Structural Damping Research Articles

VIV and forced convection response of a freely oscillating cylinder to a single impulse perturbation of varying intensity

A numerical investigation into the effects of single gust impulse on the flow physics, dynamic response, and heat transfer from an undamped two degrees of freedom circular cylinder is presented for a constant property, incompressible Newtonian fluid at a low Reynolds number range, 70≤Re≤150 and Prandtl number Pr=7. Structural natural frequency (FN) is set equal to the vortex shedding frequency of a static circular cylinder (fv) at Re=100 and is kept constant in both the degrees of freedom such that FN=FNx=FNy. Mass ratio is m*=10, while structural damping is set to zero (ζ=0). Intensities of gust impulse are varied as a function of inlet velocity while keeping the frequency of gust impulse fixed, in order to isolate the effects of intensity of gust only. Results pertaining to transient effects of gust impulse on the fluidic forces, dynamic response, and conjugate heat transfer are presented. Time histories of lift coefficient show a pattern of quicker recovery from gust in the lock-in regime. Shifting of primary and secondary frequencies due to gust impulse is observed. A very organized and regular pattern of cylinder dislodgement and recovery is observed in the lock-in regimes as opposed to chaotic behaviour out of the lock-in regime. The vortex street is seen to eventually stabilize and restore itself to its original pre-gust oscillatory behaviour. Vorticity contours depict an early vortex detachment due to the shear layers’ stretching followed by a pair of counter rotating vortices shed simultaneously. Consequent evolution of heat transfer behaviour from the cylinder surface is also recorded in the temperature contours. Furthermore, local Nusselt number at the front end of the cylinder depicts remarkable sensitivity to the gust intensity. Increase in gust intensity also causes higher peak values of local skin friction coefficient.

Učinak tla na kapacitet viskoznih prigušivača između susjednih zgrada

This study investigated the seismic pounding of two adjacent buildings considering soil–structure interaction (SSI). A comprehensive parametric study of buildings with different heights was performed to reveal the pounding-involved behaviour considering the soil effect. Wavelet transform has been conducted to gain insight into the differences in the frequency contents of the impact forces between fixed- and flexible-base adjacent structures. Linear viscous dampers (LVDs) between adjacent floors were used as pounding protection measures. The required supplemental damping ratio of the LVDs was determined through optimisation analysis under different soil types to verify the effect of the SSI on structural damping. Comparative results with and without SSI showed that incorporating the SSI worsens the pounding-involved responses during earthquakes.

Finite element dynamic modeling and attitude control for a slender flexible spacecraft

This paper aims to study the dynamic modeling and attitude control of a kind of slender flexible spacecraft. The basic structure of this kind of spacecraft is different from the traditional structure of a center rigid body with flexible appendages, but rigid bodies at both ends, which are connected by a flexible truss structure. Firstly, the finite element modeling method is adopted to establish the dynamic model of this kind of spacecraft, and the vibration frequency and damping of the flexible structure are obtained. The unconstrained modes of the flexible spacecraft are obtained through mathematical analysis, finite element analysis and frequency domain analysis. In addition, a nonlinear sliding surface with angular velocity constraint is designed. On this basis, an adaptive sliding mode (ASM) controller is proposed, stability of the close-loop system under the controller is proved, and the dynamic model is verified through mathematical simulation.

Improving the Applicability of RDM in Identification of Multimode Structural Damping with Reduced RD Signature Quality

Improving the Applicability of RDM in Identification of Multimode Structural Damping with Reduced RD Signature Quality

Energy-Based Evaluation on Soil–Structure-Interaction-Related Damping of Inelastic Bridge Pier Structure Subjected to Pulse-like Velocity Ground Motion

