Dominant Dissipation Mechanism Research Articles

Shoreline dissipation of infragravity waves

Infragravity waves (0.005–0.05Hz) have recently been observed to dissipate a large part of their energy in the short-wave (0.05–1Hz) surf zone, however, the underlying mechanism is not well understood. Here, we analyse two new field data sets of near-bed pressure and velocity at up to 13 cross-shore locations in ≲2.5m depth on a ≈1:80 and a ≈1:30 sloping beach to quantify infragravity-wave dissipation close to the shoreline and to identify the underlying dissipation mechanism. A frequency-domain Complex Eigenfunction analysis demonstrated that infragravity-wave dissipation was frequency dependent. Infragravity waves with a frequency larger than ≈0.0167–0.0245Hz were predominantly onshore progressive, indicative of strong dissipation of the incoming infragravity waves. Instead, waves with a lower frequency showed the classic picture of cross-shore standing waves with minimal dissipation. Bulk infragravity reflection coefficients at the shallowest position (water depth ≈0.7m) were well below 1 (≈0.20), implying that considerable dissipation took place close to the shoreline. We hypothesise that for our data sets infragravity-wave breaking is the dominant dissipation mechanism close to the shoreline, because the reflection coefficient depends on a normalised bed slope, with the higher infragravity frequencies in the mild-sloping regime where breaking is known to dominate dissipation. Additional numerical modelling indicates that, close to the shoreline of a 1:80 beach, bottom friction contributes to infragravity-wave dissipation to a limited extent, but that non-linear transfer of infragravity energy back to sea–swell frequencies is unimportant.

Fast Collisionless Reconnection and Electron Heating in Strongly Magnetized Plasmas

Magnetic reconnection in strongly magnetized (low-beta), weakly collisional plasmas is investigated by using a novel fluid-kinetic model [Zocco and Schekochihin, Phys. Plasmas 18, 102309 (2011)] which retains nonisothermal electron kinetics. It is shown that electron heating via Landau damping (linear phase mixing) is the dominant dissipation mechanism. In time, electron heating occurs after the peak of the reconnection rate; in space, it is concentrated along the separatrices of the magnetic island. For sufficiently large systems, the peak reconnection rate is cE(∥)(max) ≈ 0.2v(A)B(y,0), where v(A) is the Alfvén speed based on the reconnecting field B(y,0). The island saturation width is the same as in magnetohydrodynamics models except for small systems, when it becomes comparable to the kinetic scales.

Dynamic contact angle of water-based titanium oxide nanofluid

This paper presents an investigation into spreading dynamics and dynamic contact angle of TiO2-deionized water nanofluids. Two mechanisms of energy dissipation, (1) contact line friction and (2) wedge film viscosity, govern the dynamics of contact line motion. The primary stage of spreading has the contact line friction as the dominant dissipative mechanism. At the secondary stage of spreading, the wedge film viscosity is the dominant dissipative mechanism. A theoretical model based on combination of molecular kinetic theory and hydrodynamic theory which incorporates non-Newtonian viscosity of solutions is used. The model agreement with experimental data is reasonable. Complex interparticle interactions, local pinning of the contact line, and variations in solid–liquid interfacial tension are attributed to errors.

