Large Wave Numbers Research Articles

Wakefield stimulated terahertz radiation from a plasma grating

When a short laser pulse propagates in a corrugated plasma, its wakefield interacts with the density and electric field ripples of the plasma. In the present paper, the modulation of the plasma originates from two counter-propagating long laser pulses, i.e. the corrugated plasma can be assumed as a so-called plasma grating. PIC (particle in cell) simulations show electromagnetic wave radiation at a frequency just above the plasma frequency when the wakefield interacts with the grating. An electromagnetic instability is proposed as the reason for the emission process. The electrostatic driver of the electromagnetic instability is the beat of the wake and grating. That beat mode possesses a large wavenumber (originating from the small grating wavelength) and small frequency (of the order of the plasma frequency) when one concentrates on small ratios of the plasma modulation length to the wavelength of the wakefield. The latter situation occurs when the long laser pulses (which generate the grating) as well as the short laser pulse (which drives the wakefield) have similar frequency where is the plasma frequency. The coherent volume emission process lasts for a while. It is finally superseded by terahertz transition radiation at the boundaries of the original grating.

Simulating wet active polymers by multiparticle collision dynamics.

The conformational and dynamical properties of active Brownian polymers embedded in a fluid depend on the nature of the driving mechanism, e.g., self-propulsion or external actuation of the monomers. Implementations of self-propelled and actuated active Brownian polymers in a multiparticle collision (MPC) dynamics fluid are presented, which capture the distinct differences between the two driving mechanisms. The active force-free nature of self-propelled monomers requires adaptations of the MPC simulation scheme, with its streaming and collision steps, where the monomer self-propulsion velocity has to be omitted in the collision step. Comparison of MPC simulation results for active polymers in dilute solution with results of Brownian dynamics simulations accounting for hydrodynamics via the Rotne-Prager-Yamakawa tensor confirm the suitability of the implementation. The polymer conformational and dynamical properties are analyzed by the static and dynamic structure factor, and the scaling behavior of the latter with respect to the wave number and time dependence are discussed. The dynamic structure factor displays various activity-induced temporal regimes, depending on the considered wave number, which reflect the persistent diffusive motion of the whole polymer at small wave numbers, and the activity-enhanced internal dynamics at large wave numbers. The obtained simulation results are compared with theoretical predictions.

Cosmological functional renormalization group, extended Galilean invariance, and approximate solutions to the flow equations

The functional renormalisation group is employed to study the non-linear regime of late-time cosmic structure formation. This framework naturally allows for non-perturbative approximation schemes, usually guided by underlying symmetries or a truncation of the theory space. An extended symmetry that is related to Galilean invariance is studied and corresponding Ward identities are derived. These are used to obtain (formally) closed renormalisation group flow equations for two-point correlation functions in the limit of large wave numbers (small scales). The flow equations are analytically solved in an approximation that is connected to the 'sweeping effect' previously described in the context of fluid turbulence.

Photoluminescence of ytterbium doped zirconia

Raman spectra were studied for the cubic ytterbium doped zirconia with 10, 20 and 25% of dopant. This part of investigation deals with region of large wave numbers. Two different excitation sources with wavelength 785 and 532 nm are used. For first time Stokes lines that are independent of the excitation wavelength were observed together with conventional luminescence of ytterbium cation. Keywords: Zr(Yb)O2, Raman spectroscopy, photoluminescence, the crystal defects association.

Фотолюминесценция диоксида циркония, допированного иттербием

Raman spectra were studied for the cubic ytterbium doped zirconia with 10, 20 and 25% of dopant. This part of investigation deals with region of large wave numbers. Two different excitation sources with wavelength 785 and 532 nm are used. For first time Stokes lines that are independent of the excitation wavelength were observed together with conventional luminescence of ytterbium cation.

The onset of penetrative convection in a three-layered porous–air–porous system with internal heat source

