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 Application of neural networks and transfer learning to turbomachinery heat transfer(2022) Baumann, Markus; Koch, Christian; Staudacher, StephanModel-based predictive maintenance using high-frequency in-flight data requires digital twins that can model the dynamics of their physical twin with high precision. The models of the twins need to be fast and dynamically updatable. Machine learning offers the possibility to address these challenges in modeling the transient performance of aero engines. During transient operation, heat transferred between the engine’s structure and the annulus flow plays an important role. Diabatic performance modeling is demonstrated using non-dimensional transient heat transfer maps and transfer learning to extend turbomachinery transient modeling. The general form of such a map for a simple system similar to a pipe is reproduced by a Multilayer Perceptron neural network. It is trained using data from a finite element simulation. In a next step, the network is transferred using measurements to model the thermal transients of an aero engine. Only a limited number of parameters measured during selected transient maneuvers is needed to generate suitable non-dimensional transient heat transfer maps. With these additional steps, the extended performance model matches the engine thermal transients well.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 Explanation of the self-adaptive dynamics of a harmonically forced beam with a sliding mass(2020) Müller, Florian; Krack, MalteThe self-adaptive behavior of a clamped-clamped beam with an attached slider has been experimentally demonstrated by several research groups. In a wide range of excitation frequencies, the system shows its signature move: The slider first slowly moves away from the beam’s center, at a certain point the vibrations jump to a high level, then the slider slowly moves back toward the center and stops at some point, while the system further increases its high vibration level. In our previous work, we explained the unexpected movement of the slider away from the beam’s vibration antinode at the center by the unilateral and frictional contact interactions permitted via a small clearance between slider and beam. However, this model did not predict the signature move correctly. In simulations, the vibration level did not increase significantly and the slider did not turn around. In the present work, we explain, for the first time, the complete signature move. We show that the timescales of vibration and slider movement along the beam are well separated, such that the adaptive system closely follows the periodic vibration response obtained for axially fixed slider. We demonstrate that the beam’s geometric stiffening nonlinearity, which we neglected in our previous work, is of utmost importance for the vibration levels encountered in the experiments. This stiffening nonlinearity leads to coexisting periodic vibration responses and to a turning point bifurcation with respect to the slider position. We associate the experimentally observed jump phenomenon to this turning point and explain why the slider moves back toward the center and stops at some point.Item Open Access Uncertainty quantification for full-flight data based engine fault detection with neural networks(2022) Weiss, Matthias; Staudacher, Stephan; Mathes, Jürgen; Becchio, Duilio; Keller, ChristianCurrent state-of-the-art engine condition monitoring is based on a minimum of one steady-state data point per flight. Due to the scarcity of available data points, there are difficulties distinguishing between random scatter and an underlying fault introducing a detection latency of several flights. Today’s increased availability of data acquisition hardware in modern aircraft provides continuously sampled in-flight measurements, so-called full-flight data. These full-flight data give access to sufficient data points to detect faults within a single flight, significantly improving the availability and safety of aircraft. Artificial neural networks are considered well suited for the timely analysis of an extensive amount of incoming data. This article proposes uncertainty quantification for artificial neural networks, leading to more reliable and robust fault detection. An existing approach for approximating the aleatoric uncertainty was extended by an Out-of-Distribution Detection in order to take the epistemic uncertainty into account. The method was statistically evaluated, and a grid search was performed to evaluate optimal parameter combinations maximizing the true positive detection rates. All test cases were derived based on in-flight measurements of a commercially operated regional jet. Especially when requiring low false positive detection rates, the true positive detections could be improved 2.8 times while improving response times by approximately 6.9 compared to methods only accounting for the aleatoric uncertainty.Item Open Access Prediction of compressor blade erosion experiments in a cascade based on flat plate specimen(2022) Lorenz, Max; Klein, Markus; Hartmann, Jan; Koch, Christian; Staudacher, StephanErosion is an essential deterioration mechanism in compressors of jet engines. Erosion damage predictions require the determination of erosion rates through flat plate experiments. The applicability of the erosion rates is limited to conditions that are comparable to the prevailing boundary conditions of the flat plate experiment. A performed dimensional