Showing papers on "Dissipation published in 2000"

Irreversibility and heat generation in the computing process

Abstract: It is argued that computing machines inevitably involve devices which perform logical functions that do not have a single-valued inverse. This logical irreversibility is associated with physical irreversibility and requires a minimal heat generation, per machine cycle, typically of the order of kT for each irreversible function. This dissipation serves the purpose of standardizing signals and making them independent of their exact logical history. Two simple, but representative, models of bistable devices are subjected to a more detailed analysis of switching kinetics to yield the relationship between speed and energy dissipation, and to estimate the effects of errors induced by thermal fluctuations.

Theory of the lattice boltzmann method: dispersion, dissipation, isotropy, galilean invariance, and stability

Abstract: The generalized hydrodynamics (the wave vector dependence of the transport coefficients) of a generalized lattice Boltzmann equation (LBE) is studied in detail. The generalized lattice Boltzmann equation is constructed in moment space rather than in discrete velocity space. The generalized hydrodynamics of the model is obtained by solving the dispersion equation of the linearized LBE either analytically by using perturbation technique or numerically. The proposed LBE model has a maximum number of adjustable parameters for the given set of discrete velocities. Generalized hydrodynamics characterizes dispersion, dissipation (hyper-viscosities), anisotropy, and lack of Galilean invariance of the model, and can be applied to select the values of the adjustable parameters which optimize the properties of the model. The proposed generalized hydrodynamic analysis also provides some insights into stability and proper initial conditions for LBE simulations. The stability properties of some 2D LBE models are analyzed and compared with each other in the parameter space of the mean streaming velocity and the viscous relaxation time. The procedure described in this work can be applied to analyze other LBE models. As examples, LBE models with various interpolation schemes are analyzed. Numerical results on shear flow with an initially discontinuous velocity profile (shock) with or without a constant streaming velocity are shown to demonstrate the dispersion effects in the LBE model; the results compare favorably with our theoretical analysis. We also show that whereas linear analysis of the LBE evolution operator is equivalent to Chapman-Enskog analysis in the long wave-length limit (wave vector k = 0), it can also provide results for large values of k. Such results are important for the stability and other hydrodynamic properties of the LBE method and cannot be obtained through Chapman-Enskog analysis.

A scale-dependent dynamic model for large-eddy simulation: application to a neutral atmospheric boundary layer

Abstract: A scale-dependent dynamic subgrid-scale model for large-eddy simulation of turbulent flows is proposed. Unlike the traditional dynamic model, it does not rely on the assumption that the model coefficient is scale invariant. The model is based on a second test-filtering operation which allows us to determine from the simulation how the coefficient varies with scale. The scale-dependent model is tested in simulations of a neutral atmospheric boundary layer. In this application, near the ground the grid scale is by necessity comparable to the local integral scale (of the order of the distance to the wall). With the grid scale and/or the test-filter scale being outside the inertial range, scale invariance is broken. The results are compared with those from (a) the traditional Smagorinsky model that requires specification of the coefficient and of a wall damping function, and (b) the standard dynamic model that assumes scale invariance of the coefficient. In the near-surface region the traditional Smagorinsky and standard dynamic models are too dissipative and not dissipative enough, respectively. Simulations with the scale-dependent dynamic model yield the expected trends of the coefficient as a function of scale and give improved predictions of velocity spectra at different heights from the ground. Consistent with the improved dissipation characteristics, the scale-dependent model also yields improved mean velocity profiles.

Sensitivity and power deposition in a high-field imaging experiment.

