Levels Of Structural Damping Research Articles

Experimental investigations on the vortex-shedding from a highly tapered circular cylinder in smooth flow

This paper presents the results of a wind tunnel test campaign conducted to investigate the aeroelastic and aerodynamic behaviour of a tapered circular cylinder in smooth flow conditions. The model under investigation is a single-mode aeroelastic one, whose taper ratio (in terms of diameter) is 8 %. Its global dynamic response was measured through the use of an internally-mounted piezo-electric accelerometer. The sensitivity of the vortex-induced response to different levels of structural damping was explored. The harmonic content of the wake was also measured through a hot-wire anemometer positioned downstream of the wind tunnel model at different heights: this allowed the detection of the portion of the model whose local resonance was accountable for the highest vortex-induced structural response. To further expand the findings of the wake measurements, four ‘rings’ of surface-mounted pressure sensors were installed on the wind tunnel model. The Reynolds number that was covered during the experiments ranged between 9.3×103 and 3.2×104.The experimental investigations revealed that the tapering has a beneficial effect with respect to vortex-induced oscillations, reducing their magnitudes when compared against the ones of a parallel-sided circular cylinder. On the other hand, the lock-in region was found to be wider, exhibiting three distinct peak regions for the lowest level of structural damping. The combined results from the accelerometer, the hot-wire anemometer and the pressure measurements, and the application of tailored time–frequency analyses based on the continuous wavelet transform, revealed that the maximum dynamic response is linked to the local resonance between the fundamental frequency and the bottom portion of the structure. Furthermore, the magnitudes of the local mean aerodynamic drag coefficient and the local standard deviation of the (vortex-induced) lift coefficient were found to be considerably lower than those of a parallel-sided circular cylinder.

Active control of an acoustic black hole using a feedback strategy

Acoustic black holes (ABHs) are tapered structural features that can achieve high levels of structural damping within lightweight constraints. It has previously been proposed to integrate feedforward active vibration control into an ABH to enable control over a broad spectrum, however, the time-advanced information required in this control strategy is not always available. In addition, it has been shown that controlling the reflection coefficient in a beam-based ABH leads to an undesirable enhancement of the taper vibration. In this paper, a feedback control strategy is applied to an ABH terminated beam, with a piezoelectric patch providing the control actuation. The feedback strategy is a remote vibration control methodology and is used to examine the different compensators that are potentially available to control simultaneously both the local taper vibration and the reflected wave component. The investigation highlights that the taper vibration can be minimised without affecting the reflected wave, however, apart from a few isolated frequencies, it is shown that the reflected wave cannot be minimised without increasing the taper vibration relative to the passive case. Importantly, since the approach enables the passive ABH performance to be maintained whilst simultaneously minimising the vibration in the tip, the fatigue life of the structure can potentially be extended with the addition of the active control loop.

A Vibration Analysis of the Rubber Inertial Dampers Used in Electrical Vehicles.

The development of electric vehicle manufacturing, which is considered a useful new popular propulsion system, has major design differences compared to conventional vehicles. This requires a reconsideration of the main components of vehicles and an analysis of them to determine the optimal design and solutions for the new models of cars. Among the many systems that need to be reconsidered is the suspension. A cheaper solution for reducing the car’s vibrations is suspension where the damping is ensured by elastic rubber elements, which are very simple, as they have significant structural damping and a much lower price than the classic solution. The main advantage of this solution is the simplicity. The paper presents and analyzes such an element, analyzing the vibrations of this element and the way in which inertial masses (metal spheres) inserted into the volume of the rubber influence the behavior of this element. The transmissibility of such an element, and how the number of balls and the level of structural damping influences this property, is also analyzed. The results suggest possible applications in the automotive industry.

