Direction Of Energy Flow Research Articles

What if Energy Flowed from Cold to Hot? Counterfactual Thought Experiments

What if energy flowed from cold to hot regions? How would thermodynamics be different? We examine such an imagined cold-to-hot world, abbreviated here as a C2H world, and show that the unfamiliar direction of energy flow implies that two thermally interacting objects at different temperatures will not equilibrate. We find that there is no operational definition of temperature, and the first, second, and third laws of thermodynamics become meaningless.

Phase-Matching Controlled Orbital Angular Momentum Conversion in Periodically Poled Crystals.

Nonlinear interactions between light waves can exchange energy, linear momentum, and angular momentum. The direction of energy flow between frequency components is usually determined by the conventional phase-matching condition related to the linear momentum. However, the transfer law of orbital angular momentum (OAM) during frequency conversion remains to be elucidated. Here, we demonstrate experimentally that OAM transfer depends strongly on the phase-matching condition defined by both linear and orbital angular momenta. Under different phase-matching configurations, the second-harmonic wave exhibits variable OAM spectral characteristics such as the presence of just a single value or of odd orders only. Our results pave the way toward unveiling the underlying mechanism of nonlinear conversion of OAM states.

An FDTD Investigation of Orthogonality and the Backscattering of HF Waves in the Presence of Ionospheric Irregularities

AbstractOne of the most successful instruments used to investigate the Earth's ionosphere is coherent scatter radars such as the Super Dual Auroral Radar Network (SuperDARN). Their method of mapping plasma convection at high latitudes is contingent on irregularities that momentarily exist in the ionosphere and are elongated along the direction of the geomagnetic field. Under these conditions, SuperDARN transmits high‐frequency (HF) waves that are then backscattered from these plasma irregularities. In order for maximum backscatter to occur, the wave vector of the incident wave must be orthogonal to the magnetic field lines. This is called the “orthogonality condition.” Over the years, ray tracing results have generated many assumptions, where and how orthogonality occurs. However, since ray tracing only tracks the primary direction of energy flow and typically does not account for diffraction, it provides an incomplete understanding of the interaction of HF waves with small ionospheric irregularities. This work investigates the orthogonality assumptions by modeling at high‐resolution HF electromagnetic wave interactions with small‐scale irregularities in the ionosphere. Specifically, the finite‐difference time‐domain (FDTD) method is employed to determine where orthogonality occurs in the ionosphere and the number of times the condition is satisfied during the simulation. The results provide insights into the orthogonality condition as a function of source frequency and elevation angle for both unperturbed and perturbed ionospheres. From these results, small‐scale irregularities are observed to sometimes contribute significantly to the production of backscatter. A discussion is provided to highlight where three existing orthogonality condition assumptions are incorrect or misleading.

Strain-engineered widely tunable perfect absorption angle in black phosphorus from first principles

Using the density functional theory of electronic structure, we compute the anisotropic dielectric response of bulk black phosphorus subject to strain. Employing the obtained permittivity tensor, we solve Maxwell's equations and study the electromagnetic response of a layered structure comprising a film of black phosphorus stacked on a metallic substrate. Our results reveal that a small compressive or tensile strain, $\sim 4\%$, exerted either perpendicular or in the plane to the black phosphorus growth direction, efficiently controls the epsilon-near-zero response, and allows a perfect absorption tuning from low-angle of the incident beam $\theta=0^\circ$ to high values $\theta\approx 90^\circ$ while switching the energy flow direction. Incorporating a spatially inhomogeneous strain model, we also find that for certain thicknesses of the black phosphorus, near-perfect absorption can be achieved through controlled variations of the in-plane strain. These findings can serve as guidelines for designing largely tunable perfect electromagnetic wave absorber devices.

Interplay between Förster and Dexter Energy Transfer Rates in Isomeric Donor-Bridge-Acceptor Systems.

The ability to direct the flow of excitons enable molecular systems to perform highly advanced functions. Intramolecular energy transfer in donor-bridge-acceptor systems can occur by different mechanisms, and the ability to control the excited state energy pathways depends on the capacity to favor one process over another. Here, we show an anticorrelation between the rates of Förster and Dexter types of energy transfer in two isomeric donor-bridge-acceptor systems. Both dyads display intramolecular Förster triplet-to-singlet and Dexter triplet-to-triplet energy transfers. However, as the bridge-acceptor connection point changes, the rate of one energy transfer process increases at the same time as the other one decreases, allowing us to control the energy flow direction. This work shows how rational design can be used to tune excited state energy pathways in molecular dyads, which is of importance for advanced functions such as multiplicity conversion in future molecular materials.

