Showing papers on "Dissipation published in 1997"
TL;DR: It is shown theoretically that viscoelastic layers with thicknesses comparable to the biofilms studied in this work can induce energy dissipation of the same magnitude as the measured ones.
Abstract: We have measured the energy dissipation of the quartz crystal microbalance(QCM), operating in the liquid phase, when mono- or multi-layers of bio-molecules and biofilms form on the QCM electrode (with a time resolution of ca. 1 s). Examples are taken from protein adsorption, lipid vesicle adsorption and cell adhesion studies. Our results show that even very thin (a few nm) biofilms dissipate a significant amount of energy owing to the QCM oscillation. Various mechanisms for this energy dissipation are discussed. Three main contributions to the measured increase in energy dissipation are considered. (i) A viscoelastic porous structure (the biofilm) that is strained during oscillation, (ii) trapped liquid that moves between or in and out of the pores due to the deformation of the film and (iii) the load from the bulk liquid which increases the strain of the film. These mechanisms are, in reality, not entirely separable, rather, they constitute an effective viscoelastic load. The biofilms can therefore not be considered rigidly coupled to the QCM oscillation. It is further shown theoretically that viscoelastic layers with thicknesses comparable to the biofilms studied in this work can induce energy dissipation of the same magnitude as the measured ones.
667 citations
TL;DR: In this paper, a thermodynamic framework is presented for the plasticity modelling of geotechnical materials, which is capable of modelling rigorously both friction and non-associated flow, and the strong connection between these phenomena is demonstrated.
Abstract: A thermodynamic framework is presented for the plasticity modelling of geotechnical materials. The framework is capable of modelling rigorously both friction and non-associated flow, and the strong connection between these phenomena is demonstrated. The formulation concentrates on the development of constitutive models from hypotheses about the form of an energy potential and a dissipation function. The reformulation of previous work, in which the Helmholtz free energy was used, to a new approach starting from the Gibbs free energy is found to be valuable. The relationship between the new functions and classical plasticity concepts (yield surface, plastic potential, isotropic and kinematic hardening, friction, dilation) is demonstrated. Comments are made on elastic-plastic coupling. Implications of the new approach for critical state soil models are discussed.
410 citations
TL;DR: In this paper, the influence of rotation on the spectral energy transfer of homogeneous turbulence is investigated, and a model for the derivative-skewness factor is defined, which only involves a micro-Rossby number Ro(sup omega) = omega'/(2(OMEGA)).
Abstract: The influence or rotation on the spectral energy transfer of homogeneous turbulence is investigated in this paper. Given the fact that linear dynamics, e.g. the inertial waves regime tackled in an RDT (Rapid Distortion Theory) fashion, cannot Affect st homogeneous isotropic turbulent flow, the study of nonlinear dynamics is of prime importance in the case of rotating flows. Previous theoretical (including both weakly nonlinear and EDQNM theories), experimental and DNS (Direct Numerical Simulation) results are gathered here and compared in order to give a self-consistent picture of the nonlinear effects of rotation on tile turbulence. The inhibition of the energy cascade, which is linked to a reduction of the dissipation rate, is shown to be related to a damping due to rotation of the energy transfer. A model for this effect is quantified by a model equation for the derivative-skewness factor, which only involves a micro-Rossby number Ro(sup omega) = omega'/(2(OMEGA))-ratio of rms vorticity and background vorticity as the relevant rotation parameter, in accordance with DNS and EDQNM results fit addition, anisotropy is shown also to develop through nonlinear interactions modified by rotation, in an intermediate range of Rossby numbers (Ro(omega) = (omega)' and Ro(omega)w greater than 1), which is characterized by a marco-Rossby number Ro(sup L) less than 1 and Ro(omega) greater than 1 which is characterized by a macro-Rossby number based on an integral lengthscale L and the micro-Rossby number previously defined. This anisotropy is mainly an angular drain of spectral energy which tends to concentrate energy in tile wave-plane normal to the rotation axis, which is exactly both the slow and the two-dimensional manifold. In Addition, a polarization of the energy distribution in this slow 2D manifold enhances horizontal (normal to the rotation axis) velocity components, and underlies the anisotropic structure of the integral lengthscales. Finally is demonstrated the ability of a generalized EDQNM (Eddy Damped Quasi-Normal Markovian) model to predict the underlying spectral transfer structure and all the subsequent developments of classic anisotropy indicators in physical space, when compared to recent LES results. Even if the applications mainly concern developed strong turbulence, a particular emphasis is given to the strong formal analogy of this EDQNM2 model with recent weakly nonlinear approaches to wave-turbulence.
