Higher Wavenumbers Research Articles

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.

Characterization of Chitosan Films Cast from Different Solvents: FTIR, Surface and Mechanical Investigations

Chitosan (CS), a modified carbohydrate polymer derived from chitin by deacetylation, has shown various applications in a variety of industries, including food, cosmetics and packaging. In this work, CS films prepared from CS dissolved in three selected solvents i.e. formic acid (FA), acetic acid (AC) and propionic acid (PA) were investigated by employing scanning electron microscopy (SEM), atomic force microscopy (AFM), FTIR spectral analysis and mechanical tests (MT). SEM measurements showed relatively smooth and homogeneous surfaces for all films confirmed by AFM. The FTIR spectra of all CS films showed similar spectra with a little shift of some bands towards lower or higher wave numbers. In addition, CS film cast from AC recorded the highest elongation at break but the lowest tensile strength among the three types of films used.

Fixed-flux Rayleigh–Bénard convection in doubly periodic domains: generation of large-scale shear

This work studies two-dimensional fixed-flux Rayleigh–Bénard convection with periodic boundary conditions in both horizontal and vertical directions and analyses its dynamics using numerical continuation, secondary instability analysis and direct numerical simulation. The fixed-flux constraint leads to time-independent elevator modes with a well-defined amplitude. Secondary instability of these modes leads to tilted elevator modes accompanied by horizontal shear flow. For $Pr=1$ , where $Pr$ is the Prandtl number, a subsequent subcritical Hopf bifurcation leads to hysteresis behaviour between this state and a time-dependent direction-reversing state, followed by a global bifurcation leading to modulated travelling waves without flow reversal. Single-mode equations reproduce this moderate Rayleigh number behaviour well. At high Rayleigh numbers, chaotic behaviour dominated by modulated travelling waves appears. These transitions are characteristic of high wavenumber elevator modes since the vertical wavenumber of the secondary instability is linearly proportional to the horizontal wavenumber of the elevator mode. At a low $Pr$ , relaxation oscillations between the conduction state and the elevator mode appear, followed by quasi-periodic and chaotic behaviour as the Rayleigh number increases. In the high $Pr$ regime, the large-scale shear weakens, and the flow shows bursting behaviour that can lead to significantly increased heat transport or even intermittent stable stratification.

Two-parameter modified matrix splitting iteration method for Helmholtz equation

For effectively solving the nonsymmetric and indefinite linear system originating from the discretized Helmholtz equation with complex wavenumber, we propose a two-parameter modified matrix splitting iteration method. We establish the asymptotic convergence theory for this method and demonstrate its convergence under certain conditions. The proposed iteration method leads to a new preconditioner that can be efficiently inverted by using the discrete sine transform. Numerical experiments are carried out to show that the new preconditioner significantly improves the convergence rate of the GMRES method, which results in effective preconditioned GMRES method for solving the Helmholtz equation of high wavenumber.

Comparative assessment of the performance of density functionals and dispersion correction on different properties of dicationic ionic liquids - an ab initio molecular dynamics (AIMD) study.