Microwave spectroscopy of vortex dynamics in ortho-II YBa2Cu3O6.52

We present measurements of the vortex-state surface impedance, Z_s = R_s + i X_s, of a high quality, ortho-II-ordered single crystal of the cuprate high temperature superconductor YBa2Cu3O6.52 (T_c = 59K). Measurements have been made at four microwave frequencies (\omega/2\pi = 2.64, 4.51, 9.12 and 13.97 GHz), for magnetic fields ranging from 0 to 7 T. From these data we obtain the field, frequency and temperature dependence of the vortex viscosity, pinning constant, depinning frequency and flux-flow resistivity. The vortex viscosity, \eta(\omega,T), has a surprisingly strong frequency dependence and bears a striking resemblance to the zero-field quasiparticle conductivity, \sigma_qp(\omega,T), suggesting that the dominant dissipative mechanism for the flux lines is induced electric fields coupling to bulk, long-lived d-wave quasiparticles outside the vortex cores. This is in sharp contrast to the conventional Bardeen-Stephen picture, in which dissipation takes place inside quasi-normal vortex cores. The strong frequency dependence of the vortex viscosity in the microwave range requires us to treat it as a complex response function, with an imaginary part that is predicted to contribute to the apparent pinning force on the vortices. Measurements of the frequency dependence of the pinning force confirm that this term is present, and in a form consistent with the requirements of causality. At low temperatures the flux-flow resistivity, \rho_ff \propto 1/\eta, has the form \rho_ff(T) = \rho_0 + \rho_1 \ln(1/T), reminiscent of the DC resistivity of cuprates in the pseudogap regime.

The coupling between foam viscoelasticity and interfacial rheology

We study the impact of interfacial rheology on the linear viscoelastic relaxations of disordered 3D foams. Their complex shear modulus G* = G′ + iG′′ is measured as a function of frequency, bubble size, liquid viscosity and surface dilatational modulus. We show that for foams with very rigid interfaces the variations of G* with frequency f between 1 and 100 Hz deviate from the behavior previously predicted as a generic consequence of topological disorder and observed for foams with mobile or moderately rigid interfaces. Our experiments demonstrate that the relaxations slow down as the bubble size or the foaming liquid viscosity increases. We show under which conditions, depending on the interfacial rigidity, the loss factor G′′/G′ can be collapsed on master curves by applying a scale factor to the frequency. The dependence of this scale factor on the bubble size and viscosity constitutes a robust criterion helping to identify the dominant dissipation mechanism.

The influence of Langmuir turbulence on the scaling for the dissipation rate in the oceanic boundary layer

A model for estimating the turbulent kinetic energy dissipation rate in the oceanic boundary layer, based on insights from rapid‐distortion theory, is presented and tested. This model provides a possible explanation for the very high dissipation levels found by numerous authors near the surface. It is conceived that turbulence, injected into the water by breaking waves, is subsequently amplified due to its distortion by the mean shear of the wind‐induced current and straining by the Stokes drift of surface waves. The partition of the turbulent shear stress into a shear‐induced part and a wave‐induced part is taken into account. In this picture, dissipation enhancement results from the same mechanism responsible for Langmuir circulations. Apart from a dimensionless depth and an eddy turn‐over time, the dimensionless dissipation rate depends on the wave slope and wave age, which may be encapsulated in the turbulent Langmuir numberLat. For large Lat, or any Lat but large depth, the dissipation rate tends to the usual surface layer scaling, whereas when Lat is small, it is strongly enhanced near the surface, growing asymptotically as ε ∝ Lat−2 when Lat → 0. Results from this model are compared with observations from the WAVES and SWADE data sets, assuming that this is the dominant dissipation mechanism acting in the ocean surface layer and statistical measures of the corresponding fit indicate a substantial improvement over previous theoretical models. Comparisons are also carried out against more recent measurements, showing good order‐of‐magnitude agreement, even when shallow‐water effects are important.

Classical and lateral skin effect contributions estimation in strip MR coils

AbstractRadiofrequency (RF) transmitter and receiver coils in magnetic resonance imaging systems are used, respectively, to excite the nuclei in the object to be imaged and to pick up the signals emitted by the nuclei. The quality of obtained images strongly depends upon the correct choice of the coils geometry and typology. For low‐field magnetic resonance imaging systems, the dominant power dissipation mechanisms are mainly due to conductor losses and the coils performance are affected by cross‐sectional shape of RF coil conductors. This work proposes a method for the estimation of the different skin effects contribution to the total conductor resistance for coil built using strip conductor. The evaluation of coil resistance was performed at different tuning frequencies usually used in clinical scanner (21–128 MHz). © 2012 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 41B: 57–61, 2012