281 citations
TL;DR: In this article, a generalization of DPD that incorporates an internal energy and a temperature variable for each particle is presented, which can be viewed as a simplified solver of the fluctuating hydrodynamic equations and opens up the possibility of studying thermal processes in complex fluids with mesoscopic simulation technique.
Abstract: Dissipative particle dynamics (DPD) does not conserve energy and this precludes its use in the study of thermal processes in complex fluids. We present here a generalization of DPD that incorporates an internal energy and a temperature variable for each particle. The dissipation induced by the dissipative forces between particles is invested in raising the internal energy of the particles. Thermal conduction occurs by means of (inverse) temperature differences. The model can be viewed as a simplified solver of the fluctuating hydrodynamic equations and opens up the possibility of studying thermal processes in complex fluids with a mesoscopic simulation technique.
251 citations
TL;DR: In this article, a two-equation turbulence model for incompressible flow within a fluid saturated and rigid porous medium is presented, which consists of time-averaging the general (macroscopic) transport equations and closing the model with the classical eddy diffusivity concept and the Kolmogorov-Prandtl relation.
250 citations
TL;DR: In this paper, a numerical solution to the generalized Korteweg-de Vries (K-dV) equation, including horizontal variability and dissipation, is used to model the evolution of an initially sinusoidal long internal wave, representing an internal tide.
Abstract: A numerical solution to the generalized Korteweg-de Vries (K-dV) equation, including horizontal variability and dissipation, is used to model the evolution of an initially sinusoidal long internal wave, representing an internal tide. The model shows the development of the waveform to the formation of shocks and solitons as it propagates shoreward over the continental slope and shelf. The model is run using observed hydrographic conditions from the Australian North West Shelf and results are compared to current meter and thermistor observations from the shelf-break region. It is found from observations that the coefficient of nonlinearity in the K-dV equation changes sign from negative in deep water to positive in shallow water, and this plays a major role in determining the form of the internal tide transformation. On the shelf there is strong temporal variability in the nonlinear coefficient due to both background shear flow and the large amplitude of the internal tide, which distorts the density profile over a wave period. Both the model and observations show the formation of an initial shock on the leading face of the internal tide. In shallow water, the change in sign of the coefficient of nonlinearity causes the shock to evolve into a tail of short period sinusoidal waves. After further propagation a second shock forms on the back face of the wave, followed by a packet of solitons. The inclusion of bottom friction in the model is investigated along with the dependance on initial wave amplitude and variability in the coefficients of nonlinearity and dispersion. Friction is found to be important in limiting the amplitudes of the evolving waves.
231 citations
TL;DR: In this paper, a generalization of DPD that incorporates an internal energy and a temperature variable for each particle is presented, which can be viewed as a simplified solver of the fluctuating hydrodynamic equations and opens up the possibility of studying thermal processes in complex fluids with mesoscopic simulation technique.
Abstract: Dissipative particle dynamics (DPD) does not conserve energy and this precludes its use in the study of thermal processes in complex fluids. We present here a generalization of DPD that incorporates an internal energy and a temperature variable for each particle. The dissipation induced by the dissipative forces between particles is invested in raising the internal energy of the particles. Thermal conduction occurs by means of (inverse) temperature differences. The model can be viewed as a simplified solver of the fluctuating hydrodynamic equations and opens up the possibility of studying thermal processes in complex fluids with a mesoscopic simulation technique.