This study proposed a procedure of using the energy method to evaluate the SSI-related damping effect when the soil–structure interaction (SSI) was considered in a bridge pier system, which can thus be deemed a soil–foundation–superstructure (SFS) inelastic system. Firstly, the SSI is implemented by adopting a discrete-time recursive filter approach as well as frequency-dependent foundation–soil impedance functions to solve for the external soil forces exerted onto the foundation. Then, by integrating such external soil forces into the motion equations of the SSI-based SFS system, the energy equations can be formulated during the ground motions. To demonstrate the proposed procedure, an implementation study involving a bridge pier was carried out, considering two earthquake recordings. The resultant energy quantities and SSI-related damping ratio shed light on how the aspects of earthquake characteristics affected the energy dissipation mechanism of the bridge pier SSI-based SFS system. This proposed procedure renders a promising solution for quantifying the soil role in the seismic energy dissipation of arbitrary single- and multiple-degree-of-freedom systems considering the SSI effect. The results obtained show that the SSI effect was suppressed when the SFS system underwent near-fault earthquakes, which illustrated that the stiffness and damping contribution from the soil was not pronounced. Furthermore, near-fault earthquakes with large incremental velocities may lead to a low SSI-related damping ratio (SSIDR).

Vortex-induced vibration forever even with high structural damping

This study investigates the effect of structural damping on vortex-induced vibration (VIV) of a circular cylinder when the mass ratio is below its critical value. It is confirmed by water-channel experiments and a reduced-order model (ROM) that the previously identified phenomenon of VIV forever, i.e. resonance oscillations at any reduced velocity, persists even with high structural damping. Of interest, the ROM results reveal that the wake mode for VIV forever is unstable with a constant positive growth rate with increasing reduced velocity, while the experimental results suggest that VIV forever is associated with a synchronisation between the non-stationary cylinder vibration frequency and the vortex-shedding frequency.

A BSDEs approach to pathwise uniqueness for stochastic evolution equations

We prove strong well-posedness for a class of stochastic evolution equations in Hilbert spaces H when the drift term is Hölder continuous. This class includes examples of semilinear stochastic Euler-Bernoulli beam equations which describe elastic systems with structural damping, and semilinear stochastic 3D heat equations. In the deterministic case, there are examples of non-uniqueness in our framework. Strong (or pathwise) uniqueness is restored by means of a suitable additive Wiener noise. The proof of uniqueness relies on the study of related systems of infinite dimensional forward-backward SDEs (FBSDEs). This is a different approach with respect to the well-known method based on the Itô formula and the associated Kolmogorov equation (the so-called Zvonkin transformation or Itô-Tanaka trick). We deal with approximating FBSDEs in which the linear part generates a group of bounded linear operators in H; such approximations depend on the type of SPDEs we are considering. We also prove Lipschitz dependence of solutions from their initial conditions.

Optimum Design of Tuned Mass Dampers for Structures with Torsional Irregularity via Jaya Algorithm

Optimum Design of Tuned Mass Dampers for Structures with Torsional Irregularity via Jaya Algorithm

Pipe vibration attenuation through internal damping and optimal design of vibro-impact systems

Pipelines periodically supported by rack structures (PPRs) are common in chemical and petrochemical plants, among others, and conventional tools such as dampers and hysteretic absorbers are commonly used to mitigate large vibrations in these systems. In this study, we explore two alternative strategies: (i) enhancing the attenuation rate of PPR vibrations through structural internal damping, and (ii) using nonlinear vibro-impact systems (VIS) to reduce seismic vibrations in a PPR. To shed light on the first strategy, we develop analytical dispersion relations for a PPR and show how damping can improve the mitigation capabilities of the periodic system. As for the second strategy, we consider a 9-node beam, i.e., a single span (SS) of a PPR equipped with a VIS, and combine the central composite design (CCD) and Kriging metamodelling to maximize dissipation energy and minimize the number of impacts. This multi-objective optimization problem aims to find the most effective design solution for the VIS in terms of gap and coefficient of restitution (COR). Additionally, we consider the stochastic nature of seismic input and the possible chaotic behavior of the VIS. To account for the sensitive variability of the number of impacts in seismic records, we perform incremental dynamic analyses and calculate fragility functions for various engineering demand parameters, including the number of impacts. We define a 3D surface for selecting the optimal gap-COR pair. When impacts occur, transient results can be chaotic, and we compute the largest Lyapunov exponents of a few representative trajectories.