In this study, we investigated the effect of DFT density functionals and dispersion correction on an imidazolium-based dicationic ionic liquid (DIL) using ab initio molecular dynamics simulations. To achieve this purpose, the electronic structures, as well as the structural and dynamical properties of [C3(mim)2][NTF2]2 DIL, were obtained using the BLYP and PBE functionals, both with and without D3-correction, and the results were compared with experimental values. Radial distribution functions and structure factors revealed that applying D3-correction increases the interaction between the anion and hydrogen atoms of the rings and side chains. The simulation of the studied DIL with the BLYP-D3 functional depicted lower structural heterogeneity compared to the other functionals. Analysis of Voronoi tessellation and linkage chain conformations showed a reduction in the aggregation of the linkage alkyl chains in the presence of D3-correction, which is more pronounced in the BLYP functional than in PBE. Additionally, it was observed that the probability of forming a hydrogen-bond network depends on both the type of used density functionals and applying dispersion correction. The results of dynamical properties, such as the self-diffusion coefficients, velocity autocorrelation function, and van Hove correlation function, as well as ion pair, ion cage, and hydrogen bond dynamics, indicated that applying D3-correction in both density functionals leads to an increase in the dynamics of the studied DIL. Additionally, the ratio of self-diffusion coefficients of the anion to the cation in the BLYP functional is closer to experimental values compared to the PBE functional. Furthermore, the electronic structure, including dipole moment distribution, and also infrared (IR) and power spectra were studied. Applying D3-correction and the type of density functionals have a significant effect on the dipole moment distribution of ions. Moreover, the results of IR and power spectra demonstrated that only in the BLYP functional, by applying D3-correction, the hydrogen bonding between the anion and the hydrogen atoms of the cation is strengthened at high wavenumbers. Thus, we conclude that applying D3 correction to both the BLYP and PBE density functionals improves the accuracy in describing the various properties of the studied system. Overall, the evaluation of different structural, dynamical, and vibrational properties of [C3(mim)2][NTF2]2 DIL suggests that the BLYP-D3 density functional may be the best choice among the studied density functionals.

Spectral fingerprinting of cellular lipid droplets using stimulated Raman scattering microscopy and chemometric analysis.

Hyperspectral stimulated Raman scattering (SRS) microscopy is a powerful method for direct visualisation and compositional analysis of cellular lipid droplets. Here we report the application of spectral phasor analysis as a convenient method for the segmentation of lipid droplets using the hyperspectral SRS spectrum in the high wavenumber and fingerprint region of the spectrum. Spectral phasor analysis was shown to discriminate six fatty acids based on vibrational spectroscopic features in solution. The methodology was then applied to studying fatty acid metabolism and storage in a mammalian cancer cell model and during drug-induced steatosis in a hepatocellular carcinoma cell model. The accumulation of fatty acids into cellular lipid droplets was shown to vary as a function of the degree of unsaturation, whilst in a model of drug-induced steatosis, the detection of increased saturated fatty acid esters was observed. Taking advantage of the fingerprint and high wavenumber regions of the SRS spectrum has yielded a greater insight into lipid droplet composition in a cellular context. This approach will find application in the label-free profiling of intracellular lipids in complex disease models.

Efficient calculation of the three-dimensional sound pressure field around a slab track

The wavenumber domain Boundary Element (2.5D BE) method is well suited to calculate the acoustic sound field around structures with a constant cross-section along one dimension, such as noise barriers or railway track. By expressing the sound field along this dimension in wavenumber domain, the numerical model is reduced from a 3D model to 2D model at each wavenumber. A consequence of the required discrete Fourier domain representation is that the sound field is represented by periodically repeating sections, of which only one section is physically meaningful. The resolution and the number of required wavenumbers increases with the desired length and spatial discretisation of this section. Describing the sound field adequately to auralise the sound without disturbing artefacts requires a large number of wavenumbers (and thus 2D BE computations), which is not feasible for large geometries. Here, a method is introduced that allows the calculation of the 3D sound field by solving a single 2D BE problem for a dense frequency spectrum and interpolating at higher wavenumbers. The calculation efficiency is further increased by precalculating the acoustic transfer functions between each BE surface element and receiver positions. Combining these two methods allows for efficient calculation of the 3D sound field around acoustically rigid structures such as slab tracks. The numerical approach is validated by comparison with a standard 2.5D BEM calculation and an analytical solution. Precalculated transfer functions to calculate the sound radiation from railway track, which are made available online, are illustrated. An example application is presented.