230 citations
TL;DR: Durian et al. as discussed by the authors developed a microscopic model for the macroscopic deformation and flow of foam in which dimensionality, energy storage, and dissipation mechanisms, polydispersity, and the gas-liquid ratio all can be varied easily.
Abstract: By focusing on entire gas bubbles, rather than soap films or vertices, a microscopic model was recently developed for the macroscopic deformation and flow of foam in which dimensionality, energy storage, and dissipation mechanisms, polydispersity, and the gas-liquid ratio all can be varied easily [D. J. Durian, Phys. Rev. Lett. 75, 4780 (1995)]. Here, a more complete account of the model is presented, along with results for linear rheological properties as a function of the latter two important physical parameters. It is shown that the elastic character vanishes with increasing liquid content in a manner that is consistent with rigidity percolation and that is almost independent of polydispersity. As the melting transition is approached, the bubble motion becomes increasingly nonaffine and the relaxation time scale appears to diverge. Results are also presented for nonlinear behavior at large applied stress, and for the sudden avalanchelike rearrangements of bubbles from one tightly packed configuration to another at small applied strain rates. The distribution of released energy is a power law for small events, but exhibits an exponential cutoff independent of system size. This is in accord with multiple light scattering experiments, but not with other simulations predicting self-organized criticality.
219 citations
TL;DR: In this article, a method for remotely quantifying wave-breaking dynamics using an infrared imager to measure the temperature changes associated with the disruption and recovery of the surface thermal boundary layer (skin layer) is presented.
Abstract: ENERGY dissipation due to deep-water wave breaking plays a critical role in the development and evolution of the ocean surface wave field. Furthermore, the energy lost by the wave field via the breaking process is a source for turbulent mixing and air entrainment, which enhance air–sea heat and gas transfer1–3. But the current lack of reliable methods for measuring energy dissipation associated with wave breaking inhibits the quantitative study of processes occurring at ocean surfaces, and represents a major impediment to the improvement of global wave-prediction models4. Here we present a method for remotely quantifying wave-breaking dynamics which uses an infrared imager to measure the temperature changes associated with the disruption and recovery of the surface thermal boundary layer (skin layer). Although our present results focus on quantifying energy dissipation—in particular, we show that the recovery rate of the skin layer in the wakes of breaking waves is correlated with the energy dissipation rate—future applications of this technique should help to elucidate the nature of important small-scale surface processes contributing to air–sea heat5 and gas6 flux, and lead to a fuller understanding of general ocean–atmosphere interactions.
204 citations
TL;DR: In this paper, the authors report measurements of collisional properties of spheres using high-speed video analysis and compare the experimental data with the relevant models of energy dissipation and show the existence of two dissipation regimes.
Abstract: In this paper we report measurements of collisional properties of spheres using high-speed video analysis. These results agree with a simple collision operator. We study the size and velocity dependences of the coefficient of restitution in the normal direction. The experimental data are compared with the relevant models of energy dissipation and show the existence of two dissipation regimes. For large impact velocities a plastic deformation model is in good agreement with our measurements, while for smaller velocities a model of viscoelastic dissipation gives qualitative agreement.
190 citations
TL;DR: In this paper, the excess energy dissipation process of photoexcited S1 trans-stilbene in solution has been studied with picosecond time-resolved Raman spectroscopy.
