Physics Parameter Identification of Annular Tuned Liquid Damper for the Structure‐Damper‐Coupled System Using a Mechanics‐Enhanced SSI Method

Experimental study on the efficiency of tuned liquid dampers for vibration mitigation of a vertically irregular structure

Researchers have developed and applied tuned liquid dampers (TLDs) to reduce the vibration of civil structures. Previous studies have focused on using a large number of identical TLDs (single-frequency TLDs) or many multi-frequency TLDs to reduce the structural dynamic response. In those works, the optimal parameters of TLDs were determined through the optimal parameter of equivalent tuned mass dampers. In this study, a new approach is proposed to directly determine the optimal parameters of multiple identical TLDs (MITLD). Furthermore, different aspects including the best configuration control effectiveness, robustness, and sensitivity to changes in the water level of MITLD are explored. Parametric studies are also conducted to find the fundamental characteristics of MITLD and the effective domains of the MITLD’s main parameters. The results obtained from this work agree well with those of earlier research works. Based on this fundamental research, recommendations given include: (1) MITLD is effective in controlling the dynamic response of structures with low natural frequencies, (2) MITLD is very sensitive to the variation of the water level height in TLDs, and (3) a difference in the natural frequency of the main structure leads to a significant performance decrease of the MITLD.

Wind-induced responses of an airport tower — efficiency of tuned liquid damper

Floating Production Storage and Off-loading (FPSO) systems are being utilized world widely by offshore industries. The surge response of FPSO subjected to severe sea states needs to be suppressed in order to keep a particular station for operational purposes. The oil cargo tanks of FPSO can be used as passive dampers for minimizing the surge displacement response of FPSO. These cargo tanks can act as Tuned Liquid Dampers (TLDs) if the natural frequency of the liquid oscillation in containers is tuned to the surge natural frequency of FPSO or tuned to the excitation frequency. The containers can be partitioned into several tanks (as Multiple Tuned Liquid Dampers, MTLDs) with different lengths and liquid depths. The natural frequencies of MTLDs can also be distributed over a range around the natural frequency of FPSO. The present study attempts to comprehend the response control of FPSO under surge motion and so the FPSO is modeled as a single degree of freedom under random sea state. The containers can be modeled as either linear TLDs or nonlinear TLDs. Each linear TLD can be modeled by an equivalent Tuned Mass Damper (TMD) analogy based on linear wave theory. In order to account the nonlinear liquid motion in TLD, an impact damper analogy is used along with the TMD analogy. The study includes the efficiency and robustness of MTLD and it compares the response control obtained by using linear MTLDs and nonlinear MTLDs.

This paper introduces a new type of tuned liquid damper (TLD) having a relatively simple, easy-to-model behavior and high effectiveness in controlling structural vibrations. It consists of a traditional TLD with addition of a floating roof. Since the roof is much stiffer than water, it prevents wave breaking, hence making the response linear even at large amplitudes. The roof also facilitates the incorporation of supplemental devices with which the level of damping of the liquid vibration can be substantially augmented. This newly proposed TLD, denoted as tuned liquid damper with floating roof (TLD-FR), maintains the traditional advantages of TLDs (low cost, easy installation and tuning), but its numerical characterization is much simpler because the floating roof suppresses higher sloshing vibration modes, resulting in a system that can be represented by a single-degree-of-freedom model. An efficient numerical scheme, where the dynamic behavior of the TLD-FR is expressed as a second-order lineal system of equations, is discussed and validated by scaled experimental tests. The equations of motion of a structure equipped with a TLD-FR are then derived and manipulated to offer a unifying representation dependent upon only four model characteristics of the TLD-FR: The first three (mass, frequency and damping ratios) are common for all type of mass dampers, whereas the final one, termed efficiency index, is related to a similar parameter used to characterize liquid column dampers. Through this approach, the behavior of the proposed TLD-FR can be easily correlated with the behavior of other well-known linear mass damper devices. The relationship between these parameters and the geometrical characteristics of the TLD-FR is also examined. Finally, the identification of the optimal characteristic of the TLD-FR (natural frequency and damping) under stationary stochastic excitation is discussed.

