Showing papers on "Dissipation published in 2010"

Energy dissipation and transport in nanoscale devices

Abstract: Understanding energy dissipation and transport in nanoscale structures is of great importance for the design of energy-efficient circuits and energy-conversion systems. This is also a rich domain for fundamental discoveries at the intersection of electron, lattice (phonon), and optical (photon) interactions. This review presents recent progress in understanding and manipulation of energy dissipation and transport in nanoscale solid-state structures. First, the landscape of power usage from nanoscale transistors (∼10−8 W) to massive data centers (∼109 W) is surveyed. Then, focus is given to energy dissipation in nanoscale circuits, silicon transistors, carbon nanostructures, and semiconductor nanowires. Concepts of steady-state and transient thermal transport are also reviewed in the context of nanoscale devices with sub-nanosecond switching times. Finally, recent directions regarding energy transport are reviewed, including electrical and thermal conductivity of nanostructures, thermal rectification, and the role of ubiquitous material interfaces. Open image in new window

Coherent perfect absorbers: Time-reversed lasers

Abstract: An arbitrary body or aggregate can be made perfectly absorbing at discrete frequencies, if a precise amount of dissipation is added under specific conditions of coherent monochromatic illumination. This effect arises from the interaction of optical absorption and wave interference, and corresponds to moving a zero of the S-matrix onto the real wavevector axis. It is thus the time-reversed process of lasing at threshold. The effect may be demonstrated in a Si slab illuminated in the 500–900nm range. Coherent perfect absorbers form a novel class of linear optical elements—absorptive interferometers—which may be useful for controlled optical energy transfer.

Energy Dissipation and Transport in Nanoscale Devices

Abstract: Understanding energy dissipation and transport in nanoscale structures is of great importance for the design of energy-efficient circuits and energy-conversion systems. This is also a rich domain for fundamental discoveries at the intersection of electron, lattice (phonon), and optical (photon) interactions. This review presents recent progress in understanding and manipulation of energy dissipation and transport in nanoscale solid-state structures. First, the landscape of power usage from nanoscale transistors (~10^-8 W) to massive data centers (~10^9 W) is surveyed. Then, focus is given to energy dissipation in nanoscale circuits, silicon transistors, carbon nanostructures, and semiconductor nanowires. Concepts of steady-state and transient thermal transport are also reviewed in the context of nanoscale devices with sub-nanosecond switching times. Finally, recent directions regarding energy transport are reviewed, including electrical and thermal conductivity of nanostructures, thermal rectification, and the role of ubiquitous material interfaces.

Preparation and detection of a mechanical resonator near the ground state of motion

Abstract: Cold, macroscopic mechanical systems are expected to behave contrary to our usual classical understanding of reality; the most striking and counterintuitive predictions involve the existence of states in which the mechanical system is located in two places simultaneously. Various schemes have been proposed to generate and detect such states, and all require starting from mechanical states that are close to the lowest energy eigenstate, the mechanical ground state. Here we report the cooling of the motion of a radio-frequency nanomechanical resonator by parametric coupling to a driven, microwave-frequency superconducting resonator. Starting from a thermal occupation of 480 quanta, we have observed occupation factors as low as 3.8 ± 1.3 and expect the mechanical resonator to be found with probability 0.21 in the quantum ground state of motion. Further cooling is limited by random excitation of the microwave resonator and heating of the dissipative mechanical bath. This level of cooling is expected to make possible a series of fundamental quantum mechanical observations including direct measurement of the Heisenberg uncertainty principle and quantum entanglement with qubits.

Design and characterization of an electromagnetic energy harvester for vehicle suspensions

Abstract: During the everyday usage of an automobile, only 10–16% of the fuel energy is used to drive the car—to overcome the resistance from road friction and air drag. One important loss is the dissipation of vibration energy by shock absorbers in the vehicle suspension under the excitation of road irregularity and vehicle acceleration or deceleration. In this paper we design, characterize and test a retrofit regenerative shock absorber which can efficiently recover the vibration energy in a compact space. Rare-earth permanent magnets and high permeable magnetic loops are used to configure a four-phase linear generator with increased efficiency and reduced weight. The finite element method is used to analyze the magnetic field and guide the design optimization. A theoretical model is created to analytically characterize the waveforms and regenerated power of the harvester at various vibration amplitudes, frequencies, equilibrium positions and design parameters. It was found that the waveform and RMS voltage of the individual coils will depend on the equilibrium position but the total energy will not. Experimental studies of a 1:2 scale prototype are conducted and the results agree very well with the theoretical predictions. Such a regenerative shock absorber will be able to harvest 16–64 W power at 0.25–0.5 m s − 1 RMS suspension velocity.