Abstract: The excess energy dissipation process of photoexcited S1 trans-stilbene in solution has been studied with picosecond time-resolved Raman spectroscopy The peak position of the 1570-cm-1 band (CC stretch) is shown to be useful as an indicator of picosecond temperature changes; a picosecond time-resolved Raman spectrometer can be regarded as a “picosecond Raman thermometer” The cooling rates of S1 trans-stilbene thus observed in 10 different solvents show a strong correlation with the thermal diffusivities of the bulk solvents Based on this observation, a simple numerical model is proposed for the solute−solvent energy dissipation process in solution The observed cooling kinetics are analyzed with this macroscopic model It is concluded that the excess energy is first shared among the solute and the nearest solvent molecules in a few picoseconds or faster The further heat conduction to outer-sphere solvent molecules determines the whole dissipation rate, which explains the observed correlation between t
TL;DR: In this paper, a theory for calculating the fluctuations in a laminar boundary layer when the free stream is turbulent is presented and the kinetic energy equation for these fluctuations is derived and a new mechanism is revealed for their production.
Abstract: A theory is presented for calculating the fluctuations in a laminar boundary layer when the free stream is turbulent. The kinetic energy equation for these fluctuations is derived and a new mechanism is revealed for their production. A methodology is presented for solving the equation using standard boundary layer computer codes. Solutions of the equation show that the fluctuations grow at first almost linearly with distance and then more slowly as viscous dissipation becomes important. Comparisons of calculated growth rates and kinetic energy profiles with data show good agreement. In addition, a hypothesis is advanced for the effective forcing frequency and free-stream turbulence level that produce these fluctuations. Finally, a method to calculate the onset of transition is examined and the results compared to data.
TL;DR: In this article, it is shown that the efficiency of conversion of accretion energy to radiation can be much smaller than unity only for the unlikely condition that the heating of the electrons is negligible.
Abstract: Advection-dominated, high-temperature, quasi-spherical accretion flow onto a compact object, recently considered by a number of authors, assumes that the dissipation of turbulent energy of the flow heats the ions and that the dissipated energy is advected inward. It is suggested that the efficiency of conversion of accretion energy to radiation can be very much smaller than unity. However, it is likely that the flows have an equipartition magnetic field with the result that dissipation of magnetic energy at a rate comparable to that for the turbulence must occur by ohmic heating. We argue that this heating occurs as a result of plasma instabilities and that the relevant instabilities are current driven in response to the strong electric fields parallel to the magnetic field. We argue further that these instabilities heat predominantly the electrons. We conclude that the efficiency of conversion of accretion energy to radiation can be much smaller than unity only for the unlikely condition that the ohmic heating of the electrons is negligible.
TL;DR: In this paper, the effects of nonlinear interactions, reflection, and dissipation on the evolution of surface gravity waves propagating across a barred beach were investigated using field observations and numerical model predictions.
Abstract: Field observations and numerical model predictions are used to investigate the effects of nonlinear interactions, reflection, and dissipation on the evolution of surface gravity waves propagating across a barred beach. Nonlinear interactions resulted in a doubling of the number of wave crests when moderately energetic (about 0.8-m significant wave height), narrowband swell propagated without breaking across an 80-m-wide, nearly flat (2-m depth) section of beach between a small offshore sand bar and a steep (slope = 0.1) beach face, where the waves finally broke. These nonlinear energy transfers are accurately predicted by a model based on the nondissipative, unidirectional (i.e., reflection is neglected) Boussinesq equations. For a lower-energy (wave height about 0.4 m) bimodal wave field, high-frequency seas dissipated in the surf zone, but lower-frequency swell partially reflected from the steep beach face, resulting in significant cross-shore modulation of swell energy. The combined effects of reflection from the beach face and dissipation across the sand bar and near the shoreline are described well by a bore propagation model based on the nondispersive nonlinear shallow water equations. Boussinesq model predictions on the flat section (where dissipation is weak) are improved by decomposing the wave field into seaward and shoreward propagating components. In more energetic (wave heights greater than 1 m) conditions, reflection is negligible, and the region of significant dissipation can extend well seaward of the sand bar. Differences between observed decreases in spectral levels and Boussinesq model predictions of nonlinear energy transfers are used to infer the spectrum of breaking wave induced dissipation between adjacent measurement locations. The inferred dissipation rates typically increase with increasing frequency and are comparable in magnitude to the nonlinear energy transfer rates.
