PhD Defence by David Hoffmeyer - "Damping of Torsional Beam Vibrations"

Vibrations in structures is a common phenomenon. Natural loads such as wind, rain and earthquake as well as human induced loads and vibrating and moving machinery and vehicles, may all cause a structure to begin vibrating. For beam-like structures such as bridge girders, wind turbine blades, airplane wings, floor panels in buildings etc., vibrations are generally inexpedient and associated with a number of disadvantages. Fatigue damage, reduced structural lifetime and human discomfort are among the main concerns for civil engineering structures.

To mitigate or control vibrations, supplemental damping devices may be installed on the beam structure. The thesis describes a novel approach for mitigating torsional vibrations and coupled bending-torsion vibrations in thin-walled beams, which may arise due to asymmetric beam cross-sections, or when the loads are eccentrically applied. When the beam twists, out-of-plane axial warping deformations occur at the beam boundary. Restraining these deformations is associated with a potentially large shift in the natural frequency, and thereby a large damping potential.

Two different ways of restraining the warping deformations have been used, namely passive viscous dampers and active actuators with position feedback control. The active approach requires a power source, but gives more efficient damping. When investigating the structural performance with axial dampers mounted as in this case, it requires a 3D description of the structure. A full 3D model is computationally very inefficient, and simpler alternatives are thus desirable. The thesis therefore considers computationally efficient alternatives to a 3D model, and establishes an analytical approach and a numerical approach with finite beam elements. These models are calibrated to represent the full 3D model, and investigations show good agreement between the models, and large attainable damping ratios are obtained. The simpler calibrated models therefore constitute solid alternatives for initial design considerations, when investigating structures where axial dampers can be placed at the beam end.

A second part of the thesis considers the use of multiple tuned mass absorbers for damping of fully coupled beam vibrations. It is described how to tune the multiple absorbers, and it is demonstrated that the relative sizing of the absorbers should indeed not be identical. When acting in different directions on the beam, the absorbers should thus not be equal in size, but may otherwise be mounted in known ways.


Main supervisor:
Associate Professor Jan Becker Høgsberg, DTU Mechanical Engineering

Co-supervisor:
Professor Steen Krenk, DTU Mechanical Engineering

Examiners:
Professor Jakob Søndergaard Jensen, DTU Mechanical Engineering
Dr. Andrea Bergamini, EMPA Materials Science and Technology, Laboratory for Acoustics/Noise Control, Switzerland
Professor Ole Balling, Department of Engineering, Aarhus University

Chairman:
Associate Professor Niels Leergaard Pedersen, DTU Mechanical Engineering
 

Tidspunkt

fre 15 nov 19
13:00 - 17:00

Arrangør

DTU Mekanik

Hvor

Building 421, Auditorium  072
Technical University of Denmark