Semi-empirical dissipation source functions for ocean waves: Part I, definition, calibration and validation. Fabrice ArdhuinJean-Francois Filipot and Rudy Magne Service Hydrographique et Oceanographique de la Marine, Brest, France

Abstract: New parameterizations for the spectral dissipation of wind-generated waves are proposed. The rates of dissipation have no predetermined spectral shapes and are functions of the wave spectrum, in a way consistent with observation of wave breaking and swell dissipation properties. Namely, swell dissipation is nonlinear and proportional to the swell steepness, and wave breaking only affects spectral components such that the non-dimensional spectrum exceeds the threshold at which waves are observed to start breaking. An additional source of short wave dissipation due to long wave breaking is introduced, together with a reduction of wind-wave generation term for short waves, otherwise taken from Janssen (J. Phys. Oceanogr. 1991). These parameterizations are combined and calibrated with the Discrete Interaction Approximation of Hasselmann et al. (J. Phys. Oceangr. 1985) for the nonlinear interactions. Parameters are adjusted to reproduce observed shapes of directional wave spectra, and the variability of spectral moments with wind speed and wave height. The wave energy balance is verified in a wide range of conditions and scales, from the global ocean to coastal settings. Wave height, peak and mean periods, and spectral data are validated using in situ and remote sensing data. Some systematic defects are still present, but the parameterizations probably yield the most accurate overall estimate of wave parameters to date. Perspectives for further improvement are also given.

Energy variational analysis of ions in water and channels: Field theory for primitive models of complex ionic fluids

Abstract: Ionic solutions are mixtures of interacting anions and cations. They hardly resemble dilute gases of uncharged noninteracting point particles described in elementary textbooks. Biological and electrochemical solutions have many components that interact strongly as they flow in concentrated environments near electrodes, ion channels, or active sites of enzymes. Interactions in concentrated environments help determine the characteristic properties of electrodes, enzymes, and ion channels. Flows are driven by a combination of electrical and chemical potentials that depend on the charges, concentrations, and sizes of all ions, not just the same type of ion. We use a variational method EnVarA (energy variational analysis) that combines Hamilton’s least action and Rayleigh’s dissipation principles to create a variational field theory that includes flow, friction, and complex structure with physical boundary conditions. EnVarA optimizes both the action integral functional of classical mechanics and the dissipation functional. These functionals can include entropy and dissipation as well as potential energy. The stationary point of the action is determined with respect to the trajectory of particles. The stationary point of the dissipation is determined with respect to rate functions (such as velocity). Both variations are written in one Eulerian (laboratory) framework. In variational analysis, an “extra layer” of mathematics is used to derive partial differential equations. Energies and dissipations of different components are combined in EnVarA and Euler–Lagrange equations are then derived. These partial differential equations are the unique consequence of the contributions of individual components. The form and parameters of the partial differential equations are determined by algebra without additional physical content or assumptions. The partial differential equations of mixtures automatically combine physical properties of individual (unmixed) components. If a new component is added to the energy or dissipation, the Euler–Lagrange equations change form and interaction terms appear without additional adjustable parameters. EnVarA has previously been used to compute properties of liquid crystals, polymer fluids, and electrorheological fluids containing solid balls and charged oil droplets that fission and fuse. Here we apply EnVarA to the primitive model of electrolytes in which ions are spheres in a frictional dielectric. The resulting Euler–Lagrange equations include electrostatics and diffusion and friction. They are a time dependent generalization of the Poisson–Nernst–Planck equations of semiconductors, electrochemistry, and molecular biophysics. They include the finite diameter of ions. The EnVarA treatment is applied to ions next to a charged wall, where layering is observed. Applied to an ion channel, EnVarA calculates a quick transient pile-up of electric charge, transient and steady flow through the channel, stationary “binding” in the channel, and the eventual accumulation of salts in “unstirred layers” near channels. EnVarA treats electrolytes in a unified way as complex rather than simple fluids. Ad hoc descriptions of interactions and flow have been used in many areas of science to deal with the nonideal properties of electrolytes. It seems likely that the variational treatment can simplify, unify, and perhaps derive and improve those descriptions.