TL;DR: In this article, a tracking oscillator excitation scheme is proposed for cases in which the force sensor exhibits only a weak resonance enhancement, which is superior to the more standard direct feedback method.
Abstract: Dynamic force microscopy, a technique also known as non-contact force microscopy, has proved to be a powerful tool for atomic resolution imaging. A number of schemes have been developed, but recently the oscillator method has become the preferred operating mode. Here, the force sensor acts as resonator in an active feedback circuit. A practical implementation of the method is described and the underlying key concepts are discussed. It is shown that a tracking oscillator excitation scheme is superior to the more standard direct feedback method for cases in which the force sensor exhibits only a weak resonance enhancement. Furthermore, the simultaneous measurement of dissipative interaction channels is an important extension of dynamic force microscopy. It allows one to differentiate between sample materials via their plasto-mechanical response. As an example, a Cr test grating has been imaged in the constant force gradient mode. The dissipation measured on Cr-covered areas is significantly lower than that on the bare quartz glass substrate, which enables one to distinguish between the two materials with a lateral resolution comparable to that of the topographic image.
TL;DR: In this article, the mean rates of energy injection and energy dissipation in steady regimes of turbulence are measured in two types of flow confined in closed cells, one generated by counterrotating stirrers and the other by Couette-Taylor flow.
Abstract: The mean rates of energy injection and energy dissipation in steady regimes of turbulence are measured in two types of flow confined in closed cells. The first flow is generated by counterrotating stirrers and the second is a Couette-Taylor flow. In these two experiments the solid surfaces that set the fluid into motion are at first smooth, so that everywhere the velocity of the stirrers is locally parallel to its surface. In all such cases the mean rate of energy dissipation does not satisfy the scaling expected from Kolmogorov theory. When blades perpendicular to the motion are added to the stirring surfaces the Kolmogorov scaling is observed in all the large range of Reynolds numbers (10,Re,10) investigated. However, with either smooth or rough stirring the measurements of the pressure fluctuations exhibit no Reynolds number dependence. This demonstrates that, though the smooth stirrers are less efficient in setting the fluid into motion, their efficiency is independent of the Reynolds number so that the Kolmogorov scaling characterizes, in all cases, the dissipation in the bulk of the fluid. The difference in the global behaviors corresponds to a different balance between the role of the different regions of the flow. With smooth stirrers the dissipation in the bulk is weaker than the Reynoldsnumber-dependent dissipation in the boundary layers. With rough ~or inertial! stirrers the dissipation in the bulk dominates, hence the Kolmogorovian global behavior. @S1063-651X~97!11606-3#
TL;DR: In this paper, a theory is presented, which is based mainly on dimensional analysis (but also on gravity wave theory), that attempts to explain all the types of gravity wave power spectral densities (PSDs) now being measured.
Abstract: A theory is presented, which is based mainly on dimensional analysis (but also on gravity wave theory), that attempts to explain all the types of gravity wave power spectral densities (PSDs) now being measured. This theory is based on two concepts, namely, wave saturation and wave cascade. The immediate result of the simultaneous presence of these two processes is that there should exist a unique relation between the vertical (or horizontal) wavelength of a gravity wave and its period (provided the Brunt Period and dissipation rate are given and Doppler effects are omitted). This relation provides a way to derive all of the intrinsic spectra from the fundamental one which is the vertical wavenumber PSD of the horizontal winds. The most important suggestion to emerge from this theory is that e, the dissipation rate, is the main controlling independent variable for the amplitude of all but 3 of the 12 spectra predicted. It would also control the wavelength-period relations. Comparisons are made between observations and theory, and important experimental tests are proposed. This model presently appears to be useful in the analysis of gravity wave data obtained by means of lidars, radars, interferometers, and imagers. In addition, it raises a number of new scientific issues for future research.
