Christos T. Georgakis

Spherical tuned liquid damper for vibration control in wind turbines

A tuned liquid damper (TLD), which consisted of two-layer hemispherical containers, partially filled with water, was investigated as a cost-effective method to reduce the wind-induced vibration of wind turbines. A 1/20 scaled test model was designed to investigate its performance on the shaking table. Three groups of equivalent ground accelerations were inputted to simulate the wind-induced dynamic response under different load cases. The influence of rotors and nacelle was assumed to be a concentrated tip mass. A series of free and forced vibration experiments were performed on the shaking table. The experimental results indicated that the spherical TLD could effectively improve the damping capacity of the test model. The standard deviation of the dynamic response could be effectively reduced when the excitation frequency was approximately equal to its fundamental frequency. For “overspeed” and “extreme operating gust” load cases, the standard deviations of the dynamic responses were reduced more than 40% when the liquid mass was about 2% of the generalized mass; for “parking” load cases, the corresponding standard deviation was reduced more than 50% when the liquid mass was only 1% of the generalized mass. That is to say, the spherical TLD can effectively improve the anti-fatigue performance of the wind turbine tower.

SMD Model Parameters of Pedestrians for Vertical Human-Structure Interaction

Predictions of footbridge or long-span floor vibrations induced by pedestrian crowds can often prove inaccurate. One of the main deficiencies of the methods used for predicting these vibrations is the lack of consideration or erroneous representation of human-structure interaction (HSI). In this paper, the results from a series of footbridge tests designed to observe and then model HSI are presented. A laboratory footbridge was excited to three predetermined vibration amplitudes by an actuator, with and without the presence of pedestrians. In the tests with pedestrians, 4, 7 and 10 pedestrians were asked to walk repeatedly across the footbridge. Frequency response functions (FRF) of the footbridge with and without pedestrians were extracted from test data. To account for the HSI, pedestrians on the bridge were modeled as a spring-mass-damper (SMD) system attached to the footbridge. The mass, damping and stiffness of a single pedestrian were calculated by fitting the FRF obtained from the tests. It was found that the SMD models of pedestrians could adequately model HSI between a structure and its walking occupants. Furthermore, the stiffness and damping of the SMD model of a single pedestrian were found to be close to half of a standing person with two bent legs.

A Semi-active Control System for Wind Turbines

A semi-active (SA) control system based on the use of smart magnetorheological (MR) dampers to control the structural response of a wind turbine is proposed herein. The innovative approach is based on the implementation and use of a variable-properties base restraint. This is able to modify in real time its mechanical properties according to the instantaneous decision of a given control logic, the latter addressed to control one or more structural response parameters. The smart base restraint is thought to be a combination of a smooth hinge, elastic springs, large-scale adjustable MR dampers, and a control algorithm that instantaneously commands the latter during the motion, making them to modulate the reactive force as needed to achieve the performance goals. The design and operation of such a system are shown with reference to a case study consisting of an almost 100 m tall wind turbine, realized in a 1/20 scale model at the Denmark Technical University (DTU). Shaking table tests have been performed under the action of two different types of wind loads and by using two purposely written control logics, highlighting the high effectiveness of the proposed SA control technique and encouraging to further investigate in such direction.

Change in Mass and Damping on Vertically Vibrating Footbridges Due to Pedestrians

Pedestrian-induced footbridge vibrations are an issue that bridge designers often have to contend with. A plethora of research in recent years has led to the development of load models and procedures that allow for the determination of footbridge response. Nonetheless, measured footbridge responses often deviate from those predicted. One of the main deficiencies of the existing models and guidelines is the exclusion of the effect of changes in the footbridge’s dynamic properties due to the presence of pedestrians. More specifically, any change in mass and/or damping that a pedestrian might introduce to a bridge will affect the bridges overall dynamic response. This effect is an element of what is often referred to as human-structure interaction. In this paper, the results of an experimental study to determine the change in mass and damping of a vertically vibrating footbridge due to traversing pedestrians are presented.

Biomechanically Excited SMD Model of a Walking Pedestrian

AbstractThrough their biomechanical properties, pedestrians interact with the structures they occupy. Although this interaction has been recognized by researchers, pedestrians’ biomechanical properties have not been fully addressed. In this paper, a spring-mass-damper (SMD) system, with a pair of biomechanical forces, was used to model a pedestrian for application in vertical human–structure interaction (HSI). Tests were undertaken in a gait laboratory, where a three-dimensional motion-capture system was used to record a pedestrian’s walking motions at various frequencies. The motion-capture system produced the pedestrian’s center of mass (COM) trajectories from the captured motion markers. The vertical COM trajectory was approximated to be the pedestrian SMD dynamic responses under the excitation of biomechanical forces. SMD model parameters of a pedestrian for a specific walking frequency were estimated from a known walking frequency and the pedestrian’s weight, assuming that pedestrians always walk in ...

