Spectral Energy Transfer Research Articles

Spectral energy transfers in domain growth problems

Spectral energy transfers in domain growth problems

Spectral energy transfer on complex networks: a filtering approach

The spectral analysis of dynamical systems is a staple technique for analyzing a vast range of systems. But beyond its analytical utility, it is also the primary lens through which many physical phenomena are defined and interpreted. The turbulent energy cascade in fluid mechanics, a dynamical consequence of the three-dimensional Navier–Stokes equations in which energy “cascades” from large injection scales to smaller dissipation scales, is a well-known example that is precisely defined only in reciprocal space. Related techniques in the context of networked dynamical systems have been employed with great success in deriving reduced order models. But what such techniques gain in analytical tractability, they often lose in interpretability and locality, as the lower degree of freedom system frequently contains information from all nodes of the network. Here, we demonstrate that a network of nonlinear oscillators exhibits spectral energy transfer facilitated by an effective force akin to the Reynolds stress in turbulence, an example of an emergent higher order interaction. Then, introducing a filter-based decomposition motivated by large eddy simulation, we show that such higher order interactions can be localized to individual nodes and study the effects of local topology on such interactions.

A europium-based MOF fluorescent probe for efficiently detecting Co2+ and CrO42−

Cobalt is an essential trace element in the human body that is vital for metabolism in living organisms. Insufficient or excessive intake of cobalt can lead to adverse effects. We prepared a europium-based metal–organic framework (Eu-MOF), NH4[Eu(sal)4(phen)2] (abbreviated as Eu-sal, sal = salicylic acid group, phen = 1,10-phenanthroline). Fluorescence investigations revealed that Eu-sal can efficiently and selectively identify Co2+ through a fluorescence quenching effect, with a quenching efficiency K SV and limit of detection (LOD) of 2.7 × 104 M−1 and 0.37 μM, respectively. In addition, Eu-sal also shows highly selective for CrO42− (K SV: 3.6 × 104 M−1; LOD: 0.27 μM). The quenching of Eu-sal fluorescence emission by Co2+ and CrO42− ions may be caused by spectral overlap, energy transfer, and competition. Furthermore, Eu-sal has good thermal and chemical stability. These results indicate that Eu-sal is a promising fluorescence probe for highly selective detection of Co2+ and CrO42− ions.

Coupled Short-Ig Wave Dunamics Over A Shallow Barrier Reef

Reef barriers play a major role many coral islands, by sheltering the lagoon from the ocean wave energy and then creating a unique habitat for many species. This filtering action becomes increasingly crucial for ecosystems health and shoreline protection in the context of climate change and related sea level rise, degradation of coral systems and modification of wave conditions. A strong research effort has therefore been engaged by the coastal oceanographers community for the last two decades to improve our knowledge and prediction skills of wave dynamics over coral reef systems. A widely reported observation is the importance of infragravity waves (IG) over wave-driven reef systems, whether fringing or barrier reefs. IG are primarily forced by groups in the incoming short-wave (SW) field, either by the release of bound waves or the breakpoint oscillations (Bertin et al. 2018). IG period typically ranges between 30 and 200s, which makes them prone to excite or interact with natural seiching modes in reef-lagoon systems often ranging in the Very Low Frequency (VLF) band. Further research efforts are now necessary to better understand the interaction between long IG/VLF oscillations and SW field. In particular, long waves are expected to play a dynamic depth-filtering role on SW energy, acting as long carrier wave able to promote the propagation of larger SW groups by IG/VLF crests. More generally, the spectral energy transfers over the reef crest-flat system and their relative importance w.r.t. frictional and breaking dissipation are not fully understood over the complete range of surface waves. The aim of the present study is to analyse and to discuss a series of field observations performed on the barrier reef of Maupiti Island, French Polynesia. A particular focus is placed on the interaction between SW and IG wave fields across the reef crest-flat system.

Resonant decay of kinetic Alfvén waves and implication on spectral cascading

A general equation describing the resonant nonlinear mode-coupling among kinetic Alfvén waves (KAWs) is derived using nonlinear gyrokinetic theory, which can be applied to study the potentially strong spectral energy transfer of KAWs. As a first application, the parametric decay of a pump KAW into two sideband KAWs is studied, with particular emphasis on cascading in the perpendicular wavenumber. It is found that, for the “co-propagating” cases with all three KAWs propagating in the same direction along the equilibrium magnetic field line, it exhibits a dual cascading character in the perpendicular wavenumber space; while for the “counter-propagating” cases with one sideband propagating in the opposite direction with respect to the pump wave, it instead, can exhibit both dual and inverse cascading behaviors. Furthermore, the implications of these findings on shear Alfvén waves regarding instability, nonlinear saturation, and charged particle transport in fusion plasmas are also discussed.