Abstract: Image signal-to-noise ratio and power dissipation are investigated theoretically up to 400 MHz. While the text is mathematical, the figures give insights into predictions. Hertz potential is introduced for probe modeling where charge separation cannot be ignored. Using a spherical geometry, the potential from current loops that would produce a homogeneous static B1 field is calculated; at high frequency it is shown to create an unnecessarily inhomogeneous field. However, a totally homogeneous field is shown to be unattainable. Boundary conditions are solved for circularly polarized fields, and strategies for limited shimming of the sample B1 field are then presented. A distinction is drawn between dielectric resonance and spatial field focusing. At high frequency, the region of maximum specific absorption is shown to move inside the sample and decrease. From the fields in both rotating frames, the signal-to-noise ratio is derived and compared with the traditional, low-frequency formulation. On average, it is mostly found to be slightly larger at high frequency. Nevertheless, the free induction decay is sometimes found to be annulled.

A thermodynamic frame work for rate type fluid models

Abstract: In this paper, we develop a thermodynamic approach for modeling a class of viscoelastic fluids based on the notion of an `evolving natural configuration'. The material has a family of elastic (or non-dissipative) responses governed by a stored energy function that is parametrized by the `natural configurations'. Changes in the current natural configuration result in dissipative behavior that is determined by a rate of dissipation function. Specifically, we assume that the material possesses an infinity of possible natural (or stress-free) configurations. The way in which the current natural configuration changes is determined by a `maximum rate of dissipation' criterion subject to the constraint that the difference between the stress power and the rate of change of the stored energy is equal to the rate of dissipation. By choosing different forms for the stored energy function ψ and the rate of dissipation function ξ, a whole plethora of energetically consistent rate type models can be developed. We show that the choice of a neo-Hookean type stored energy function and a rate of dissipation function that is quadratic, leads to a Maxwell-like fluid response. By using this procedure with a different choice for the rate of dissipation, we also derive a model that is similar to the Oldroyd-B model. We also discuss several limiting cases, including the limit of small elastic strains, but arbitrarily large total strains, which leads to the classical upper convected Maxwell model as well as the Oldroyd-B model.

Static compaction techniques to control scan vector power dissipation

Abstract: Excessive switching activity during scan testing can cause average power dissipation and peak power during test to be much higher than during normal operation. This can cause problems both with heat dissipation and with current spikes. Compacting scan vectors greatly increases the power dissipation for the vectors (generally the power becomes several times greater). The compacted scan vectors often can exceed the power constraints and hence cannot be used. It is shown here that by carefully selecting the order in which pairs of test cubes are merged during static compaction, both average power and peak power for the final test set can be greatly reduced. A static compaction procedure is presented that can be used to find a minimal set of scan vectors that satisfies constraints on both average power and peak power. The proposed approach is simple yet effective and can be easily implemented in the conventional test vector generation flow used in industry today.

Enhanced semi-passive damping using continuous switching of a piezoelectric device on an inductor

Abstract: The SSD technique proposed here addresses the problem of resonance damping on a mechanical structure. SSD stands for Synchronized Switch Damping. Apart from active techniques, passive ones consist in connecting a piezoelectric insert attached to the structure to a passive electric network in which the energy generated by the piezoelectric inserts is degraded. In the semi passive approach, the piezoelectric inserts are continuously switched from open circuit to short circuit synchronously to the structure motion. Due to this switching mechanism, a phase shift appears between the piezoelectric strain and the resulting voltage, thus creating energy dissipation. For the new technique proposed here, instead of discharging the piezoelectric inserts during a brief short circuit, they are connected on a small inductor, allowing the inversion of the voltage and then released to open circuit. In this case the voltage amplitude is optimized and is 90 degrees out of phase with the motion then enhancing the damping mechanism. The technique is applicable at any frequency without the need for a large tuned inductor, especially for low frequency applications. There is no need for external power supply unless for the low power circuitry of the switch device. The implementation of the switch drive with a very cheap micro-controller is described. Experimental results measured on cantilever beams made with different materials are proposed. Damping ability ranges from 6 dB on a very viscoelastic epoxy beam to nearly 20 dB on a steel beam. Harmonic excitation and transient results are both proposed and compared. Finally, an electromechanical model is proposed, giving an interpretation of the damping mechanism. Theoretical predictions are in good agreement with the experiments.

