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    Item type:Publication, Access status: with files ,
    Towards a network perspective on the nonlinear dynamics of mechanical structures
    (2026)
    Geier, Charlotte
    ;
    Hoffmann, Norbert
    The nonlinear dynamics of mechanical structures present significant challenges in engineering today, particularly as these structures become more intricate and environmental conditions grow increasingly unpredictable. In this work, different perspectives on the dynamics of mechanical structures are explored and compared. The balance between interpretability and generalizability is studied in the context of classical, physics-based approaches, data-driven methods and new network-based techniques. Network science offers novel views by interpreting mechanical systems as networks composed of nodes and edges. This thesis focusses on the applicability and potential of network-based methods for analyzing mechanical structures.
    Publicationtype: Doctoral Thesis
      70  45
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    Item type:Publication, Access status: with files ,
    Ice-induced vibrations of vertical structures
    (2021-12-09)
    Ziemer, Gesa
    ;
    Hoffmann, Norbert
    Vertical structures exposed to drifting level ice can exhibit significant ice-induced vibrations. However, their fundamental physical process is not sufficiently understood to date. Therefore, novel model tests in ice have been conducted utilizing local pressure mapping techniques to assess the influence of local failure processes on the occurrence of ice-induced vibrations. Analysis of the acquired data reveals a significant impact of the relative velocity between ice and structure on the local failure and contact development. Based on this finding, a prediction method and a scaling approach are developed.
    Publicationtype: Doctoral Thesis
      195  304
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    Item type:Publication, Access status: with files ,
    Data-driven techniques for the nonlinear dynamics of mechanical structures
    (2020-11)
    Stender, Merten
    ;
    Hoffmann, Norbert
    In structural vibrations, several modeling approaches have helped to develop better, i.e. safer, less-vibrating, and more controllable designs for machines and structures. However, complex nonlinear vibrations of multi-physics, multi-component, and multi-scale systems still represent challenges to today’s identification and vibration prediction approaches. Particularly, damping and friction can play a crucial role for vibration mitigation while being inherently difficult to characterize, quantify, or even approximate. At the same time, the data sciences have become omnipresent not only in different fields of science but also in society. This thesis introduces a rigorous framework and discusses chances and limitations for using machine learning in complex structural dynamics. Special focus is put on the physical peculiarities of the vibration signals and physics-informed learning. In the context of case studies, new methodologies are presented for several systems ranging from single-degree-of-freedom oscillators to complete automotive disk brake systems.
    Publicationtype: Doctoral Thesis
      1099  3177
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    Inducing predefined nonlinear rogue waves on basis of breather solutions
    (2019)
    Witt, Andy
    ;
    Hoffmann, Norbert
    The Peregrine Breather, an analytical solution of the Nonlinear Schroedinger Equation, is modified to induce locally and temporally accurate, predefined extreme wave events in an arbitrary directed sea state. To this end, a parameter study of the distortion term of the unstable Peregrine modulation is performed to enforce form, height, and steepness of the rogue wave. In addition, the phase modulation is identified to be the crucial perturbation for the nonlinear Breather dynamic. With this knowledge, the inducing of a predefined rogue wave in non-uniform carrier waves and even in irregular, directed sea states is presented, its limits determined, and the Breather dynamics analyzed by spectral and phase evolution analysis in time and space. The results are compared to the dynamics of real occurred extreme waves and a reverse engineering as well as a forecast model for nonlinear real rogue waves are depicted briefly.