The study is devoted to a linear stability problem for a system of two horizontal porous sublayers separated by an air interlayer under the influence of the gravity field. The porous sublayers are capable of generating heat with a constant volumetric strength of heat source proportional to the solid volume fraction. Equal temperatures are maintained at the external boundaries of the three-layered system. The onset value for internal convection is numerically determined in the limiting case of a thin air sublayer. The critical wave number is also found, upon reaching which the equilibrium stability is lost in the system, and convective patterns begin to arise. The influence on the convective stability of the ratio of the permeabilities of the layers, the relative thickness of the lower porous sublayer, and the parameter of the air interlayer is analyzed. This parameter is the ratio of the air sublayer thickness raised to the third power to the Darcy number. New results are obtained that take into account the localization of the convective flow. It is shown that the permeability ratio can both destabilize and stabilize the motionless state. An increase in the interlayer parameter unambiguously reduces the convection onset value. A map for the local and large-scale convective regimes has been constructed in the coordinates of permeability ratio versus the air interlayer parameter. A demarcation line of the transition from one regime to another on the map corresponds to the parameters at which the critical internal Rayleigh–Darcy numbers for the two minima of the bimodal neutral curve have equal values. One of the minima belongs to the region of small wave numbers (a long-wave minimum) and defines the onset value for large-scale convection, and the other one is in the region of large wave numbers (a short-wave minimum) and indicates the onset value for local convection. It is shown that with an increase in the air interlayer parameter, the ratio of permeability required for a transition from large-scale to local convective regimes decreases.

Distribution of energy in propagation for ocean extreme wave generation in hydrodynamics laboratory

BACKGROUND AND OBJECTIVES: The hydrodynamic uncertainty of the ocean is the reason for testing marine structures as an initial consideration. This uncertainty has an impact on the natural structure of the topography as well as marine habitats. In the hydrodynamics laboratory, ships and offshore structures are tested using mathematical models as input to the wave marker. For large wavenumbers, Benjamin Bona Mahony's equation has a stable direction and position in the wave tank. During their propagation, the generated waves exhibit modulation instability and phase singularity phenomena. These two factors refer to Benjamin Bona Mahony as a promising candidate for generating extreme waves in the laboratory. The aim of this research is to investigate the distribution of energy in each modulation frequency change. The Hamiltonian formula that describes the phenomenon of phase singularity is used to observe energy. This data is critical in determining the parameters used to generate extreme waves.METHODS: The envelope of the Benjamin Bona Mahony wave group can be used to study the Benjamin Bona Mahony wave. The Benjamin Bona Mahony wave group is known to evolve according to the Nonlinear Schrodinger equation. The Hamiltonian governs the dynamics of the phase amplitude and proves the Nonlinear Schrodinger equation's singularity for finite time. The Hamiltonian is derived from the appropriate Lagrangian for Nonlinear Schrodinger and then transformed into the Hamiltonian with the displaced phase-amplitude variable.FINDINGS: Potential energy is related to wave amplitude and kinetic energy is related to wave steepness in the study of surface water waves. When , the maximum wave amplitude and steepness are obtained. When , extreme waves cannot be formed due to steepness. This is due to the possibility of breaking waves into smaller waves on the shore. In terms of position, the energy curve is symmetrical.CONCLUSION: According to Hamiltonian's description of the energy distribution, the smaller the modulation frequency, the greater the potential and kinetic energy involved in wave propagation, and vice versa. While the wave's amplitude and steepness will be greatest for a low modulation frequency, and vice versa. The modulation frequency considered as an extreme wave generator is , because the resulting amplitude is quite high and the energy in the envelope is also quite large.

Unusual high frequency EMIC waves: Detailed analysis of EMIC wave excitation and energy coupling between EMIC and magnetosonic waves

• Testing high frequency EMIC waves generation by anisotropic suprathermal protons. • Cyclotron resonance with magnetosonic waves is inefficient in thermal proton heating. • Quantitative understanding of the magnetosonic-EMIC energy coupling process is needed. A recent event study suggested a coupling process whereby suprathermal protons (10–100 eV), as a result of perpendicular heating by pre-existing magnetosonic waves, can excite electromagnetic ion cyclotron (EMIC) waves with frequency near the local proton cyclotron frequency ( f ~ 0.95 f cp ). The present study tests this coupling process. First, one-dimensional hybrid (kinetic ions/massless fluid electrons) simulations of parallel-propagating EMIC waves initialized with the plasma conditions derived from the event prove that high frequency EMIC waves can be generated by anisotropic suprathermal protons instead of more energetic (10s of keV) ring current protons. Calculation of the quasilinear pitch-angle diffusion coefficient for electrons using the simulated waves suggests that these high frequency EMIC waves outside the plasmasphere can play a role in the pitch-angle scattering of ~ MeV electrons due to the large wavenumber of the excited EMIC waves. Second, the role of the pre-existing magnetosonic waves in the suprathermal proton heating is examined using quasilinear diffusion theory and simple test particle calculation. It is found that the quasilinear diffusion becomes ineffective in energies relevant to the suprathermal protons, indicating that the low-energy ( < 10 eV) protons cannot be efficiently energized to the observed level through multi-harmonic cyclotron resonances with the magnetosonic waves observed. Rather, some kind of non-resonant process may have been at play, if the magnetosonic waves were actually involved in the heating. This result suggests that more quantitative understanding of the suggested energy coupling process through magnetosonic waves is needed.