Vibration control with state-switched piezoelectric materials

Abstract: In this paper, a semi-active control law is used to switch the electrical shunt circuit of a piezoelectric actuator for energy dissipation in a simple mechanical system. Switching is carried out between open-circuit (high stiffness) and short- or resistive-circuit (low stiffness) states. The actuator is held in its high stiffness state when the system is moving such that energy can be stored in the actuator. When the system’s motion would cause it to receive energy back from the actuator, the actuator is switched to a low stiffness state, dissipating the energy. In this paper, the concept is developed starting with the governing piezoelectric equations. Numerical simulation results are presented which show that the technique provides energy dissipation that is comparable to other piezoelectric shunt mechanisms, using optimal shunt resistance in each case. Finally, a brief analysis is presented in which the piezoelectric element is used in a flexible beam element to illustrate the effects of a parallel str...

Effect of viscous, viscoplastic and friction damping on the response of seismic isolated structures

Abstract: In this paper the efficiency of various dissipative mechanisms to protect structures from pulse-type and near-source ground motions is examined. Physically realizable cycloidal pulses are introduced, and their resemblance to recorded near-source ground motions is illustrated. The study uncovers the coherent component of some near-source acceleration records, and the shaking potential of these records is examined. It is found that the response of structures with relatively low isolation periods is substantially affected by the high-frequency fluctuations that override the long duration pulse. Therefore, the concept of seismic isolation is beneficial even for motions that contain a long duration pulse which generates most of the unusually large recorded displacements and velocities. Dissipation forces of the plastic (friction) type are very efficient in reducing displacement demands although occasionally they are responsible for substantial permanent displacements. It is found that the benefits by hysteretic dissipation are nearly indifferent to the level of the yield displacement of the hysteretic mechanism and that they depend primarily on the level of the plastic (friction) force. The study concludes that a combination of relatively low friction and viscous forces is attractive since base displacements are substantially reduced without appreciably increasing base shears and superstructure accelerations. Copyright © 2000 John Wiley & Sons, Ltd.

The Eddy Dissipation Turbulence Energy Cascade Model

Abstract: The turbulence energy cascade model used in the Eddy Dissipation Concept for combusting flow is presented and discussed in relation to existing knowledge of relevant turbulent flows. The cascade consists of a stepwise model for energy transfer from larger to smaller scales and for energy dissipation from each scale level by viscous forces. The cascade model makes a connection between the viscous fine structures, where combustion takes place, and the larger transporting eddies which are simulated by turbulence models. Thus, fine-structure quantities are expressed in terms of turbulence energy and dissipation. The model is compared to turbulence-energy-spectrum data for the inertial subrange and the dissipative range for nonreacting and reacting flows. The model is also discussed in relation to isotropic decaying turbulence in the transition from initial to final periods of decay. It is concluded that the energy cascade model captures important features of the turbulence structural interaction and ...

Information-theoretic limits of control

Abstract: Fundamental limits on the controllability of physical systems are discussed in the light of information theory. It is shown that the second law of thermodynamics, when generalized to include information, sets absolute limits to the minimum amount of dissipation required by open-loop control. In addition, an information-theoretic analysis of control systems shows feedback control to be a zero sum game: each bit of information gathered from a dynamical system by a control device can serve to decrease the entropy of that system by at most one bit additional to the reduction of entropy attainable without such information. Consequences for the control of discrete state systems and chaotic maps are discussed.

Theoretical evaluation of the distributed power dissipation in biological cells exposed to electric fields.