Simulating a conical tuned liquid damper

Recently, accidents at critical infrastructure facilities due to seismic activity, military explosions and accidents have become more frequent. The importance of studying such events in terms of the preservation and improvement of the environment and the sustainable development of any country is beyond doubt. These man-made effects negatively affect the environment. The hazardous materials released pollute soil, and surface and groundwater. Dust and emissions reduce the quality of the atmosphere. Destroyed objects could disrupt and degrade ecosystems resulting in a reduction in biodiversity. In this regard, a comprehensive analysis of the system to increase the environmental safety level of buildings and different technogenic objects under seismic loads, military explosions and emergencies, the development of theoretical models and practical solutions to ensure their stability and reduce environmental threats, is an urgent scientific task. The aim of this paper is to study the effectiveness of tuned liquid dampers in mitigating structural damage when subjected to vibrations. Tuned liquid dampers are strong tanks, partially filled with liquid, which are strongly integrated into a flexible structure. A numerical model that incorporates the interaction between a structure and a tuned liquid damper was developed. The structure is assumed to be a single degree of freedom system. To obtain the fundamental frequencies of a tuned liquid damper, the boundary element method was used. The novelty of the proposed method is that it defines the mitigation of vibrations in a structure with a tuned liquid damper.

Tuned liquid dampers (TLDs) are introduced in structural systems to suppress vibration of the structure due to dynamic actions. Using a grid-based Finite Volume Method-Finite Element Method (FVM/FEM), efficiency of TLDs of cylindrical and rectangular geometries was evaluated by performing a two-way Fluid-Structure Interaction (FSI) analysis. The nonlinear sloshing response of fluid was simulated by employing a three-dimensional Navier–Stokes equation as the governing equation and occurrence of wave breaking was accounted by using Volume of Fluid (VOF) method for free surface tracking. Utilizing the developed numerical model, a series of three-dimensional transient analysis were conducted on a structure with and without TLDs aiming to investigate effect of base excitation amplitude. The result show that maximum performance of TLDs at resonance is obtained when the forcing ratio is limited to 0.1, and for this case, both cylindrical and rectangular TLDs displayed comparable efficiency. Effect of excitation frequency fe was also investigated under a wide range of the forcing ratio showing that as fe increases, the natural frequency of coupled TLD-structural system approaches the natural frequency of un-damped structural system.

For some large structures, structural damping alone has been insufficient to limit the dynamic motions to acceptable levels for serviceability considerations (e.g. the review paper by Kareem et al. 1999). Thus, auxiliary dampers have been added. Designers are faced with the task of understanding complex fluid-structure interactions when attempting to estimate energy dissipation performance of, for example, tuned sloshing dampers. To this end, a numerical wave tank can provide useful information on the free surface motions, resonant frequencies, etc. In the present paper, the nonlinear effects of sloshing motions in moving liquid tanks (Frandsen 2002) are investigated in connection with using tuned liquid dampers (TLD) to suppress structural vibrations in tall buildings or other structures to along-wind or cross-wind forces. The current, fully nonlinear model simulates two-dimensional, non-overturning waves during structural vibrations. Numerical solutions of the governing nonlinear potential flow equations are obtained using a finite-difference time-stepping scheme on adaptively mapped grids. Simulations of sloshing motions in forced excited tanks are carried out in which tank size and fluid-to-structure mass ratio is varied. Natural frequencies of the structure-TLD system are extracted for small to steep amplitude waves. All results obtained show that the coupling of a liquid storage tank to a structure can change the behaviour of the entire system considerably. An optimum TLD- structure system is identified relative to shift in resonance frequency. Good agreement for small horizontal forcing amplitude is achieved between the numerical model and first order small perturbation theory. The dependence of the nonlinear behaviour of the solution on the wave steepness is discussed. It is found that nonlinear effects become important when the steepness reaches about 0.1.

Structural health monitoring (SHM) system is an essential part of large structures to allow for early detection of possible damages or defects and avoid human or economic losses. In this paper, stochastic subspace identification (SSI) is employed as a means to obtain the main characteristics of the structure i.e., mode shapes, natural frequencies, etc. Features obtained from SSI are used to assess the health status of the structure. The proposed method is implemented on a scaled three-story laboratory structure where structural damping is altered using a novel magnetic approach. Finite-element analysis is employed to validate the experimental results.

In this study, the natural frequency of the water sloshing wave in a rectangular tank with various sloped bottom shapes is derived from its equivalent flat-bottom tank using the linear wave theory. In this equivalency, the bottom wet length of the tank was used to replace the flat length and the deepest water depth was taken as the water depth in the natural frequency formula of a flat-bottom tank. The calculated natural frequency was validated with experimental data. The rationale for the use of the bottom wet length in the frequency evaluation of a rectangular tank of various sloped bottom shapes is demonstrated based on the finite element coordinate transformation. Different bottom lengths change the travel distance and thus the damping ratio of the water sloshing wave in the tank.