AERODYNAMIC STABILITY OF BRIDGES WITH VARIOUS LEVELS OF STRUCTURAL DAMPING

Introduction: Structural damping is one of the most important parameters affecting the aerodynamic stability of bridge structures. Purpose of the study: We aimed to assess the effect that structural damping of a bridge structure has on its stability in a wind current. Methods: In the course of the study, we performed experimental studies of the aerodynamic stability in typical girder bridge structures (with two and four main girders) with different levels of structural damping, facilitated by a unique experimental unit: Large Research Gradient Wind Tunnel, courtesy of the National Research Moscow State University of Civil Engineering (NRU MGSU). Results: The results of the experimental studies show that, despite the general trend towards the decrease in the amplitude of bridge span structure oscillations as the structural damping level increases, the dependence between these parameters is nonlinear. When providing R&D support in the design of real-life structures, in case it is necessary to increase the aerodynamic stability of the superstructure by increasing the level of structural damping (changing the type of joints in structural elements, using mechanical damping devices), it is recommended to conduct experimental studies in wind tunnels to assess the effectiveness of a given solution.

Validation of variable helix milling instability islands

During machining, it is well-known that unstable self-excited vibrations known as regenerative chatter can limit productivity. There has been a great deal of research that has sought to understand regenerative chatter, and to avoid it through modifications to the machining process. One promising approach is the use of variable helix tools. Here, the time delay between successive tooth passes is intentionally modified, in order to improve the boundary of instability. Previous research has predicted that such tools can offer significant performance improvements whereby islands of instability occur in the stability lobe diagram. By avoiding these islands, it is possible to avoid regenerative chatter, at depths of cut that are orders of magnitude higher than for traditional tools. However, to the authors’ knowledge, these predictions have not been experimentally validated, and there is limited understanding of the parameters that can give rise to these improvements. The present study seeks to address this shortfall. A recent approach to analysing regenerative chatter stability is modified, and its numerical convergence is shown to outperform alternative methods. It is then shown that islands of instability only emerge at relatively high levels of structural damping, and that they are particularly susceptible to model convergence effects. The model predictions are validated against detailed experimental data that uses a specially designed configuration to minimise experimental error. To the authors’ knowledge, this provides the first experimentally validated study of unstable islands in variable helix milling, whilst also demonstrating the importance of structural damping and numerical convergence on the prediction accuracy.

Active feedforward control of flexural waves in an Acoustic Black Hole terminated beam

Acoustic Black Holes (ABHs) are structural features that are typically realised by introducing a tapering thickness profile into a structure that results in local regions of wave-speed reduction and a corresponding enhancement in the structural damping. In the ideal theoretical case, where the ABH tapers to zero thickness, the wave-speed reaches zero and the wave entering the ABH can be perfectly absorbed. In practical realisations, however, the thickness of the ABH taper and thus the wave-speed remain finite. In this case, to obtain high levels of structural damping, the ABH is typically combined with a passive damping material, such as a viscoelastic layer. This paper investigates the potential performance enhancements that can be achieved by replacing the complementary passive damping material with an active vibration control (AVC) system in a beam-based ABH, thus creating an active ABH (AABH). The proposed smart structure thus consists of a piezo-electric patch actuator, which is integrated into the ABH taper in place of the passive damping, and a wave-based, feedforward AVC strategy, which aims to minimise the broadband flexural wave reflection coefficient. To evaluate the relative performance of the proposed AABH, an identical AVC strategy is also applied to a beam with a constant thickness termination. It is demonstrated through experimental implementation, that the AABH is able to achieve equivalent broadband performance to the constant thickness beam-based AVC system, but with a lower computational requirement and a lower control effort, thus offering significant practical benefits.

Determinación del amortiguamiento independiente de la frecuencia en estructuras civiles incluso en la situación de modos de vibración muy próximos aplicando técnicas de optimización

Specific computer programs for identification in modal analysis of structures are not economical, and they may not always be available. This paper presents a technique for simple optimization, getting fast and quality results, in the estimation of the damping ratios for each separate mode, and that can be resolved with a demo version (downloadable for free from the internet) of commercial optimization software. It is suitable for low levels of structural damping (or non-frequency-dependent damping), which are those that appear in large structures. Likewise, it is not necessary for the modes to be separate, or even that the magnitude of the near modes be less than that of those with which we are working; within the algorithm movable residual complements optimize their position according to the frequency and vertical axes (the response magnitude), to take into account the influence of the modes outside the considered range. In addition, it is not very sensitive to the presence of noise in the signal.