Nonlinear Contact Stiffness and Damping Identification of a Tenon Connection Structure Based on Its Dynamic Characteristics

A tenon connection structure is widely used for the blade-disk connection in turbomachinery, and its contact stiffness and contact damping have caused nonlinear dynamic characteristics. The nonlinear dynamic characteristics of a blade-disk structure are mainly affected by its contact stiffness and damping characteristics, which are difficult to accurately predict and affected by many factors such as the operating speed and the centrifugal force caused by the rotation speed. Based on a single blade and tenon groove structure and the use of the dynamic characteristics experiments and simulations, the influence law of the contact stiffness and damping of the tenon connection structure with the change of the blade root compression force on the dynamic characteristics of the structure is obtained. The average relative error of identifying the contact stiffness is 0.48%; thus, identification of the nonlinear contact stiffness and damping of multiple pairs of contact curved surfaces is realized.

МОДЕЛИРОВАНИЕ СОБСТВЕННЫХ ЧАСТОТ И АМПЛИТУДЫ ВЫНУЖДЕННЫХ КОЛЕБАНИЙ ВЕРТИКАЛЬНО-ФРЕЗЕРНОГО СТАНКА

The article demonstrate the technique for modeling the form of natural oscillations, as well as the amplitude and mode of forced oscillations of the HAAS VF-1 vertical milling machine. As initial data, a publicly available 3D model of load-bearing elements, information on the mass and dimensions of the spindle motor are used. In preparation for the simulation, the original solid model is idealized by removing small chamfers, fillets, and small diameter holes. Movable joints with linear rolling guides and ball bearings are replaced by rigid joints, since vibration damping in these units is not considered. During the first simulation, ten natural frequencies with the largest values of the oscillation amplitude are obtained. The second simulation shows, the influence of the spindle drive layout (coaxial installation of the electric motor with the spindle through a cam clutch or connection using a toothed belt transmission) and additional devices for dissipating vibration energy (mass and structural dampers in the form of structural elements articulated through plastic bushings) is revealed by the magnitude of forced steady-state oscillations that may occur during the processing of the part.

Sandwich designs for high levels of noise and vibrations

Lightweight materials with high structural damping and reasonably well flexural performance are demanded in some components of aerial vehicles such as interior part of helicopters where high levels of noise and vibrations exist. One of the best choices for this is through the use of a polymer matrix composite with suitable fiber orientations and a structural foam core. In this study, face sheets from a woven carbon fiber reinforced polymer plies and a foam core with three different thicknesses (10.7 mm, 25.4 mm, and 49.8 mm) have been used to manufacture sandwich structures to yield high dynamic values (damping and flexural rigidity). To measure these values, the sandwich beams with free-free boundary conditions were subjected to free vibration tests using a non-contact response measuring system. The measurements were made for the first, second and third natural frequencies, and compared to numerical modal analysis via the ANSYS package program. The results from experimental and numerical methods are found to be reasonably good in agreement and show that the sandwich structures are able to give high damping values, in general. It seems the specimens with relatively low thickness give relatively high damping values compared to the others, whereas the thicker ones are useful for high frequency domains and flexural rigidity. The behaviors of the beams have been found frequency dependent, due to the viscoelastic behavior of the foam cores at high frequency levels.

Re-use of jute fiber hybrid nonwoven breather within laminated composite applications: A case study

Jute/polyester hybrid nonwoven mats were developed as an alternative to the conventional nylon or polyester breather to reduce the use of synthetic polymers and associated waste generated during composite manufacturing. However, end-of-life breathers still face the challenge of waste management. In this study, a circular economy strategy for multiple applications of waste hybrid nonwoven breathers was thus proposed to study their recycling possibilities. Hybrid nonwoven breathers were first used to facilitate the autoclave forming of carbon fiber-reinforced plastics, and non-destructive and mechanical tests were performed to determine the quality. The used breathers were then recycled and reused in three ways: directly as reinforcing material and in hybrid combinations with glass fibers and ramie fibers for the production of new composites. Results of quality checks show that the hybrid nonwoven breather is a viable alternative to commercial breathers. The thermal, mechanical, and damping properties of the composites incorporating reused material were investigated. The hybrid nonwoven breather/Glass fiber composites show superior thermal and mechanical properties as well as improved damping properties. The hybrid nonwoven breather/Ramie fiber composites have the advantages of low density and high loss factor reaching up to 0.1572. The product of specific modulus and loss factor was then used as the figure of merit to estimate the structural damping properties of the composites. Both glass fiber and ramie fiber hybrid composites presented a higher figure of merit with 0.86 and 0.83 GPa/(g/cm3), respectively. Therefore, hybrid nonwoven breathers can be adapted to replace synthetic materials in semi-structural applications.