analysis enables the correct transfer of the flat plate erosion rates to the presented physical calculation model through limits in spatial and time resolution. This efficient approach avoids computationally intensive single-impact computations. The approach features a re-meshing procedure that adheres to the limits derived by the dimensional analysis. The computation approach is capable of describing local geometry changes on cascade compressor blades which are exposed to erosive particles. A linear erosion cascade experiment performed on NASA Rotor 37 provides validation data for the calculated erosion-induced shape change. Arizona Road Dust particles are used to deteriorate Ti-Al6-4V compressor blades. The experiment is performed at an incidence of i = 7°and Ma = 0.76 representing ground idle conditions. The presented parametric study for element size and time step revealed preferable values for the presented computation. Calculations performed with the determined values showed that the erosion prediction is within the measurement tolerance of the experiment and, therefore, high accordance between the computation and the experiment is achieved. To extend the current state of the art, it is demonstrated that the derived discretization is decisive for the correct reproduction of the eroded geometries and fitting parameters are no longer needed. The good agreement between the experimental measurements and the calculated results confirms the correct application of the physical model to the phenomenology of erosion. Thus, the presented physical model offers a novel approach to adapting deterioration mechanisms caused by erosion to any compressor blade geometry.Item Open Access Steady-state fault detection with full-flight data(2022) Weiss, Matthias; Staudacher, Stephan; Becchio, Duilio; Keller, Christian; Mathes, JürgenAircraft engine condition monitoring is a key technology for increasing safety and reducing maintenance expenses. Current engine condition monitoring approaches use a minimum of one steady-state snapshot per flight. Whilst being appropriate for trending gradual engine deterioration, snapshots result in a detrimental latency in fault detection. The increased availability of non-mandatory data acquisition hardware in modern airplanes provides so-called full-flight data sampled continuously during flight. These datasets enable the detection of engine faults within one flight by deriving a statistically relevant set of steady-state data points, thus, allowing the application of machine-learning approaches. It is shown that low-pass filtering before steady-state detection significantly increases the success rate in detecting steady-state data points. The application of Principal Component Analysis halves the number of relevant dimensions and provides a coordinate system of principal components retaining most of the variance. Consequently, clusters of data points with and without engine fault can be separated visually and numerically using a One-Class Support Vector Machine. High detection rates are demonstrated for various component faults and even for a minimum instrumentation suite using synthesized datasets derived from full-flight data of commercially operated flights. In addition to the tests conducted with synthesized data, the algorithm is verified based on operational in-flight measurements providing a proof-of-concept. Consequently, the availability of continuously sampled in-flight measurements combined with machine-learning methods allows fault detection within a single flight.Item Open Access Experimental and numerical investigation into the effect of surface roughness on particle rebound(2022) Altmeppen, Johannes; Sommerfeld, Heike; Koch, Christian; Staudacher, StephanErosion damage and particle deposition are crucial wear phenomena in gas turbine engines. As a result, compressor efficiency decreases, stability margin reduces, and maintenance cost increases. Hence, predicting these phenomena in an accurate manner is of paramount importance for a cost-efficient, safe, and sustainable operation. Erosion and particle deposition in the annulus are affected by particle transportation in the fluid and particle-wall interaction. The latter involves the particle impact, the potential damage of the surface and/or the particle, and the particle rebound. Particle rebounds are statistical in nature due to the target surface roughness, the variability in particle sizes, and superimposed effects caused by particle shapes as well as particle rotation and particle break-up during contact. Multiple studies investigated the statistics of particle rebound, providing empirical-based models for median and spread. However, modeling the particle-wall interaction and its data spread on a transparent physical basis allows separating the effect of target roughness from superimposed effects. The presented article pursues this objective by assessing the statistical spread of particle rebound data through multiple techniques and utilizing their interdependencies. It combines experimental, numerical, and analytical considerations. For the first time, coherent boundary conditions for the experimental, numerical and analytical setup allow the distinction of the effect of roughness from the integral effect of the superimposed phenomena. A sandblast test rig equipped with laser measurement equipment was used to measure particle rebound from flat titanium and stainless steel plates at different angles. The numerical setup is developed under OpenFOAM 6 using a RANS solver for