Imperial College London

Professor Norbert Hoffmann

Faculty of EngineeringDepartment of Mechanical Engineering

Visiting Professor
 
 
 
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Contact

 

n.hoffmann

 
 
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Location

 

557City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

171 results found

Yang Z, Pan J, Chen J, Zi Y, Oberst S, Schwingshackl CW, Hoffmann Net al., 2021, A novel unknown-input and single-output approach to extract vibration patterns via a roving continuous random excitation., ISA Trans

Operating deflection shape analysis allows investigating the dynamic behaviour of a structure during operation. It normally requires simultaneous, multi-point measurements to capture the response from an unknown excitation source (unknown-input and multiple-output), which can complicate its usage for structures without ease of access. A novel vibration pattern testing method is proposed based on a roving continuous random excitation employing a small robotic Hexbug device and a single-point measurement. The Hexbug introduces a random excitation in consecutive locations while roaming over the structure. The resulting multi-modal, time and location dependent response of the system is captured in a single location, and then analysed with a newly developed method based on empirical wavelet transform, multiscale morphological filtering and optimization to extract the excited vibration patterns. The efficiency of the proposed method is experimentally demonstrated on a free-free and a cantilevered beam with comparison to mode shapes extracted by hammer test. The validation highlights its ability to extract several vibration patterns from a long slender structure with good accuracy and robustness, with the general ability to expand the usability of an operating deflecting shape analysis.

Journal article

Kimmoun O, Hsu H-C, Hoffmann N, Chabchoub Aet al., 2021, Experiments on uni-directional and nonlinear wave group shoaling, OCEAN DYNAMICS, Vol: 71, Pages: 1105-1112, ISSN: 1616-7341

Journal article

Stender M, Hoffmann N, 2021, bSTAB: an open-source software for computing the basin stability of multi-stable dynamical systems, NONLINEAR DYNAMICS, Vol: 107, Pages: 1451-1468, ISSN: 0924-090X

Journal article

Chabchoub A, Slunyaev A, Hoffmann N, Dias F, Kibler B, Genty G, Dudley JM, Akhmediev Net al., 2021, The Peregrine Breather on the Zero-Background Limit as the Two-Soliton Degenerate Solution: An Experimental Study, FRONTIERS IN PHYSICS, Vol: 9, ISSN: 2296-424X

Journal article

Neidhardt M, Ohlsen J, Hoffmann N, Schlaefer Aet al., 2021, Parameter Identification for Ultrasound Shear Wave Elastography Simulation, Current Directions in Biomedical Engineering, Vol: 7

Elasticity of soft tissue is a valuable information to physicians in treatment and diagnosis of diseases. The elastic properties of tissue can be estimated with ultrasound (US) shear wave imaging (SWEI). In US-SWEI, a force push is applied inside the tissue and the resulting shear wave is detected by high-frequency imaging. The properties of the wave such as the shear wave velocity can be mapped to tissue elasticity. Commonly, wave features are extracted by tracking the peak of the shear wave, estimating the phase velocity or with machine learning methods. To tune and test these methods, often simulation data is employed since material properties and excitation can be accurately controlled. Subsequent validation on real US-SWEI data is in many cases performed on tissue phantoms such as gelatine. Clearly, validation performance of these procedures is dependent on the accuracy of the simulated tissue phantom and a thorough comparison of simulation and experimental data is needed. In this work, we estimate wave parameters from 400 US-SWEI data sets acquired in various homogeneous gelatine phantoms. We tune a linear material model to these parameters. We report an absolute percentage error for the shear wave velocity between simulation and phantom experiment of <2.5%. We validate our material model on unknown gelatine concentrations and estimate the shear wave velocity with an error <3.4% for in-range concentrations indicating that our material model is in good agreement with US-SWEI measurements.