Theoretical and applied considerations in depth-integrated currents for third-generation wave models

A new derivation of ray equations is proposed in the framework of a current uniform with depth, differentiating the contributions from the current-induced wave phase speed and wave group velocity. It is useful when substituting higher order expressions for those quantities that account for a sheared current. The new formulation is easy to implement in third-generation spectral wave models even if this task is not performed in the present paper. The numerical vertical integration of the current profile evaluating the current-induced corrections is also studied. We show that a large numerical error can potentially be induced if one uses the vertical resolution normally encountered in ocean modeling. We offer a simple solution to reduce this error and demonstrate its impact on the computation of the current-induced phase speed and wave group corrections using both theoretical and realistic profiles. Discrepancies can induce changes up to 40% of the current-induced speed. We observe these strong discrepancies for large wavenumbers, associated with wind waves, when current effects are the most prevalent.

Detection of spongy pulp in guava using light properties and near infrared spectroscopy

Spongy pulp defect in guava is recognised by a dry-looking pulp with a brown colour, and not acceptable to consumers which causes substantial loss in value. Current detection of spongy pulp uses visual assessment of the flesh, which is half cut from the sample. The present work aimed to develop a classifying model based on non-destructive technique for the detection of spongy guava. Guava samples harvested at full maturity were determined for visible light properties, visible light reflectance, and near infrared reflectance. The light properties and light reflectance of guava peel were used to derive a classification model which was then compared with a near infrared reflectance model, which in turn provided absorbance of the flesh and peel using stepwise discriminant analysis. The models were used to classify the guavas into normal and spongy flesh groups, which were assigned with reference to the visual assessment on half cut samples. The classification accuracy for the model using gloss and light reflectance at 650 nm (chlorophyll b) was 90.4%. However, the model developed from the near infrared absorbance provided better accuracy (92.7%). It appeared that the largest wavenumber at 4,721 cm-1 contributed to the total sugar content, which implied that spongy and normal guavas had different total sugar contents in the flesh. The present work demonstrated the potential of near infrared spectroscopy to discriminate spongy from normal guavas. However, the accuracy of the classification could be further improved by analysing more samples from the next season.

Partial structure factors derived from coarse-grained phase-separation dynamics on a disordered lattice with density fluctuations

The simplest and most time-efficient phase-separation dynamics simulations are carried out on a disordered lattice to calculate the partial structure factors of coarse-grained A-B binary mixtures. The typical coarse-grained phase-separation models use regular lattices and can describe the local concentrations but cannot describe both local density and concentration fluctuations. To introduce fluctuation for local density in the model, the particle positions from a hard sphere fluid model are determined as disordered lattice points for the model. Then we place the local order parameter as the difference of the concentrations of A and B components on each lattice point. The concentration at each lattice point is time-evolved by discrete equations derived from the Cahn-Hilliard equation. From both fluctuations, Bhatia and Thornton’s structure factor can be accurately calculated. The structure factor for concentration fluctuations at the large wavenumber region gives us the correct mean concentrations of the components. Using the mean concentrations, partial structure factors can be converted from three of Bhatia and Thornton’s structure factors. The present model and procedures can provide a means of analysing the structural properties of many materials that exhibit complex morphological changes.

Instability of coaxial viscoelastic jets under a radial electric field

In this paper we investigate temporal linear instability and absolute/convective instability of coaxial viscoelastic jets in the presence of a radial electric field. With a leaky dielectric model to describe the finite conductivity of the outer liquid, both axisymmetric instability and nonaxisymmetric instability are studied and compared to determine the predominant mode in the jet instability. For relatively small wavenumbers, the electric field suppresses the axisymmetric instability, while it enhances axisymmetric instability for relatively large wavenumbers. As the electric field increases, the instability mode tends to be a short wave. The elasticity of the inner and outer liquid is found to contribute to the coaxial jet instability, and the increase of the inner-liquid elasticity is discovered to allow the para-sinuous mode to predominate over the helical mode. Furthermore, the spatiotemporal instability analysis indicates that under weaker electric fields the axisymmetric mode is convectively unstable, but becomes absolutely unstable when the electric field is relatively strong, when the outer liquid has a certain degree of elasticity. While the nonaxisymmetric mode is found to be always absolutely unstable. A boundary line between absolute and convective instability is given on the parameter space of the Deborah number (relative relaxation time) of the outer liquid and the electrical Euler number. Our results in temporal and spatiotemporal instability analysis can be useful for understanding the origin of the different modes and desired mode control in coaxial electrospinning.