Evolution of micro/nano-structural arrays on crystalline silicon carbide by femtosecond laser ablation

An experimental study is conducted to investigate the multi-length scale evolution from micro/nano-structural arrays to V-grooves by femtosecond laser-irradiations, considering the effects of laser polarization and the number of scans on the periodic features and the micro/nano-structures. It is found that the evolution from micro/nano-structural arrays to V-grooves is gradual as the laser fluence, scan speed or the number of scans is increased. Laser-induced surface instability, hatching effects, Coulomb explosions, melting, and evaporation are found to be the underlying mechanisms for the micro/nano structural formation. As the laser fluence increases, the agglomerated nanoparticles change from sparse to dense, so that the boundary between the LIPSS (laser-induced periodic surface structures) becomes less obvious. The oxygen (O) elements attached to the surface of nanoparticles increase with an increase in laser fluence. A high-spatial-frequency LIPSS (HSFL) can transform to low-spatial-frequency LIPSS (LSFL) as the laser fluence excesses the ablation threshold, while the spatial period of the HSFL and LSFL is independent of fluence. It is shown that the irradiated surface evolves from a near-damage-free zone to one with a recast layer and thermal-induced micro-cracks, defined as heat-affected zones (HAZ). Near thermal damage-free micro/nano-structural arrays with different orientations on the sidewalls of the grooves are fabricated at close to the laser ablation threshold by multi-pass scanning. A self-organizing model is developed which shows that laser polarization leads to the asymmetry of energy distribution and the structural array is arranged along the main direction of energy flow. The model analysis shows a good agreement with experimental results and provides an efficient means to understand the interaction between femtosecond laser and silicon carbide (SiC).

Negative Thermal Transport in Conduction and Advection*Supported by the National Natural Science Foundation of China (Grant No. 11725521).

Negative refractive index has drawn a great deal of attention due to its unique properties and practical applications in wave systems. To promote the related physics in thermotics, here we manage to coin a complex thermal conductivity whose imaginary part corresponds to the real part of complex refractive index. Therefore, the thermal counterpart of negative refractive index is just negative imaginary thermal conductivity, which is featured by the opposite directions of energy flow and wave vector in thermal conduction and advection, thus called negative thermal transport herein. To avoid violating causality, we design an open system with energy exchange and explore three different cases to reveal negative thermal transport. We further provide experimental suggestions with a solid ring structure. All finite-element simulations agree with theoretical analyses, indicating that negative thermal transport is physically feasible. These results have potential applications such as designing the inverse Doppler effect in thermal conduction and advection.

Long- and Short-Term Comparative Analysis of Renewable Energy Sources

Network working conditions are influenced noticeably by the connection of renewable energy sources to distribution networks. This becomes more and more important due to the increase in renewable energy source penetration over the last few years. This in turn can lead to a mass effect. As a result, the classical open network model with simple unidirectional direction of energy flow has been replaced with an active model that includes many local energy sources. This paper deals with the analysis of long- and short-term changes in power and energy generated by three types of renewable energy sources with similar rated power and which operate in the same region (i.e., located no more than tens of kilometers away). The obtained results can be a starting point for a broader evaluation of the influence of renewable energy sources on power quality in power systems, which can be both positive (supply reliability) and negative (voltage fluctuations and higher harmonics in current and voltage waveforms). It is important not only to correctly place but also to assure the diversity of such sources as it has been confirmed by the source variability coefficient. The long-term analysis allows us also to estimate the annual repeatability of energy production and, furthermore, the profitability of investment in renewable sources in a given region.

Study on phonon spectra and heat capacities of CL-20/MTNP cocrystal and co-formers by density functional theory method.

The phonon spectra and heat capacities of 2, 4, 6, 8, 10, 12-hexanitrohexaazaisowurtzitane/1-methyl-3, 4, 5-trinitropyrazole (CL-20/MTNP) cocrystal and co-formers were calculated in the framework of DFT. By analyzing the phonon density of states (DOS), the energy flow directions and trigger bonds of cocrystal and co-formers have been obtained and the microscopic physical nature was revealed for thermal decomposition mechanism, detonation performance, and sensitivity. For CL-20/MTNP cocrystal, the phonon number of "doorway" modes and the characteristic vibrational frequencies Δωd are between those of its co-formers, which can provide the microscopic understanding for the ordering of impact sensitivity at experiment, ε-CL-20 > CL-20/MTNP > MTNP. In CL-20/MTNP cocrystal, more phonons and stronger phonon DOS peaks of CL-20 molecules than those of MTNP molecules mean cocrystal's detonation performance is mainly dominated by CL-20 molecules. The heat capacities obtained by the Debye model rise with elevated temperatures at 0-600K and the order is ε-CL-20 > CL-20/MTNP > MTNP. Graphical abstract The phonon spectra and heat capacities of CL-20/MTNP cocrystal and co-formers were calculated by density functional theory (DFT). In CL-20/MTNP cocrystal, the detonation performance and impact sensitivity are mainly dominated by CL-20 molecules. The broken bonds caused by energy transfer may undergo a multi-phonon pumping process.