Journal article

Klein M, Clauss GF, Hoffmann N, 2021, Introducing envelope soliton solutions for wave-structure investigations, OCEAN ENGINEERING, Vol: 234, ISSN: 0029-8018

Journal article

Papangelo A, Putignano C, Hoffmann N, 2021, Critical thresholds for mode-coupling instability in viscoelastic sliding contacts, NONLINEAR DYNAMICS, Vol: 104, Pages: 2995-3011, ISSN: 0924-090X

Journal article

Niedergesass B, Papangelo A, Grolet A, Vizzaccaro A, Fontanela F, Salles L, Sievers AJ, Hoffmann Net al., 2021, Experimental observations of nonlinear vibration localization in a cyclic chain of weakly coupled nonlinear oscillators, Journal of Sound and Vibration, Vol: 497, Pages: 1-10, ISSN: 0022-460X

Experimental results on nonlinear vibration localization in a cyclic chain of weakly coupled oscillators with clearance nonlinearity are reported. Numerical modelling and analysis complements the experimental study. A reduced order model is derived and numerical analysis based on the harmonic balance method demonstrates the existence of multiple classes of stable spatially localized nonlinear vibration states. The experiments agree very well with the numerical results. The findings suggest that vibration localization due to fundamentally nonlinear effects may also arise in mechanical structures with relevance in engineering.

Journal article

Fontanela F, Vizzaccaro A, Auvray J, Niedergesäß B, Grolet A, Salles L, Hoffmann Net al., 2021, Nonlinear vibration localisation in a symmetric system of two coupled beams, Nonlinear Dynamics, Vol: 103, Pages: 3417-3428, ISSN: 0924-090X

We report nonlinear vibration localisation in a system of two symmetric weakly coupled nonlinear oscillators. A two degree-of-freedom model with piecewise linear stiffness shows bifurcations to localised solutions. An experimental investigation employing two weakly coupled beams touching against stoppers for large vibration amplitudes confirms the nonlinear localisation.

Journal article

Stender M, Adams C, Wedler M, Grebel A, Hoffmann Net al., 2021, Explainable machine learning determines effects on the sound absorption coefficient measured in the impedance tubea), JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA, Vol: 149, Pages: 1932-1945, ISSN: 0001-4966

Journal article

Stender M, Tiedemann M, Spieler D, Schoepflin D, Hoffmann N, Oberst Set al., 2021, Deep learning for brake squeal: Brake noise detection, characterization and prediction, MECHANICAL SYSTEMS AND SIGNAL PROCESSING, Vol: 149, ISSN: 0888-3270

Journal article

Nitti A, Stender M, Hoffmann N, Papangelo Aet al., 2021, Spatially localized vibrations in a rotor subjected to flutter, NONLINEAR DYNAMICS, Vol: 103, Pages: 309-325, ISSN: 0924-090X

Journal article

Stender M, Wedler M, Hoffmann N, Adams Cet al., 2021, Explainable machine learning: A case study on impedance tube measurements

Machine learning techniques allow for finding hidden patterns and signatures in data. Currently, these methods are gaining increased interest in engineering in general and in vibroacoustics in particular. Although ML methods are successfully applied, it is hardly understood how these black-box-type methods make their decisions. Explainable machine learning aims at overcoming this issue by deepen the understanding on the decision making process through perturbation-based model diagnosis. This paper introduces machine learning methods and reviews recent techniques for explainability and interpretability. These methods are exemplified on sound absorption coefficient spectra of one sound absorbing foam material measured in an impedance tube. Variances of the absorption coefficients measurements as a function of the specimen thickness and the operator are modeled by univariate and multivariate machine learning models. In order to identify the driving patterns, i.e., how and in which frequency regime the measurements are affected by the setup specifications, Shapley additive explanations are derived for the ML models. It is demonstrated how explaining machine learning models can be used to discover and express complicated relations in experimental data, thereby paving the way to novel knowledge discovery strategies in evidence-based modeling.

Conference paper

Lünser H, Hartmann M, Desmars N, Behrendt J, Hoffmann N, Klein Met al., 2021, Influence of sea state parameters on the accuracy of wave simulations of different complexity