Extraction of Nonlinear Aerodynamic Damping of Crosswind-Excited Tall Buildings from Aeroelastic Model Tests

AbstractThis paper presents a new approach for extracting nonlinear aerodynamic damping of crosswind-excited tall buildings from response statistics at different levels of structural damping obtain...

Optimal design of damping layers in SMA/GFRP laminated hybrid composites

This work describes the optimization of the shape profiles for shape memory alloys (SMA) sheets in hybrid layered composite structures, i.e. slender beams or thinner plates, designed for the passive attenuation of flexural vibrations. The paper starts with the description of the material and architecture of the investigated hybrid layered composite. An analytical method, for evaluating the energy dissipation inside a vibrating cantilever beam is developed. The analytical solution is then followed by a shape profile optimization of the inserts, using a genetic algorithm to minimize the SMA material layer usage, while maintaining target level of structural damping. Delamination problem at SMA/glass fiber reinforced polymer interface is discussed. At the end, the proposed methodology has been applied to study the hybridization of a wind turbine layered structure blade with SMA material, in order to increase its passive damping.

Design of a Super Multispan Continuous Girder Bridge Using Enhanced Seismic Response Modification Technologies

This paper reports the state-of-theart seismic design concept utilizing an extensive combined application of seismic response modification devices, including elastomeric bearings, seismic dampers, and low-friction slide bearings with a particular layout for the super multispan continuous girder bridges, used in the Imagiregawa Bridge project in Japan. The baseline of the design concept is the reduction of the resilient force introduced by the bearing system, while a high level of structural damping is introduced with the use of high-capacity seismic dampers to utilize the seismic force reduction effect of the seismic isolation concept. There are other great advantages in the present design concept for long super multispan continuous girder bridges: statically indeterminate forces that act on multispan continuous girders supported by elastomeric bearings induced by temperature changes, desiccation shrinkage, and creep can be reduced so that any unfavorable effects to the seismic response are reduced. It also eliminates the necessity for adjustment of elastomeric bearing installation with the “post slide” procedure during construction. Since the elastomeric bearings and seismic dampers are installed on a limited number of piers, 22% reduction of the projected construction cost of the substructures is achieved as compared with conventional seismic isolation design.

One-way and two-way couplings of CFD and structural models and application to the wake-body interaction

The current study focuses on the wake-body interaction of a circular cylinder, whose transverse free vibration is modeled by a mass-spring-damper system coupled to a computational fluid dynamics (CFD) model for the flow and wake. We first simulate the free vibration of the elastically-mounted cylinder and the wake, and analyze the transverse load it exerts on the cylinder and its phase with the vibration. We vary the damping by three orders of magnitude and examine the difference in the wake-body interaction for slightly-damped and highly-damped systems. We then use the spectral properties of the free vibration and use them to construct two different types of forced vibrations: one consists only of the fundamental component of the free vibration, and the other accounts for all spectral properties of it. We compare the wake load for each type to that corresponding to the free vibration. The forced vibrations correspond to a one-way coupling and the information is communicated from the CFD model to the structural model, whereas the free vibration corresponds to a two-way coupling of the models. By comparing the spectral properties of the wake load, including the phase relation of its components with the vibration, which we obtained for the free vibration and for the equivalent forced vibration, we identify the effects of the wake feedback. The findings show that a forced vibration does not reproduce exactly the wake load at small and intermediate levels of structural damping. As the damping increases, the vibration changes from being in-phase with the wake load to being 90° out-of-phase with it, corresponding to two different wake states, and the forced vibration gives wake load that is very close to the one occurring in the case of full wake-body interaction.