Experimental studies on carbon nanotube strain sensors

Conventional metal foil strain sensors only measure strain on the structural surface in predetermined directions and locations. These are made of Nichrome (Ni-Cr alloy) and Constantan (Cu-Ni alloy) and have gauge factor of 2. A carbon nanotube (CNT) as a strain sensing material is gaining research interest due to its outstanding electrical and mechanical properties. These sensors apart from multidirectional sensing are capable of making additional contributions such as structure strengthening or structure damping. Bucky paper (BP) alternatively called CNT film or CNT network made from single-walled carbon nanotube or multiwalled carbon nanotubes (SWCNT or MWCNT) has been used by some researchers in the past as strain sensor. However, they have observed nonlinear relationship between strain and change in resistance, and lower gauge factor. Literature review in the field indicates that there is scope for improvement in the performance of Bucky paper-based strain sensors. The objective of this experimental work is to investigate effect of CNT film constituents (MWCNT, SWCNT and mix of both), purification of CNTs and effect of preparation method on sensitivity. The work involves preparation of film from CNTs by vacuum filtration method and preparing the strain sensors from the same. These sensors are loaded along with conventional foil strain gauges on opposite side. The change in resistance of sensor due to applied load is measured to evaluate sensitivity or gauge factor of sensor for various cases. It was observed that the sensitivity of CNT strain sensor is dependent on the preparation and purification methods as well as on the aspect ratio.

New Exponential Stability Result for Thermoelastic Laminated Beams with Structural Damping and Second Sound

New Exponential Stability Result for Thermoelastic Laminated Beams with Structural Damping and Second Sound

A novel tuned inerter eddy current damper: modeling, optimization, and evaluation

This study proposes a novel tuned inerter eddy current damper using a rack and gear mechanism (TIECD), in which the configuration and mechanical model are introduced in detail. The TIECD design procedure for the seismic control of primary structures is established based on the effective damping ratio enhancement (EDRE) effect. A closed-form solution for TIECD optimal design under white noise excitation is presented using stochastic linearization techniques (SLT) with force-based and energy-based equivalence criteria. The control performance of the TIECD obtained by the closed-form solution, including the EDRE effect and effective damping ratio, is in good agreement with the numerical results obtained from the Monte Carlo simulation when a suitable TIECD critical velocity is selected; a large critical velocity is conducive to improving the accuracy of the closed-form solution. Note that the effect of a low structural damping level on the optimal design of TIECD can be ignored, which is beneficial in practical engineering applications because structural damping cannot be precisely known in most cases. Furthermore, several numerical studies have been conducted to investigate the availability of TIECD under real seismic excitations. The results indicate that the effectiveness of TIECD in mitigating the seismic response of primary structures is approximately the same as that of a tuned viscous mass damper (TVMD) and much greater than that of the viscous and eddy current dampers with the same damping parameters. It can be considered that the maximum damping force and maximum deformation of TIECD and TVMD are the key design factors for short-period and long-period structures, respectively, and the advantages of TIECD over TVMD are owing to the damping force limiting characteristics and small deformation.

Energy research of three-adjacent structures connected by Kelvin-type dampers based on power flow

The control effect of a three-adjacent structural system connected by Kelvin-type dampers and its energy transfer and distribution under uncertain and deterministic excitations are investigated in this study. A simplified model of multi-degree-of-freedom mechanics is used for establishing structural equations; the power flow equations of this structural system are derived based on the power flow theory. The influence law of a structural parameter changes on the input power flow when the damper that is not connected is investigated; the damper parameters are optimized using the NSGA-II (Non-dominated Sorting Genetic Algorithm-Ⅱ) algorithm after connecting the dampers, and the damping control effect is evaluated. Next, the variation law of the input power flow of the structure and damper is analyzed with the structure and damper parameters as study variables. These results indicate that an increase in structural stiffness has a positive excitation effect on the seismic performance improvement of the structure; the influence of the structural damping ratio on the input power flow is sufficiently small. The influence of change in damper parameters on the input power flow is closely related to the structural parameter setting. The three-adjacent damping structure system connected to optimal parameter dampers exhibits a good damping performance when the structural parameters are determined.