Advancing the temporal direct deconvolution method with spatial regularization

This study continues the exploration of temporal large-eddy simulation, particularly the extension of the temporal direct deconvolution method (TDDM) with a regularization term based on spatial dissipation. Furthermore, we aim to put insight stemming from previous work to test. Specifically, the hypothesis is that the temporal residual-stress leads to a reduction of the required artificial dissipation in under-resolved simulations. Moreover, this work seeks corroborate earlier discoveries with a posteriori results. We perform a numerical examination of two different spatial regularization terms in conjunction with TDDM: a spatial variant of selective frequency damping, functioning as a relaxation term that gradually drifts the velocity toward the filtered velocity, and the dynamic Smagorinsky model incorporating a prefactor. We test various cases, including the Taylor–Green vortex flow with a Reynolds number of Re = 3000, forced homogeneous isotropic turbulence with Reλ=200, turbulent channel flow at Reτ=590, and the flow over a periodic hill with Re = 10 935. Additionally, we also analyze the various dissipation contributions in TDDM as well as their interrelations. We also discuss grid artifacts and energy budget errors using these to compare the different models. Our results confirm the hypothesis that residual-stress dissipation reduces the necessary artificial dissipation. Because of the numerical ill-conditioning of deconvolution, whether temporal or spatial, there are practical limitations in the size of the filter width. Due to these limitations, the impact remains relatively minor. The a posteriori results of the new spatial regularization term show it to be effective in eliminating energy from the high wavenumber range.

Pre-resonance effects in deep UV Raman spectra of normal and deuterated water.

We have investigated the shape of the OH/OD stretching Raman band of water as a function of the excitation wavelength in the deep UV region (200-266 nm). By analyzing the spectral profiles, we highlighted selective pre-resonance effects in the high wavenumber component of the OH/OD stretching band, associated to distorted H-bonded water configurations. A van't Hoff treatment of the temperature-dependent Raman spectra provides an estimate of the thermal energy associated to the change from ordered (ice-like) to disordered configurations that agrees with values obtained by related methods based on a two-state model of water. These results open the possibility of exploiting the observed pre-resonance deep-UV signal enhancement to investigate H-bonding properties in aqueous media.

Statistical Characteristics of Mesoscale Fluctuations of Wind Velocity, Temperature, and Gas Concentrations Obtained from Aircraft Measurements in the Troposphere of the Arctic Region

Abstract The results of airborne measurements and statistical characteristics of mesoscale fluctuations of wind velocity, temperature, and concentrations of gas constituents at different heights of a stably stratified troposphere are presented. The measurements were carried out in September 2022 in the Arctic region of Russia with the aircraft laboratory Tu-134 “Optik.” The obtained spectra and structure functions of the fluctuations are interpreted with the theoretical model of formation of the spectrum of mesoscale wind velocity and temperature fluctuations described in the paper. The presence at high wavenumbers of a steep section in the obtained horizontal wavenumber spectra of the fluctuations of wind velocity and greenhouse gas concentration with a slope close to −3 is discussed. The fluctuation spectra along different slanted tracks of the aircraft crossing the tropospheric layer between altitudes of 1 and 9 km are also obtained and analyzed with the theoretical model.

Forensic Pathological Diagnosis of Acute and Old Myocardial Infarction Using Fourier Transform Infrared Spectroscopy.