Optimising subgrid-scale closures for spectral energy transfer in turbulent flows

Subgrid-scale (SGS) modelling is formulated using a local transport of spectral kinetic energy estimated by a wavelet multiresolution analysis. Using a spectrally and spatially local decomposition by wavelet, the unresolved inter-scale energy transfer and modelled SGS dissipation are evaluated to enforce explicitly and optimally their balance a priori over a range of large-eddy simulation (LES) filter widths. The formulation determines SGS model constants that optimally describe the spectral energy balance between the resolved and unresolved scales at a given cutoff scale. The formulation is tested for incompressible homogeneous isotropic turbulence (HIT). One-parameter Smagorinsky- and Vreman-type eddy-viscosity closures are optimised for their model constants. The algorithm discovers the theoretical prediction of Lilly (The representation of small-scale turbulence in numerical simulation experiments. In Proceedings of the IBM Scientific Computing Symposium on Environmental Sciences, pp. 195–210) at a filter cutoff scale in the inertial subrange, whereas the discovered constants deviate from the theoretical value at other cutoff scales so that the spectral optimum is achieved. The dynamic Smagorinsky model used a posteriori shows a suboptimal behaviour at filter scales larger than those in the inertial subrange. A two-parameter Clark-type closure model is optimised. The optimised constants provide evidence that the nonlinear gradient model of Clark et al. (J. Fluid Mech., vol. 91, issue 1, 1979, pp. 1–16) is prone to numerical instability due to its model form, and combining the pure gradient model with a dissipative model such as the classic Smagorinsky model enhances numerical stability but the standard mixed model is not optimal in terms of spectral energy transfer. A posteriori analysis shows that the optimised SGS models produce accurate LES results.

Probing the Nonlinear Interactions of Supertidal Internal Waves Using a High-Resolution Regional Ocean Model

Abstract The internal wave (IW) continuum of a regional ocean model is studied in terms of the vertical spectral kinetic energy (KE) fluxes and transfers at high vertical wavenumbers. Previous work has shown that this model permits a partial representation of the IW cascade. In this work, vertical spectral KE flux is decomposed into catalyst, source, and destination vertical modes and frequency bands of nonlinear scattering, a framework that allows for the discernment of different types of nonlinear interactions involving both waves and eddies. Energy transfer within the supertidal IW continuum is found to be strongly dependent on resolution. Specifically, at a horizontal grid spacing of 1/48°, most KE in the supertidal continuum arrives there from lower-frequency modes through a single nonlinear interaction, whereas at 1/384° and with sufficient vertical resolution KE transfers within the supertidal IW continuum are comparable in size to KE transfer from lower-frequency modes. Additionally, comparisons are made with existing theoretical and observational work on energy pathways in the IW continuum. Induced diffusion (ID) is found to be associated with a weak forward frequency transfer within the supertidal IW continuum. ID is also limited to the highest vertical wavenumbers and is more sensitive to resolution relative to spectrally local interactions. At the same time, ID-like processes involving high-vertical-wavenumber near-inertial and tidal waves as well as low-vertical-wavenumber eddy fields are substantial, suggesting that the processes giving rise to a Garrett–Munk-like spectra in the present numerical simulation and perhaps the real ocean may be more varied than in idealized or wave-only frameworks.

Energy cascade in the Garrett–Munk spectrum of internal gravity waves

We study the spectral energy transfer due to wave–triad interactions in the Garrett–Munk spectrum of internal gravity waves based on a numerical evaluation of the collision integral in the wave kinetic equation. Our numerical evaluation builds on the reduction of the collision integral on the resonant manifold for a horizontally isotropic spectrum. We evaluate directly the downscale energy flux available for ocean mixing, whose value is in close agreement with the finescale parameterization. We further decompose the energy transfer into contributions from different mechanisms, including local interactions and three types of non-local interactions, namely parametric subharmonic instability, elastic scattering (ES) and induced diffusion (ID). Through analysis on the role of each mechanism, we resolve two long-standing paradoxes regarding the mechanism for forward cascade in frequency and zero ID flux for the GM76 spectrum. In addition, our analysis estimates the contribution of each mechanism to the energy transfer in each spectral direction, and reveals new understanding of the importance of local interactions and ES in the energy transfer.

