Extra Damping Research Articles

Rotating skyrmion lattices by spin torques and field or temperature gradients

Chiral magnets like MnSi form lattices of skyrmions, i.e., magnetic whirls, which react sensitively to small electric currents $j$ above a critical current density ${j}_{c}$. The interplay of these currents with tiny gradients of either the magnetic field or the temperature can induce a rotation of the magnetic pattern for $j>{j}_{c}$. Either a rotation by a finite angle of up to ${15}^{\phantom{\rule{0.16em}{0ex}}\ensuremath{\circ}}$ or---for larger gradients---a continuous rotation with a finite angular velocity is induced. We use Landau-Lifshitz-Gilbert equations extended by extra damping terms in combination with a phenomenological treatment of pinning forces to develop a theory of the relevant rotational torques. Experimental neutron scattering data on the angular distribution of skyrmion lattices suggest that continuously rotating domains are easy to obtain in the presence of remarkably small currents and temperature gradients.

Natural sloshing frequencies and modes in a rectangular tank with a slat-type screen

Being installed in tanks, screens play the role of slosh-suppressing devices which may strongly change resonant sloshing frequencies and yield an extra nonlinear damping due to cross-flow resulting in either flow separation or jet flow. Employing the linear sloshing theory and domain decomposition method, we construct an accurate analytical approximation of the natural sloshing modes in a rectangular tank with a slat-type screen at the tank middle. Two-dimensional irrotational flow of an ideal incompressible liquid is assumed. Because the considered flow model does not account for flow separation and jet flow at the screen, the velocity field is locally singular at the sharp edges. The constructed solution captures this singularity. Analyzing this solution establishes a complex dependence of the natural sloshing frequencies on the solidity ratio, the number of submerged screen gaps, the liquid depth, and the position of perforated openings relative to the mean free surface. Results are compared with experimental data. Natural surface wave profiles are discussed in the context of a jump of the velocity potential at the screen and the local inflow component to the screen.

10.1007/s11451-008-2007-y

A dispersion relation is derived for magnetostatic waves in a ferrite film with inhomogeneous layers near the interfaces. It is shown that the transition layers can be taken into account using a single integrated parameter, namely, the effective film thickness. It is established that the layered structure of the film causes an extra wave damping. The relations obtained can be used to analyze the dispersion and damping of the main modes of magnetostatic waves.

The Damping of a Quartz Tuning Fork in Superfluid 3He-B at Low Temperatures

We have measured the damping on a quartz tuning fork in the B-phase of superfluid He-3 at low temperatures, below 0.3T (c). We present extensive measurements of the velocity dependence and temperature dependence of the damping force. At the lowest temperatures the damping is dominated by intrinsic dissipation at low velocities. Above some critical velocity an extra temperature independent damping mechanism quickly dominates. At higher temperatures there is additional damping from thermal quasiparticle excitations. The thermal damping mechanism is found to be the same as that for a vibrating wire resonator; Andreev scattering of thermal quasiparticles from the superfluid back-flow leads to a very large damping force. At low velocities the thermal damping force varies linearly with velocity, but tends towards a constant at higher velocities. The thermal damping fits very well to a simple model developed for vibrating wire resonators. This is somewhat surprising, since the quasiparticle trajectories through the superfluid flow around the fork prongs are more complicated due to the relatively high frequency of motion. We also discuss the damping mechanism above the critical velocity and compare the behaviour with other vibrating structures in superfluid He-3-B and in superfluid He-4 at low temperatures. In superfluid He-4 the high velocity response is usually dominated by vortex production (quantum turbulence), however in superfluid He-3 the response may either be dominated by pair-breaking or by vortex production. In both cases the critical velocity in superfluid He-3-B is much smaller and the high velocity drag coefficient is much larger, compared to equivalent measurements in superfluid He-4.

Asymptotic stabilization via control by interconnection of port-Hamiltonian systems

We study the asymptotic properties of control by interconnection, a passivity-based controller design methodology for stabilization of port-Hamiltonian systems. It is well-known that the method, in its basic form, imposes some unnatural controller initialization to yield asymptotic stability of the desired equilibrium. We propose two different ways to overcome this restriction, one based on adaptation ideas, and the other one adding an extra damping injection to the controller. The analysis and design principles are illustrated through an academic example.

Uniform Boundary Controllability of a Semidiscrete 1-D Wave Equation with Vanishing Viscosity

This article deals with the approximation of the boundary control of the linear one-dimensional wave equation. It is known that the high frequency spurious oscillations that the classical methods of finite difference and finite element introduce lead to nonuniform controllability properties (see [J. A. Infante and E. Zuazua, M2AN Math. Model. Numer. Anal., 33 (1999), pp. 407–438]. A space-discrete scheme with an added numerical vanishing viscous term is introduced and analyzed. The extra numerical damping filters out the high numerical frequencies and ensures the convergence of the sequence of discrete controls to a control of the continuous conservative wave equation when the mesh size tends to zero.