Study on Material and Energy Flow in Steel Forging Production Process

Forging production is widely used in aerospace, automobile, shipbuilding and other industries because of its high mechanical properties. Due to the high energy consumption and high pollution in steel and iron enterprises, with the promotion of green manufacturing and energy saving and emission reduction, more and more attention has been paid to the study of energy conservation problem. In this paper, combined with the process of forging production, the material flow of steel, fuel and other related accessories in the production process is studied and analyzed. Based on the material flow model, the energy flow direction of the forging process is qualitatively determined with the material as the carrier, and the energy flow model of the whole forging production process is carried out.

Waveguide modes in Weyl semimetals with tilted dirac cones.

We theoretically study unattenuated electromagnetic guided wave modes in centrosymmetric Weyl semimetal layered systems. By solving Maxwell's equations for the electromagnetic fields and using the appropriate boundary conditions, we derive dispersion relations for propagating modes in a finite-sized Weyl semimetal. Our findings reveal that for ultrathin structures and proper Weyl cones tilts, extremely localized guided waves can propagate along the semimetal interface over a certain range of frequencies. This follows from the anisotropic nature of the semimetal where the diagonal components of the permittivity can exhibit a tunable epsilon-near-zero response. From the dispersion diagrams, we determine experimentally accessible regimes that lead to high energy-density confinement in the Weyl semimetal layer. Furthermore, we show that the net system power can vanish all together, depending on the Weyl cone tilt and frequency of the electromagnetic wave. These effects are seen in the energy transport velocity, which demonstrates a substantial slowdown in the propagation of electromagnetic energy near critical points of the dispersion diagrams. Our results can provide guidelines in designing Weyl semimetal waveguides that can offer efficient control in the velocity and direction of energy flow.

Extraction of optimal SAGSHP sizing based on the various control logic in specific climate condition

In this paper, the optimal performance configurations of Solar Assisted Ground Source Heat Pump (SAGSHP) are proposed to provide the heat load requirements of a building in Tehran. Optimization has been done, on one side, based on the energy flow directions, introduced as control logic, and on the other side, using the optimal amount of energy from the solar and ground heat sources. Five scenarios are presented based on the control logic. In the following, the optimum mode in each scenario has been obtained according to the optimal percentage of energy available from considered sources in the studied area. In each scenario, the optimal mode is introduced based on the system's coefficient of performance (COP) which leads the optimal system sizing. For each scenario, the required heat is produced by the solar collector for the building, which varies from 20% to 60%, and the rest of the required heat demand is generated by the heat pump. The modeling results showed the fifth scenario as the best one for winter, where the working fluid from the ground heat exchanger and was directed into the solar collector, and its temperature increased through absorbing the heat of the sun. For summer, the suitable scenario where the solar collector and the thermal storage tank operate parallel to the heat pump is the fourth one. Finally and in general, the fifth scenario is considered as the optimal performance scenario, in which the COP in January has been 5.359.

Pseudospin-Polarized Topological Line Defects in Dielectric Photonic Crystals

Electromagnetic topological insulators have been explored extensively due to the robust edge states they support. In this work, we propose a topological electromagnetic system based on a line defect in topologically nontrivial photonic crystals (PCs). With a finite-difference supercell approach, modal analysis of the PCs structure is investigated in detail. The topological line-defect states are pseudospin polarized and their energy flow directions are determined by the corresponding pseudospin helicities. These states can be excited by using two spatially-symmetric line-source arrays carrying orbital angular momenta. The feature of the unidirectional propagation is demonstrated and it is stable when disorders are introduced to the PCs structure.

