Accurate structural modeling of blisk mistuning is critical for the analysis of forced response in turbomachinery. Apart from intentional mistuning, mistuning can be due to the manufacturing tolerances, corrosion, foreign object damage, and in-service wear in general. It has been shown in past studies that mistuning can increase the risk of blade failure due to energy localization. For weak blade to blade coupling, this localization has been shown to be critical and higher amplitudes of vibration are expected in few blades. This paper presents a comparison of three reduced order models (ROMs) for the structural modeling of blisks. Two of the models assume cyclic symmetry, while the third model is free of this assumption. The performance of the reduced order models for cases with small and large amount of mistuning will be examined. The benefits and drawbacks of each reduction method will be discussed.

Integrating a fan with a boundary layer ingestion (BLI) configuration into an aircraft fuselage can improve propulsion efficiency by utilizing the lower momentum airflow in the boundary layer developed due to the surface drag of the fuselage. As a consequence, velocity and total pressure variations distort the flow field entering the fan in both the circumferential and radial directions. Such variations can negatively affect fan aerodynamics and give rise to vibration issues. A fan configuration to benefit from BLI needs to allow for distortion without large penalties. Full annulus unsteady computational fluid dynamics (CFD) with all blades and vanes is used to evaluate the effects on aerodynamic loading and forcing on a fan designed to be mounted on an adapted rear fuselage of a Fokker 100 aircraft, i.e., a tail cone thruster. The distortion pattern used as a boundary condition on the fan is taken from a CFD analysis of the whole aircraft with a simplified model of the installed fan. Detailed simulations of the fan are conducted to better understand the relation between ingested distortion and the harmonic forcing. The results suggest that the normalized harmonic forcing spectrum is primarily correlated to the circumferential variation of inlet total pressure. In this study, the evaluated harmonic forces correlate with the total pressure variation at the inlet for the first 12 engine orders, with some exceptions where the response is very low. At higher harmonics, the distortion content as well as the response become very low, with amplitudes in the order of magnitude lower than the principal disturbances. The change in harmonic forcing resulting from raising the working line, thus, increasing the incidence on the fan rotor, increases the forcing moderately. The distortion transfers through the fan resulting in a non-axisymmetric aerodynamic loading of the outlet guide vane (OGV) that has a clear effect on the aerodynamics. The time average aerodynamic load and also the harmonic forcing of the OGV vary strongly around the circumference. In particular, this is the case for some of the vanes at higher back pressure, most likely due to an interaction with separations starting to occur on vanes operating in unfavorable conditions.

Boundary Layer Ingestion (BLI) is a concept with the potential to reduce energy consumption needed for aircraft propulsion. For the fan this results in a need to work well in an environment of increased continuous distortion. The objective of this study is to increase the understanding of unsteady aerodynamic forces due to total pressure distortion in a BLI fan. This is done by using computational fluid dynamics to analyze a fan design intended for a BLI installation. Results include low subsonic to transonic operating speeds and distortion in a wide range of wave numbers. The forcing exhibits a significant dependency on the aeroacoustic cut-on/cut-off condition. This is in particular true at part speed where the normalized unsteady force rises sharply before dropping suddenly as the corrected speed or engine order increases. Unsteady pressure in the blade passage is observed to exhibit the same increase in level followed by a sudden drop once the cut-on limit is passed and undamped pressure waves propagating out from the blade row appear. The unsteady forces on the first three modes exhibit different sensitivities to the distortion wavelength but all are affected by the acoustic condition. By comparing results for the reference fan to a similar fan design with a higher blade count the cascade properties of the blade row are found to dominate the interaction. A result of this is that a higher blade count fan may be less affected by the distortion, and be less prone to propagate noise due to low engine order distortion.

A distorted air stream entering an aeroengine fan or compressor leads to harmonic forces on the rotating blades. These aerodynamically induced forces are well-known causes of blade vibration and associated fatigue problems. Significant levels of distortion arise from different sources like side wind and high angles of attack that occur at specific operating conditions. For aircraft with the engine closely integrated with the fuselage, the engine will be exposed to distortion during the entire flight cycle. With a focus on understanding the aeroacoustic interaction the computational fluid dynamics (CFD) analyses used here consider harmonics of the distortion. Harmonic responses are calculated from low to transonic speeds for a range of cases. Major phenomena and driving parameters affecting the forcing strength and pressure amplitudes in the blade passage are identified from the analyses. It is demonstrated that the forcing strength is strongly affected by the cut-on/cutoff conditions upstream and downstream of the blades. Also, depending on design parameters of the blade, the aeroacoustics of the blade passage is important for the resulting forcing. All analyses are made in two-dimensional over a wide range of flow conditions as well as geometric variations. The results of the study provide an increased understanding of the harmonic forcing of blades. A simple model is proposed that can identify conditions where increased pressure amplitudes in the blade passage may be expected. The sensitivities to parameters may also give some guidance in how design and operation can be adapted to reduce the aerodynamic forcing.

The BLI (boundary layer ingestion) concept for propulsion seeks to improve the energy efficiency of aircraft propulsion. This is achieved by accelerating low momentum flow ingested from boundary layers and wakes developed over the fuselage through the fan. A major challenge that needs to be overcome to realise the benefits is that the fan needs to work efficiently in distorted flow. Understanding the effects of distortion on the aerodynamic performance and the distortion transfer through the fan is therefore essential to future designs. A BLI fan, designed at reduced scale, is used for analytic modelling and experiments in a rig designed for this purpose. The test rig replicates BLI conditions for a fan installed at the aircraft tail cone. An unsteady model that includes all blades and vanes of the fan, as well as the nacelle and the by-pass duct of the test rig is used for CFD (computational fluid dynamics) simulations. Test results are used to confirm that the CFD model is representative of the aerodynamics of the fan. The tests are conducted using varying fan operating conditions but also tests with an added distortion screen. Analysis results are then used to investigate the effects of distortion on the fan efficiency, as well as on the overall efficiency. The fan efficiency is found to be moderately decreased depending on the level of and extent of inlet circumferential distortion. In terms of overall energy efficiency, a net improvement over a similar fan in clean inlet flow is found.