Abstract: The paper deals with the power dissipation caused by exposure of biological cells to electric fields of various frequencies. With DC and sub-MHz AC frequencies, power dissipation in the cell membrane is of the same order of magnitude as in the external medium. At MHz and GHz frequencies, dielectric relaxation leads to dielectric power dissipation gradually increasing with frequency, and total power dissipation within the membrane rises significantly. Since such local increase can lead to considerable biochemical and biophysical changes within the membrane, especially at higher frequencies, the bulk treatment does not provide a complete picture of effects of an exposure. In this paper, we theoretically analyze the distribution of power dissipation as a function of field frequency. We first discuss conductive power dissipation generated by DC exposures. Then, we focus on AC fields; starting with the established first-order model, which includes only conductive power dissipation and is valid at sub-MHz frequencies, we enhance it in two steps. We first introduce the capacitive properties of the cytoplasm and the external medium to obtain a second-order model, which still includes only conductive power dissipation. Then we enhance this model further by accounting for dielectric relaxation effects, thereby introducing dielectric power dissipation. The calculations show that due to the latter component, in the MHz range the power dissipation within the membrane significantly exceeds the value in the external medium, while in the lower GHz range this effect is even more pronounced. This implies that even in exposures that do not cause a significant temperature rise at the macroscopic, whole-system level, the locally increased power dissipation in cell membranes could lead to various effects at the microscopic, single-cell level.

Tsunamis: Non-Breaking and Breaking Solitary Wave Run-Up

Abstract: This study considers the run-up of non-breaking and breaking solitary waves on a smooth sloping beach. A non-linear theory and a numerical model solving the non-linear shallow water equations (NLSW) were developed to model this physical process. Various experiments to obtain wave amplitude time-histories, water particle velocities, wave free-surface profiles, and maximum run-up were conducted and the results were compared with the analytical and numerical models. A higher order theoretical solution to the non-linear shallow water equations, which describes the non-breaking wave characteristics on the beach, was sought and presented in this study. The solution was obtained analytically by using the Carrier and Greenspan (1958) hodograph transformation. It was found that the non-linear theory agreed well with experimental results. The maximum run-up predicted by the non-linear theory is larger than that predicted by Synolakis (1986) at the order of the offshore relative wave height for a given slope. This correction for non-breaking waves on beach decreases as the beach slope steepens, and increases as the relative incident solitary wave height increases. A unique run-up gage that consists of a laser and a photodiode camera was developed in connection with this study to measure the time-history of the tip of the run-up tongue of a non-breaking solitary wave as it progresses up the slope. The results obtained with this run-up gage agree well with other measurements and provides a simple and reliable way of measuring run-up time histories. The run-up of breaking solitary waves was studied experimentally and numerically since no fully theoretical approach is possible. The wave characteristics such as wave shape and shoaling characteristics, and, for plunging breakers, the shape of the jet produced are presented. The experimental results show that wave breaking is such a complicated process that even sophisticated numerical models cannot adequately model its details. Two different plunging wave breaking and resultant run-up were found from the experiments. The point, where the tip of the incident jet produced by the plunging breaking wave impinges determines the characteristics of the resulting splash-up. If the jet impinges on a dry slope, no splash-up occurs and the plunging breaker simply collapses. If the impingement point is located on the free-surface, splash-up including a reflected jet is formed, which further increases the turbulence and energy dissipation associated with wave breaking. It is hypothesized that both clockwise and counter clockwise vortices may be generated by the impinging plunging jet and the reflected jet associated with the splash-up when the jet impinges on the front face of a breaking wave or on the still water surface in front of the wave. If only the run-up process and maximum run-up are of interest, the wave and the water flow produced after breaking can be simplified as a propagating bore, which is analogous to a shock wave in gas dynamics. A numerical model using this bore structure to treat the process of wave breaking and propagation was developed. The non-linear shallow water equations were solved using the weighted essentially non-oscillatory (WENO) shock capturing scheme employed in gas dynamics. Wave breaking and propagation is handled automatically 1w this scheme and no ad-hoc term is required. A computational domain mapping technique proposed by Zhang (1996) is used in the numerical scheme to model the shoreline movement. This numerical scheme is found to provide a somewhat simple and reasonably good prediction of various aspects of the run-up process. The numerical results agree well with the experiments corresponding to the run-up on a. relatively steep slope (1:2.08) as well as on a more gentle slope (1:19.85). A simple empirical estimate of maximum run-up based on energy conservation considerations is also presented where the energy dissipation associated with wave breaking was estimated using the results from the numerical model. This approach appears to be useful and the maximum run-up predicted agrees reasonably well with the experimental results. The splash-up of a solitary wave on a vertical wall positioned at different locations on a gentle slope was also investigated in this study to understand the degree of protection from tsunamis afforded by seawalls. It was found that the effect of breaking wave kinematics offshore of the vertical wall on the splash-up is of critical importance to the maximum splash-up. The maximum slope of the front face of the wave upon impingement of the wave on the wall, which represents the maximum water particle acceleration, was important in defining the maximum sheet splash-up as well as the trend for splash-up composed of drops and spray.

