Structural and Multidisciplinary Optimization, 2011
The paper presents an efficient 88 line MATLAB code for topology optimization. It has been develo... more The paper presents an efficient 88 line MATLAB code for topology optimization. It has been developed using the 99 line code presented by Sigmund (Struct Multidisc Optim 21(2):120–127, 2001) as a starting point. The original code has been extended by a density filter, and a considerable improvement in efficiency has been achieved, mainly by preallocating arrays and vectorizing loops. A speed improvement with a factor of 100 is obtained for a benchmark example with 7,500 elements. Moreover, the length of the code has been reduced to a mere 88 lines. These improvements have been accomplished without sacrificing the readability of the code. The 88 line code can therefore be considered as a valuable successor to the 99 line code, providing a practical instrument that may help to ease the learning curve for those entering the field of topology optimization. The paper also discusses simple extensions of the basic code to include recent PDE-based and black-and-white projection filtering methods. The complete 88 line code is included as an appendix and can be downloaded from the web site www.topopt.dtu.dk.
Ground vibrations induced by railway traffic at grade and in tunnels are often studied by means o... more Ground vibrations induced by railway traffic at grade and in tunnels are often studied by means of two-and-half dimensional (2.5D) models that are based on a Fourier transform of the coordinate in the longitudinal direction of the track. In this paper, the need for 2.5D coupled finite element-boundary element models is demonstrated in two cases where the prediction of railway
Vibrations induced by road and rail traffic are a common source of discomfort to people. Numerica... more Vibrations induced by road and rail traffic are a common source of discomfort to people. Numerical models have been developed for the prediction of traffic induced vibrations in the free field or in the built environment. These models consist of a finite element formulation for the vehicles and the buildings and a boundary element formulation that accounts for the wave propagation in the soil. The latter is based on the Green’s functions of a horizontally layered halfspace. The experimental validation of these models reveals a discrepancy between the predicted and measured response in the higher frequency range. Given the crucial role of the Green’s functions in the prediction model, the dynamic soil characteristics governing these functions are a possible source of the discrepancy. Common techniques for the in-situ measurement of the dynamic soil characteristics such as the spectral analysis of surface waves (SASW) test and the seismic cone penetration test (SCPT) are based on local averages of the soil characteristics and have a limited resolution. The small scale variations of the soil characteristics are not revealed. In this paper, the influence of the small scale variations of the dynamic shear modulus on the Green’s functions of a vertically inhomogeneous soil is studied. A probabilistic approach is followed where the mean soil is modelled using the results of the aforementioned measurement techniques. Superimposed on the mean profile is a zero mean random process that represents the small scale variations of the dynamic shear modulus. This process is characterized by a marginal probability density function and a correlation function, estimated by means of a cone penetration test (CPT). The resolution of the CPT test is high as compared to the SASW and the SCPT tests. The non-Gaussian random process is discretized by means of a Hermite polynomial expansion and a Karhunen-Loeve decomposition [1]. The methodology of the stochastic finite element method [2] is applied to a hybrid thin layer - direct stiffness formulation [3] in order to assemble the stochastic system equations. These are solved by means of a Monte Carlo simulation to obtain the stochastic Green’s functions. The results of the simulation are in good correspondence with the discrepancy observed in the validation of the deterministic vibration prediction models. In the low frequency range, the small scale variations of the dynamic shear modulus are not resolved by the waves and all realizations of the stochastic Green’s functions tend to the Green’s functions of the mean soil. In the high frequency range, the waves do resolve the small scale variations. As a result, phase shifts and variations of the amplitude occur between different realizations of the stochastic Green’s functions.
A recent development in operational modal analysis (OMA) is the possibility of using measured, ar... more A recent development in operational modal analysis (OMA) is the possibility of using measured, artificial loads in addition to the unmeasured, ambient excitation, while the ratio between forced and ambient excitation can be low compared to classical experimental modal analysis (EMA). Most of these so-called OMAX algorithms lack the intuitiveness of their EMA and OMA counterparts, since they fit a system model that takes both the measured and the operational excitation into account directly to the measured signals. A more physically intuitive subspace algorithm for OMAX, that starts with an accurate decomposition of the measured joint response in a forced and an ambient part, was recently introduced. In this paper, the performance of this algorithm, which is called CSI-ic/ref, is assessed by means of a case study, where a two-span steel arch footbridge is tested in operational conditions, with and without using additional actuators. From a comparison of the modal parameters with results from a finite elementmodel, an OMA algorithm, and an alternative OMAX algorithm, it can be concluded that CSI-ic/ref yields accurate modal parameter estimates.