Fourier transform infrared spectroscopy (FTIR) was used to analyze myocardial infarction tissues at different stages of pathological change to achieve the forensic pathology diagnosis of acute and old myocardial infarction. FTIR spectra data of early ischemic myocardium, necrotic myocardium, and myocardial fibrous tissue in the left ventricular anterior wall of the sudden death group of atherosclerotic heart disease and the myocardium of the normal control group were collected using hematoxylin-eosin (HE) and immunohistochemistry (IHC) staining as a reference, and the data were analyzed using multivariate statistical analysis. The mean normalized spectra of control myocardium, early ischemic myocardium and necrotic myocardium were relatively similar, but the mean second derivative spectra were significantly different. The peak intensity of secondary structure of proteins in early ischemic myocardium was significantly higher than in other types of myocardium, and the peak intensity of the α-helix in necrotic myocardium was the lowest. The peaks of amide Ⅰ and amide Ⅱ in the mean normalized spectra of myocardial fibrous tissue significantly shifted towards higher wave numbers, the peak intensities of amide Ⅱ and amide Ⅲ were higher than those of other types of myocardium, and the peak intensities at 1 338, 1 284, 1 238 and 1 204 cm-1 in the mean second derivative spectra were significantly enhanced. Principal component analysis (PCA) and partial least square-discriminant analysis (PLS-DA) showed that FTIR could distinguish different types of myocardium. FTIR technique has the potential to diagnose acute and old myocardial infarction, and provides a new basis for the analysis of the causes of sudden cardiac death.

Investigation of guided wave propagation in nanoscale layered periodic piezoelectric plates based on Eringen's nonlocal and strain gradient theory

This paper investigates the dynamic propagation behavior of guided waves in nanoscale media composed of alternating layers of piezoelectric phases. The study employs Eringen's nonlocal and strain gradient theories to derive equations for symmetric wave propagation and the scattering relations of guided waves perpendicular to these structures. Dispersion curves are calculated and plotted to compare these theories, considering the influence of nanoscale size-effect, volume ratio of constituent layers, and material length scale. Using the strain gradient theory, the paper explores the impact of length scale parameters on the propagation behavior of horizontally polarized guided waves in an alternating layer structure of nanoscale piezoelectric material, specifically nanoscale piezoelectric laminates. The analysis involves calculating localization factors and dispersion curves to analyze the effects of size-effect and length scale parameters on wave propagation and band structures. Additionally, the study plots and analyzes changes in mechanical displacement and electrical potential, followed by an investigation and discussion of nanoscale size effects and the effects of the material's length scale parameter on dispersion curves and conversion modes within specific regions. The results highlight that the length scale effect parameter significantly impacts the mode conversion zones and band gap structures, more so than the NLPE continuum theory suggests. As the strain gradient parameter increases, these zones shift towards higher wavenumbers and to the right . Furthermore, introducing the length scale factor results in a substantial decrease in both the formation of conversion zones and the density of dispersion curves.

High-speed k-linear swept laser using acousto-optic deflectors with Doppler shift compensation for optical coherence tomography.

Swept laser based on the acousto-optic deflector (AOD) is a promising swept source in optical coherence tomography (OCT) applications for its high wavenumber linear sweep without mechanical motion. However, the poor coherence length and the elongated cavity of the laser imposed limitations on the acquisition of high-quality images with adequate imaging depth and high imaging speed. In this Letter, we demonstrate a compact high-speed wavenumber linear swept laser based on AOD using Doppler shift compensation, achieving a high linearity of Pearson's R of 0.999991, a duty cycle of ∼100%, an extended coherence length of 5.7 mm, an output power of 18 mW, and excellent phase stability at a sweep speed of 500 kHz. OCT structural images with a system sensitivity of 103.2 dB and OCT angiography (OCTA) of human palm in vivo have been successfully performed, serving as a compelling demonstration of the excellent performance of this swept laser. We believe that the proposed laser will be of high potential in various clinical and industrial applications in the future.

Q-compensated wavefield depth extrapolation-based migration using a viscoacoustic wave equation

