The damping properties of the foam-filled shaft of primary feathers of the pigeon Columba livia

The avian feather combines mechanical properties of robustness and flexibility while maintaining a low weight. Under periodic and random dynamic loading, the feathers sustain bending forces and vibrations during flight. Excessive vibrations can increase noise, energy consumption, and negatively impact flight stability. However, damping can alter the system response, and result in increased stability and reduced noise. Although the structure of feathers has already been studied, little is known about their damping properties. In particular, the link between the structure of shafts and their damping is unknown. This study aims at understanding the structure-damping relationship of the shafts. For this purpose, laser Doppler vibrometry (LDV) was used to measure the damping properties of the feather shaft in three segments selected from the base, middle, and tip. A combination of scanning electron microscopy (SEM) and micro-computed tomography (µCT) was used to investigate the gradient microstructure of the shaft. The results showed the presence of two fundamental vibration modes, when mechanically excited in the horizontal and vertical directions. It was also found that the base and middle parts of the shaft have higher damping ratios than the tip, which could be attributed to their larger foam cells, higher foam/cortex ratio, and higher percentage of foam. This study provides the first indication of graded damping properties in feathers.

A Simplified Dynamic Analysis of Existing Pedestrian Bridges, a Possibility to get new Experience for Designing

<p>Technology gives us new instruments that can help us in our daily work. There is a rapid development of different kinds of sensors used in buildings, bridges, towers and other structures. These sensors can help us gather much information from our existing structures, which gives us the possibility to use a large amount of data obtained from real structures to evaluate our theoretical models used in the design and refine them.</p><p>In the topic of pedestrian bridges, it is interesting to evaluate their dynamic properties since they are sensible to the dynamic vertical and horizontal loads from people. The fundamental data for the dynamic behaviour of a pedestrian bridge are the eigenfrequencies, which can be obtained easily using an accelerometer. Today there is almost always one available since the mobile phones are equipped with them. With an application, it is easy to obtain the fundamental vibration frequency for a bridge and it is also possible to analyse the higher vibration modes in a frequency spectrum. Furthermore, it is easy to obtain a measurement of acceleration in the time domain from walking or running. Also the damping has a very important effect on the vibration level, and it can also be evaluated with measurements.</p><p>This tool can be used for evaluating a similar structure as the one to be designed and it could provide us with valuable insight in the dynamic behaviour of the structure. This paper presents some examples, where the theoretical results from models are compared with measurements of the finished bridges. The results from measurements are validated with a simple, well known structure.</p>

A strain-wavelength modelling of low-frequency cantilever fibre Bragg grating accelerometer

The accuracy of single-degree-of-freedom (SDOF) model in describing the beam motion of low-frequency cantilever fibre Bragg grating (FBG) accelerometer can be further explored, since the SDOF model is limited to fundamental vibration modes. Therefore, this paper addresses the aforementioned limitation by introducing a modal model of the cantilever Euler-Bernoulli (EB) beam into the wavelength shift equation. This modal model (FBG-MM) considered five vibration modes. The convergent series of eigenfunction for cantilevered EB beam was solved using a standard modal expansion theory. The curvature of the cantilevered beam resulted from dual differentiation of the eigenfunction (with respect to x) is then related to the strain and wavelength of the FBG. The computed wavelength shift using FBG-MM was compared with the SDOF model. The experimental results where the harmonic base excitation occurring at five different frequencies were also discussed. The simulation results showed that the wavelength shift exhibited more reasonable behaviour along the beam particularly when the excitation frequency exceeded the second bending mode (596.67 Hz). The FBG-MM and experimental wavelength shift showed convincing correlation only when the excitation frequency came close to the fundamental frequency. On the other hand, there was no agreement at low excitation frequencies due to stiffness issues of the cantilever beam and the capability of the optical spectrum analyser. In future, the improvement of this study will focus on introducing a tip mass on the cantilever beam for increasing the accelerometer sensitivity and representing the cantilever beam using Timoshenko model.

A Method for Multihole Blasting Seismic Wave Prediction and Its Application in Pillar Recovery

Long-hole blasting in mines is likely to cause strong vibration of surficial infrastructure, greatly damage the rock mass surrounding goaf near explosion center, and possibly induce blast vibration disasters. In this article, an improved method for multihole blasting seismic wave prediction is proposed to estimate far-field blast vibration. In this method, the fundamental vibration waveforms are firstly measured through the field blast with a single deck at an underground pilot area. The fundamental vibration waveforms are then used to simulate the vibration waveforms for a single-deck case in the production blast by considering the difference of the equivalent distances from the production blast site and the pilot area to the surface measuring point. The vibration waveforms for the single-deck case are linearly superposed to predict the possible vibration waveforms in production blast with multiple long holes and decks according to the designed delay time between decks. Based on these predicted waveforms, the blast vibration can be estimated and the blast design can be optimized to determine a rational delay time in accordance with the vibration limit. The proposed method was applied in pillar recovery of Hongling Polymetallic Mine to optimize the long-hole blast design to manage blast vibration. The rational delay time for the 716 production blast design was recommended as 26 ms. The practice showed that the blast vibration induced by the 716 production blast has been managed, and the predicted and the measured waveforms agree well. It provides an effective method for multihole blast design to control blast vibration.