The Connection between Bubble Size Spectra and Energy Dissipation Rates in the Upper Ocean

Abstract: A formula for the maximum size of a bubble for which surface tension forces can prevent bubble breakup by inertial forces, combined with the observed sizes of air bubbles in breaking waves, implies an energy dissipation rate. One dataset from the surf zone gives a dissipation rate of the order of 0.1 W kg−1, but the large number of small bubbles, and the bubble size spectrum generally, are puzzling. A simple dimensional cascade argument suggests that injected air beneath a breaking wave is rapidly broken up by turbulence, producing an initial size spectrum proportional to (radius)−10/3 before modification by dissolution and rising under buoyancy. This spectral slope is comparable with data from the surf zone. The cascade argument does, however, predict that for a constant dissipation rate there is a rapid accumulation of a large number of bubbles at the scale at which surface tension prevents further breakup; it is possible that the observed size spectrum reflects the range of turbulent energy di...

Ion Cyclotron Wave Dissipation in the Solar Corona: The Summed Effect of More than 2000 Ion Species

Abstract: In this paper the dissipation of ion cyclotron resonant Alfven waves in the extended solar corona is examined in detail. For the first time, the wave damping arising from more than 2000 low-abundance ion species is taken into account. Useful approximations for the computation of coronal ionization equilibria for elements heavier than nickel are presented. Also, the Sobolev approximation from the theory of hot-star winds is applied to the resonant wave dissipation in the solar wind, and the surprisingly effective damping ability of "minor" ions is explained in simple terms. High-frequency (10-10,000 Hz) waves propagating up from the base of the corona are damped significantly when they resonate with ions having charge-to-mass ratios of about 0.1, and negligible wave power would then be available to resonate with higher charge-to-mass ratio ions at larger heights. This result confirms preliminary suggestions from earlier work that the waves that heat and accelerate the high-speed solar wind must be generated throughout the extended corona. The competition and eventual equilibrium between wave damping and wave replenishment may explain observed differences in coronal O VI and Mg X emission line widths.

Electroporation dynamics in biological cells subjected to ultrafast electrical pulses: a numerical simulation study.

Abstract: A model analysis of electroporation dynamics in biological cells has been carried out based on the Smoluchowski equation. Results of the cellular response to short, electric pulses are presented, taking account of the growth and resealing dynamics of transient aqueous pores. It is shown that the application of large voltages alone may not be sufficient to cause irreversible breakdown, if the time duration is too short. Failure to cause irreversible damage at small pulse widths could be attributed to the time inadequacy for pores to grow and expand beyond a critical threshold radius. In agreement with earlier studies, it is shown that irreversible breakdown would lead to the formation of a few large pores, while a large number of smaller pores would appear in the case of reversible breakdown. Finally, a pulse width dependence of the applied voltage for irreversible breakdown has been obtained. It is shown that in the absence of dissipation, the associated energy input necessary reduces with decreasing pulse width to a limiting value. However, with circuit effects taken into account, a local minima in the pulse dependent energy function is predicted, in keeping with previously published experimental reports.