transient simulations with compressible media in combination with one-way coupled particle flows. The numerical model includes the rebound spread model proposed by Altmeppen et al. combined with the quasi-analytical rebound model proposed by Bons et al. The statistical spread of particle rebound is investigated for roughness levels that are similar to the ones of deteriorated high-pressure compressor blades as discussed by Gilge et al. The measured surface roughness of the experimentally investigated targets is used as input parameters to the numerical framework. The rebound statistics obtained in the numerical simulation are compared to the rebound data measured in the experiment. Based on this study, conclusions are drawn about which part of the rebound spread is attributable to surface roughness and which is caused by superimposed effects. It was found that the effect of surface roughness as characterized by Altmeppen et al. contributes in the order of 46 % to the rebound spread for small impact angles. However, the share in spread due to roughness gradually decreases with increasing global impact angles to a level of 13 % for angles close to 90°. The remaining percentage of rebound spread is attributed to superimposing phenomena. In addition to the rolling and sliding of aspherical particles, further phenomena such as plastic deformation and erosion of the roughness peaks during contact and the associated dissipation of energy gain in importance for steeper impact angles. Therefore, the effect of surface roughness should not be neglected in numerical simulations of particle-laden flows. Modeling the superimposed phenomena which are observed to be dominating at high impact angles opens up a further field of research.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 Preliminary design and analysis of supersonic business jet engines(2022) Schlette, Timo; Staudacher, StephanCurrently projected supersonic business jets target selected supersonic flight missions with Mach numbers of about 1.4 and a larger number of long-range subsonic flight missions. They form a new type of aircraft that is specially tailored to these requirements. The question arises as to which engine configurations and technology levels are required to support these new applications. This is addressed firstly by exploring the design space of potential working cycles. An aircraft model is used to translate the results of the cycle study into an expected aircraft range. An optimal core engine and fan configuration result from the cycle study and the derived mission ranges. The preliminary design of the low-pressure components is investigated in the second step based on the optimal core configuration. The highest non-dimensional parameters are encountered in subsonic flight conditions. The highest dimensional parameters are encountered in supersonic high-altitude flight conditions. High-overall-efficiency configurations do not result in optimal aircraft ranges. There is an optimal number of two fan stages and a specific thrust of about 300 m/s, resulting in a maximum aircraft range that is 11% superior to that achievable with a single-stage fan. A fan hub-to-tip ratio range that is comparable to that of military fans is desirable, with an aerodynamic lower limit around 0.37. The low-pressure turbine stage count is a compromise between turbine mass and size.Item Open Access Analysis of the non-periodic oscillations of a self-excited friction-damped system with closely spaced modes(2021) Woiwode, Lukas; Vakakis, Alexander F.; Krack, MalteIt is widely known that dry friction damping can bound the self-excited vibrations induced by negative damping. The vibrations typically take the form of (periodic) limit cycle oscillations. However, when the intensity of the self-excitation reaches a condition of maximum friction damping, the limit cycle loses stability via a fold bifurcation. The behavior may become even more complicated in the presence of any internal resonance conditions. In this work, we consider a two-degree-of-freedom system with an elastic dry friction element (Jenkins element) having closely spaced natural frequencies. The symmetric in-phase motion is subjected to self-excitation by negative (viscous) damping, while the symmetric out-of-phase motion is positively damped. In a previous work, we showed that the limit cycle loses stability via a secondary Hopf bifurcation, giving rise to quasi-periodic oscillations. A further increase in the self-excitation intensity may lead to chaos and finally divergence, long before reaching the fold bifurcation point of the limit cycle. In this work, we use the method of complexification-averaging to obtain the slow flow in the neighborhood of the limit cycle. This way, we show that chaos is reached via a cascade of period-doubling bifurcations on invariant tori. Using perturbation calculus, we establish analytical conditions for the emergence of the secondary Hopf bifurcation and approximate analytically its location. In particular, we show that non-periodic oscillations are the typical case for prominent nonlinearity, mild coupling (controlling the proximity of the modes), and sufficiently light damping. The range of validity of the analytical results presented herein is thoroughly assessed numerically. To the authors’ knowledge, this is the first work that shows how the challenging Jenkins element can be treated formally within a consistent perturbation approach in order to derive closed-form analytical results for limit cycles and their bifurcations.