The fundamental mechanisms of blade flutter in modern aircraft engines are very complex. Flutter is a self-excited aeroelastic instability phenomenon which can finally cause material fatigue and, in the worst case, leads to blade failure within a very short time. The risk of flutter has to be considered during the design process and it is necessary to avoid that safety risk. The aeroelastic stability has to be ensured over the whole operating range especially near operating limits or typical flutter boundaries, like at stall or choke conditions. Topic of this paper are inlet distortions, which can have an additional influence on the flutter stability of the fan and the first compressor stages of jet engines. For this purpose a sinusoidal steady total pressure inlet distortion was defined. The influence of this inlet distortion on the flow field and the flutter stability of a highly loaded transonic fan rotor (NASA rotor 67) is investigated. The static deflection of the manufactured blade was considered using an accurate mesh morphing algorithm to update the fan performance characteristic considering the deformed blade structure. The fan rotor interacts with the upstream distorted flow which leads to different blade loading between the adjacent blades. A decoupled flutter stability analysis using the three-dimensional viscous flow solver TBLOCK and the open-source software package CalculiX for pre-stressed modal analyses is carried out. The flutter stability analyses with TBLOCK are performed using the so-called energy method which was introduced by Carta. In order to predict the flutter stability under clean inflow conditions, two different formulations, the Influence Coefficient Method (ICM) and Traveling Wave Mode (TWM) formulation, are taken into account, whereas both formulations are compared to each other. The influence coefficients were directly calculated from the TWM formulation to determine the required number of passages for the ICM. It can be seen that the stability curves obtained with the ICM are in a good agreement to the TWM-method. The use of ICM reduces substantially the number of unsteady CFD calculations because of the fact that only one unsteady CFD calculation is needed to reconstruct the stability curve for each eigenmode and operating point. The effect of inlet distortion on flutter stability is investigated applying the TWM formulation only. Indeed, it was established that such flow disturbances have also for specific blades, considering the operating point, eigenmode and nodal diameter a destabilising impact on their aeroelastic behavior and can cause flutter, which is mostly determined by the time-averaged stability parameter. Just in the same manner a positive effect was observed for certain blades in the blade row.

A major technical challenge for modern aero engines is the development of designs which reduce noise and emission whilst increasing aerodynamic efficiency and ensuring aeroelastic stability of low-temperature engine components such as fans and low-pressure compressors. Composites are used in aviation due to their excellent stiffness and strength properties, which also enable additional flexibility in the design process. The weight reduction of the turbomachine components, due to composite materials and lighter engines, is especially relevant for the design and developments of hybrid-electric or distributed propulsion systems [1]. To accomplish this, a representative volume element (RVE) of a glass-fiber reinforced polymer is created, describing the geometrical arrangement of the textile reinforcement structure within the polymer matrix. For both phases, realistic linear elastic properties are assumed. This RVE will be investigated with the finite element method under various loading conditions to assess its anisotropic elastic properties and also its damping behaviour for elastic waves. To study the influence of delamination on the mechanical properties, small defects will be introduced into the model at the interface between reinforcement and matrix. Based on this micromechanical approach, a constitutive model for the composite will be formulated that describes the anisotropic properties as well as the damping behaviour. This constitutive model is then used to describe the material response in a macro-mechanical model, which serves as the basis for an aeroelastic analysis of a 1/3-scaled high-speed fan using a conventional (Ti-6Al-4V) and fiber composite material.

Circumferential grooves in the casing of an axial compressor rotor or fan are known to be beneficial by extending the operating range of the machine. The goal of this paper is to analyze, if such grooves have a significant effect on the flutter stability, too. Generally, flutter should always be avoided as these self-excited blade vibrations can lead to high-cycle fatigue and therefore may damage the blades. In the present paper, the flutter behavior of a nominal fan is analyzed by performing a unidirectional Fluid-Structure-Interaction (FSI) simulation. To model the traveling wave arising during flutter, three different possibilities are available for computational fluid dynamics (CFD): the traveling wave mode method (TWM), the Fourier transformation method (FT) and the influence coefficient method (INFC). The TWM and INFC will be used within this investigation. At first, the computed flutter stability of the commercial CFD solver ANSYS CFX is compared to the results of the academic CFD solver TBLOCK. Therefore, a MATLAB code is introduced to be able to use the very efficient INFC method in combination with ANSYS CFX. The main part of this paper deals with the examination of three different circumferential grooves. Two of them had been optimized regarding aerodynamics and aeroacoustics in a joint research project and produce a minor change in flutter behavior. The third groove is of an arbitrary chosen design and it is discussed how its axial position has an impact on the vibration characteristic of the fan. All CFD simulations are conducted for two different operating points at 100% speed and the first two eigenmodes of the fan blade.

Aircraft propulsion will continue to rely on gas turbine technology for the next decades to come. Thus, to achieve environmental agreements, ensure engine safety and retain economic competitiveness, ongoing development with a multidisciplinary design approach is indispensable. In the present study the multi criteria analysis of the fan, a decisive component in modern aero engines, is examined. In particular the interaction of the fan blades with the fan casing is analyzed and an appropriate design approach including automatic optimization is used. As one part of the disciplines conjunction an automated aeroacoustic approach is realized. The aerodynamic and acoustic fitness functions and constraints are based on Reynolds-Averaged Navier-Stokes (RANS) simulations of the fan stage. A fast analytical prediction tool for fan noise, PropNoise, which has been under development since recent years and already validated on several test cases, is used. Preliminary studies have shown that the flow in the rotor tip region is a major contributor to the broadband noise emission. Based on this, the optimization process focuses on the variation of the casing contour around the fan blades. The impact of the modified flow field in the rotor tip region concerning the aeroelastic behavior is also investigated. As aeroelastic evaluation requires a high level of know-how and is very time consuming, it is linked to the optimization process chain by a discrete evaluation of selected members. This allows a simultaneous adjustment of the design in case of aeroelastic issues. Furthermore, the impact of the fan modifications regarding the overall engine performance is evaluated. Offdesign cycle calculations allow incorporating such detailed studies in a global engine optimization.

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