Measurement of Local Deformations in Steel Monostrands Using Digital Image Correlation

The local deformation mechanisms in steel monostrands have a significant influence on their fatigue life and failure mode. However, the observation and quantification of deformations in monostrands experiencing axial and transverse deformations is challenging because of their complex geometry, difficulties with the placement of strain gauges in the vicinity of the anchorage, and, most importantly, the relatively small magnitude of deformation occurring in the monostrand. This paper focuses on the measurement of localized deformations in high-strength steel monostrands using the digital image correlation (DIC) technique. The presented technique enables the measurement of individual wire strains along the length of the monostrand and also provides quantitative information on the relative movement between individual wires, leading to a more in-depth understanding of the underlying fatigue mechanisms. To validate the proposed image-based measurement method, two different tests were performed, with the one correlation method showing good agreement. Data collected from the DIC technique creates a basis for the analysis of the fretting and localized bending behavior of the monostrand and provides relevant information on the internal state of displacement of the monostrand under bending load.

Evaluation of Damping Using Frequency Domain Operational Modal Analysis Techniques

Operational Modal Analysis (OMA) techniques provide in most cases reasonably accurate estimates of structural frequencies and mode shapes. In contrast though, they are known to often produce uncertain structural damping estimates, which is mainly due to inherent random and/or bias errors. In this paper a comparison is made of the effectiveness of two existing OMA techniques in providing accurate damping estimates for random stationary loading, varying levels of signal noise, number of added measurement channels and level of structural damping. The investigation is focusing on the two frequency domain techniques, the Frequency Domain Decomposition (FDD) and the Frequency Domain Polyreference (FDPR). The response of a two degree-of-freedom (2DOF) system is numerically established with specified modal parameters subjected to white noise loading. The system identification is evaluated with well separated and closely spaced modes. Finally, the results of the numerical study are presented, in which the error of the structural damping estimates obtained by each OMA technique is shown for a range of damping levels. From this, it is clear that there are notable differences in accuracy between the different techniques.

Estimation of Damping for one of the New European Court Towers in Luxembourg

The two new high rise buildings for the European Court of Justice in Luxembourg have been tested by harmonic shakers and by Operational Modal Analysis. The background for the tests is to estimate the influence on the damping of one of the towers from an array of Tuned Liquid Dampers (TLDs) placed on top of the building. The TLDs have been designed to minimise the response of the buildings to wind loading, resulting in an increase in occupancy comfort. The harmonic excitation is performed to estimate the damping at a response level corresponding to moderate wind loading whereas the OMA has been performed under minimum wind loading. The paper presents the testing programme and the main results of which one is a clear non-linear behaviour of the introduced viscous damping system. To the authors’ knowledge, no other structural excitation of this scale has been undertaken, before and after the application of tuned damping systems, in which the damping system is completely independent of the excitation system.

Tuned Liquid Dampers for the New European Court of Justice, Luxembourg

As a consequence of the unique positioning and 3, 5:1 plan ratio, the proposed twin buildings (103 m) for the latest expansion of the European Court of Justice (Luxembourg) led to the commissioning of a comprehensive set of wind-tunnel tests. Experimental testing and numerical analyses showed the buildings to be susceptible to unacceptably large wind-induced accelerations at the top levels. To mitigate these vibrations, a Tuned Liquid Damper (TLD) array is proposed and designed for both the buildings. With an optimal design of the TLD array, total maximum reduction in top-level acceleration is found to be in the range of 35–40%, reducing building accelerations below acceptable limits for human comfort. Experimental verification of the performance of the dampers is undertaken through 1:2 scale shaking table tests.

Scenario Based Approach for Load Identification

In output only analysis the load identification has been a puzzle for several years. Different techniques have been purposed to cope with the inversion problem that lies within this field. However it has been shown, that most methods struggle to obtain robust and consistent results in cases of modal truncation and noise contaminated signals. In the light of these challenges, a scenario based method is proposed. This approach utilizes model updating along with mode shape expansion to obtain a reliable numerical model of the given structure. Then, by evaluating a series of rational load scenarios, it is possible to obtain a reasonable input identification – both the spatial distribution and the temporal variation of the load. The method is demonstrated numerically and experimentally.