Walk partitions of flow in Ecological Network Analysis: Review and synthesis of methods and indicators

Abstract Ecological Network Analysis (ENA) has provided insights into the structure, function, and transformation of ecosystems for more than forty years. Key insights from ENA focus on how the patterns of directed weighted transactions among system components (e.g., species, functional groups, economic sectors) create emergent and often unexpected relationships in ecosystems that affect system function and sustainability. Flow analysis, also called throughflow analysis, is one of several core techniques in ENA. Generally, it traces the flux of energy or matter through the network from inputs to outputs. During the forty-years of development, flow analysis has accreted multiple extensions and modifications. In this concept and synthesis paper, we review four flow analyses and show how they are conceptually linked by partitioning flows across subsets of pathways within networks. These flow analyses include: (1) the definition of throughflow, a measure of the total processing power of a network; (2) Leontief’s decomposition based on walk length, indicating the direction and distance of energy or matter flow; (3) Finn’s measure of recycling of matter in networks; and (4) five mode analysis, characterizing flows according to their origin and destination. Presenting these techniques side-by-side with a common conceptual framework reveals overlaps and distinctive elements among the analytic products. This synthesis clarifies the flow analyses tools and their applications to ecological and socio-economic networks and provides example applications. Further, new insights are presented by combining existing flow analyses to calculate novel indices that further characterize the flow structure of networks. For example, both indirect flows in networks and cycling are highly important features in networks. In order to determine the proportion of indirect flows generated through cycling, we can use the ratio of Cycled Flow identified from Finn’s analysis and the indirect flows identified in the Leontief analysis. As ENA matures through additional analysis development and applications, it will continue to provide insights into ecosystems and contribute to the broader area of network science.

Taking a Stab at Quantifying the Energetics of Biological Puncture.

An organism's ability to control the timing and direction of energy flow both within its body and out to the surrounding environment is vital to maintaining proper function. When physically interacting with an external target, the mechanical energy applied by the organism can be transferred to the target as several types of output energy, such as target deformation, target fracture, or as a transfer of momentum. The particular function being performed will dictate which of these results is most adaptive to the organism. Chewing food favors fracture, whereas running favors the transfer of momentum from the appendages to the ground. Here, we explore the relationship between deformation, fracture, and momentum transfer in biological puncture systems. Puncture is a widespread behavior in biology requiring energy transfer into a target to allow fracture and subsequent insertion of the tool. Existing correlations between both tool shape and tool dynamics with puncture success do not account for what energy may be lost due to deformation and momentum transfer in biological systems. Using a combination of pendulum tests and particle tracking velocimetry (PTV), we explored the contributions of fracture, deformation and momentum to puncture events using a gaboon viper fang. Results on unrestrained targets illustrate that momentum transfer between tool and target, controlled by the relative masses of the two, can influence the extent of fracture achieved during high-speed puncture. PTV allowed us to quantify deformation throughout the target during puncture and tease apart how input energy is partitioned between deformation and fracture. The relationship between input energy, target deformation and target fracture is non-linear; increasing impact speed from 2.0 to 2.5 m/s created no further fracture, but did increase deformation while increasing speed to 3.0 m/s allowed an equivalent amount of fracture to be achieved for less overall deformation. These results point to a new framework for examining puncture systems, where the relative resistances to deformation, fracture and target movement dictate where energy flows during impact. Further developing these methods will allow researchers to quantify the energetics of puncture systems in a way that is comparable across a broad range of organisms and connect energy flow within an organism to how that energy is eventually transferred to the environment.

Development of a Stand-Alone Photovoltaic System Considering Shaded Effect for Energy Storage and Release

The purpose of this study was to develop a photovoltaic system that stores energy for use in direct current micro-grid systems or to supply electric power to consumers living in remote areas. If the photovoltaic module array is shaded, the signals of conventional maximum power point trackers (MPPT) may be trapped at the local power maxima. Therefore, this study developed a smart maximum power point tracker to track the maximum power point (MPP). The control method adopted a teaching learning based optimization (TLBO) algorithm. To adjust the energy flow direction of the direct current load terminal, this study proposed an energy accumulation and release strategy that used a high-boost/buck-ratio bidirectional converter to control the battery charge and discharge for energy accumulation and release. In addition, this study developed an inverter to convert direct current into alternating current for alternating current loads.

