MS2.5 Advances in Control of Structural Vibrations

Vibration absorption has been studied for over a century by systems and dynamics specialists from variety of different perspectives. Actively tuned class occupies a large place in the literature. The tuning operation imparts increased efficacy of the absorber when removing the undesired oscillations by “sensitizing” the absorber substructure to attract vibratory energy upon itself while leaving the main body of the structure relatively quiet. The concept of “actively tuned absorber” has been implemented to date with considerable success. It has been shown and is broadly accepted that an ideal absorber for tonal excitation needs to be “resonant” at the frequency of the excitation. With that point in mind, one can build a tuned resonant absorber which could remove the oscillations at its point of attachment to the main structure. That is, a “collocated” vibration absorber at the point of suppression. The main theme of the present study arises when the absorber to main structure attachment point and the point of desired suppression happen to be separated from one another, i.e., “non collocated” vibration suppression operation. Similar absorber tuning philosophy as described above would lead to an undesired consequence, namely the inclusion of a part of the main structure (together with the attached absorber) to be jointly “resonant” to execute complete removal of the oscillations. In recent literature this partition is named “resonant substructure”. This philosophy brings about two critical challenges: (a) How to identify this “resonant substructure” (b) How to actively and properly sensitize it for the incoming excitation frequencies? Both of these topics have attracted attention in recent years from various angles especially on the lumped mass constructs. However, the scientific findings are very weak, to say the least, when it comes to implementing the same concepts to continua (such as beams, plates etc.). This conceptual dichotomy between the lumped mass structures and the continua is the main topic of this paper. So, we wish to extend the well understood “non collocated vibration absorption (NCVA)” operation to be deployed on a simple continuum (say an oscillating beam). This seemingly straightforward task being harnessed with the lumped mass experience of the NCVA’s suddenly presents a dichotomy in the form of uncertainty for both problems (a) and (b). The authors wish to lay out the complexities of the problem and establish the non triviality of the lumped mass to continua transition of the NCVA procedure. On this occasion, we also wish to formulate a generalized benchmark problem to be solved. In brief, main difficulty arises from the fact that when a point is designated for suppression on a lumped mass construct, the main objective is to create a negating force via a sensitized “resonant absorber subsection”. Once this mission is accomplished the point of suppression acts like a fictitious ground, and the entire system becomes decoupled with that ground being in the middle. In a continuum, however, such a decoupling effect disappears as the “fictitious ground” (with zero transverse oscillations) still transmits some influence from one side to the

Offshore Wind Turbines (OWT), nowadays, are designed with greater energy power capacities and thus they are comprised of components of larger dimensions in accordance with the increase in the energy demands worldwide. In order to install these larger OWTs, floating heavy lift vessels, operating under dynamic positioning, have been employed, increasing the efficiency of the operations. The methods of dissipating the undesired motion of the hanging loads play a decisive role in the safety and efficiency of installation. Various motion compensation and position control methods have been employed and tested in situ over the years, namely active tugger lines, gripper frames, crane motion compensators, etc. However, all of the current methods require mechanical equipment attached to payload as well as active human intervention. This fact, coupled with the delicate nature of positioning OWT components, the small error tolerances and the harsh offshore environment further highlight the paucity in research for a non-contact motion compensation technique for OWT installation. Such a technique would both contribute to the safety of the crew (by reducing the risk associated with human interventions offshore) and potentially reduce the required time for the overall procedure, a major factor for the efficiency and cost of the operation. The proposed technique is based on the magnetic interaction between the component and an electromagnetic actuator, involving both attractive and repulsive forces. To demonstrate the concept in a controlled environment, the dynamic behaviour of a magnetically controlled simple pendulum is studied both in a laboratory experiment and via numerical modelling. Special attention is paid to the study of the efficiency of the attained control and to the parameters of influence on the dynamic behaviour of the system. The system under consideration is strongly non-linear due to the distance-depended nature of the magnetic interaction force and the saturation control limits imposed by the physical actuator. This paper studies these effects and aims to optimize the performance of the contactless technique accordingly.

In recent decades, height-wise urban development relying on slender high-rise buildings with rectangular floorplans is booming in all major cities worldwide. The construction of such buildings is expected to intensify even more over the next years to accommodate pressing demands for residential and office space due to increasing urbanization trends in ever more congested urban environments with high-premium land. However, as their slenderness (height-to-width) ratio increase, rectangular buildings become susceptible to wind-borne vibrations in the crosswind direction leading to excessive floor accelerations, above building code standards, causing discomfort to the occupants and, ultimately, serviceability failure. This is due partly to the low inherent damping of slender buildings and partly to aerodynamic vortex shedding (VS) effects, that is, resonance of the fundamental natural period of the structure with the period of alternating vortices generated at the sharp corner edges of the building. To this end, tuned mass dampers and, more recently, lightweight inerter-based vibration absorbers (IVAs) have been considered in the literature for vibrations serviceability performance improvement in wind-sensitive tall buildings. Still, for routine slender buildings, serviceability floor acceleration thresholds mandated by building code standards are commonly met in practice by stiffening the lateral load-resisting structural system (LLRS) as the incorporation of dampers and vibration absorbers is considered to be “exotic”. However, LLRS stiffening results in higher material use which increases upfront costs and embodied carbon emissions. In this context, this study investigates the potential of using IVAs for LLRS self-weight reduction in buildings whose design is governed by serviceability floor acceleration (occupant comfort) criteria, ultimately leading to embodied carbon emission savings. This is achieved through a novel multi-objective optimization-driven framework with dual objectives for the integrated design of LLRS equipped with an IVA. The framework utilizes an optimality criteria (OC)-based structural sizing algorithm for weight minimization of the LLRS, and a pattern search algorithm for optimal IVA tuning to meet the code-specified floor acceleration criteria for occupant comfort with least structural weight (objective 1) and IVA inertance (objective 2) possible. The latter is the inertia property endowed to the IVA through the use of an inerter device which resists relative acceleration. The applicability of the framework is illustrated by considering a 15-storey steel moment resisting frame building equipped with a tuned inerter damper (TID) under stochastic crosswind excitation accounting for VS effects. The TID is a well-studied IVA in the literature. It is shown that significant reductions in LLRS self-weight are achieved by the proposed integrated design framework as the inertance property of the TID increases. This further results in considerable embodied carbon emission reduction quantified using the latest data applicable in UK. Thus, the bi-objective optimization formulation provides optimal pareto fronts trading structural self-weight to inertance which is quite promising and innovative approach for reducing the embodied carbon footprint of future slender buildings.

The study aims to investigate the effectiveness of tuned mass damper inerter (TMDI) for vibration enhancement of offshore wind turbine subjected to wind and wave forces. H2 optimization method is used to find the optimal parameters of the TMDI. Wind velocity is varied to examine the performances of TMDI under varying wind loads, where the wave is kept the same. In addition, the uncertainty of parent structure is considered to test the robustness of the TMDI. Its performance is compared with an optimal TMD system. It is found the designed TMDI is far better than the TMD system. In addition to vibration mitigation, TMDI has much higher effectiveness in energy harvesting.