In a viscoacoustic medium, intrinsic attenuation causes seismic wavefields to attenuate in amplitude and become dispersed in phase, leading to distorted structural imaging and inaccurate migrated amplitudes. To address this problem, viscoacoustic reverse time migration corrects for dispersion and amplitude attenuation effects in wavefield propagation in forward and backward directions. To provide a parallel alternative approach, the time fractional derivative viscoacoustic wave equation in the depth domain is solved and a Q-compensated wavefield depth extrapolation scheme is established to compensate for viscoacoustic effects during recursive wavefield depth extrapolation. This approach decouples the amplitude attenuation and phase dispersion effects from the viscoacoustic vertical wavenumber. To suppress high wavenumber components and address the problem of exponential amplitude growth during wavefield depth extrapolation, we limit the imaginary part of the vertical wavenumber in the frequency wavenumber domain and use an adaptive stabilization solution. Numerical experiments of impulse responses in a viscoacoustic isotropic medium demonstrate that our proposed scheme can observe calculated wavefields exhibiting amplitude attenuation and phase dispersion effects in comparison to wavefields of the acoustic medium, indicating good agreement with theoretical expectations. We also demonstrate the capability of our proposed scheme to recover imaging amplitudes through a variety of numerical experiments, such as imaging of three-layer, Marmousi, and BP gas reservoir models. The results indicate that our proposed scheme can recover amplitude attenuation and phase dispersion effects more accurately than acoustic migration. We also use marine seismic data featuring natural gas hydrates to indicate that our proposed Q-compensated scheme can generate an enhanced imaging result, especially for the bottom simulating reflectors, compared with the conventional imaging algorithm without Q-compensation.

Nonlocal effective medium theory for phononic temporal metamaterials

We have developed a nonlocal effective medium theory (EMT) for phononic temporal metamaterials using the multiscale technique. Our EMT yields closed-form expressions for effective constitutive parameters and reveals these materials as reciprocal media with symmetric band dispersion. Even without spatial symmetry breaking, nonzero Willis coupling coefficients emerge with time modulation and broken time-reversal symmetry, when the nonlocal effect is taken into account. Compared to the local EMT, our nonlocal version is more accurate for calculating the bulk band at high wavenumbers and essential for understanding nonlocal effects at temporal boundaries. This nonlocal EMT can be a valuable tool for studying and designing phononic temporal metamaterials beyond the long-wavelength limit.

Quantitative studies on the microstructure evolution and its impact on the viscosity of a molten Al2O3-Na3AlF6 system by Raman spectroscopy and theoretical simulations

The microstructure of the molten alumina-cryolite system is an important aspect in the dissolution behavior of alumina (α-Al2O3) in cryolite (Na3AlF6) molten salt and impacts the physical properties of the molten system. In this work, the symmetric stretching vibrational wavenumbers of aluminum nonbridging fluorine (AlFx3-x) and aluminum-oxygen-fluorine (Al2OFy4-y, and Al2O2Fz2-z) bonds in the high wavenumber range, as well as their corresponding Raman scattering cross sections (RSCS), were first analyzed and determined by quantum chemistry (QC) ab initio calculations. The species in binary Al2O3-Na3AlF6 melts with varying Al2O3 contents from 0 to 6.0 wt% were then qualitatively and quantitatively analyzed using in situ high-temperature Raman spectroscopy in conjunction with first principles calculation from room temperature to melt. The species of AlF4-, AlF52- and AlF63- were found to be present and predominant in the melts with Al2O3 contents ranging from 0 to 6.0 wt%. The content of Al2OF62- species tended to increase with Al2O3 content from 0 to 3.0 wt%, while Al2O2F42- species only existed in the melts with an Al2O3 concentration of more than 3.0 wt%. Finally, the abundance of various species in the molten Al2O3-Na3AlF6 system was further correlated to the melt viscosity. The contribution to viscosity was primarily determined by the distribution of AlFx3-x (x = 4, 5, and 6) and Al2OF62- species, and the contribution ability of corresponding species to viscosity was Al2OF62-, AlF63-, AlF52- and AlF4- from largest to smallest.