Seismic excitation of offshore wind turbines and transition piece response

AbstractExpansion of the offshore wind industry in seismically active areas has raised concerns regarding the structural integrity of offshore wind turbines under earthquake loading. This paper details a 3D finite element study to investigate the behaviour of the structure, and in particular the transition piece (TP), under seismic loads. The work focuses on equivalent grouted connection and TP‐less designs, selected as promising design solutions for seismic zones. The numerical model is validated against a medium‐scale 4‐point bending laboratory test, and scaled up to a representative 8 MW turbine. The results show that cracking of the grout occurs due to earthquake excitation at the top of the TP; a location that is typically undamaged during monotonic loading. This can lead to excessive settlement of the transition piece and loss of axial capacity caused by deterioration of the grout‐steel bond and water ingress. The residual hub displacement after earthquake excitation is around 0.1 m. The global monotonic response post‐earthquake excitation is not significantly altered, with apparent stiffness and ultimate strength maintained. An equivalent TP‐less design is shown to have a 5% lower natural frequency than an equivalent grouted connection design, suggesting a reduced global stiffness, with reduced structural damping due to the absence of grout. However, TP‐less design eliminates the risk of grout deterioration and settlement and may therefore be a safer design option for offshore wind turbines installed in seismic zones in the future.

An optical method to monitor transparent contact interfaces during high frequency shear vibration cycles

Contacting interfaces provide frictional damping in jointed structures subjected to high dynamic loads. Predicting this frictional damping during vibration cycles is highly important since it strongly affects the dynamic response of the assembly and hence the lifetime of parts. Since frictional damping is heavily influenced by the contact condition at the interface, the most direct and insightful approach is thereby to actively monitor the contact interface. Although several methods have already been proposed to monitor the contact interfaces quasi-statically or in pre-sliding, such as digital image correlation, X-Ray and ultrasound, only limited data is available of the frictional interface behaviour during high frequency vibration.To provide a better insight into the contact interface behaviour during high frequency cyclic motion, an optical method is here developed based on transparent friction specimens and total internal reflection, and applied to an existing friction test rig. The resulting measurements across the whole interface show a large variation in the real area of contact during each vibration cycle, which could be linked to the kinematics of the contact interface. This large variation is observed for the first time in high frequency oscillating contacts and is attributed to ageing effect and fracture of asperities. These two effects dominate the contact mechanism at different sliding velocities and induce variations in the real area of contact during each vibration cycle. These results suggest that the mechanisms behind high frequency contact behaviour are more complex than what commonly assumed in dynamics simulations.

Updated heat capacities of \(^{161-164}\)Dy nuclei

This work presents the updated heat capacities of \(^{161-164}\)Dy nuclei in the nuclear temperature region from 0 to 1 MeV. The updated heat capacities are obtained within the canonical ensemble method making use of the most recent and recommended experimental nuclear level density (NLD) data together with those calculated within the back-shifted Fermi gas (BSFG) model with energy-dependent parameters. By comparing the updated heat capacities with the un-updated ones, which are obtained by using the old experimental NLD data and the BSFG with energy-independent parameters, we found that the updated and un-updated heat capacities are almost identical at low temperature, but differ from each others at high temperature. This discrepancy can be interpreted by the damping of nuclear shell structure with increasing the excitation energy. Besides, we observe that the S-shape in the updated heat capacities is much more pronounced in even-even Dy isotopes than in even-odd ones, whereas the un-updated heat capacities do not clearly exhibit this S-shape. Therefore, the updated heat capacities should provide a more convincing evidence for the signature of pairing phase transition in nuclear systems.