Direct numerical simulation of hypersonic turbulent boundary layers. Part 2. Effect of wall temperature

Abstract: In this paper, we perform direct numerical simulation (DNS) of turbulent boundary layers at Mach 5 with the ratio of wall-to-edge temperature Tw/Tδ from 1.0 to 5.4 (Cases M5T1 to M5T5). The influence of wall cooling on Morkovin's scaling, Walz's equation, the standard and modified strong Reynolds analogies, turbulent kinetic energy budgets, compressibility effects and near-wall coherent structures is assessed. We find that many of the scaling relations used to express adiabatic compressible boundary-layer statistics in terms of incompressible boundary layers also hold for non-adiabatic cases. Compressibility effects are enhanced by wall cooling but remain insignificant, and the turbulence dissipation remains primarily solenoidal. Moreover, the variation of near-wall streaks, iso-surface of the swirl strength and hairpin packets with wall temperature demonstrates that cooling the wall increases the coherency of turbulent structures. We present the mechanism by which wall cooling enhances the coherence of turbulence structures, and we provide an explanation of why this mechanism does not represent an exception to the weakly compressible hypothesis.

A turbulence-driven model for heating and acceleration of the fast wind in coronal holes

Abstract: A model is presented for generation of fast solar wind in coronal holes, relying on heating that is dominated by turbulent dissipation of MHD fluctuations transported upward in the solar atmosphere. Scale-separated transport equations include large-scale fields, transverse Alfvenic fluctuations, and a small compressive dissipation due to parallel shears near the transition region. The model accounts for proton temperature, density, wind speed, and fluctuation amplitude as observed in remote sensing and in situ satellite data.

The relationship between near-surface turbulence and gas transfer velocity in freshwater systems and its implications for floating chamber measurements of gas exchange

Abstract: We performed a series of gas exchange measurements in 12 diverse aquatic systems to develop the direct relationship between near-surface turbulence and gas transfer velocity. The relationship was log-linear, explained 78% of the variation in instantaneous gas transfer velocities, and was valid over a range of turbulent energy dissipation rates spanning about two orders of magnitude. Unlike wind-based relationships, our model is applicable to systems ranging in size from less than 1 km2 to over 600 km2. Gas fluxes measured with our specific model of floating chambers can be grossly overestimated (by up to 1000%), particularly in low-turbulence conditions. In high-turbulence regimes, flux overestimation decreases to within 50%. Direct measurements of turbulent energy dissipation rate provide reliable estimation of the associated gas transfer velocity even at short temporal and spatial scales.

Balanced and Unbalanced Routes to Dissipation in an Equilibrated, Eady Flow

Abstract: The oceanic general circulation is forced at large scales and is unstable to mesoscale eddies. Large-scale currents and eddy flows are approximately in geostrophic balance. Geostrophic dynamics is characterized by an inverse energy cascade except for dissipation near the boundaries. In this paper, we confront the dilemma of how the general circulation may achieve dynamical equilibrium in the presence of continuous large-scale forcing and the absence of boundary dissipation. We do this with a forced horizontal flow with spatially uniform rotation, vertical stratification and vertical shear in a horizontally periodic domain, i.e. a version of Eady's flow carried to turbulent equilibrium. A direct route to interior dissipation is presented that is essentially non-geostrophic in its dynamics, with significant submesoscale frontogenesis, frontal instability and breakdown, and forward kinetic energy cascade to dissipation. To support this conclusion, a series of simulations is made with both quasigeostrophic and Boussinesq models. The quasigeostrophic model is shown as increasingly inefficient in achieving equilibration through viscous dissipation at increasingly higher numerical resolution (hence Reynolds number), whereas the non-geostrophic Boussinesq model equilibrates with only weak dependence on resolution and Rossby number.