Turbulent kinetic energy dissipation rate estimation from PIV velocity vector fields

Abstract: Particle image velocimetry (PIV) offers a new possibility for turbulence kinetic energy dissipation rate measurements. It can be used for the estimation of the turbulent kinetic energy dissipation rate using its definitions. Dissipation rate definitions include spatial derivatives of the velocity fluctuation components. Two different estimates for two-dimensional and isotropic definitions calculated from PIV results have been compared with direct numerical simulations in one case. Results show that the spatial resolution achieved is a critical factor in the accuracy of the computed dissipation rate. An error in velocity measurements can dominate the measured dissipation rate. The optimal result for the estimate will be achieved if the interrogation area is neither too large (under-sampling phenomenon) nor too small (noise in the measurement data/physical limits of the PIV system).

Toward achieving energy efficiency in presence of deep submicron noise

Abstract: Presented in this paper are: 1) information-theoretic lower bounds on energy consumption of noisy digital gates and 2) the concept of noise tolerance via coding for achieving energy efficiency in the presence of noise. In particular, lower bounds on a) circuit speed f/sub c/ and supply voltage V/sub dd/; b) transition activity t in presence of noise; c) dynamic energy dissipation; and d) total (dynamic and static) energy dissipation are derived. A surprising result is that in a scenario where dynamic component of power dissipation dominates, the supply voltage for minimum energy operation (V/sub dd, opt/) is greater than the minimum supply voltage (V/sub dd, min/)for reliable operation. We then propose noise tolerance via coding to approach the lower bounds on energy dissipation. We show that the lower bounds on energy for an off-chip I/O signaling example are a factor of 24/spl times/ below present day systems. A very simple Hamming code can reduce the energy consumption by a factor of 3/spl times/, while Reed-Muller (RM) codes give a 4/spl times/ reduction in energy dissipation.

On the Efficiency of Internal Shocks in Gamma-Ray Bursts

Abstract: The fraction of a fireball kinetic energy that is radiated by internal shocks is sensitive to the amplitude of initial fluctuations in the fireball. We give a simple analytical description for the dissipation of modest-amplitude fluctuations and confirm it with direct numerical simulations. At high amplitudes, the dissipation occurs in a nonlinear regime with efficiency approaching 100%. Most of the fireball energy can then be radiated away by the prompt gamma-ray burst, and only a fraction remains for the afterglow.

Scalar mixing and dissipation rate in large-eddy simulations of non-premixed turbulent combustion

Abstract: Predictions of scalar mixing and the scalar dissipation rate from large-eddy simulations of a piloted nonpremixed methane/air diffusion flame (Sandia flame D) using the Lagrangian-type flamelet model are presented. The results obtained for the unconditionally filtered scalar dissipation rate are qualitatively compared with general observations of scalar mixing from experiments in non-reactive and reactive jets. In agreement with experimental data, provided the reaction zone has an inward direction, regions of high scalar dissipation rate are organized in layerlike structures, inwardly inclined to the mean flow and aligned with the instantaneous reaction zone. The analysis of single-point time records of the mixture fraction reveals ramplike structures, which have also been observed experimentally and are believed to indicate large-scale turbulent structures. The probability density function (pdf) of the instantaneous resolved scalar dissipation rate at stoichiometric mixture evaluated at cross sections normal to the the nozzle axis is shown to be described accurately by a lognormal pdf with σ=1. A new model for the conditionally averaged scalar dissipation rate has been proposed and is shown to account for local deviations from the simple mixing layer structure. The stabilizing effect of the pilot flame in the present configuration is also discussed. Finally, the influence of the resolved fluctuations of the scalar dissipation rate on the flame structure is investigated, revealing only a weak influence on temperature and nitric oxide predictions. However, the model requires further refinement for situations in which local extinction events become important.