ICE modes with high wave numbers

This work deals with high-k modes of the ion cyclotron emission (ICE)—the modes with frequencies close to ion cyclotron harmonics and wave numbers well exceeding those of fast magnetoacoustic modes (FMM). These modes exist due to finite Larmor radius of the ions. They are responsible for the ICE recently observed in the DIII-D tokamak [N. A. Crocker et al., Nucl. Fusion 62, 026023 (2022)]. The work is aimed at knowing the radial structure of high-k eigenmodes. Its analysis is relied on a comprehensive study carried out by employing a general dispersion equation for traveling waves, which allowed to group modes and determine conditions of their existence. Differential equations for various eigenmodes (standing waves) were obtained and analyzed. In particular, electrostatic modes and electromagnetic modes with various ratios of the longitudinal phase velocity to the electron thermal velocity were considered. A numerical code solving these equations was developed. It is found that high-k modes with sufficiently large poloidal mode numbers are located at the plasma periphery, and their amplitudes in the plasma core are very small. It is concluded that high-k modes are not an exceptional case, although presumably FMMs can explain most ICE experiments. They exist in plasmas with various magnitudes of β, representing different wave branches associated with finite Larmor radius of the ions. The mentioned experiment on DIII-D is discussed.

Theoretical and numerical investigation into the suppressing mechanism of radiated noise in an ABH-cavity system

The problem of radiated noise of a coupling system between acoustic space and flexible structure is very common in practical applications. In this paper, the acoustic black hole (ABHs) structure is used for reducing the radiated noise of the plate-cavity system as an efficient passive control technology. The suppression mechanism of the ABH-cavity system is investigated. Theoretical analysis shows that enhanced radiated noise suppressing effect can be obtained by rearranging the numbers and locations of ABHs compared to its uniform counterpart at cavity modal frequencies. Due to this suppression mechanism, ABH structure exhibits a reduced radiation efficiency of the panel, which can be explained by the wavenumber analysis that lots of high wavenumber components are generated and effective wavenumbers in the radiation region are reduced.

Window into the mind: Advanced handheld spectroscopic eye-safe technology for point-of-care neurodiagnostic.

Traumatic brain injury (TBI), a major cause of morbidity and mortality worldwide, is hard to diagnose at the point of care with patients often exhibiting no clinical symptoms. There is an urgent need for rapid point-of-care diagnostics to enable timely intervention. We have developed a technology for rapid acquisition of molecular fingerprints of TBI biochemistry to safely measure proxies for cerebral injury through the eye, providing a path toward noninvasive point-of-care neurodiagnostics using simultaneous Raman spectroscopy and fundus imaging of the neuroretina. Detection of endogenous neuromarkers in porcine eyes' posterior revealed enhancement of high-wave number bands, clearly distinguishing TBI and healthy cohorts, classified via artificial neural network algorithm for automated data interpretation. Clinically, translating into reduced specialist support, this markedly improves the speed of diagnosis. Designed as a hand-held cost-effective technology, it can allow clinicians to rapidly assess TBI at the point of care and identify long-term changes in brain biochemistry in acute or chronic neurodiseases.

Simultaneous bond-selective deuterium-based isotopic labeling sensing with disposable ultra-miniature CARS fiber probe.

Deuterium-based isotopic labeling is an important technique for tracking cellular metabolism with the Raman signals analysis of low-wavenumber (LW) C-D bonds and high-wavenumber (HW) C-H bonds. We propose and demonstrate a disposable ultra-miniature fiber probe to detect LW and HW coherent anti-Stokes Raman scattering (CARS) spectra for deuterated compounds simultaneously and bond-selectively sensing. The 10.78 µm diameter disposable fiber probe, comprised of focusing taper as fiber probe head and time-domain walk-off eliminating fiber section with designed length, realizes wide-frequency-interval dual Stokes pulse delivering and focusing. The fiber probe enables quantitative concentration determination with resolution down to 11 mM. The chemical vibration modes of LW region C-D bonds and HW region C-H bonds of the mixture samples of organic compounds and their deuterated counterparts in a simulated cell are simultaneously excited and characterized. The CARS disposable fiber probe introduces a promising handle for in vivo biochemical detection based on isotopic labeling sensing.