TL;DR: In this article, a new model for the computation of unsteady friction losses in transient flow is developed and verified for laminar and turbulent flows and the comparison of measured and computed pressure heads shows excellent agreement.
Abstract: A new model for the computation of unsteady friction losses in transient flow is developed and verified in this study. The energy dissipation in transient flow is estimated from the instantaneous velocity profiles. The ratio of the energy dissipation at any instant and the energy dissipation obtained by assuming quasi-steady conditions defines the energy dissipation factor. This is a nondimensional, time-varying parameter that modifies the friction term in the transient flow governing equations. The model was verified for laminar and turbulent flows and the comparison of measured and computed pressure heads shows excellent agreement. This model can be adapted to an existing transient program that uses the well-known method of characteristics for the solution of the continuity and momentum equations.
TL;DR: In this article, the disordered drop-push process and asymmetric exclusion process were studied in a driven lattice gas system with quenched spatial randomness, and it was shown that time-dependent correlation functions which monitor dissipation of kinematic waves behave as in the pure system if the wave speed is nonzero.
Abstract: We study driven lattice gas systems with quenched spatial randomness: the disordered drop-push process, for which the steady state is shown to have inhomogeneous product measure form, and the disordered asymmetric exclusion process. We conjecture that time-dependent correlation functions which monitor dissipation of kinematic waves behave as in the pure system if the wave speed is nonzero, and support this with simulations. This speed vanishes close to half filling in the disordered exclusion process where macroscopic regions with different densities coexist.
TL;DR: In this article, the authors performed direct numerical simulations of an externally driven two-dimensional magnetohydrodynamic system over extended periods of time to simulate the dynamics of a transverse section of a solar coronal loop.
Abstract: We perform direct numerical simulations of an externally driven two-dimensional magnetohydrodynamic system over extended periods of time to simulate the dynamics of a transverse section of a solar coronal loop. A stationary and large-scale magnetic forcing was imposed, to model the photospheric motions at the magnetic loop footpoints. A turbulent stationary regime is reached, which corresponds to energy dissipation rates consistent with the heating requirements of coronal loops. The temporal behavior of quantities such as the energy dissipation rate shows clear indications of intermittency, which are exclusively due to the strong nonlinearity of the system. We tentatively associate these impulsive events of magnetic energy dissipation (from 5 × 1024 to 1026 ergs) to the so-called nanoflares. A statistical analysis of these events yields a power-law distribution as a function of their energies with a negative slope of 1.5, which is consistent with those obtained for flare energy distributions reported from X-ray observations.
TL;DR: In this paper, a path integral formulation for the dynamic Casimir effect is developed, which allows us to study arbitrary deformations in space and time of the perfectly reflecting (conducting) boundaries of a cavity.
Abstract: A path integral formulation is developed for the dynamic Casimir effect. It allows us to study arbitrary deformations in space and time of the perfectly reflecting (conducting) boundaries of a cavity. The mechanical response of the intervening vacuum is calculated to linear order in the frequency--wave-vector plane. For a single corrugated plate we find a correction to mass at low frequencies, and an effective shear viscosity at high frequencies, both anisotropic. For two plates there is resonant dissipation for all frequencies greater than the lowest optical mode of the cavity.
TL;DR: In this paper, an algebraic model for the dissipation rate tensor of turbulence is derived using integrity bases methods and tensor symmetry properties, which is based on an analysis of the exact transport equation for the Dissipation Rate tensor.
