06 Fakultät Luft- und Raumfahrttechnik und Geodäsie
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/7
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Item Open Access Coupled simulation of turbomachinery flutter and forced response blade vibrations using nonlinear frequency domain methods(2024) Berthold, Christian; Krack, Malte (Prof. Dr.)The central topic of this work is the simulation of nonlinear blade vibrations in turbomachinery. Two main causes of blade vibrations are flutter, denoting self-excited vibrations of the blades, and forced response due to e.g. aerodynamic rotor-stator interactions. During operation, the vibration levels of the blades must not exceed critical values in order to prevent high cycle fatigue or immediate failure of the engine. This motivates the development of numerical methods for the prediction of blade vibrations in order to evaluate the robustness of mechanical designs against flutter and forced response. In this work, the focus is laid on bladed turbine disks with interlocked shrouds, which represent a challenging task for numerical simulation. While interlocked shrouds introduce friction (and thus damping) into the structural system, possibly reducing the level of vibrations, they can alter the vibration shape and vibration frequency with increasing amplitude. This in turn makes the aerodynamic damping of the blade motion a nonlinear function of the vibration amplitude. Thus, the mechanical system is bidirectionally coupled, since the two physical domains (fluid and solid) interact with each other. Current numerical analysis tools like the energy method or the use of influence coefficients have deficits in resolving these nonlinear fluid-structure interactions. This motivates the development of improved numerical methods for the simulation of nonlinear blade vibrations. In this work, a refined energy method and a bidirectionally coupled fluid-structure solver are suggested for this purpose. For both approaches, the Harmonic Balance method is employed, which approximates a periodic motion of the blades very efficiently in the frequency domain. The novel methods are applied to numerical test cases of low pressure turbines to demonstrate the methods' capabilities and to investigate the potential influence of nonlinear contact forces on the blade vibrations. Here, the refined energy method allows to gain valuable insight on the impact of shroud contact interfaces on the aerodynamic damping. It is found, that the nonlinear structural contact forces can give rise to stable limit cycle oscillations as well as stability limits, which mark the amplitude level where blade vibrations become unstable if it is exceeded. Furthermore, the coupled solver reveals the complex interaction between a vibrating blade with shroud contact interfaces and a shock motion. For the analysis of forced response, the coupled solver is embedded into a path continuation procedure with a sequential and a parallel variant. The coupled method not only demonstrates the influence of nonlinear friction on the forced response but also reveals, that the superposition assumption regarding the aerodynamic wake excitation and the blade vibration induced aerodynamic forces can lead to inaccurate results.Item Open Access A model to assess the importance of runway and taxiway particles to aircraft engine compressor deterioration(2024) Scarso, Stefano; Staudacher, Stephan; Mathes, Jürgen; Schwarz, NormanDuring service, civil turbofans experience environmentally induced deterioration. Predicting this in a digital service twin model is computationally challenging due to the need to model both deterioration mechanisms and environmental conditions. For compressor erosion, a key challenge is to model particle ingestion throughout a flight mission (FM). During ground operations, these particles may be airborne or deposited on runways and taxiways. This work assesses the impact of the latter on turbofan core compressor deterioration during a mission. The airflow field in front of the engine intake is approximated using potential flow theory. Comparisons with measurements show that the predicted air velocity near the engine is underestimated since the inlet ground vortices generated from viscous effects are neglected. The forces acting on the particles are derived from the flow field. It turns out that most particles are lifted from the ground during take-off (TO). Yet only smaller particles below ≈50 µm are ingested into the engine intake. A deterioration model based on flat plate erosion experiments is used to compute mission severity, assuming all particles are similar to medium Arizona Road Dust. The results indicate that the engine’s distance from the ground, power setting, and the number of particles on the ground are key parameters influencing the impact of runway and taxiway particles. Considering the underestimation of the airflow field and thus the number of particles ingested, it is concluded that runway and taxiway particles play a major role in turbofan compressor deterioration.Item Open Access Numerical prediction of frictional vibro-impacts : combining massless boundaries and component mode synthesis(2025) Monjaraz Tec, Carlo Daniel; Krack, Malte (Prof. Dr.