The accurate description of the complex genesis and evolution of ocean waves as well as the associated kinematics and dynamics is indispensable for the design of offshore structures and assessment of marine operations. In the majority of cases, the water wave problem is reduced to potential flow theory on a somehow simplified level. However, the non-linear terms in the surface boundary conditions and the fact that they must be fulfilled on the unknown water surface make the boundary value problem considerably complex. On the one hand, the use of complex methods for solving the boundary value problem may give, at the expense of computational time, a very accurate representation of reality. On the other hand, simplified methods are numerically efficient but may only provide sufficient accuracy for a limited range of applications. This paper investigates the influence of different characteristic sea state parameters on the accuracy of irregular wave field simulations (based on a JONSWAP spectrum) by applying the high-order spectral method. Hereby, the underlying Taylor series expansion is truncated at different orders so that numerical simulations of different complexity can be investigated. The wave steepness, spectral-peak enhancement factor as well as directional spreading are systematically varied and truncation at fourth order serves as reference. It is shown that, for specific parameters in terms of wave steepness, enhancement factor and simulation time, the boundary value problem can be significantly reduced while providing sufficient accuracy.

Conference paper

Desmars N, Hartmann M, Behrendt J, Klein M, Hoffmann Net al., 2021, Reconstruction of ocean surfaces from randomly distributed measurements using a grid-based method

In view of deterministic ocean wave prediction, we introduce and investigate a new method to reconstruct ocean surfaces based on randomly distributed wave measurements. Instead of looking for the optimal parameters of a wave model through the minimization of a cost function, our approach directly solves the free surface dynamics - coupled with an interpolation operator - for the quantities of interest (i.e., surface elevation and velocity potential) at grid points that are used to compute the relevant operators. This method allows a high flexibility in terms of desired accuracy and ensures the physical consistency of the solution. Using the linear wave theory and unidirectional wave fields, we validate the applicability of the proposed method. In particular, we show that our grid-based method is able to reach similar accuracy than the wave-model parameterization method at a reasonable cost.

Conference paper

Ohlsen J, Neidhardt M, Schlaefer A, Hoffmann Net al., 2021, Modelling shear wave propagation in soft tissue surrogates using a finite element‐ and finite difference method, PAMM, Vol: 20, ISSN: 1617-7061

Journal article

Stender M, Hoffmann N, Papangelo A, 2020, The Basin Stability of Bi-Stable Friction-Excited Oscillators, LUBRICANTS, Vol: 8

Journal article

Tonazzi D, Passafiume M, Papangelo A, Hoffmann N, Massi Fet al., 2020, Numerical and experimental analysis of the bi-stable state for frictional continuous system, NONLINEAR DYNAMICS, Vol: 102, Pages: 1361-1374, ISSN: 0924-090X

Journal article

Papangelo A, Putignano C, Hoffmann N, 2020, Self-excited vibrations due to viscoelastic interactions, MECHANICAL SYSTEMS AND SIGNAL PROCESSING, Vol: 144, ISSN: 0888-3270

Journal article

Stender M, Jahn M, Hoffmann N, Wallaschek Jet al., 2020, Hyperchaos co-existing with periodic orbits in a frictional oscillator, JOURNAL OF SOUND AND VIBRATION, Vol: 472, ISSN: 0022-460X

Journal article

Hartmann MCN, Polach FVBU, Ehlers S, Hoffmann N, Onorato M, Klein Met al., 2020, Investigation of Nonlinear Wave-Ice Interaction Using Parameter Study and Numerical Simulation, JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING-TRANSACTIONS OF THE ASME, Vol: 142, ISSN: 0892-7219

Journal article

Klein M, Dudek M, Clauss GF, Ehlers S, Behrendt J, Hoffmann N, Onorato Met al., 2020, On the Deterministic Prediction of Water Waves, FLUIDS, Vol: 5

Journal article

Jahn M, Stender M, Tatzko S, Hoffmann N, Grolet A, Wallaschek Jet al., 2020, The extended periodic motion concept for fast limit cycle detection of self-excited systems, COMPUTERS & STRUCTURES, Vol: 227, ISSN: 0045-7949

Journal article

Stender M, Hoffmann N, 2020, Deep learning for predicting brake squeal, International Conference on Noise and Vibration Engineering (ISMA) / International Conference on Uncertainty in Structural Dynamics (USD), Publisher: KATHOLIEKE UNIV LEUVEN, DEPT WERKTUIGKUNDE, Pages: 3327-3337

Conference paper

Wollmann T, Dannemann M, Langkamp A, Modler N, Gude M, Salles L, Hoffmann N, Filippatos Aet al., 2020, Combined experimental-numerical approach for the 3D vibration analysis of rotating composite compressor blades: An introduction