This paper presents the new type of a Tuned Liquid Damper (TLD) which utilizes water in a U-shaped tank as a mass of dynamic damper. The authors carried out model tests and vibration experiments in ship structures and in building structures. The effectiveness of a mechanism of the TLD developed by authors was proved good in a model test, as designed. A natural frequency of the damper was controlled by changing air pressure in the tank. The experiments in the building as well as in the ship revealed that the reduction in structural vibration by the TLD was verified. The mechanism of the TLD with a semi-active control system was proved effective in simulation tests, as expected. From the result of an experiment onboard the ship, it has became obvious that the semi-active control system was applicable to practical use.

An onshore wind turbine is considered as a solution for energy transition for many economies due to its low installation, operation and maintenance costs. A wind turbine structure is a low frequency structure due to the mass and stiffness distribution (along its length) and is therefore sensitive to dynamics. These structures are being planned in seismic regions and it is therefore necessary to understand the responses during multi-frequency excitation. Furthermore, it is necessary to verify the efficacy of any mitigation measures such as dampers. In this article, a scaled model of an onshore wind turbine is designed for the purpose of evaluating the effectiveness of a rectangular-shaped tuned liquid damper (TLD) in controlling the seismic responses to prolong the fatigue life. A series of shaking table tests is performed to examine the performance of the TLD under various conditions, encompassing harmonic forced vibrations with varying motion frequencies and earthquake-induced vibrations. This study incorporated two distinct tower base scenarios: (a) one with a fixed base as the base case and also to simulate rock-socketed condition; (b) another considering the flexibility offered by the foundation, i.e. taking account of soil–structure interaction (SSI). Five carefully chosen earthquake motions were utilized to comprehensively assess the performance of the TLD. The experimental results highlighted that the efficiency of the TLD in controlling peak nacelle accelerations and peak bending strains at the base of the wind tower in particular when the excitation frequencies closely matched the natural frequency of the test model. The use of the TLD resulted in a significant reduction in tower base bending strains and nacelle accelerations for both fixed base and SSI scenarios. Furthermore, the implementation of the TLD significantly contributed to structural damping and expedited the decay of motion in both nacelle accelerations and tower base bending strains. It is concluded that the efficacy of the TLD is dependent on spectral characteristics of the earthquakes.

SummaryThe hydrodynamic characteristics of rectangular tanks with internal damping devices, including baffles and paddles, are investigated using shaking table tests excited by colored noise. The complex second‐order blind identification method is employed to decouple fluid sloshing responses and to obtain mode shapes. The modal frequency and damping ratio are also identified by the modified Bayesian spectral density approach. The influence of baffle angle, paddle width, liquid depth, and excitation amplitude on the natural frequency and damping ratio of tuned liquid dampers (TLDs) is analyzed. Results show that the natural frequency is reduced due to the added mass produced by the damping devices, and the reduction increases gradually with the increase in the size of the damping devices, indicating that the influence of the added mass should be considered in the design. This result is essentially different from the conclusion of the existing finding that the modal frequency of TLD increases continuously with the increase in the baffle angle. Therefore, the tuning of TLD cannot be realized by rotating the baffles. Moreover, the damping ratio of TLD with paddles shows a nonlinear relationship with the fluid response. The nonlinear empirical model fitted by the test results can predict the damping ratio with high accuracy and can be used in the preliminary design of TLD for the wind‐induced vibration control of super‐tall buildings.

The excessive lateral vibrations of Londons Millenium Bridge and the Toda Park Bridge in Japan due to a large number of crossing pedestrians have raised an unexpected problem in footbridge constructions. Secondary tuned structures, like the conventional tuned mass damper (TMD) or the tuned liquid damper (TLD) were installed to the bridge in order to suppress these vibrations. In the present investigation it is proposed to apply the more efficient and more economic tuned liquid column damper (TLCD), which relies on the motion of a liquid mass in a sealed tube to counteract the external motion, while a built‐in orifice plate induces turbulent damping forces that dissipate kinetic energy. For optimal tuning of TLCDs the natural frequency and equivalent linear damping coefficient have to be chosen suitable, likewise to the conventional TMDs, as given in Den Hartog [1]. The advantages of TLCDs are: simple tuning of natural frequency and damping, low cost of design and maintenance and a simple construction. A mathematical model of a three degree‐of‐freedom (DOF) bridge coupled with an optimal tuned TLCD is derived and analyzed numerically. Furthermore, a small scale experimental model set‐up has been constructed in the laboratory of the TU‐Insitute. The experimental results are in good agreement with the theoretical predictions and indicate that TLCDs are effective damping devices for the undesired pedestrian induced footbridge vibrations. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)