Parameterization of bottom friction under combined wave-tide action in the Hooghly estuary, India

A comprehensive knowledge of the wave climate is an essential pre-requisite for practical applications in coastal and near-shore region. In coastal areas the dominant dissipative mechanism appears to be bottom friction where the relative strength depends on bottom characteristics, sediment type and bottom ripple geometry. The present state-of-art version SWAN wave model uses three popular formulations for dissipative mechanism due to bottom interaction. The bottom friction coefficient as in SWAN cannot be a constant and needs to be tuned based on prevailing hydrodynamic conditions when applied for a tidal dominant region. A combined wave-current interaction study should take into account varying water levels, reversal of current system and associated shear on the sea-bed. The present work reports on development of a new resistance law for bottom friction under the combined action of waves and currents. This new formulation has been implemented in the SWAN model and further its implication on variability of significant wave height for the Hooghly estuary located in the Bay of Bengal, East coast of India was investigated by comparison with ENVISAT satellite measurement. Based on this study, it could be ascertained that new friction formulation has potential for wind-wave modeling studies in near-shore and coastal waters.

Ultrathin Single Crystal Diamond Nanomechanical Dome Resonators

We present the first nanomechanical resonators microfabricated in single-crystal diamond. Shell-type resonators only 70 nm thick, the thinnest single crystal diamond structures produced to date, demonstrate a high-quality factor (Q ≈ 1000 at room temperature, Q ≈ 20 000 at 10 K) at radio frequencies (50-600 MHz). Quality factor dependence on temperature and frequency suggests an extrinsic origin to the dominant dissipation mechanism and methods to further enhance resonator performance.

High-Q micromechanical resonators for mass sensing in dissipative media

Single crystal silicon-based micromechanical resonators are developed for mass sensing in dissipative media. The design aspects and preliminary characterization of the resonators are presented. For the suggested designs, quality factors of about 20 000 are typically measured in air at atmospheric pressure and 1000–2000 in contact with liquid. The performance is based on a wine-glass-type lateral bulk acoustic mode excited in a rectangular resonator plate. The mode essentially eliminates the radiation of acoustic energy into the sample media leaving viscous drag as the dominant fluid-based dissipation mechanism in the system. For a mass loading distributed over the central areas of the resonator a sensitivity of 27 ppm ng−1 is measured exhibiting good agreement with the results of the finite element method-based simulations. It is also shown that the mass sensitivity can be somewhat enhanced, not only by the proper distribution of the loaded mass, but also by introducing shallow barrier structures on the resonator.

Molecular transport and flow past hard and soft surfaces: computer simulation of modelsystems

The equilibrium and flow of polymer films and drops past a surface are characterized bythe interface and surface tensions, viscosity, slip length and hydrodynamic boundaryposition. These parameters of the continuum description are extracted from molecularsimulations of coarse-grained models. Hard, corrugated substrates are modelled by aLennard-Jones solid while polymer brushes are studied as prototypes of soft, deformablesurfaces. Four observations are discussed. (i) If the surface becomes strongly attractive or iscoated with a brush, the Navier boundary condition fails to describe the effect of thesurface independently of the strength and type of the flow. This failure stems from theformation of a boundary layer with an effective, higher viscosity. (ii) In the case ofbrush-coated surfaces, flow induces a cyclic, tumbling motion of the tethered chainmolecules. Their collective motion gives rise to an inversion of the flow in the vicinityof the grafting surfaces and leads to strong, non-Gaussian fluctuations of themolecular orientations. The flow past a polymer brush cannot be described byBrinkman’s equation. (iii) The hydrodynamic boundary condition is an importantparameter for predicting the motion of polymer droplets on a surface under theinfluence of an external force. Their steady-state velocity is dictated by a balancebetween the power that is provided by the external force and the dissipation.If there is slippage at the liquid–solid interface, the friction at the solid–liquidinterface and the viscous dissipation of the flow inside the drop will be the dominantdissipation mechanisms; dissipation at the three-phase contact line appears to be lessimportant on a hard surface. (iv) On a soft, deformable substrate like a polymer brush,we observe a lifting-up of the three-phase contact line. Controlling the graftingdensity and the incompatibility between the brush and the polymer liquid we canindependently tune the softness of the surface and the contact angle and therebyidentify the parameters for maximizing the deformation at the three-phase contact.

