Local Dissipation Research Articles

Global Calculations of Local and Remote Near-Inertial-Wave Dissipation

AbstractGlobal estimates are presented of the fraction q of wind-generated near-inertial wave power available for local turbulent dissipation under the assumption that modes 1–3 propagate “far” and the higher modes remain to eventually break. Using climatological stratification profiles and mixed layer depth, the modal distribution of near-inertial energy flux is computed following Gill's classic 1984 work by projecting a slab flow in the mixed layer onto the dynamical modes. Global maps and zonal-mean profiles are presented, which show a global-mean value of q = 0.63 and 0.75 for winter and summer profiles, respectively. The simplicity of the calculation makes it of potential use in parameterizations of near-inertial breaking in climate simulations.

Precise Correlation of Contact Area and Forces in the Unstable Friction between a Rough Fluoroelastomer Surface and Borosilicate Glass.

Stick-slip friction of elastomers arises due to adhesion, high local strains, surface features, and viscous dissipation. In situ techniques connecting the real contact area to interfacial forces can reveal the contact evolution of a rough elastomer surface leading up to gross slip, as well as provide high-resolution dynamic contact areas for improving current slip models. Samples with rough surfaces were produced by the same manufacturing processes as machined seals. In this work, a machined fluoroelastomer (FKM) hemisphere was slid against glass, and the stick-slip behavior was captured optically in situ. The influence of sliding velocity on sliding behavior was studied over a range of speeds from 1 µm/s to 100 µm/s. The real contact area was measured from image sequences thresholded using Otsu’s method. The motion of the pinned region was delineated with a machine learning scheme. The first result is that, within the macroscale sticking, or pinned phase, local pinned and partial slip regions were observed and modeled as a combined contact with contributions to friction by both regions. As a second result, we identified a critical velocity below which the stick-slip motion converted from high frequency with low amplitude to low frequency with high amplitude. This study on the sliding behavior of a viscoelastic machined elastomer demonstrates a multi-technique approach which reveals precise changes in contact area before and during pinning and slip.

Real-time hybrid simulation on high strength steel frame with Y-shaped eccentric braces

In recent years, real-time hybrid simulation (RTHS) has been widely examined by researchers as a new, economical, and reliable seismic testing technology for structural analysis. In this paper, RTHS is applied to the space structure experiment of a high strength steel frame with Y-shaped eccentric braces (HSYE). First, the proposed RTHS method utilized the upper stories of the prototype as the experimental substructure, and the lower stories was taken as the numerical substructure and simulated in finite element software. Then, the OpenFresco test platform and xPC-Target with an adaptive feed-forward delay compensator were used to realize real-time loading. Before conducting the RTHS, the method of adding an adaptive stage before seismic acceleration record was proposed to compensate the initial delay. On this basis, the seismic behavior of the HSYE was tested using selected seismic acceleration records. The results indicated that the RTHS results are stable and reliable. Under an earthquake simulated load, the HSYE structure can give full play to the local energy dissipation characteristics of the shear link and ensure the elasticity of the main frame, which has vast engineering application prospects.

An efficient targeted ENO scheme with local adaptive dissipation for compressible flow simulation

High fidelity numerical simulation of compressible flow requires the numerical method being used to have both stable shock-capturing capability and high spectral resolution. Recently, a family of Targeted Essentially Non-Oscillatory (TENO) schemes is developed to fulfill such requirements. Although TENO has very low dissipation for smooth flow, it introduces a cutoff value CT to maintain the non-oscillatory shock-capturing property. As CT actually controls the dissipation property of TENO, the choice of CT for better shock-capturing capability also means higher dissipation for small structures. To overcome this, in this paper, a new local adaptive method is proposed for the choice of CT. By introducing a novel adaptive function based on the WENO smoothness indicators, CT is dynamically adjusted from 1.0×10−10 for lower dissipation to 1.0×10−4 for stable capturing of shock according to the smoothness of the reconstruction stencil. The numerical results of the new method are compared with those of the original TENO method and an adaptive TENO method in Fu et al. (2019) [49]. It reveals that the new method is capable of suppressing numerical oscillations near discontinuities while further improving the resolution of TENO at a low extra computational cost.