DMA testing of Ni–Mn–Ga/polymer composites

Ni–Mn–Ga/polymer composites are an interesting group of materials with new potential for applications such as vibration damping. Polymer selection is critical for Ni–Mn–Ga/polymer composites. The transition temperatures of both components should be selected carefully considering the future application, the contact between Ni–Mn–Ga and polymer is important and stiffness of the polymer matrix is one of the influencing factors. The adhesion between Ni–Mn–Ga and the polymer matrix was studied in shear stress tests. Ni–Mn–Ga adhered strongly to the tested epoxies but considerably less so to silicones. The damping properties of Ni–Mn–Ga/polymer composites were studied by dynamic mechanical analysis (DMA). Compared to the pure polymer sample, DMA measurements confirmed extra damping in the martensite–austenite transition temperature region of the Ni–Mn–Ga after a soft epoxy matrix had been applied.

Theory of magnetostatic waves in a ferrite film with transition layers

A dispersion relation is derived for magnetostatic waves in a ferrite film with inhomogeneous layers near the interfaces. It is shown that the transition layers can be taken into account using a single integrated parameter, namely, the effective film thickness. It is established that the layered structure of the film causes an extra wave damping. The relations obtained can be used to analyze the dispersion and damping of the main modes of magnetostatic waves.

Effect of coherence property of incident light on prism waveguide coupling

The accurate reflection coefficient of prism waveguide coupler that takes coherence property of incident light into account is obtained at first. Based on the reflection coefficient, the effect of coherence property of incident light on prism coupling is discussed in this work. It is shown that extra damping in waveguide caused by the coherence property is larger for high-order resonant mode than that for low-order mode. For 100fs Gaussian pulses incidence, the reflected spectra at different resonant modes are obtained theoretically and experimentally, which show that coherence property has a strong effect on prism coupling at high-order modes.

Ultrasonic bubbles in medicine: Influence of the shell

Ultrasound contrast agents consist of microscopically small bubbles encapsulated by an elastic shell. These microbubbles oscillate upon ultrasound insonification, and demonstrate highly nonlinear behavior, ameliorating their detectability. (Potential) medical applications involving the ultrasonic disruption of contrast agent microbubble shells include release-burst imaging, localized drug delivery, and noninvasive blood pressure measurement. To develop and enhance these techniques, predicting the cracking behavior of ultrasound-insonified encapsulated microbubbles has been of importance. In this paper, we explore microbubble behavior in an ultrasound field, with special attention to the influence of the bubble shell. A bubble in a sound field can be considered a forced damped harmonic oscillator. For encapsulated microbubbles, the presence of a shell has to be taken into account. In models, an extra damping parameter and a shell stiffness parameter have been included, assuming that Hooke’s Law holds for the bubble shell. At high acoustic amplitudes, disruptive phenomena have been observed, such as microbubble fragmentation and ultrasonic cracking. We analyzed the occurrence of ultrasound contrast agent fragmentation, by simulating the oscillating behavior of encapsulated microbubbles with various sizes in a harmonic acoustic field. Fragmentation occurs exclusively during the collapse phase and occurs if the kinetic energy of the collapsing microbubble is greater than the instantaneous bubble surface energy, provided that surface instabilities have grown big enough to allow for break-up. From our simulations it follows that the Blake critical radius is not a good approximation for a fragmentation threshold. We demonstrated how the phase angle differences between a damped radially oscillating bubble and an incident sound field depend on shell parameters.

Piezoresistive accelerometers for MCM-package - Part II:The packaging

This paper is the second part of a twofold work related to the full integration of piezoresistive accelerometers in a multichip modules-deposited (MCM-D) type with flip-chip interconnection package. This part is fully devoted to the packaging of an accelerometer with movable parts in an MCM so a description of the package fabrication is given. The accelerometer was designed taking into account its final package and, therefore, some modifications to the standard piezoresistive accelerometer were done. In particular, a sealing ring was defined around the movable parts of the sensor in order to protect them from the underfill used during the final packaging process. This underfill is being studied at the moment so the tests performed have been without taking into account its effects. After the manufacturing step the MCM packaged accelerometers are attached to a PLCC44 substrate and characterized. The results show an improvement on the sensors performance when using the MCM solution in comparison to the accelerometers packaged in standard TO-8. The results show that the influence of the packaging technique on the final stress of the sensors is negligible. They also show an extra damping due to the confined air volume around the swinging proof mass of the accelerometers. The quality factor of the TO-8 packaged accelerometers is of 12 whereas for the MCM packaged devices is of 1, 2. This low-quality factor is of great interest for applications where the devices can suffer large acceleration changes. Finally, the results are in good agreement with the FEM simulations previously done and they demonstrate the viability of this technology.