Radiation and Dissipation of Internal Waves Generated by Geostrophic Motions Impinging on Small-Scale Topography: Theory

Abstract: Observations and inverse models suggest that small-scale turbulent mixing is enhanced in the Southern Ocean in regions above rough topography. The enhancement extends O(1) km above the topography, suggesting that mixing is supported by the breaking of gravity waves radiated from the ocean bottom. In this study, it is shown that the observed mixing rates can be sustained by internal waves generated by geostrophic motions flowing over bottom topography. Weakly nonlinear theory is used to describe the internal wave generation and the feedback of the waves on the zonally averaged flow. Vigorous inertial oscillations are driven at the ocean bottom by waves generated at steep topography. The wave radiation and dissipation at equilibrium is therefore the result of both geostrophic flow and inertial oscillations differing substantially from the classical lee-wave problem. The theoretical predictions are tested versus two-dimensional high-resolution numerical simulations with parameters representative of ...

A Method for Estimating the Turbulent Kinetic Energy Dissipation Rate from a Vertically Pointing Doppler Lidar, and Independent Evaluation from Balloon-Borne In Situ Measurements

Abstract: A method of estimating dissipation rates from a vertically pointing Doppler lidar with high temporal and spatial resolution has been evaluated by comparison with independent measurements derived from a balloon-borne sonic anemometer. This method utilizes the variance of the mean Doppler velocity from a number of sequential samples and requires an estimate of the horizontal wind speed. The noise contribution to the variance can be estimated from the observed signal-to-noise ratio and removed where appropriate. The relative size of the noise variance to the observed variance provides a measure of the confidence in the retrieval. Comparison with in situ dissipation rates derived from the balloon-borne sonic anemometer reveal that this particular Doppler lidar is capable of retrieving dissipation rates over a range of at least three orders of magnitude. This method is most suitable for retrieval of dissipation rates within the convective well-mixed boundary layer where the scales of motion that the D...

Estimating dissipation from single stationary trajectories.

Abstract: In this Letter we show that the time reversal asymmetry of a stationary time series provides information about the entropy production of the physical mechanism generating the series, even if one ignores any detail of that mechanism. We develop estimators for the entropy production which can detect nonequilibrium processes even when there are no measurable flows in the time series.

Iterative linearized density matrix propagation for modeling coherent excitation energy transfer in photosynthetic light harvesting.

Abstract: Rather than incoherent hopping between chromophores, experimental evidence suggests that the excitation energy transfer in some biological light harvesting systems initially occurs coherently, and involves coherent superposition states in which excitation spreads over multiple chromophores separated by several nanometers. Treating such delocalized coherent superposition states in the presence of decoherence and dissipation arising from coupling to an environment is a significant challenge for conventional theoretical tools that either use a perturbative approach or make the Markovian approximation. In this paper, we extend the recently developed iterative linearized density matrix (ILDM) propagation scheme [E. R. Dunkel et al., J. Chem. Phys. 129, 114106 (2008)] to study coherent excitation energy transfer in a model of the Fenna–Matthews–Olsen light harvesting complex from green sulfur bacteria. This approach is nonperturbative and uses a discrete path integral description employing a short time approxim...

Cyclic behavior of precast segmental concrete bridge columns with high performance or conventional steel reinforcing bars as energy dissipation bars

Abstract: The cyclic behavior of precast segmental concrete bridge columns with high performance (HP) steel reinforcing bars and that with conventional steel reinforcing bars as energy dissipation (ED) bars were investigated. The HP steel reinforcing bars are characterized by higher strength, greater ductility, and superior corrosion resistance compared with the conventional steel reinforcing bars. Three large-scale columns were tested. One was designed with the HP ED bars and two with the conventional ED bars. The HP ED bars were fully bonded to the concrete. The conventional ED bars were fully bonded to the concrete for one column, whereas unbonded for a length to delay fracture of the bars and to increase energy dissipation for the other column. Test results showed that the column with the HP ED bars had greater drift capacity, higher lateral strength, and larger energy dissipation than that with fully bonded conventional ED bars. The column with unbonded conventional ED bars achieved the same drift capacity and similar energy dissipation capacity as that with the HP ED bars. All the three columns showed good self-centering capability with residual drifts not greater than 0.4% drift. An analytical model referred to as joint bar-slip rotation method for pushover analysis of segmental columns with ED bars is proposed. The model calculates joint rotation from the slip of the ED bars from two sides of the joint. Good agreement was found between analytical predictions and the envelope responses of the three columns. Copyright © 2010 John Wiley & Sons, Ltd.