Dissipation of Slow Magnetosonic Waves in Coronal Plumes

Abstract: Recently, slow magnetosonic waves were identified in polar plumes, at heights up to about 1.2 R☉ using the Extreme Ultraviolet Imaging Telescope (EIT) observations of quasi-periodic EUV intensity fluctuations, and higher in the corona using the Ultraviolet Coronagraph Spectrometer (UVCS) white-light channel. First, we derive the linear dispersion relation for the slow waves in the viscous plasma. Next, we derive and solve an evolutionary equation of the Burgers type for the slow waves, incorporating the effects of radial stratification, quadratic nonlinearity, and viscosity. Finally, we model the propagation and dissipation of slow magnetosonic waves in polar plumes using one-dimensional and two-dimensional MHD codes in spherical geometry. The waves are launched at the base of the corona with a monochromatic source. We find that the slow waves nonlinearly steepen as they propagate away from the Sun into the solar wind. The nonlinear steepening of the waves leads to enhanced dissipation owing to compressive viscosity at the wave fronts. The efficient dissipation of the slow wave by compressive viscosity leads to damping of the waves within the first solar radii above the surface. We investigate the parametric dependence of the wave properties.

Microfluid Dynamics and Acoustics of Resonant Liners

Abstract: It is known that most of the acoustic dissipation associated with a resonant liner takes place around the openings of the resonators. However, because the openings are physically very small, there has not been any direct experimental observation of the flow and acoustic fields in this region. As a result, current understanding of liner dissipation mechanisms are either completely theoretical or are based on experiments using much larger physical models. Inasmuch as large openings were used in these experiments, the true Reynolds numbers were unfortunately not reproduced. The flow around and inside a typical liner resonator under the excitation of an incident acoustic field is investigated by direct numerical simulation (DNS). There are two distinct advantages in using DNS. First, by the use of a carefully designed grid, even very small-scale features of the flowfield can be resolved and observed. Second, the correct Reynolds number can be imposed in the simulations. Numerical experiments reveal that at low sound intensity, acoustic dissipation comes mainly from viscous losses in the jetlike unsteady laminar boundary layers adjacent to the walls of the resonator opening. At high sound intensity, dissipation is due to the shedding of microvortices at the mouth of the resonator. The energy dissipation rate associated with the shedding of microvortices is found to be very high. Results of a parametric study of this phenomenon are reported.

Impact of nonlinear waves on the dissipation of internal tidal energy at a shelf break

Abstract: The vertical and temporal structure of the dissipation of turbulent kinetic energy within the internal tide at a location 5 km shoreward of the shelf break on the Malin Shelf has been determined using a combination of the free-falling light yo-yo profiler and acoustic doppler current profilers. Two distinct internal wave regimes were encountered: period I in which large-amplitude high-frequency nonlinear internal waves (NIWs) occurred (around neap tides) and period II in which the internal wave spectral continuum was not dominated by any particular frequency band (around spring tides). Empirical orthogonal function analysis shows that for the low-frequency waves, 76% of the variance was described by mode 1, rising to 95% for the high-frequency waves. During period I the dissipation and vertical mixing were characterized by the NIWs, and 70% of the dissipation occurred in the bottom boundary layer. During period II the depth-integrated dissipation was more evenly distributed throughout the tidal cycle, whereas vertical mixing was greatly enhanced during a single hour long episode of elevated thermocline dissipation coincident with weakened stratification. During both periods I and II ∼30% of the total measured dissipation occurred within the thermocline when averaged over 12.4 hours; the remainder occurred within the bottom boundary layer(BBL). Tidal average values for depth-integrated dissipation and vertical eddy diffusivity for period I (II) were 1.1×10−2 W m−2 (4.0×10−2 W m−2) and 5 cm2 s−1 (12 cm2 s−1), respectively. Decay rates and internal damping are discussed, and vertical heat fluxes are estimated. Observed dissipation rates are compared with a simple model for BBL dissipation.

Temperature-dependent internal friction in silicon nanoelectromechanical systems

Abstract: We report the temperature-dependent mechanical properties of nanofabricated silicon resonators operating in the megahertz range. Reduction of temperature leads to an increase of the resonant frequencies of up to 6.5%. Quality factors as high as 1000 and 2500 are observed at room temperature in metallized and nonmetallized devices, respectively. Although device metallization increases the overall level of dissipation, internal friction peaks are observed in all devices in the T=160–180 K range.