Subgrid Parameterizations of Ocean Mesoscale Eddies Based on Germano Decomposition

AbstractOcean models at intermediate resolution (1/4°), which partially resolve mesoscale eddies, can be seen as Large eddy simulations of the primitive equations, in which the effect of unresolved eddies must be parameterized. In this work, we propose new subgrid models that are consistent with the physics of two‐dimensional flows. We analyze subgrid fluxes in barotropic decaying turbulence using Germano (1986, https://doi.org/10.1063/1.865568) decomposition. We show that Leonard and Cross stresses are responsible for the enstrophy dissipation, while the Reynolds stress is responsible for additional kinetic energy (KE) backscatter. We utilize these findings to propose a new model, consisting of three parts, that is compared to a baseline dynamic Smagorinsky model. The three‐component model accurately simulates the spectral transfer of energy and enstrophy and improves the representation of KE spectrum, resolved KE and enstrophy decay in a posteriori experiments. The backscattering component of the new model (Reynolds stress) is implemented both in quasi‐geostrophic and primitive equation ocean models and improves statistical characteristics, such as the vertical profile of eddy KE, meridional overturning circulation and cascades of kinetic and potential energy.

Spectral study and energy transfer analysis of Er:YAlO3 crystals

Er:YAP crystals have been proved promising mid-infrared gain materials but there is few systematic study focused on the relationship between the doping concentration and spectral properties. In this work, Er:YAP crystals with a series of doping concentrations(0.1 at.%, 1 at.%, 5 at.%, 10 at.%, 15 at.%, 30 at.%) were fast grown successfully by a modified laser-heated pedestal growth (LHPG) method. Through investigating doping concentration dependent spectral characteristics in detail, we found that the cross relaxation process (Er3+: 4S3/2+4I15/2 → 4I9/2+4I13/2) gets significantly enhanced with the doping concentration when it is below 5 at.%, and eventually tends to saturate. And the cooperative up-conversion process (Er3+: 4I13/2+4I13/2 → 4I15/2+4I9/2) conducive to high-efficiency ∼3 μm laser starts to dominate when the doping concentration is 5 at.% or even less. This result demonstrates the potential of Er:YAP crystals to achieve high-power and high-efficiency laser output under relatively low Er-doping concentration, and offers a valuable guidance for doping optimization of Er:YAP systems.

Spectral characterization and energy transfer study of Nd3+/Yb3+ in borosilicate glasses

Very sharp emission in near infrared region is observed for Nd3+/Yb3+ codoped 60B2O3-10SiO2-5PbO-10Al2O3-10Li2O-5ZnO (BSPbLAZ) glasses which were prepared by melt-quenching method along with Nd3+ doped BSPbLAZ glass. Photoluminescence excitation (PLE) spectra of 1 mol% of Nd2O3 doped BSPbLAZ glass shows intense band at 584 nm at an emission wavelength of 1062 nm. Photoluminescence emission (PL) spectra for 1 mol% of Nd2O3 doped BSPbLAZ glass at an excitation wavelength of 584 nm shows three characteristics emission bands in NIR range, 4F3/2→4I9/2, 4I11/2 and 4I13/2, most intense among these being the one at 1062 nm corresponding to 4F3/2→4I11/2. In the codoped system (Nd2O3/Yb2O3), prominent emission bands for Nd3+ are observed at 887, 1062 and 1336 nm corresponding to 4F3/2→4I9/2, 4I11/2 and 4I13/2 respectively, while for Yb3+ ions transition is observed at 980 nm corresponding to 2F5/2→2F7/2. Detailed investigation of characteristics like NIR emission, lifetime and energy transfer efficiency on increment of Yb2O3 doping concentration helps in understanding the utility of these glasses in the fields like photonic and optical devices. In BSPbLAZ glass Yb3+ can be easily sensitized by Nd3+ ions on account of its 4f-4f transitions. These factors provide healthy possibility for usage of Nd3+/Yb3+codoped BSPbLAZ glasses as multiple pump channel sources for laser fiber system based on Yb3+ ions.

(Invited) control of supercontinuum generation due to soliton propagation in fibers with varying dispersion

We demonstrate experimental and numerical studies of supercontinuum generation for silica fibers with longitudinally varying diameter and dispersion. The significant difference in the spectral and temporal transformations of the pump pulse depending on the direction of propagation in the researched fiber samples is shown. Numerical simulations demonstrate the possibility of the supercontinuum spectra management by controlling the longitudinal profile of the fiber. Ways to optimize the output in terms of spectral flatness and efficient energy transfer to the desired wavelength region are presented.