Attempts to improve energy absorption characteristics of circular metal tubes subjected to axial loading

Abstract: In this paper, experimental investigation of two new structural design solutions with the aim of improving crashworthiness characteristics of cylindrical metal tubes is performed. In the first design method, a rigid steel ring is press-fitted on top of circular aluminum tubes. When this arrangement of dissipating energy is subjected to axial compression, the rigid ring is driven into the cylindrical tube and expands its top area; then, plastic folds start shaping along the rest of the tube length as the compression of the structure continues. In the second design method, wide grooves are cut from the outer surface of steel thick-walled circular tubes. In fact, this method converts thick-walled tubes into several thin-walled tubes of shorter length, being assembled together coaxially. When this energy absorbing device is subjected to axial compression, plastic deformation occurs within the space of each wide groove, and thick portions control and stabilize collapsing of the whole structure. In the present study, several specimens of each developed design methods with various geometric parameters are prepared and compressed quasi-statistically. Also, some ordinary tubes of the same size of these specimens are compressed axially to investigate efficiency of the presented structural solutions in energy absorption applications. Experimental results show the significant efficiency of the presented design methods in improving crashworthiness characteristics and collapse modes of circular tubes under axial loading.

Statistical properties of visibility graph of energy dissipation rates in three-dimensional fully developed turbulence

Abstract: We study the statistical properties of complex networks constructed from time series of energy dissipation rates in three-dimensional fully developed turbulence using the visibility algorithm. The degree distribution is found to have a power-law tail with the tail exponent α = 3.0 . The exponential relation between the number of the boxes N B and the box size l B based on the edge-covering box-counting method illustrates that the network is not self-similar, which is also confirmed by the hub-hub attraction according to the visibility algorithm. In addition, it is found that the skeleton of the visibility network exhibits excellent allometric scaling with the scaling exponent η = 1.163 ± 0.005 .

Unsteady friction and visco-elasticity in pipe fluid transients

Abstract: The current waterhammer models coupled with friction models cannot adequately represent the pressure wave attenuation observed in real-world pipe systems, because the pressure wave damping is affected by additional effects not accounted for. One such effect is pipe visco-elasticity from the material behaviour of pipe-wall to be investigated herein. The numerical results indicate that the pressure head attenuation attributable to unsteady friction is comparable to the visco-elastic effect during the initial transient stage, while the visco-elastic effect becomes dominant both in terms of damping and phase shift at later stages. An analytical analysis shows that the visco-elastic effect is more critical if the visco-elastic retardation time is less than the wave travel time along the entire pipeline length. In addition, it is demonstrated that the visco-elastic term in waterhammer models is wrongly referred to in the literature as energy dissipation instead of energy transfer between fluid and pipe-wall by ...

Self-centering structural systems with combination of hysteretic and viscous energy dissipations

Abstract: SUMMARY 7 This paper presents an innovative set of high-seismic-resistant structural systems termed Advanced FlagShaped (AFS) systems, where self-centering elements are combined in series and/or in parallel with 9 alternative forms of energy dissipation (yielding, friction and viscous damping). AFS systems is developed using the rationale of combining velocity-dependent with displacement-dependent energy dissipation for 11 self-centering systems, particularly to counteract near-fault earthquakes. Non-linear time-history analyses (NLTHA) on a set of four single-degree-of-freedom (SDOF) systems under a suite of 20 far-field and 13 20 near-fault ground motions are used to compare the seismic performance of AFS systems with the conventional systems. It is shown that AFS system with a combination of hysteretic and viscous energy 15 dissipations achieved greater performance in terms of the three performance indices. The use of friction slip in series of viscous energy dissipation is shown to limit the peak response acceleration and induced 17 base-shear. An extensive parametric analysis is carried out to investigate the influence of two design parameters, 1 and 2 on the response of SDOF AFS systems with initial periods ranging from 0.2 to 3.0s 19 and with various strength levels when subjected to far-field and near-fault earthquakes. For the design of self-centering systems with combined hysteretic and viscous energy dissipation (AFS) systems, 1 is 21 recommended to be in the range of 0.8−1.6 while 2 to be between 0.25 and 0.75 to ensure sufficient self-centering and energy dissipation capacities, respectively. Copyright q 2009 John Wiley & Sons, Ltd. 23