Modified Cross-Correlation Method for the Blind Identification of Structures

Recently, blind source separation (BSS) methods have gained significant attention in the area of signal processing. Independent component analysis (ICA) and second-order blind identification (SOBI) are two popular BSS methods that have been applied to modal identification of mechanical and structural systems. Published results by several researchers have shown that ICA performs satisfactorily for systems with very low levels of structural damping, for example, for damping ratios of the order of 1% critical. For practical structural applications with higher levels of damping, methods based on SOBI have shown significant improvement over ICA methods. However, traditional SOBI methods suffer when nonstationary sources are present, such as those that occur during earthquakes and other transient excitations. In this paper, a new technique based on SOBI, called the modified cross-correlation method, is proposed to address these shortcomings. The conditions in which the problem of structural system identification can be posed as a BSS problem is also discussed. The results of simulation described in terms of identified natural frequencies, mode shapes, and damping ratios are presented for the cases of synthetic wind and recorded earthquake excitations. The results of identification show that the proposed method achieves better performance over traditional ICA and SOBI methods. Both experimental and large-scale structural simulation results are included to demonstrate the applicability of the newly proposed method to structural identification problems.

Dynamic Serviceability in Lightweight Engineered Timber Floors

Vibration of lightweight engineered timber floors (ETFs) is a problem occasionally reported by occupants of dwellings constructed with such floors. Vibration of timber floors has been studied extensively in the past resulting in the development of design criteria to prevent dynamic serviceability problems. Nevertheless, unsatisfactory serviceability continues to be reported. The present experimental investigation has examined several lightweight ETF configurations and found that most satisfy existing design criteria intended to preclude bodily oscillations under foot traffic. However, these same floors failed to display satisfactory performance with respect to vibrational disturbances such as drumminess and shaking. Common variations in design such as changes in fasteners or the inclusion of blocking have little effect on vibrational disturbances. Instead, experimental evidence suggests that minimum levels of structural damping may be required to control such phenomena.

Instability of a cantilevered flexible plate in viscous channel flow

The stability of a flexible cantilevered plate in viscous channel flow is studied as a representation of the dynamics of the human upper airway. The focus is on instability mechanisms of the soft palate (flexible plate) that cause airway blockage during sleep. We solve the Navier–Stokes equations for flow with Reynolds numbers up to 1500 fully coupled with the dynamics of the plate motion solved using finite-differences. The study is 2-D and based upon linearized plate mechanics. When both upper and lower airways are open, the plate is found to lose its stability through a flutter mechanism and a critical Reynolds number exists. When one airway is closed, the plate principally loses its stability through a divergence mechanism and a critical flow speed exists. However, below the divergence-onset flow speed, flutter can exist for low levels of structural damping in the flexible plate. Our results serve to extend understanding of flow-induced instability of cantilevered flexible plates and will ultimately improve the diagnosis and treatment of upper-airway disorders.

Estimating the frequency response of a structure from static loading

The noise radiated from a structure is directly related to the level of surface vibration or the structural response at the excitation point. That is an estimate of the response of the structure is an important parameter for the prediction of the radiated noise energy, or surface vibration level. The response of a structure can be divided into two sections: the low‐frequency regime and the high‐frequency regime. The low‐frequency regime is that below the first natural frequency of the structure. In this regime the response is directly related to the static stiffness. Above the first resonant frequency the structure goes through a series of resonances and antiresonances. At high frequencies, the bandwidth separating successive modes becomes less than the bandwidth associated with each mode, for typical machine structures, with a high level of structural damping. The frequency at which this occurs is sometimes less than 1 kHz and the response approaches a mean level. A direct relation exists between the high‐frequency mean level and the response level in the low‐frequency regime. This relationship is also dependent on the value of the structural damping. Thus, the mean structural response level can be estimated from knowledge of the static response or static stiffness.