The Effects of Upper-Hybrid Waves on Energy Dissipation in the Electron Diffusion Region.

Using a two‐dimensional particle‐in‐cell simulation, we investigate the effects and roles of upper‐hybrid waves (UHW) near the electron diffusion region (EDR). The energy dissipation via the wave‐particle interaction in our simulation agrees with measured by magnetospheric multiscale (MMS) spacecraft. It means that UHW contributes to the local energy dissipation. As a result of wave‐particle interactions, plasma parameters which determine the larger‐scale energy dissipation in the EDR are changed. The y‐directional current decreases while the pressure tensor increases/decreases when the agyrotropic beam density is low/high, where ‐coordinates correspond the ‐boundary coordinates. Because the reconnection electric field comes from , our result implies that UHW plays an additional role in affecting larger‐scale energy dissipation in the EDR by changing plasma parameters. We provide a simple diagram that shows how the UHW activities change the profiles of plasma parameters near the EDR comparing cases with and without UHW.

Magnetic Energy Conversion and Transport in the Terrestrial Magnetotail Due to Dipolarization Fronts

AbstractDipolarization front (DF) is an important region involved with energy conversion and flux transport in the planet's magnetotail. Using observation data from the recent Magnetospheric Multiscale (MMS) mission, we have identified and analyzed 215 DF events. Significant magnetic energy conversion and transport are observed at the DFs, which show strong dawn‐dusk asymmetry. We find that the transport process dominates the change of local magnetic energy. Nearly 70% of the events show net Poynting flux flowing into the DF region. Such effect is more significant than local magnetic energy dissipation, averagely. When DF approaches the near‐Earth region, the net flow‐in Poynting flux decreases and magnetic energy dissipation increases. Our results suggest that the downstream magnetic energies of transient magnetic reconnections in the midtail may be transported to the near‐Earth region by one DF event after another. These intensive strikes to the magnetosphere in the near‐planet region may drive stronger storms, compared with the previous knowledge about DF.

Thermo-Mechanical Response of an Additively Manufactured Energetic Material Simulant to Dynamic Loading

The mesoscale thermo-mechanical behavior of an additively manufactured energetic material (AMEM) simulant under dynamic loading is studied. The material is unidirectionally printed using direct ink writing (DIW) of a high solid-loaded photopolymer and cured under ultraviolet (UV) light exposure. Experiments and multi-physics computations are performed to relate localized deformation, dissipation mechanisms, and temperature rises to the print structure. Simultaneous high-speed visible light and infrared imaging is used to obtain deformation and temperature fields over the same area of samples with micrometer spatial and microsecond temporal resolutions. Loading along different directions relative to the print structure of the material is achieved using a split-Hopkinson pressure bar (SHPB) or Kolsky bar at the average strain rate of ~ 310 s−1. Shear banding and shear failure are observed. Simulations accounting for the geometry and print structure of the samples are performed. The microstructural heterogeneities are found to significantly affect the orientation-dependent deformation, damage, and heating, with the damage and heating most pronounced when the loading direction and print orientation are non-collinear. The heating is attributed to both constituent inelastic dissipation and internal friction. Depending upon the strain rate level and the loading orientation, the contribution of frictional dissipation to the overall heating is 0.9–4.5%. Despite this relatively low fraction in the overall heating, friction is localized at fracture sites and plays an important role in the development of local temperature spikes called hotspots which are of great interest for energetic materials.