Structural and Multidisciplinary Optimization, 2011
The paper presents an efficient 88 line MATLAB code for topology optimization. It has been develo... more The paper presents an efficient 88 line MATLAB code for topology optimization. It has been developed using the 99 line code presented by Sigmund (Struct Multidisc Optim 21(2):120–127, 2001) as a starting point. The original code has been extended by a density filter, and a considerable improvement in efficiency has been achieved, mainly by preallocating arrays and vectorizing loops. A speed improvement with a factor of 100 is obtained for a benchmark example with 7,500 elements. Moreover, the length of the code has been reduced to a mere 88 lines. These improvements have been accomplished without sacrificing the readability of the code. The 88 line code can therefore be considered as a valuable successor to the 99 line code, providing a practical instrument that may help to ease the learning curve for those entering the field of topology optimization. The paper also discusses simple extensions of the basic code to include recent PDE-based and black-and-white projection filtering methods. The complete 88 line code is included as an appendix and can be downloaded from the web site www.topopt.dtu.dk.
Ground vibrations induced by railway traffic at grade and in tunnels are often studied by means o... more Ground vibrations induced by railway traffic at grade and in tunnels are often studied by means of two-and-half dimensional (2.5D) models that are based on a Fourier transform of the coordinate in the longitudinal direction of the track. In this paper, the need for 2.5D coupled finite element-boundary element models is demonstrated in two cases where the prediction of railway
Vibrations induced by road and rail traffic are a common source of discomfort to people. Numerica... more Vibrations induced by road and rail traffic are a common source of discomfort to people. Numerical models have been developed for the prediction of traffic induced vibrations in the free field or in the built environment. These models consist of a finite element formulation for the vehicles and the buildings and a boundary element formulation that accounts for the wave propagation in the soil. The latter is based on the Green’s functions of a horizontally layered halfspace. The experimental validation of these models reveals a discrepancy between the predicted and measured response in the higher frequency range. Given the crucial role of the Green’s functions in the prediction model, the dynamic soil characteristics governing these functions are a possible source of the discrepancy. Common techniques for the in-situ measurement of the dynamic soil characteristics such as the spectral analysis of surface waves (SASW) test and the seismic cone penetration test (SCPT) are based on local averages of the soil characteristics and have a limited resolution. The small scale variations of the soil characteristics are not revealed. In this paper, the influence of the small scale variations of the dynamic shear modulus on the Green’s functions of a vertically inhomogeneous soil is studied. A probabilistic approach is followed where the mean soil is modelled using the results of the aforementioned measurement techniques. Superimposed on the mean profile is a zero mean random process that represents the small scale variations of the dynamic shear modulus. This process is characterized by a marginal probability density function and a correlation function, estimated by means of a cone penetration test (CPT). The resolution of the CPT test is high as compared to the SASW and the SCPT tests. The non-Gaussian random process is discretized by means of a Hermite polynomial expansion and a Karhunen-Loeve decomposition [1]. The methodology of the stochastic finite element method [2] is applied to a hybrid thin layer - direct stiffness formulation [3] in order to assemble the stochastic system equations. These are solved by means of a Monte Carlo simulation to obtain the stochastic Green’s functions. The results of the simulation are in good correspondence with the discrepancy observed in the validation of the deterministic vibration prediction models. In the low frequency range, the small scale variations of the dynamic shear modulus are not resolved by the waves and all realizations of the stochastic Green’s functions tend to the Green’s functions of the mean soil. In the high frequency range, the waves do resolve the small scale variations. As a result, phase shifts and variations of the amplitude occur between different realizations of the stochastic Green’s functions.
A recent development in operational modal analysis (OMA) is the possibility of using measured, ar... more A recent development in operational modal analysis (OMA) is the possibility of using measured, artificial loads in addition to the unmeasured, ambient excitation, while the ratio between forced and ambient excitation can be low compared to classical experimental modal analysis (EMA). Most of these so-called OMAX algorithms lack the intuitiveness of their EMA and OMA counterparts, since they fit a system model that takes both the measured and the operational excitation into account directly to the measured signals. A more physically intuitive subspace algorithm for OMAX, that starts with an accurate decomposition of the measured joint response in a forced and an ambient part, was recently introduced. In this paper, the performance of this algorithm, which is called CSI-ic/ref, is assessed by means of a case study, where a two-span steel arch footbridge is tested in operational conditions, with and without using additional actuators. From a comparison of the modal parameters with results from a finite elementmodel, an OMA algorithm, and an alternative OMAX algorithm, it can be concluded that CSI-ic/ref yields accurate modal parameter estimates.
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Papers by Mattias Schevenels