Low-Field MR Coils: Comparison between Strip and Wire Conductors

This work describes how the cross-sectional shape of radio-frequency coil conductors affects coils performance. This is of particular importance at low Larmor frequencies such as those of low-field magnetic resonance imaging systems where conductor and capacitor losses are the dominant power dissipation mechanisms. We demonstrate that conductors having a circular cross section allow the coil to achieve significantly better performance than the one built using flat strips. The change in coil quality factor due to conductor geometry was verified to be due only to changes in the conductors’ resistance and not their inductance. The results are not limited to low-field proton imaging but they are equally applicable to other situations where the Larmor frequency is in the megahertz range, including nuclear magnetic resonance of other nuclear species at intermediate fields.

A computational and experimental study of resonators in three dimensions

In a previous work by the present authors, a computational and experimental investigation of the acoustic properties of two-dimensional slit resonators was carried out. The present paper reports the results of a study extending the previous work to three dimensions. This investigation has two basic objectives. The first is to validate the computed results from direct numerical simulations of the flow and acoustic fields of slit resonators in three dimensions by comparing with experimental measurements in a normal incidence impedance tube. The second objective is to study the flow physics of resonant liners responsible for sound wave dissipation. Extensive comparisons are provided between computed and measured acoustic liner properties with both discrete frequency and broadband sound sources. Good agreements are found over a wide range of frequencies and sound pressure levels. Direct numerical simulation confirms the previous finding in two dimensions that vortex shedding is the dominant dissipation mechanism at high sound pressure intensity. However, it is observed that the behavior of the shed vortices in three dimensions is quite different from those of two dimensions. In three dimensions, the shed vortices tend to evolve into ring (circular in plan form) vortices, even though the slit resonator opening from which the vortices are shed has an aspect ratio of 2.5. Under the excitation of discrete frequency sound, the shed vortices align themselves into two regularly spaced vortex trains moving away from the resonator opening in opposite directions. This is different from the chaotic shedding of vortices found in two-dimensional simulations. The effect of slit aspect ratio at a fixed porosity is briefly studied. For the range of liners considered in this investigation, it is found that the absorption coefficient of a liner increases when the open area of the single slit is subdivided into multiple, smaller slits.

DAMPING OF MAGNETOHYDRODYNAMIC TURBULENCE IN PARTIALLY IONIZED GAS AND THE OBSERVED DIFFERENCE OF VELOCITIES OF NEUTRALS AND IONS