-Ing.)Vibro-impact processes are a subset of nonlinear vibrations. They involve the essential interaction of instant contacts and vibrations. Their correct prediction and experimental characterization, using dedicated simulation and testing methods, enlarge the design space and robustness of technologies such as Impact Energy Scatterers. Numerical predictions of vibro-impact have contradicting requirements. A fine spatial and time discretization is needed; however, this yields large model problem sizes which impede long-term simulation. Frequency domain solutions are not applicable for non-periodic states, which are likely to be present in these vibrations. Furthermore, the existing time-domain methods rely on empirical parameters for contact modeling which reduces the predictive nature of the simulation. This thesis aims to develop and validate a time-domain simulation method for frictional vibro-impact, which is useful for industrial applications. The novel core concept of the method is to unite the concept of massless boundary (originating from computational mechanics) and the idea of component mode synthesis (standard in structural dynamics) by exploiting the key advantages of each. The massless boundaries allow a quasi-static solution of the contact forces, leading to stable contact enforcement, and are numerically more robust than conventional approaches. Until now, massless boundaries have been implemented in finite element models, but such models are still too large, and thus unsuitable to simulate long steady-states. Therefore, model-order reduction with component mode synthesis is quintessential. The developed method is based on four key components: an underlying finite element model, a time integration scheme compatible with massless boundaries, a solution for dynamic normal and frictional contact enforcement, and a component mode synthesis method compatible with massless boundaries. The semi-explicit time integration scheme is developed to work with singular mass matrices, resulting in a scheme that solves contacts quasi-statically and prioritizes energy conservation. Normal and frictional contacts are modeled as set-valued laws and imposed locally within the spatially resolved contact domain. The finite element model is reduced using the MacNeal method and a mass-boundary compatible variant of the Craig-Bampton method derived in this work. This framework limits the method to linear elasticity and kinematics, while addressing nonsmooth (and therefore nonlinear) contact behavior. Numerical benchmarks are used to evaluate the method. The kinematics, energy conservation, and computational cost of the conventional mass-carrying models and the proposed method are compared. A variant of the Moreau time integration scheme and the harmonic balance method in conjunction with a dynamic Lagrangian formulation are considered. The results show that the massless boundary models have better energy conservation and convergence properties while reducing the computational time by at least one order of magnitude. As a first validation step, experimental measurements are used to evaluate the predicted post-impact velocity response and its modal energy distribution. A metal sphere impacts a steel beam, where the velocity response of the beam is measured for a single collision. The measurements are compared with predictions obtained using two different approaches: state-of-the-art finite element analysis and the proposed method. The proposed method reduces the numerical effort by 3-4 orders of magnitude compared to the finite element model without compromising the excellent agreement with the measurements. Finally, for a second validation step, two cantilever beams subjected to frictional impacts at the free end are measured experimentally. The beams have similar geometries and close but unequal natural frequencies. The underlying linear model is updated based on the natural frequencies and damping ratios identified in the non-impact regime. The nonlinear simulation of the steady-state response to forward and backward stepped-sine excitation is compared with measurements. The results are in excellent agreement with respect to amplitude response, frequency content, and contact activity, especially considering the uncertainty associated with the observed material loss in the contact region and the nonlinear behavior of the clamping.Item Open Access Parametric study for model calibration of a friction-damped turbine blade with multiple test data(2024) Ferhatoglu, Erhan; Botto, Daniele; Zucca, StefanoModel updating using multiple test data is usually a challenging task for frictional structures. The difficulty arises from the limitations of nonlinear models which often overlook the uncertainties inherent in contact interfaces and in actual test conditions. In this paper, we present a parametric study for the model calibration process of a friction-damped turbine blade, addressing the experimentally measured response variability in computational simulations. On the experimental side, a recently developed test setup imitating a turbomachinery application with mid-span dampers is used. This setup allows measuring multiple responses and contact forces under nominally identical macroscale conditions. On the computational side, the same system is modeled in a commercial finite element software, and nonlinear vibration analyses are performed with a specifically developed in-house code. In numerical simulations, the multivalued nature of Coulomb’s law, which stems from the inherent variability range of static friction forces in permanently sticking contacts, is considered to be the main uncertainty. As the system undergoes vibration, this uncertainty propagates into the dynamic behavior, particularly under conditions of partial slip in contacts, thus resulting in response variability. A deterministic approach based on an optimization algorithm is pursued to predict the limits of the variability range. The model is iteratively calibrated to investigate the sensitivity of response limits to contact parameters and assembly misalignment. Through several iterations, we demonstrate how uncertain initial contact conditions can be numerically incorporated into dynamic analyses of friction-damped turbine blades. The results show a satisfactory level of accuracy between experiments and computational simulations. This work offers valuable insights for understanding what influences test rig response and provides practical solutions for numerical simulations to improve agreement with experimental results.