Abstract: The modelling of anisotropies in the dissipation rate of turbulence is considered based on an analysis of the exact transport equation for the dissipation rate tensor. An algebraic model is systematically derived using integrity bases methods and tensor symmetry properties. The new model differs notably from all previously proposed models in that it depends nonlinearly on the mean velocity gradients. This gives rise to a transport equation for the scalar dissipation rate that is of the same general form as the commonly used model with one major exception: the coefficient of the production term is dependent on the invariants of both the rotational and irrotational strain rates. The relationship between the new model and other recently proposed models is examined in detail. Some basic tests and applications of the model are also provided along with a discussion of the implications for turbulence modelling.
TL;DR: In this article, a control law for friction dampers is proposed to maximize energy dissipation in an instantaneous sense by modulating the normal force at the friction interface, considering the effect of displacement-and velocity-induced friction dynamics.
TL;DR: In this paper, the role of the relaxation time in a diffusion process was studied for spherically symmetric fluid distributions undergoing dissipation in the form of a radial heat flow, and the temperature gradient appearing as a result of perturbations, and thereby the subsequent evolution of the system, is highly dependent on the product of relaxation time by the period of oscillation of the sphere.
Abstract: We study, for spherically symmetric fluid distributions undergoing dissipation in the form of a radial heat flow, the role played by the relaxation time in a diffusion process. We show that the temperature gradient appearing as a result of perturbations, and thereby the subsequent evolution of the system, is highly dependent on the product of relaxation time by the period of oscillation of the· sphere.
TL;DR: In this article, a wave packet description is employed for the primary system while the bath is represented by an array of two-level systems and the number of bath modes determines the period during which the surrogate Hamiltonian reproduces the dynamics of a primary system.
Abstract: Dissipative dynamics of an adsorbate near a metal surface is formulated consistently by replacing the infinite system-bath Hamiltonian by a finite surrogate Hamiltonian This finite representation is designed to generate the true short time dynamics of a primary system coupled to a bath A detailed wave packet description is employed for the primary system while the bath is represented by an array of two-level systems The number of bath modes determines the period the surrogate Hamiltonian reproduces the dynamics of the primary system The convergence of this construction is studied for the dissipating Harmonic oscillator and the double-well tunneling problem Converged results are obtained for a finite duration by a bath consisting of 4–11 modes The formalism is extended to dissipation caused by electron-hole-pair excitations The stopping power for a slow moving proton is studied showing deviations from the frictional limit at low velocities Vibrational line shapes of hydrogen and deuterium on nickel were studied In the bulk the line shape is mostly influenced by nonadiabatic effects The interplay between two baths is studied for low temperature tunneling between two surface sites of hydrogen on nickel A distinction between lattice modes that enhance the tunneling and ones that suppress it was found
TL;DR: In this paper, the authors calculated the turbulent dissipation heating rate employing two models for the interstellar turbulence, a superposition of magnetohydrodynamic waves and a fluid-turbulence-like model based on the ideas of Higdon, and found that the fluid-like turbulence model is an acceptable and realistic model of the turbulence in the interstellar medium once the effects of ion-neutral collisions are included in the model.
Abstract: We have recently published observations that specify most of the turbulent and mean plasma characteristics for a region of the sky containing the interstellar diffuse ionized gas (DIG). These observations have provided virtually all of the information necessary to calculate the heating rate from dissipation of turbulence. We have calculated the turbulent dissipation heating rate employing two models for the interstellar turbulence. The first is a customary modeling as a superposition of magnetohydrodynamic waves. The second is a fluid-turbulence-like model based on the ideas of Higdon. This represents the first time that such calculations have been carried out with full and specific interstellar turbulence parameters. The wave model of interstellar turbulence encounters the severe difficulty that plausible estimates of heating by Landau damping exceed the radiative cooling capacity of the interstellar DIG by 3-4 orders of magnitude. Clearly interstellar turbulence does not behave like an ensemble of obliquely propagating fast magnetosonic waves. The heating rate due to two other wave dissipation mechanisms, ion-neutral collisional damping and the parametric decay instability, are comparable to the cooling capacity of the diffuse ionized medium. We find that the fluid-like turbulence model is an acceptable and realistic model of the turbulence in the interstellar medium once the effects of ion-neutral collisions are included in the model. This statement is contingent on an assumption that the dissipation of such turbulence because of Landau damping is several orders of magnitude less than that from an ensemble of obliquely propagating magnetosonic waves with the same energy density. Arguments as to why this may be the case are made in the paper. Rough parity between the turbulent heating rate and the radiative cooling rate in the DIG also depends on the hydrogen ionization fraction being in excess of 90% or on a model-dependent lower limit to the heating rate being approximately valid. We conclude that the dissipation of turbulence is capable of providing a substantial and perhaps major contribution to the energy budget of the diffuse ionized medium.