Properties of time integration with first order filter damping

Introduction of algorithmic damping by increasing the parameter values in the Newmark algorithm leads to undesirable low-frequency damping and reduced order of accuracy. It is demonstrated, how these effects can be removed by introducing an extra damping term in the form of a first order linear filter. When the linear filter is discretized in time, the state variable associated with the filter can be eliminated, leading to a weighted average of the equations of motion at two consecutive times. The filter procedure contains the known versions of alpha weighted Newmark methods as special cases, but gives a different and improved weighting of the excitation terms. A complete analysis of the properties of the algorithm when used on linear systems is given, including the frequency response properties. It is demonstrated that the effect of ‘overshoot’ is the consequence of a conservation relation that operates on a modified form of the mechanical energy of the system, and analytic results are presented for the magnitude of the effect. Copyright © 2005 John Wiley & Sons, Ltd.

Constant temperature molecular dynamics simulations of energetic particle–solid collisions: comparison of temperature control methods

Appropriate temperature control methods must be incorporated into simulations that maintain constant temperature. A specific role played by these methods in modeling energetic particle–solid collisions is to absorb the excess energy wave generated by the collision that, if left unchecked, propagates through the material and reflects from the system boundaries. In this study, five temperature control methods are investigated for use in molecular dynamics simulations of carbon cluster deposition on diamond surfaces. These five methods are the Nosé–Hoover thermostat, the generalized Langevin equation (GLEQ) approach, the Berendsen method, a modified GLEQ approach where an extra damping mechanism is introduced, and a combination of the GLEQ and Berendsen methods. The temperature control capability and the effectiveness of these methods at reducing the amplitude of the reflected energy wave produced in these systems are compared and discussed. It is found that the realistic performance of these methods depends on the specifics of the system, including incident energy and substrate size. Among the five methods considered, the Berendsen method is found to be effective at removing excess energy in the early stages of the deposition process, but the resultant final temperatures are relatively high in most cases. Additionally, the performance of the Nosé–Hoover thermostat is similar to that of the Berendsen method. If the diamond substrate is large enough and the incident energy is not too high, the GLEQ approach is found to be sufficient for removal of excess energy. However, the modified GLEQ approach and the new, combined thermostat perform moderately better than the other approaches at high incident energies.

Modelling the static stiffness and dynamic frequency response characteristics of a leaf spring truck suspension

The suspension characteristics of a 10 ton truck with a highly non-linear leaf spring suspension have been investigated. The acceleration transmissibility frequency response characteristics of the truck were first investigated. Results showed that whereas the bounce (sprung mass) resonant frequency was quite close to that expected, the wheel-hop (unsprung mass) resonant frequencies were roughly twice that expected. As no obvious mechanism for this effect was evident, the static stiffness characteristics of the suspension were measured in an attempt to understand this phenomenon. A simple plot of the resulting suspension-generated force against suspension deflection showed that two distinct stiffness regimes were present; the first, which was associated with small suspension deflections of between 2 and 4 mm, was significantly greater than the second, which was associated with larger suspension deflections. A state-variable linear 5 degree-of-freedom spring mass damper model of the vehicle was created within MATLAB in an attempt to predict the frequency response characteristics of the vehicle. The state-variable model predictions were close to those seen experimentally for the sprung mass at frequencies below 3 Hz, but were quite incorrect for both the sprung and unsprung masses above this frequency. The state-variable model was then used again with increased values of suspension stiffness and results showed that using these higher values led to a more accurate predicted response for both the sprung and unsprung masses above 3 Hz, whereas below this frequency their responses were clearly incorrect. Comparison of experimental and predicted results showed that the vehicle frequency response was dominated above 3 Hz by the low suspension deflection-high stiffness regime identified in the static stiffness results, whereas below this frequency, the high suspension deflection- low stiffness regime tended to dominated the response. To overcome the problem of the transition from the low to the high stiffness regimes when predicting the frequency response characteristics, a non-linear model was created using MATLAB and SIMULINK. Results showed that the model was able to capture this transition, with resulting frequency response curves for both the sprung and unsprung masses being very similar to the experimental results. Further refinements to the model were able to account for additional discrepancies between predictions and experimental results. The non-linear nature of the spring stiffnesses, resulting in a very useful hysteresis damping effect at the sprung mass resonant frequency, meant that for most normal road conditions the vehicle would ride on a near solid suspension. The apparent advantages of the extra damping would therefore be o set by a significant decrease in ride quality, and this was especially evident in respect to road noise and vibration-a finding borne out in practice. This paper establishes the principal characteristics of the truck suspension and goes on to describe the linear state-variable and non-linear models created to simulate the frequency response characteristics of the vehicle suspension.