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. As compressor blades are subjected to highly dynamic loads, there is a particular interest in determining their modal properties under operating condition. Furthermore, intensive research is conducted for the development of fibre-reinforced epoxy blades due to the high specific stiffness and strength as well as the high damping of composite materials. Traditional modal analysis techniques are state of the art to determine the vibration behaviour of non-rotating/stationary blades, where some new approaches show the vibration analysis of rotating blades. These approaches for rotating structures have the disadvantage, that either the excitation or the measurement method are influencing the dynamic behaviour of the investigated structure or the method itself cannot be applied for composite materials. Other techniques do not allow a continuous or full-field measurement of the rotating structure. To determine the vibration behaviour of rotating composite compressor blades, a combined experimental-numerical approach is introduced. Therefore, an experimental system for the vibration excitation and a 3-dimensional determination of the vibration behaviour of rotating components are presented. An overview of the main addressed research topics is given.

Conference paper

Wollmann T, Dannemann M, Langkamp A, Modler N, Gude M, Salles L, Hoffmann N, Filippatos Aet al., 2020, Combined experimental-numerical approach for the 3D vibration analysis of rotating composite compressor blades: An introduction

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. As compressor blades are subjected to highly dynamic loads, there is a particular interest in determining their modal properties under operating condition. Furthermore, intensive research is conducted for the development of fibre-reinforced epoxy blades due to the high specific stiffness and strength as well as the high damping of composite materials. Traditional modal analysis techniques are state of the art to determine the vibration behaviour of non-rotating/stationary blades, where some new approaches show the vibration analysis of rotating blades. These approaches for rotating structures have the disadvantage, that either the excitation or the measurement method are influencing the dynamic behaviour of the investigated structure or the method itself cannot be applied for composite materials. Other techniques do not allow a continuous or full-field measurement of the rotating structure. To determine the vibration behaviour of rotating composite compressor blades, a combined experimental-numerical approach is introduced. Therefore, an experimental system for the vibration excitation and a 3-dimensional determination of the vibration behaviour of rotating components are presented. An overview of the main addressed research topics is given.

Conference paper

Wollmann T, Dannemann M, Langkamp A, Modler N, Gude M, Salles L, Hoffmann N, Filippatos Aet al., 2020, Combined experimental-numerical approach for the 3D vibration analysis of rotating composite compressor blades: An introduction

As compressor blades are subjected to highly dynamic loads, there is a particular interest in determining their modal properties under operating condition. Furthermore, intensive research is conducted for the development of fibre-reinforced epoxy blades due to the high specific stiffness and strength as well as the high damping of composite materials. Traditional modal analysis techniques are state of the art to determine the vibration behaviour of non-rotating/stationary blades, where some new approaches show the vibration analysis of rotating blades. These approaches for rotating structures have the disadvantage, that either the excitation or the measurement method are influencing the dynamic behaviour of the investigated structure or the method itself cannot be applied for composite materials. Other techniques do not allow a continuous or full-field measurement of the rotating structure. To determine the vibration behaviour of rotating composite compressor blades, a combined experimental-numerical approach is introduced. Therefore, an experimental system for the vibration excitation and a 3-dimensional determination of the vibration behaviour of rotating components are presented. An overview of the main addressed research topics is given.

Conference paper

Shiroky IB, Papangelo A, Hoffmann N, Gendelman OVet al., 2020, Nucleation and propagation of excitation fronts in self-excited systems, PHYSICA D-NONLINEAR PHENOMENA, Vol: 401, ISSN: 0167-2789

Journal article

Stender M, Di Bartolomeo M, Massi F, Hoffmann Net al., 2019, Revealing transitions in friction-excited vibrations by nonlinear time-series analysis, NONLINEAR DYNAMICS, Vol: 98, Pages: 2613-2630, ISSN: 0924-090X

Journal article

Stender M, Tiedemann M, Hoffmann N, 2019, Energy harvesting below the onset of flutter, JOURNAL OF SOUND AND VIBRATION, Vol: 458, Pages: 17-21, ISSN: 0022-460X

Journal article

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