Numerical modal analysis of a 850 KW wind turbine steel tower

The study deals with the numerical analysis aspects that are necessary for identifying of modal parameters of the tower structure as the most important part of the horizontal axis wind turbine, which are basic for the dynamic response analysis. In the present study, the modal behavior of an actual 55-m-high steel tower of 850 KW wind turbine (GAMESA G52/850 model) is investigated by using three-dimensional (3D) Finite Element (FE) method. The model was used to identify natural frequencies, their corresponding mode shapes and mass participation ratios, and the suggestions to avoid resonance for tower structure under the action wind. The results indicate that there is a very good agreement with the fundamental vibration theory of Euler-Bernoulli beam with lamped masse in bending vibration modes. When the rotor of the wind turbine runs at the speed of less than or equal to 25.9 rpm it will not have resonant problems (stiff–stiff tower design). Furthermore, in case the rotor runs at the speed of between 25.9 and 30.8 rpm, the adequate controller is necessary in order to avoid the corresponding resonant susceptible area of the tower structure (soft–stiff tower design).

Prediction equation for the fundamental vibration period of concrete gravity dams with impounded water

The design of dam structures requires advanced analysis techniques due to the dam body’s complicated interaction with its surroundings. Consequently, most of the time, it is a must to perform response history analysis during the design and assessment of dam structures. The target fundamental vibration period is necessary to generate ground motion histories compatible with the site-specific response spectrum. In this study, an equation was developed to predict the fundamental vibration period of concrete gravity dams based on an extensive database. First, a wide range of parameters to describe gravity dams’ three-dimensional (3D) geometry and their valleys are determined. A total of 19,440 different 3D numerical models with fluid and solid elements were formed, and the fundamental vibration period values were determined to create the necessary database. Then, multiple nonlinear regression analysis was conducted to propose an equation to predict the fundamental vibration period. The proposed equation could be used to accurately estimate the required fundamental vibration period during the preliminary design stage or the implementation of simplified procedures or during the spectrum matching operations.

Influence of Wind-Induced Antenna Oscillations on Radar Observations and Its Mitigation

As Typhoon Goni (2015) passed over Ishigaki Island, a maximum gust speed of 71 m s−1 was observed by a surface weather station. During Typhoon Goni’s passage, mountaintop radar recorded antenna elevation angle oscillations, with a maximum amplitude of ~0.2° at an elevation angle of 0.2°. This oscillation phenomenon was reflected in the reflectivity and Doppler velocity fields as Typhoon Goni’s eyewall encompassed Ishigaki Island. The main antenna oscillation period was approximately 0.21–0.38 s under an antenna rotational speed of ~4 rpm. The estimated fundamental vibration period of the radar tower is approximately 0.25–0.44 s, which is comparable to the predominant antenna oscillation period and agrees with the expected wind-induced vibrations of buildings. The reflectivity field at the 0.2° elevation angle exhibited a phase shift signature and a negative correlation of −0.5 with the antenna oscillation, associated with the negative vertical gradient of reflectivity. FFT analysis revealed two antenna oscillation periods at 0955–1205 and 1335–1445 UTC 23 August 2015. The oscillation phenomenon ceased between these two periods because Typhoon Goni’s eye moved over the radar site. The VAD analysis-estimated wind speeds at a range of 1 km for these two antenna oscillation periods exceeded 45 m s−1, with a maximum value of approximately 70 m s−1. A bandpass filter QC procedure is proposed to filter out the predominant wavenumbers (between 40 and 70) for the reflectivity and Doppler velocity fields. The proposed QC procedure is indicated to be capable of mitigating the major signals resulting from antenna oscillations.

Mitigation of vibrations in sandwich-type structures by a controllable constrained layer

This study presents and tests a method for semi-active control of vibrations in sandwich-type beam structures. This method adapts a strategy called prestress accumulation release. The prestress accumulation release strategy is based on structural reconfiguration: it uses short time, impulsive and localised changes of actuator properties (such as stiffness or damping), which are applied to a part of the system in the moments, when its strain energy attains a local maximum. The method has been earlier applied as a global control scheme to mitigate the fundamental vibration mode of a cantilever beam (by stiffness control) and in the task of mitigating the first four modes of a frame structure (by damping control). This study proposes a prestress accumulation release strategy and tests its effectiveness for the case of a three-layered sandwich structure, with the internal layer fabricated from a material with dissipative characteristic locally controllable through the material damping coefficient. In contrast to the earlier research, the control is applied thus at the level of material characteristics instead of a discrete set of dedicated actuators. Based on the finite element method, a numerical experiment involving a passively damped, as well as prestress accumulation release-controlled, three-layered cantilever beam excited by initial displacements was performed. The effectiveness of the approach was studied for a broad range of internal layer damping parameters. The presented results revealed a high potential of the prestress accumulation release strategy in semi-active damping of vibrations of sandwich-type structures.