne. Since this facility has no turbulence control screen, the rotor and stator ingestion noise were found to be significant, particularly at lower frequencies. Staggat et al. [87] scaled this fan and performed an analytical parameter study to investigate rotor boundary layer noise. They showed that the predicted rotor boundary layer noise increases with boundary layer thickness and shape factor. For an integrated turbofan, it was therefore postulated that the rotor boundary layer noise has the potential to be dominant compared to RSI noise at low and mid frequencies for approach and take-off conditions. The relevance of interaction sources like ingestion or boundary layer noise depends on the fan configuration, on test rig conditions and on the integration of an engine into the aircraft architecture. But even for these cases, RSI noise due to the wake turbulence is still an important contributor to the overall fan broadband noise levels. Its accurate prediction using analytical or numerical tools is therefore of great interest.