Dissipation coefficients from scalar and fermion quantum field interactions

Abstract: Dissipation coefficients are calculated in the adiabatic, near thermal equilibrium regime for a large class of renormalizable interaction configurations involving a two-stage mechanism, where a background scalar field is coupled to heavy intermediate scalar or fermion fields which in turn are coupled to light scalar or fermion radiation fields. These interactions are typical of warm inflation microscopic model building. Two perturbative regimes are shown where well defined approximations for the spectral functions apply. One regime is at high temperature, when the masses of both intermediate and radiation fields are less than the temperature scale and where the poles of the spectral functions dominate. The other regime is at low temperature, when the intermediate field masses are much bigger than the temperature and where the low energy and low three-momentum regime dominate the spectral functions. The dissipation coefficients in these two regimes are derived. However, due to resummation issues for the high temperature case, only phenomenological approximate estimates are provided for the dissipation in this regime. In the low temperature case, higher loop contributions are suppressed and so no resummation is necessary. In addition to inflationary cosmology, the application of our results to cosmological phase transitions is also discussed.

Enhancing performance of network-on-chip architectures with millimeter-wave wireless interconnects

Abstract: In a traditional Network-on-Chip (NoC), latency and power dissipation increase with system size due to its inherent multi-hop communications. The performance of NoC communication fabrics can be significantly enhanced by introducing long-range, low power, high bandwidth direct links between far apart cores. In this paper a design methodology for a scalable hierarchical NoC with on-chip millimeter (mm)-wave wireless links is proposed. The proposed wireless NoC offers significantly higher throughput and lower energy dissipation compared to its conventional multi-hop wired counterpart. It is also demonstrated that the proposed hierarchical NoC with long range wireless links shows significant performance gains in presence of various application-specific traffic and multicast scenarios.

Energy dissipation in two-dimensional unsteady plunging breakers and an eddy viscosity model

Abstract: An experimental study of energy dissipation in two-dimensional unsteady plunging breakers and an eddy viscosity model to simulate the dissipation due to wave breaking are reported in this paper. Measured wave surface elevations are used to examine the characteristic time and length scales associated with wave groups and local breaking waves, and to estimate and parameterize the energy dissipation and dissipation rate due to wave breaking. Numerical tests using the eddy viscosity model are performed and we find that the numerical results well capture the measured energy loss. In our experiments, three sets of characteristic time and length scales are defined and obtained: global scales associated with the wave groups, local scales immediately prior to breaking onset and post-breaking scales. Correlations among these time and length scales are demonstrated. In addition, for our wave groups, wave breaking onset predictions using the global and local wave steepnesses are found based on experimental results. Breaking time and breaking horizontal length scales are determined with high-speed imaging, and are found to depend approximately linearly on the local wave steepness. The two scales are then used to determine the energy dissipation rate, which is the ratio of the energy loss to the breaking time scale. Our experimental results show that the local wave steepness is highly correlated with the measured dissipation rate, indicating that the local wave steepness may serve as a good wave-breaking-strength indicator. To simulate the energy dissipation due to wave breaking, a simple eddy viscosity model is proposed and validated with our experimental measurements. Under the small viscosity assumption, the leading-order viscous effect is incorporated into the free-surface boundary conditions. Then, the kinematic viscosity is replaced with an eddy viscosity to account for energy loss. The breaking time and length scales, which depend weakly on wave breaking strength, are applied to evaluate the magnitude of the eddy viscosity using dimensional analysis. The estimated eddy viscosity is of the order of 10 −3 m 2 s −1 and demonstrates a strong dependence on wave breaking strength. Numerical simulations with the eddy viscosity estimation are performed to compare to the experimental results. Good agreement as regards energy dissipation due to wave breaking and surface profiles after wave breaking is achieved, which illustrates that the simple eddy viscosity model functions effectively.