허니콤 구조물의 차음특성에 관한 연구

As a test specimen. an aluminum extruded panel with a dimension of 640 mm<TEX>$\times$</TEX>740 mm<TEX>$\times$</TEX>40mm is considered. This plate has 9 mm thickness if mass is concerned. Based on the FEM modeling in rigidity. the specimen turns out to be 32 mm and 12 mm thickness In isotropic steel plate. Also, the characteristics of transmission loss on the honeycomb structure have been examined experimentally with reverberation chamber. A honeycomb structure follows mass law in above 800 Hz. In order to improve the noise transmission effect in lower frequency, extra damping treatment is suggested. As a conclusion. the examined honeycomb structure Is designed to Improve the bending rigidity, not for the noise reduction.

Theory of the de Haas–van Alphen effect in type-II superconductors

Theories of quasiparticle spectra and the de Haas--van Alphen (dHvA) oscillation in type-II superconductors are developed based on the Bogoliubov--de Gennes equations for vortex-lattice states. As the pair potential grows through the superconducting transition, each degenerate Landau level in the normal state splits into quasiparticle bands in the magnetic Brillouin zone. This brings Landau-level broadening, which in turn leads to the extra dHvA oscillation damping in the vortex state. We perform extensive numerical calculations for three-dimensional systems with various gap structures. It is thereby shown that (i) this Landau-level broadening is directly connected with the average gap at $H=0$ along each Fermi-surface orbit perpendicular to the field $\mathbf{H},$ (ii) the extra dHvA oscillation attenuation is caused by the broadening around each extremal orbit. These results imply that the dHvA experiment can be a unique probe to detect band- and/or angle-dependent gap amplitudes. We derive an analytic expression for the extra damping based on the second-order perturbation with respect to the pair potential for the Luttinger-Ward thermodynamic potential. This formula reproduces all our numerical results excellently, and is used to estimate band-specific gap amplitudes from available data on ${\mathrm{NbSe}}_{2},$ ${\mathrm{Nb}}_{3}\mathrm{Sn},$ and ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}.$ The obtained value for ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ is fairly different from the one through a specific-heat measurement, indicating presence of gap anisotropy in this material.

REDUCED ORDER ADAPTIVE OBSERVER BACKSTEPPING

It is the intent of this paper to add to, and improve the existing design tools for nonlinear output feedback systems. Considered here are systems, exhibiting parametric uncertainty, and containing nonlinearities dependent on the output alone. A lower triangular state transformation is utilised, equivalent to that obtained via the traditional and recursive back stepping algorithm. Extra damping is introduced to counter the uncertainty and observer error effects. The benefits of this approach lie in the significant reduction in total controller dynamic order, as well as the simplicity and clarity provided by implementing back stepping from a purely structural perspective.

Gap Anisotropy and de Haas-van Alphen Effect in Type-II Superconductors

We present a theoretical study on the de Haas-van Alphen (dHvA) oscillation in the vortex state of type-II superconductors, with a special focus on the connection between the gap anisotropy and the oscillation damping. Numerical calculations for three different gap structures clearly indicate that the average gap along extremal orbits is relevant for the magnitude of the extra damping, thereby providing a support for experimental efforts to probe gap anisotropy through the dHvA signal. We also derive an analytic formula for the extra damping which gives a good fit to the numerical results.

Polyolefin fiber-reinforced concrete composites: Part II. Damping and interface debonding

Two damping mechanisms are suggested to explain the high damping found from preliminary dynamic testing of polyolefin fiber-reinforced concrete composites (FRC). A significant interfacial relative displacement, which may cause debonding and corresponding interfacial friction, is suggested as the fundamental energy dissipation mode during vibration. Such a displacement may easily take place over a great fraction of an interface between low-aspect ratio fiber and cementitious matrix, resulting in high damping in crimped FRC. Debonding at the intersections of microcracks in matrix and fiber due to the crack opening under tensile strain also consumes vibration energy, producing extra damping in fine FRC. In order to gain high damping without seriously degrading other mechanical properties of FRC, a combination of reinforcing fibers with different geometric features is suggested.

Separate determination of liquid density and viscosity with sagittally corrugated Love-mode sensors

A novel microacoustic sensor is reported for separate determination of liquid density and viscosity. The device is based on a layered quartz/SiO 2-system with sagittal corrugations, supporting acoustic waveguide Love-modes. The sensor combines the merits of Love-mode devices, e.g., robustness and high sensitivity, with liquid trapping as an effective approach to separate density and viscosity influences on acoustic shear modes. The outstanding features are extraordinary high and adjustable sensitivity to density changes with inherent large differences between density and viscosity responses. A model is discussed to describe the frequency response to liquid loading. Furthermore, studies of attenuation changes have shown that only little extra damping occurs due to the corrugations. Moreover, strategies are discussed for device optimisation in terms of temperature stability and sensitivity enhancement.