Spreading of a nanodroplet over isothermally heated smooth and nanostructured surfaces: A molecular dynamics study

Owing to scale effects, spreading of nanodroplets over heated textured surfaces displays distinct features from that of millimetric-sized droplets. In this work, spreading behaviors of a water nanodroplet on isothermally heated smooth and nanostructured surfaces are investigated for various surface temperatures, wettability, roughness via molecular dynamics (MD) simulations. The simulations show that, in the early spreading stage, the spreading radius of the nanodroplet on the textured surface is smaller than that on the smooth surface, whereas a reverse result was reported for millimetric-sized droplets. Because the nanodroplet size is comparable to the feature size of nanostructures, the pinning effect becomes prominent, leading to a large local dissipation in the vicinity of the contact line and hence hindering the nanodroplet spreading over the nanostructured surface. However, more drastic evaporation on the nanostructured surface in the later spreading stage facilitates the adsorption and desorption of water molecules, weakening the pinning effect. As a result, the difference in the spreading radius over the two surfaces becomes less obvious in the later spreading stage. The simulations also demonstrate that a higher surface temperature increases the spreading coefficient and promotes the evaporation, thereby accelerating the nanodroplet spreading; the nanodroplet spreading is significantly suppressed when the intrinsic wettability of the nanostructured surface changes from hydrophilic to moderate wettability; because of stronger pinning effect, the nanostructured surface with a larger aspect ratio exhibits a slower spreading. Moreover, owing to the influence of disjoining pressure, an abnormal evaporation phenomenon is observed for the intrinsically moderate wettability surface, on which the nanodroplet shows a much faster evaporation rate in the later spreading stage than that on intrinsically hydrophilic surfaces.

Finite differential hot-wire method for localized heat transfer characterization at microscale

In this work, we develop a finite differential hot-wire method to uncover the localized heat dissipation physics along microwires. The localized convection effect can be decoupled at the finite differential element with a spatial resolution of 10 microns. The heat convection coefficient is characterized from 314 W m−2 K to 1116 W m−2 K along different positions of microwires with different diameters, which decreases from wire ends to its middle point. The slope of temperature gradient at wire surface versus temperature is determined from 20150 m−1 to 8640 m−1, which can be used to calculate the boundary layer thickness around microwires.

Prediction of slurry grinding due to media and coarse rock interactions in a 3D pilot SAG mill using a coupled DEM + SPH model

A coupled DEM + SPH model can be used to predict the motion and breakage of resolved coarser particles within a SAG mill. The fine product from coarse particle fracture can then be included in the slurry phase modelled using SPH. This allows, in principle, the prediction of the breakage and transport of coarser material and the transport of the finer material within the grinding and pulp chambers of a SAG mill including discharge performance of the mill. It also allows the effect of the changing solids loading on the slurry rheology to be included. In this paper we will explore the development of an extension of this model that also allows prediction of the grinding of the finer particles embedded in the slurry phase due to the collisions and shear of the coarser particles (rocks and grinding media). The size distribution of the slurry fines is discretised into a set of size fractions so that its change due to grinding can be tracked at each point in the slurry. This is formulated as a system of coupled advection-diffusion equations. An SPH discretisation of this system is then developed. The resulting coupled SPH ODE’s are solved using the SPH method in a way that is fully coupled to the DEM and SPH parts of the model. The proposed model includes a diffusive component that allows for the shear induced dispersion of the slurry size fractions and allows prediction of the spatial distribution of these fine size fractions within the slurry phase. The advection of the slurry is automatically accounted for by the motion of the SPH particles which is an important benefit of using the SPH method for such wet mill modelling. The local fine grinding behaviour arising from the coarse DEM resolved components of the charge are characterised at each location in terms of the local energy dissipation rate. This information is used in conjunction with a first order grinding law to predict the grinding of each slurry size fraction at each location in the mill due to the collisional action of the coarser particles. The ability of this new model to predict fine particle grinding and transport within the slurry phase is demonstrated for an industry standard 1.8 m diameter by 0.6 m long AG/SAG pilot mill.