Design and evaluation of a quasi-monochromatic and high-energy flow thermophotovoltaic test system

The energy conversion efficiency of thermophotovoltaic (TPV) cells is quite different for different spectral energy and related to the spectral energy intensity. To obtain the energy conversion relationship of the thermophotovoltaic cell for various spectral energy with high-density flux, a test system was designed, which consists of a blackbody radiation source, spectrum control device, temperature control device, thermophotovoltaic cells, and cooling device. To evaluate the performance of the test system, the Monte Carlo ray tracing method and the finite volume method is applied to analyze the spectral energy transfer and temperature of the primary devices. The quasi-monochromatic and high-energy flow that reaches the cell surface can be obtained. The energy conversion performance of thermophotovoltaic cells under a single high-energy spectrum can be thoroughly investigated through this designed test system. The results show that the energy transfer efficiency of this newly designed test system is greater than 14 %. When the radiation source temperature is 1000 K, 1500 K, 2000 K, and 2500 K, the design efficiency of the testing system is 14.11 %, 14.38 %, 14.58 %, and 14.87 %, respectively. When the temperature of the blackbody radiation source is 2000 K and 2500 K, the energy conversion efficiency of the GaSb cell is 20.37 % and 22.85 %, respectively. This work provides theoretical guidance for developing a quasi-monochromatic and high-energy flow thermophotovoltaic test system.

Nonlinear energy dissipation and transfers in coarsening systems.

During coarsening, small structures disappear, leaving behind only large ones. Here we study the spectral energy transfers in Model A, where the order parameter ϕ evolves via nonconserved dynamics. We show that the nonlinear interactions dissipate fluctuations and facilitate energy transfers among the Fourier modes so that only ϕ(k=0), where k is the wave number, survives at the end and approaches the asymptotic value +1 or -1. We contrast the coarsening evolution for the initial conditions with 〈ϕ(x,t=0)〉=0 and with uniformly positive or negative ϕ(x,t=0).

Spectral splitting solar energy transfer in small-diameter multimode optical fiber based on two-stage concentration

A two-stage concentration system (TSC) is proposed for transfer spectral splitting solar energy via a small-diameter multimode optical fiber. The TSC includes a Fresnel lens that concentrates the energy before passing it through UV700 for spectral splitting. The system employs an optical fiber coupler for the second concentration stage and a single-diameter multimode fiber for energy transfer. We established a mathematical model of TSC, coupled optical fiber transmission, and analyzed it via simulation and experimentation. Simulation results demonstrate that the TSC reduces the spot's energy by 42% of the thermal radiation spectrum, decreases the focal spot radius by 58%, and improves the energy flux by 10.93 times with UV700. Additionally, experimental findings reveal that the TSC reduces the focal spot energy by 40.46% of the thermal radiation spectrum, decreases the fiber end temperature by 59.52%, identifies the optimal coupling distance at 308.75 mm, and yields a coupling efficiency of 60.7%.

A new approach to atmospheric turbulence measurements using HOSE on radar backscattered echoes

A Novel approach has been proposed for the measurement of turbulent energy dissipation rate (ε) from Nonlinear index (NLI) based measurements using the Higher Order Spectral Estimation (HOSE) technique. The higher order spectral analysis, also known as Bispectrum, is useful for identifying nonlinearities present in the signals. Earlier studies used Bispectrum measurements to experimentally study the spectral energy transfer due to wavenumber interactions in a turbulent flow, which is briefly discussed in the manuscript. This is one of the main motivations for considering HOSE on Atmospheric signals. The Bicoherence obtained by this method has been applied to the backscattered signals received from the Mesosphere, Stratosphere, and Troposphere (MST) radar, Gadanki. Here we considered both convective and clear air atmospheric observations for the analysis and calculated the Nonlinear Index (which represents the amount of nonlinearity in signals) obtained from the Bicoherence for each range bin. We observed that the nonlinearity index indirectly provides information about turbulent intensity. An empirical relationship between the nonlinear index and turbulent energy dissipation rate has been established through regression models. The results of the turbulence energy dissipation rate (ε) obtained from the nonlinear index based method have been compared with the turbulence energy dissipation rate (ε) obtained from the spectrum width based method.