Theoretical and observational studies on the turbulence of the interstellar medium developed fast in the past decades. The theory of supersonic magnetized turbulence, as well as the understanding of projection effects of observed quantities, are still in progress. In this work we explore the characterization of the turbulent cascade and its damping from observational spectral line profiles. We address the difference of ion and neutral velocities by clarifying the nature of the turbulence damping in the partially ionized. We provide theoretical arguments in favor of the explanation of the larger Doppler broadening of lines arising from neutral species compared to ions as arising from the turbulence damping of ions at larger scales. Also, we compute a number of MHD numerical simulations for different turbulent regimes and explicit turbulent damping, and compare both the 3-dimensional distributions of velocity and the synthetic line profile distributions. From the numerical simulations, we place constraints on the precision with which one can measure the 3D dispersion depending on the turbulence sonic Mach number. We show that no universal correspondence between the 3D velocity dispersions measured in the turbulent volume and minima of the 2D velocity dispersions available through observations exist. For instance, for subsonic turbulence the correspondence is poor at scales much smaller than the turbulence injection scale, while for supersonic turbulence the correspondence is poor for the scales comparable with the injection scale. We provide a physical explanation of the existence of such a 2D-3D correspondence and discuss the uncertainties in evaluating the damping scale of ions that can be obtained from observations. However, we show that the statistics of velocity dispersion from observed line profiles can provide the spectral index and the energy transfer rate of turbulence. Also, comparing two similar simulations with different viscous coefficients it was possible to constrain the turbulent cut-off scale. This may especially prove useful since it is believed that ambipolar diffusion may be one of the dominant dissipative mechanism in star-forming regions. In this case, the determination of the ambipolar diffusion scale may be used as a complementary method for the determination of magnetic field intensity in collapsing cores. We discuss the implications of our findings in terms of a new approach to magnetic field measurement proposed by Li & Houde (2008).

Experimental and theoretical investigation of the energy dissipation of a rolling disk during its final stage of motion

This paper is concerned with the dominant dissipation mechanism for a rolling disk in the final stage of its motion. The aim of this paper is to present the various dissipation mechanisms for a rolling disk which are used in the literature in a unified framework. Furthermore, new experiments on the ‘Euler disk’ using a high-speed video camera and a novel image analysis technique are presented. The combined experimental/theoretical approach of this paper sheds some more light on the dominant dissipation mechanism on the time-scale of several seconds.

Damping mechanisms of single-clamped and prestressed double-clamped resonant polymer microbeams

In this article, an investigation of the damping mechanisms of resonant single- and double-clamped polymer microbeams for a frequency range from 10 kHz to 5 MHz is presented. The suspended structures are made of SU-8, an epoxy-type photoresist, by means of a sacrificial layer technique. The vibration was measured with a laser-Doppler vibrometer in high vacuum at different temperatures and at atmospheric pressure. The influence of air damping in rarefied air was investigated and the intrinsic damping mechanisms were determined in high vacuum (p<0.05 Pa). After excluding a variety of possible damping factors, the dominant intrinsic dissipation mechanism of the single-clamped microbeams was understood to be the material damping with maximum quality factors (Q) of around 70 at 20 °C. Quality factors of up to 720 at 20 °C were measured for stringlike double-clamped microbeams, which suggest a different intrinsic damping mechanism than material loss. It is shown that internal damping mechanisms due to flexure and elongation have a small impact on the damping of stretched strings. Modeling the clamping loss based on the wave transmission into the suspended anchor plates indicates that it is the dominant intrinsic dissipation in the prestressed double-clamped microresonators. At atmospheric pressure it was shown that at low frequencies the quality factors of single-clamped and stringlike double-clamped microbeams are limited by the squeeze-film air damping. At high frequencies the quality factors are limited by the specific intrinsic damping. In between the two particular regions with a specific dominant damping mechanism the quality factors show a maximum.

MODELING OF KNEE ARTICULAR CARTILAGE DISSIPATION DURING GAIT ANALYSIS

Articular cartilage dissipates contact loads according to three dissipative mechanisms: frictional drag, intrinsic viscoelasticity, and surface friction. Estimation of dissipation due to these three mechanisms during gait is required to understand the dissipative properties of articular cartilage. Fourteen healthy subjects performed a gait analysis on treadmill. Tibiofemoral contact forces were estimated from inverse dynamic analysis and from a reductionist knee contact model. These contact forces and the results obtained from a preloading creep simulation were introduced into a biphasic poroviscoelastic articular cartilage model, and a one-dimensional confined compression was performed. Articular dissipation from each dissipative mechanism was estimated. Sensitivity analysis was performed to determine the effects of material parameters and length of the preloading simulation on the patterns of the dissipative mechanisms. Dissipative force patterns for all dissipative mechanisms were found to be similar to those of tibiofemoral contact forces. Frictional drag was found to be the dominant dissipative mechanism. The initial permeability and the viscoelastic spectrum parameters were found to have an important impact on the magnitude of the peaks of dissipative patterns. If appropriate material parameters are introduced, this model could be used to compare the difference between healthy and osteoarthritic human articular cartilage.