Tailoring the Switching Dynamics in Yttrium Oxide‐Based RRAM Devices by Oxygen Engineering: From Digital to Multi‐Level Quantization toward Analog Switching

AbstractThis work investigates the transition from digital to gradual or analog resistive switching in yttrium oxide‐based resistive random‐access memory devices. It is shown that this transition is determined by the amount of oxygen in the functional layer. A homogeneous reduction of the oxygen content not only reduces the electroforming voltage, allowing for forming‐free devices, but also decreases the voltage operation window of switching, thereby reducing intra‐device variability. The most important effect as the dielectric becomes substoichiometric by oxygen engineering is that more intermediate (quantized) conduction states are accessible. A key factor for this reproducibly controllable behavior is the reduced local heat dissipation in the filament region due to the increased thermal conductivity of the oxygen depleted layer. The improved accessibility of quantized resistance states results in a semi‐gradual switching both for the set and reset processes, as strongly desired for multi‐bit storage and for an accurate definition of the synaptic weights in neuromorphic systems. A theoretical model based on the physics of mesoscopic structures describing current transport through a nano‐constriction including asymmetric potential drops at the electrodes and non‐linear conductance quantization is provided. The results contribute to a deeper understanding on how to tailor materials properties for novel memristive functionalities.

Effect of Structure on Response of a Three-Dimensional-Printed Photopolymer-Particulate Composite Under Intermediate Strain Rate Loading

Abstract The thermo-mechanical response of an additively manufactured photopolymer-particulate composite under conditions of macroscopic uniaxial compression without lateral confinement at overall strain rates of 400–2000 s−1 is studied. The material has a direct-ink-written unidirectional structure. Computations are performed to quantify the effects of microstructure attributes including anisotropy, defects, and filament size on localized deformation, energy dissipations, and temperature rises. To this effect, an experimentally informed Lagrangian finite element framework is used, accounting for finite-strain elastic–plastic deformation, strain-rate effect, failure initiation and propagation, post-failure internal contact and friction, heat generation due to friction and inelastic bulk deformation, and heat conduction. The analysis focuses on the material behavior under overall compression. Despite relatively low contribution to overall heating, friction is localized at fracture sites and plays an essential role in the development of local temperature spikes unknown as hotspots. The microstructural attributes are found to significantly affect the development of the hotspots, with local heating most pronounced when loading is transverse to the filaments or when the material has higher porosities, stronger inter-filament junctions, or smaller filament sizes. Samples with smaller filament sizes undergo more damage, exhibit higher frictional dissipation, and develop larger hotspots that occur primarily at failure sites.

S−d model for local and nonlocal spin dynamics in laser-excited magnetic heterostructures

We discuss a joint microscopic theory for the laser-induced magnetization dynamics and spin transport in magnetic heterostructures based on the $s$-$d$ interaction. Angular momentum transfer is mediated by scattering of itinerant $s$ electrons with the localized ($d$ electron) spins. We use the corresponding rate equations and focus on a spin one-half $d$ electron system, leading to a simplified analytical expression for the dynamics of the local magnetization that is coupled to an equation for the non-equilibrium spin accumulation of the $s$ electrons. We show that this description converges to the microscopic three-temperature model in the limit of a strong $s$-$d$ coupling. The equation for the spin accumulation is used to introduce diffusive spin transport. The presented numerical solutions show that during the laser-induced demagnetization in a ferromagnetic metal a short-lived spin accumulation is created that counteracts the demagnetization process. Moreover, the spin accumulation leads to the generation of a spin current at the interface of a ferromagnetic and non-magnetic metal. Depending on the specific magnetic system, both local spin dissipation and interfacial spin transport are able to enhance the demagnetization rate by providing relaxation channels for the spin accumulation that is build up during demagnetization in the ferromagnetic material.