Layer-resolved characterization of subsurface of materials usmg ultrafast two-dimensional electronic spectroscopy

Surface of matter normally contains sublayers with molecular or electronic structure different from the outmost surface and the bulk, which may play a critical role in surface energy and/or charge transfer processes. Therefore, the development of layer-resolved characterization methods is of great importance for surface science and techniques. Although optical spectroscopy methods are very sensitive to structure, their spatial resolution is often much larger than the inter-layer distance of the sublayers, resulting in the inability to achieve laminar resolution. In this work, we discuss the possibilities of utilizing two-dimensional (2D) electronic spectroscopy to distinguish spectral information and energy transfer between different layers, which cannot otherwise be obtained from linear spectroscopy methods owing to lineshape broadening. By theoretical 2D spectral simulations, we investigated two layered systems by numerical simulations, material surface:subsurface:bulk and molecule:surface:subsurface:bulk. The directional energy transfer rates from the bulk to the surface layer owing to the surface-bulk coupling was preset. Due to the fact that the energy transfer between the subsurface and the outermost surface layer is faster than that between the bulk and the outermost surface layer, there will be special time periods during which only the former takes place and gives rise to the distinguishable dynamics of off-diagonal peaks. Our simulation results showed that by analyzing the difference between the projections of the off-diagonal peak, the excitation energy of the surface, subsurface and bulk layers can be determined. Furthermore, for the four-layer system, the off-diagonal peaks can be kept away from the interference of diagonal peaks, providing a better chance for realizing laminar resolution compared with the three-layer system.

Synthesis and real-time monitoring of the morphological evolution of luminescent Eu(TCPP) MOFs.

Real-time acquisition of the morphological information of nanomaterials is crucial to achieving morphological controllable synthesis, albeit being challenging. A novel device was designed, which integrated dielectric barrier discharge (DBD) plasma synthesis and simultaneous in situ spectral monitoring of the formation of metal-organic frameworks (MOFs). Important dynamic luminescence behaviors such as coordination induced emission (CIE), antenna effect (AE), and red-blue shift were continuously captured to reveal the spectral emission mechanism and energy transfer progress and verify the correlation with the morphological evolution of the MOFs. The prediction and control of morphology were successfully achieved with Eu(TCPP) as a model MOF. The proposed method will shed new light on exploring the spectral emission mechanism, energy conversion and in situ morphology monitoring of other luminescent materials.

Spectral properties and energy transfer at kinetic scales in collisionless plasma turbulence

Context. Recent satellite observations in the solar wind and in the Earth’s magnetosheath have shown that the turbulent magnetic field spectrum, which is know to steepen around ion scales, has another break at electron scales where it becomes even steeper. The origin of this second spectral break is not yet fully understood, and the shape of the magnetic field spectrum below electron scales is still under debate. Aims. By means of a fully kinetic simulation of freely decaying plasma turbulence, we study the spectral properties and the energy exchanges characterizing the turbulent cascade in the kinetic range. Methods. We started by analyzing the magnetic field, electron velocity, and ion velocity spectra at fully developed turbulence. We then investigated the dynamics responsible for the development of the kinetic scale cascade by analyzing the ion and electron filtered energy conversion channels, represented by the electromagnetic work J ⋅ E, pressure–strain interaction −P : ∇ u, and the cross-scale fluxes of electromagnetic (e.m.) energy and fluid flow energy, accounting for the nonlinear scale-to-scale transfer of energy from large to small scales. Results. We find that the magnetic field spectrum follows the k−α exp(−λ k) law at kinetic scales with α ≃ 2.73 and λ ≃ ρe (where ρe is the electron gyroradius). The same law with α ≃ 0.94 and λ ≃ 0.87ρe is observed in the electron velocity spectrum, but not in the ion velocity spectrum that drops as a steep power law ∼k−3.25 before reaching electron scales. By analyzing the filtered energy conversion channels, we find that electrons play a major role with respect to the ions in driving the magnetic field dynamics at kinetic scales. Our analysis reveals the presence of an indirect electron-driven mechanism that channels the e.m. energy from large to sub-ion scales more efficiently than the direct nonlinear scale-to-scale transfer of e.m. energy. This mechanism consists of three steps. In the first step the e.m. energy is converted into electron fluid flow energy at large scales; in the second step the electron fluid flow energy is nonlinearly transferred toward sub-ion scales; in the final step the electron fluid flow energy is converted back into e.m. energy at sub-ion scales. This electron-driven transfer drives the magnetic field cascade up to fully developed turbulence, after which dissipation becomes dominant and the electrons start to subtract energy from the magnetic field and dissipate it via the pressure–strain interaction at sub-ion scales.

New Travelling Wave Solutions of Conformable Cahn-Hilliard Equation

In this article, two methods are proposed to solve the fractional Cahn-Hilliard equation. This model describes the process of phase separation with nonlocal memory effects. Cahn-Hilliard equations have numerous applications in real-world scenarios, e.g., material sciences, cell biology, and image processing. Different types of solutions have been obtained. For this, the fractional complex transformation has been used to convert fractional differential equation to ordinary differential equation of integer order. As a result, these solutions are new solutions that do not exist in the literature.