Energy dissipation in non-uniform mangrove forests of arbitrary depth

This study aims to develop a predictive model of wave propagation through non-uniform forest in water of arbitrary depth. The theoretical model solves a full boundary value problem for wave propagation with dissipation for non-uniform mangrove forest and arbitrary water depth. In particular, wave−trunk interaction and wave breaking were found to be the dominant dissipation mechanisms. A modified mild-slope equation including dissipation is applied for wave model over changing water depth within the mangrove forest. The non-linear governing equations for wave−trunk interactions are linearised using the concept of stochastic minimalisation. The numerical results of the model are verified and compared with the experimental wave data in Can Gio Mangrove Biosphere Reserve, Southern Vietnam. The results for wave height, wave spectrum and wave-induced velocities as well as for the coefficients of transmission and reflection suggest that most of the energy is dissipated within the mangrove forest even at relatively small distance. The effect of wave breaking plays a more important role on wave attenuation in sparse forest, however, it is smaller compared to the effect of wave–trunk interactions in the denser forest.

Evolution and morphology of saucer-shaped sills in analogue experiments

AbstractThe growth of shallow sills is studied in analogue experiments performed in polymethyl methacrylate (PMMA) and glass. The experimental fractures curve towards the surface to become saucer-shaped, which is consistent with many field observations of dolerite sills. The curvature of the saucer is shown to decrease as the in situ stress acting parallel to the surface increases relative to an estimate of the strength of the fracture-induced stress field. The initially circular fractures also elongate in plan view to become egg-shaped, a tendency that decreases with increasing importance of viscous dissipation in the growth process. Sill emplacement is further examined mathematically by considering a shallow, circular, fluid-driven fracture propagating in a homogeneous brittle elastic material. The fractures are shown to undergo three transitions related to the mechanics of sill growth. Each transition is associated with a characteristic time that is derived from analysis of the governing equations using scaling methods. These characteristic times provide an estimate of how long viscous flow is the dominant energy dissipation mechanism, how long significant lag between the fluid and fracture fronts is expected to persist, and how long the sill will take to attain an extent that is of the same order as its depth.

Influence of Boundary Conditions on the Dynamic Characteristics of Squeeze Films in MEMS Devices

Micromechanical structures that have squeeze-film damping as the dominant energy dissipation mechanism are of interest in this paper. For such structures with narrow air gap, the Reynolds equation is used for calculating squeeze-film damping, which is generally solved with trivial pressure boundary conditions on the side walls. This procedure, however, fails to give satisfactory results for structures under two important conditions: 1) for an air gap thickness comparable to the lateral dimensions of the microstructure and 2) for nontrivial pressure boundary conditions such as fully open boundaries on an extended substrate or partially blocked boundaries that provide side clearance to the fluid flow. Several formulas exist to account for simple boundary conditions. In practice, however, there are many micromechanical structures such as torsional microelectromechanical system (MEMS) structures that have nontrivial boundary conditions arising from partially blocked boundaries. Such boundaries usually have clearance parameters that can vary due to fabrication. These parameters, however, can also be used as design parameters if we understand their role on the dynamics of the structure. We take a MEMS torsion mirror as an example device that has large air gap and partially blocked boundaries due to static frames. We actuate the device and experimentally determine the quality factor Q from the response measurements. Next, we model the same structure in ANSYS and carry out computational fluid dynamics analysis to evaluate the stiffness constant K, the damping constant D, and the quality factor Q due to the squeeze film. We compare the computational results with experimental results and show that without taking care of the partially blocked boundaries properly in the computational model, we get unacceptably large errors.