Dynamics after quenches in one-dimensional quantum Ising-like systems

We study the out-of-equilibrium dynamics of one-dimensional quantum Ising-like systems, arising from sudden quenches of the Hamiltonian parameter $g$ driving quantum transitions between disordered and ordered phases. In particular, we consider quenches to values of $g$ around the critical value $g_c$, and mainly address the question whether, and how, the quantum transition leaves traces in the evolution of the transverse and longitudinal magnetizations during such a deep out-of-equilibrium dynamics. We shed light on the emergence of singularities in the thermodynamic infinite-size limit, likely related to the integrability of the model. Finite systems in periodic and open boundary conditions develop peculiar power-law finite-size scaling laws related to revival phenomena, but apparently unrelated to the quantum transition, because their main features are generally observed in quenches to generic values of $g$. We also investigate the effects of dissipative interactions with an environment, modeled by a Lindblad equation with local decay and pumping dissipation operators within the quadratic fermionic model obtainable by a Jordan-Wigner mapping. Dissipation tends to suppress the main features of the unitary dynamics of closed systems. We finally address the effects of integrability breaking, due to further lattice interactions, such as in anisotropic next-to-nearest neighbor Ising (ANNNI) models. We show that some qualitative features of the post-quench dynamics persist, in particular the different behaviors when quenching to quantum ferromagnetic and paramagnetic phases, and the revival phenomena due to the finite size of the system.

Conditional scalar dissipation rate modeling for turbulent spray flames using artificial neural networks

Machine learning techniques have been used for the closure of the conditional scalar dissipation rate in turbulent spray flames. Statistical data are extracted from carrier-phase direct numerical simulation (CP-DNS) results for the generation of artificial neural networks that are trained to predict the conditional scalar dissipation rate such that they could serve as a sub-grid model for large eddy simulations of spray combustion. A quantitative error assessment suggests that predictions of the conditionally averaged dissipation rate are in excellent agreement with CP-DNS data. Further comparison with commonly used models for the unconditionally filtered dissipation rate promises significant improvements if ANNs are used for closure. The artificial neural networks also help to identify the important features that affect the local dissipation rates. The results suggest that few - mostly droplet related - parameters suffice as input features for accurate ANNs. This is in contradiction to standard modeling techniques that are solely based on gas phase properties and highlights the need to revisit scalar dissipation rate modeling for spray flames if analytical expressions are to be used.

A novel continuum damage model to simulate quasi-brittle failure in mode I and mixed-mode conditions using a continuous or a continuous-discontinuous strategy

This paper formulated and implemented a novel local continuum damage model to be used with a continuous or a continuous-discontinuous strategy to simulate crack growth in quasi-brittle failure for mode I and mixed-mode conditions. The damage model is based on an evolution law using only physical parameters, which can be obtained through fracture and resistance tests without the need for further calibration. The local energy dissipation is controlled through regularization by the fracture energy and element length ensuring that different mesh densities dissipated the same energy during the damage evolution. The proposed damage model is employed in a finite element framework in two strategies: (i) a continuous strategy in which only the continuum damage model is used to predict crack propagation failure; and (ii) a continuous-discontinuous strategy in which a discontinuous enrichment is used to insert a crack within the elements after the total damage state is reached with no ad-hoc threshold value for the transition from continuous to the discontinuous approach. Comprehensive validation is demonstrated through comparisons with experimental results for various types of tests in mode I and mixed-mode conditions: single edge notch beam, unnotched four-point beam, anti-symmetric four-point shear beam, and double-edge-notched specimen. The results demonstrated that the proposed damage model and both strategies successfully simulated the crack propagation and failure of all simulated tests. Good agreement with the experimental results is observed for the crack path, maximum load, and entire load–displacement softening curves with coarser meshes than other models of the literature. Furthermore, the 3D results of the mixed-mode double-edge-notched specimen show that the proposed model provided good results even with coarser meshes using 16 times fewer elements than recent and similar results from the literature.

Real-Time Microscale Temperature Imaging by Stimulated Raman Scattering.