    Publicationtype: Doctoral Thesis
      596  550
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    Fluid-structure interaction of flexible lifting bodies with multi-body dynamics of order-reduced models and the actuator-line method
    (epubli, 2016)
    Neumann, Sönke
    ;
    Hoffmann, Norbert
    The development of complex energy converters generates currently unresolved demands on simulation tools. In this thesis, a model-based method to represent fluid-structure interaction is developed using the example of vertical-axis turbines. The kinematics of a turbine are simulated with a multi-body approach supporting flexible bodies. The dynamics of the flexible bodies are based on order-reduced models. The fluid domain is represented by Reynolds-averaged Navier-Stokes equations combined with an actuator-line method, a reduced model of the blade approximated by external forces in the momentum equations. Coupling of mechanical and fluid solver is achieved by a two-way iterative approach. The convergence rate is increased by usage of a step prediction based on quasi-Newton methods. An acceleration using a proper orthogonal decomposition to decompose the system dynamics achieves the fastest convergence rates. As an alternative to the partitioned approach a monolithic approach is outlined and its restrictions by using the selected solvers are identified. The simulation method is validated with experimental data and applied to vertical-axis turbines, both in air and water domains. Extensive validation of uncoupled flow problems and coupled dynamic response of turbines is accomplished. The results are in good agreement with experimental data and analytic solutions. Comparison to potential flow and streamtube methods shows the enhanced accuracy of the developed tool chain. Compared to available, fully meshed solutions, the computational costs are significantly lower. Conclusively, a vertical-axis tidal turbine with flexible blades is simulated and deflections are compared to a linear solution. The local blade deflections are examined and linked to wake-body interaction.
    Publicationtype: Doctoral Thesis
      681  1604
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    Time and frequency optimal motion control of CNC machine tools
    (2013)
    Alkafafi, Loay
    ;
    Hoffmann, Norbert
    This dissertation introduces a new approach to design time and frequency optimal motion profiles for multi-axis CNC machine tools moving along given contours. The proposed approach modifies time optimal motion profiles and uses the concept of time-scaling to locally reshape the resulting profiles in order to achieve the frequency attenuation. The reshaping process targets the path trajectories and only the dynamics of the path trajectories, i.e. the velocity, acceleration and jerk. Simulation and experimental studies showed that the new motion profiles are capable of driving the machine with any practically relevant speed and accuracy.
    Publicationtype: Doctoral Thesis
      845  1725
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    Friction interface mechanics and self-induced vibrations
    (2013)
    Wernitz, Boris
    ;
    Hoffmann, Norbert
    Vibrations in braking systems have been studied since the beginning of the last century and despite several insights, still many phenomena, particularly in the area of friction induced vibrations, are not fully understood. The objective of the actual study was the identification of the complex dynamics in the friction interface of a dry friction brake system. In this context, particular consideration was given to the generation of instabilities and brake squeal. In work presently being undertaken, irregular vibration data of a dry friction brake in non-squealing condition were collected with sampling rates above 200 kHz. The resulting time series were subjected to linear, nonlinear and statistical analyses and in particular with regard to the involved multi-scale dynamics. At first, topological examination of the friction surfaces disclosed the microscale characteristics; visual inspections of the coating, generated on the brake disk insights in to the friction interface transformations. Secondly, linear investigations of the vibration data in the frequency domain provided first impressions of the friction interface: while the spectral frequency distribution remains mostly independent from mechanic parameters, it is essentially controlled by the pad formulation and physical and chemical interface transformations during the experiment. Thirdly, the data were explored on the basis of recurrence analysis and together with the estimation of dimensionality the phase space was reconstructed. Evidently, the irregular vibration states of friction brakes in non-squealing condition are strongly controlled both by apparently steady and disruptive phenomena (intermittency). The duration between the states was typically in the range of milliseconds. Phase space reconstruction and largest Lyapunov coefficient estimation indicated thereby that these phenomena are dominated more by low-dimensional chaotic deterministic dynamics than by high-dimensional stochastic processes. In the following, characteristic measures of the recurrence quantification analysis have been extracted which disclosed an interrelation with the noise propensity of the overall brake system. Finally, statistical studies of the distribution of vibration increments on the basis of probability density functions concluded the evaluation of the experiments. The distribution analyses disclosed the non-Gaussian characteristic of the vibration signals under steady sliding whereby the deviation from the normal distribution depends on the corresponding scale. Furthermore, the vibration during sliding in non-squealing condition is effectively generated by dynamics on different scales. And beyond that, the difference between the measured and the Gaussian distribution suggests a correlation between the squeal propensity and the deviation under consideration.
    Publicationtype: Doctoral Thesis
      822  1247