Jens Malmborg presenterar sin licentiatavhandling
The population is growing, and an increasing proportion of the population live in urban areas. As a consequence, human exposure to noise and vibrations is increasing. Larger and denser cities lead to a higher amount of traffic close to where people work and live. Land close to railways and heavily trafficked roads, previously left unexploited, are now being used for dwellings and offices. Vibrations are often accompanied by noise, to which longterm exposure is known to have serious health effects. Furthermore, some buildings such as hospitals and research facilities contain instruments that are highly sensitive to vibrations, and require proper vibration isolation to ensure safe operation. To address the problems of noise and vibrations, their generation and propagation need to be understood.
In this thesis, numerical modeling strategies for predicting groundborne vibrations from a surface railway track have been studied. Focus have been on the vibration transmission from the track to the freefield, and to a smaller extent on the actual load generation due to a train running on an uneven rail.
The wave propagation in the ground resulting from the dynamic loads on the track can be calculated using numerous numerical techniques. The finite element method offers a large flexibility regarding modeling capabilities in terms of geometrical conditions and material properties. However, the need for discretizing a large soil volume, under and between the track and the receiver, can generate very large systems of equations that are timeconsuming or practically impossible to solve. Computational savings can be made by introducing a coordinate transformation into the governing equations, so that the computational model is formulated in a moving frame of reference following the vehicle. Furthermore, if a horizontally layered viscoelastic halfspace is assumed, a so called Green’s function (a fundamental solution) for the ground dynamic response can be found very efficiently by employing a semianalytical solution procedure in frequency–wavenumber domain. Here, the Green’s function in a moving reference frame was used for establishing a dynamic stiffness matrix for a set of points in the track–soil interface, to which a finite element representation of the track was coupled. After solving the coupled track–soil problem, the Green’s function was used again to obtain the freefield ground vibrations resulting from the forces in the track–soil interface. The influence of different modeling strategies regarding the railway track was investigated, and further the change in response due to a mitigation measure under the track was studied using this model.
Additional efficiency may be obtained by applying a so called 2.5D procedure, in which a Fourier transform with regards to the track direction coordinate is performed. Instead of solving one large 3D problem, a sequence of 2D problems is solved for a set of discrete wavenumbers, after which the 3D response is recovered by an inverse Fourier transform. In the thesis, a very time efficient model is formulated that employs a 2.5D finite element representation of the railway track, coupled to a dynamic stiffness matrix of the layered ground obtained using the aforementioned semianalytical approach.
Finally, a 2.5D model employing finite elements for both the track and the surrounding soil was implemented and compared to the two previously mentioned coupled models, showing very good agreement.
Fredagen den 13 mars, 2020, kl 13.00 i sal V:D, V-huset, LTH, John Ericssons väg 1, Lund.
Jens Nielsen, Forskningsprofessor, Chalmers.
Professor Per-Erik Austrell, Avdelningen för byggnadsmekanik, LTH.
Professor Kent Persson, Avd för byggnadsmekanik, LTH.
Universitetslektor Peter Persson, Avd för byggnadsmekanik, LTH.