Microscale thermometry of aqueous solutions is essential to understand the dynamics of local heat generation and dissipation in chemical and biological systems. A wide variety of fluorescent probes have been developed to map temperature changes with submicrometer resolution, but they often suffer from the uncertainty associated with microenvironment-dependent fluorescent properties. In this work, we develop a label-free ratiometric stimulated Raman scattering (SRS) microscopy technique to quantify microscale temperature by monitoring the O-H Raman stretching modes of water. By tracking the ratio changes of the hydrogen-bonding O-H band and the isosbestic band, we can directly quantify the temperature of water-based environments in real time without exogenous contrast agents. We demonstrate real-time measurement of localized intracellular and extracellular temperature changes due to laser absorption. This high-speed nonlinear optical imaging technique has the potential for in situ microscale imaging of thermogenesis in both chemical and biological systems.

Vanadium oxide coatings to self-regulate current sharing in high-temperature superconducting cables and magnets

High-temperature superconductors such as REBa2Cu3O7 − δ (REBCO, RE = rare earth) enable high-current cables and high-field magnets. By removing the turn-to-turn insulation in a magnet application, recent experiments demonstrated that REBCO magnets can self-protect against catastrophic damage during a superconducting-to-normal transition (quench), i.e., when the stored magnetic energy rapidly converts to heat. The current can bypass the hot spot during a quench, thereby reducing the localized heat dissipation. The removal of the insulation between turns, however, leads to excessive eddy currents during current ramping, thereby forcing a much-prolonged magnet charging time. To address this issue, we investigate vanadium oxide (VOx) coatings as a temperature-dependent self-switching medium that automatically manages current sharing. VOx coatings (with 1.70 ≤ × ≤ to 2.07) were deposited by reactive cathodic arc deposition, initially on insulating glass to determine the electrical properties, and later on commercial REBCO tapes. The coatings are x-ray amorphous but with a short-range crystalline ordering according to Raman spectrometry. The resistivity of VOx decreased by at least three orders of magnitude when the temperature increased from 80 to 300 K. The coating process is compatible with commercial REBCO tapes as evidenced by the negligible change in the critical current caused by the coating process. The results from current sharing experiments and circuit analysis suggest that the VOx coating can effectively self-regulate current sharing in REBCO magnets, suppress excessive eddy currents, and enable self-protection during quenches.

Turbulence, pseudo-turbulence, and local flow topology in dispersed bubbly flow

Bubbly turbulent flow in a channel is investigated using interface-resolved direct numerical simulation. An efficient coupled level-set volume-of-fluid solver based on a fast Fourier transform algorithm is implemented to enable a high resolution and fast computation at the same time. Up to 384 bubbles are seeded in the turbulent channel flow corresponding to 5.4% gas volume fraction. Bubbles are clustered in the channel center due to the downward flow direction. The bubbles induce additional pseudo-turbulence in the channel center and are also able to attenuate the energy in the boundary layer by reducing the shear production. Turbulent kinetic energy budget indicates a significant buoyancy production in the channel center. A local equilibrium between buoyancy production and dissipation is observed here besides the shear production peak in the boundary layer. Comparing the local production and dissipation indicates a coexistence of boundary layer turbulence near the wall and bubble-induced pseudo-turbulence in the channel center. The liquid phase and gas phase are coupled through the complex liquid–gas interface. Local flow topology analysis is depicted in the liquid phase around the bubbles as well as in the gas phase. The flow topology of the liquid phase and the gas phase differs from each other significantly. Local dissipation is more dominant in the liquid phase near the bubble interface, whereas local enstrophy is preferred in the gas phase. In the liquid phase, a high dissipation event is preferred close to the interface, whereas a high enstrophy event is dominant away from the interface.

Budden’s Paradox Resolved

The Budden energy nonconservation paradox is dispelled herein by recognizing that the pole approach to the spatial origin from below in the complex plane can be resolved into a real principal value minus $i\pi $ times a Dirac delta, the imaginary coefficient whereof supplies just the right amount of localized dissipation to equilibrate the energy budget precisely, regardless of whether wave incidence be up or down. Only the reflectionless downward wave incidence remains as a counterintuitive challenge to physical anticipation, but at least a challenge liberated from its former onus of energy nonconservation.