Determination of the crystal-melt interface kinetic coefficient from molecular dynamics simulations

Abstract: The generation and dissipation of latent heat at the moving solid–liquid boundary during non-equilibrium molecular dynamics (MD) simulations of crystallization can lead to significant underestimations of the interface mobility. In this work we examine the heat flow problem in detail for an embedded atom description of pure Ni and offer strategies to obtain an accurate value of the kinetic coefficient, μ. For free-solidification simulations in which the entire system is thermostated using a Nose–Hoover or velocity rescaling algorithm a non-uniform temperature profile is observed and a peak in the temperature is found at the interface position. It is shown that if the actual interface temperature, rather than the thermostat set point temperature, is used to compute the kinetic coefficient then μ is approximately a factor of 2 larger than previous estimates. In addition, we introduce a layered thermostat method in which several sub-regions, aligned normal to the crystallization direction, are indepently thermostated to a desired undercooling. We show that as the number of thermostats increases (i.e., as the width of each independently thermostated layer decreases) the kinetic coefficient converges to a value consistent with that obtained using a single thermostat and the calculated interface temperature. Also, the kinetic coefficient was determined from an analysis of the equilibrium fluctuations of the solid–liquid interface position. We demonstrate that the kinetic coefficient obtained from the relaxation times of the fluctuation spectrum is equivalent to the two values obtained from free-solidification simulations provided a simple correction is made for the contribution of heat flow controlled interface motion. Finally, a one-dimensional phase field model that captures the effect of thermostats has been developed. The mesoscale model reproduces qualitatively the results from MD simulations and thus allows for an a priori estimate of the accuracy of a kinetic coefficient determination for any given classical MD system. The model also elucidates that the magnitude of the temperature gradients obtained in simulations with a single thermostat depends on the length of the simulation system normal to the interface; the need for the corrections discussed in this paper can thus be gauged from a study of the dependence of the calculated kinetic coefficient on system size.

Sources and sinks of climate entropy

Abstract: A satisfactory climate model must have similar sources and sinks of entropy to those of the atmosphere itself. This paper examines some of the theoretical and numerical issues involved in entropy sources and sinks. A balance equation is developed that involves a balance between entropy destruction by emission and absorption of radiation and entropy production by irreversible molecular processes. Estimating the thermal molecular source from turbulent enthalpy fluxes is shown to be consistent with the second law of thermodynamics only for free convection. Four entropy sources connected with the dissipation of kinetic energy are discussed. Moist convection is treated in terms of the mixing of warm/dry and cool/moist air masses. Questions are raised about the validity of treating latent heat as an external heat source acting upon a dry atmosphere. Theoretical estimates of entropy sources are compared with earlier estimates by Peixoto and others, and to the sources and sinks of entropy in a Goddard Institute of Space Studies climate control run. There are many discrepancies, particularly for moist convection. Our knowledge of atmospheric sources and sinks of entropy is evidently poor and, even if they were known, it would be difficult to construct a climate model with sources and sinks as precise as Johnson considers to be necessary.

Energy Production, Frictional Dissipation, and Maximum Intensity of a Numerically Simulated Tropical Cyclone*

Abstract: A tropical cyclone (TC) viewed as a heat engine converts heat energy extracted from the ocean into the kinetic energy of the TC, which is eventually dissipated due to surface friction. Since the energy production rate is a linear function while the frictional dissipation rate is a cubic power of surface wind speed, the dissipation rateisgenerallysmaller thantheproductionrateinitiallybutincreasesfasterthantheproductionrate asthestormintensifies.Whenthedissipationrateeventuallyreachestheproductionrate,theTChasnoexcess energy to intensify. Emanuel hypothesized that a TC achieves its maximum potential intensity (E-MPI) when the surface frictional dissipation rate balances the energy production rate near the radius of maximum wind (RMW). Although theE-MPIagreeswell withthemaximumintensityofnumericallysimulated TCsinearlier axisymmetricmodels,thebalancehypothesisneartheRMWhasnotbeenevaluated.Thisstudyshowsthatthe frictional dissipation rate in a numerically simulated mature TC is about 25% larger than the energy production rate near the RMW, while the dissipation rate is lower than the energy production rate outside the eyewall. This finding implies that the excess frictional dissipation under the eyewall should be partially balanced by the energy production outside the eyewall and thus the local balance hypothesis underestimates the TC maximum intensity. Both Lagrangian and control volume equivalent potential temperature (ue) budget analysesdemonstratethattheenergygainedbyboundarylayerinflowairduetosurfaceentropyfluxesoutside of and prior to interaction with the eyewall contributes significantly to the energy balance in the eyewall through the lateral inward energy flux. This contribution is further verified using a sensitivity experiment in whichthesurfaceentropyfluxesareeliminatedoutsidearadiusof30‐45 km,whichleadstoa13.5%reduction in the maximum sustained near-surface wind speed and a largely reduced size of the model TC.