Effects of mount positions on vibrational energy flow transmission characteristics in aero-engine casing structures

The main goal of the study is to apply the structural intensity method to analyze the effects of positions of the main-mount and the sub-mount on the vibrational energy flow transmission characteristics in aero-engine casing structures, so as to attenuate the vibration of the casing and the whole aero-engine. Structural intensity method, indicating magnitude and direction of the vibrational energy flow, is a powerful tool to study vibration problems from the perspective of energy. In this paper, a casing-support-rotor coupling model subjected to the rotor unbalanced forces is established by the finite element method. Formulations of the structural intensity of a shell element and the structural intensity streamline are given. A simulation system consisting of the finite element tool and the in-house program is developed to carry out forced vibration analysis and structural intensity calculation. The structural intensity field of the casing is visualized in the forms of vector diagram and streamline representation. The vibrational energy flow behaviors of the casing at the rotor design rotating speed are analyzed, and the vibrational energy flow transmission characteristics of the casing with different axial positions of the main-mount and the sub-mount are investigated. Moreover, some measures to attenuate the vibration of the casing are obtained from the numerical results, and their effectiveness is verified in the frequency domain and the time domain. The results shed new light on the effects of the mount positions on the vibration energy transmission behaviors of the casing structure. The structural intensity method is a more advanced tool for solving vibration problems in engineering. Furthermore, it may provide some guidance for the vibration attenuation of the casing and the whole aero-engine.

Flapping wing energy harvesting: aerodynamic aspects

Aerodynamic forces on flapping wings create forward thrust. Natural flyers like birds and insects apply these forces effectively. Extensive studies have shown that the efficiency of flapping wings can be improved by various means. Of importance is the development and control of leading edge vortices (LEV’s). Thrust production means that energy is transferred from the flapping system into the fluid. In a similar way it can be achieved that energy flows from the fluid into the flapping system, i.e. fluid energy may be harvested. Responsible for the direction of energy flow is the ratio of pitching amplitude versus amplitude of the induced incidence of the plunging motion. If this ratio is smaller than unity thrust energy is produced; if it is larger than unity energy is transferred into the flapping system. In the present paper, emphasis is placed on the detailed study of the aerodynamic effects and on some ideas of optimization of energy harvesting of a flapping system. It will be shown that similar to the thrust production mode also in the energy harvesting mode, the influence of LEV’s is of major concern. The control of these vortices by airfoil deformation is shown to be beneficial for optimizing the efficiency of energy harvesting.

Manipulating the magnetic energy density and energy flux by cylindrically symmetric state of polarization

Using the Richards and Wolf formulas for an arbitrary cylindrical vector (CV) beam, we obtain explicit expressions for all components of the electric and magnetic field strength vectors near the focus, as well as expression for the energy flux in an aplanatic optical system. Based on such analytical models, it reveals that the intensity pattern of the magnetic field at the focus can be tailored by appropriately adjusting the initial phase, peak-centered, doughnut, and flat-topped magnetic fields can be achieved using this method. For the energy flows, in contrast, they are almost the same for arbitrary CV beams, which exhibit hollow shaped patterns for both the transverse and longitudinal components. Unlike the longitudinal component, however, the hollow shaped pattern of the transverse component is separated into two regions, arriving from the null transverse energy flow in the focal plane. Besides, the directions of the transverse energy flow are reversed between these two regions, which are directed inwardly and outwardly along the radial direction, respectively.

Charge Transfer Dynamics of Phase-Segregated Halide Perovskites: CH3NH3PbCl3 and CH3NH3PbI3 or (C4H9NH3)2(CH3NH3) n-1Pb nI3 n+1 Mixtures.

Lead halide perovskites present a versatile class of solution-processable semiconductors with highly tunable bandgaps that span ultraviolet, visible, and near-infrared portions of the spectrum. We explore phase-separated chloride and iodide lead perovskite mixtures as candidate materials for intermediate band applications in future photovoltaics. X-ray diffraction and scanning electron microscopy reveal that deposition of precursor solutions across the MAPbCl3/MAPbI3 composition space affords quasi-epitaxial cocrystallized films, in which the two perovskites do not alloy but instead remain phase-segregated. First-principle calculations further support the formation of an epitaxial interface and predict energy offsets in the valence band and conduction band edges that could result in intermediate energy absorption. The charge dynamics of variable mixtures of the relatively narrow bandgap (1.57 eV) MAPbI3 perovskite and wide bandgap (3.02 eV) MAPbCl3 are probed to map charge and energy flow direction and kinetics. Time-resolved photoluminescence and transient absorption measurements reveal charge transfer of photoexcited carriers in MAPbCl3 to MAPbI3 in tens of picoseconds. The rate of quenching can be further tuned by replacing MAPbI3 with two-dimensional Ruddlesden-Popper (BA)2(MA) n-1Pb nI3 n+1 ( n = 3, 2, and 1) perovskites, which also remain phase-separated.