TL;DR: In this paper, the energy equation for the boundary layer flow of an electrically conducting fluid under the influence of a constant transverse magnetic field over a linearly stretching non-isothermal flat sheet is considered.
Abstract: This paper presents solutions of the energy equation for the boundary layer flow of an electrically conducting fluid under the influence of a constant transverse magnetic field over a linearly stretching non-isothermal flat sheet. Effects due to dissipation, stress work and heat generation are considered. Analytical solutions of the resulting linear nonhomogeneous boundary value problems, expressed in terms of Kummer's functions, are presented for the case of prescribed surface temperature as well as the case of prescribed wall heat flux, both of which are assumed to be quadratic functions of distance. The boundary value problems are also solved by direct numerical integration yielding results in excellent agreement with the analytical solutions.
TL;DR: In this article, the authors perform direct numerical simulations of an externally driven two-dimensional magnetohydrodynamic system over extended periods of time to simulate the dynamics of a transverse section of a solar coronal loop.
Abstract: We perform direct numerical simulations of an externally driven two-dimensional magnetohydrodynamic system over extended periods of time to simulate the dynamics of a transverse section of a solar coronal loop. A stationary and large-scale magnetic forcing was imposed, to model the photospheric motions at the magnetic loop footpoints. A turbulent stationary regime is reached, which corresponds to energy dissipation rates consistent with the heating requirements of coronal loops. The temporal behavior of quantities such as the energy dissipation rate show clear indications of intermittency, which are exclusively due to the strong nonlinearity of the system. We tentatively associate these impulsive events of magnetic energy dissipation to the so-called nanoflares. A statistical analysis of these events yields a power law distribution as a function of their energies with a negative slope of 1.5, which is consistent with those obtained for flare energy distributions reported from X-ray observations.
TL;DR: In this article, the inertial range scalings of local averages of energy dissipation rate and enstrophy (vorticity squared ) were studied using high resolution direct numerical simulation data for homogeneous and isotropic turbulence.
Abstract: The inertial range scalings of local averages of energy dissipation rate and enstrophy (vorticity squared ) are studied using high resolution direct numerical simulation data for homogeneous and isotropic turbulence. The Taylor microscale Reynolds number is 216. It is found that the enstrophy is more intermittent than dissipation, consistent with previous one-dimensional surrogate measurements at high Reynolds numbers. Contrary to some recent expectations, enstrophy and dissipation have different exponents. [S0031-9007(97)03795-2] PACS numbers: 47.27.Gs
TL;DR: In this article, the conservative evolution of weakly nonlinear narrowbanded gravity waves in deep water is investigated numerically with a modified nonlinear Schrodinger equation, for application to wide wave tanks.
Abstract: The conservative evolution of weakly nonlinear narrow-banded gravity waves in deep water is investigated numerically with a modified nonlinear Schrodinger equation, for application to wide wave tanks. When the evolution is constrained to two dimensions, no permanent shift of the peak of the spectrum is observed. In three dimensions, allowing for oblique sideband perturbations, the peak of the spectrum is permanently downshifted. Dissipation or wave breaking may therefore not be necessary to produce a permanent downshift. The emergence of a standing wave across the tank is also predicted.