Primordial perturbations in kinetically dominated regimes of general relativity and hybrid quantum cosmology

Scalar fields with an energy density dominated by its kinetic part may have played a relevant role in the very early stages of the Universe. Compared to the standard inflationary paradigm, they may lead to modifications in observable quantities, e.g. the anisotropies found in the cosmic microwave background. Kinetically dominated regimes arise in classical fast-roll scenarios as well as in quantum bouncing cosmologies. For instance, kinetic dominance is typical in interesting preinflationary phases of Loop Quantum Cosmology. In this work, we analyze the leading-order effects that the presence of a scalar field potential causes on the primordial cosmological perturbations in these kinetically dominated epochs. These effects can be grouped in two sets, namely, those that affect the effective mass of the perturbations and those that affect the choice of their vacuum state. The effective mass is modified directly by terms that include the potential, but also indirectly by the change in the background dynamics and the relation between the parameterization of these dynamics and the conformal time, usually employed to describe the evolution of the perturbations. On the other hand, away from de Sitter inflation, the Bunch-Davies state is no longer the most natural vacuum at all scales. Recent proposals suggest to modify it by carrying out certain Hamiltonian diagonalization with a suitable asymptotic behavior at large wavenumber scales. Both this diagonalization condition and the imposed asymptotic behavior depend on the effective mass of the perturbations, and therefore the selected vacuum state varies in the presence of the scalar field potential.

Sub-100-nm periodic nanostructure formation induced by short-range surface plasmon polaritons excited with few-cycle laser pulses

We have demonstrated that intense 7-fs, ∼810-nm laser pulses can produce a much finer periodic nanostructure on a diamond-like carbon film through ablation in air than that formed with intense 100-fs, ∼800-nm laser pulses. The minimum period size is ∼60 nm, corresponding to ∼1/13 of the center wavelength. To understand the physical mechanism responsible for the finer nanostructuring, we have observed a bonding structural change in the nanostructure with micro-Raman spectroscopy and a scanning transmission electron microscope. It has been found that the modified layer thickness with the 7-fs pulses is much thinner than that with the 100-fs pulses. The results show that the 7-fs pulses create a few-nm-thick layer with high-density electrons and excite short-range surface plasmon polaritons, which have a large wave number around the layer and induce the plasmonic near-field nanoablation. The period size estimated by using a model target reproduces well the observed size of nanostructures.

Implicit approximate decoupling procedure withCrank–Nicolsonscheme for bandpass symmetric rotationally geophysical problems

To efficient simulate symmetric rotationally geophysical problems, implicit Crank–Nicolson (CN) scheme incorporated with approximate decoupling procedure is proposed in the body of revolution (BOR) finite-difference time-domain (FDTD) algorithm. For further absorption of large number of low-frequency waves in bandpass simulation, complex envelope method is incorporated with the perfectly matched layer implementation. To be more specific, the proposed implementation shows advantages in terms of improved efficiency, considerable accuracy, and remarkable absorption. To demonstrate effectiveness and efficiency of the proposed implementation, geophysical numerical examples are carried out in the BOR-FDTD domain. Through different numerical examples, it can be concluded from simulation results that the proposed implementation can obtain admirable entire performance in bandpass geophysical simulation. In addition, it can also be illustrated that it can keep stable when time step surpasses far beyond the Courant–Friedrichs- Levy condition.

Development of a Monsoon Depression and Its Interaction with the Large-Scale Background: A Case Study

Abstract Structure and time evolution of the large-scale background and an embedded synoptic-scale monsoon depression and their interactions are studied. The depression formation is preceded by a cyclonic circulation around 400 hPa. The Fourier-based scale separation technique is used to isolate large (wavenumbers 0–8) and synoptic-scale (wavenumbers 12–60). The wavelength and depression center is determined objectively. The synoptic-scale depression has an average longitudinal wavelength of around 1900 km and a north–south size of 1100 km; it is most intense with a vorticity of 20.5 × 10 −5 s −1 at 900 hPa. The strongest cold core of −3.0°C below 850 hPa and the above warm core of around 2.0°C are evident. The depression is tilted southwestward in the midtroposphere with no significant vertical tilt in the lower troposphere. The mean maximum intensity and upward motion over the life cycle of depression are in close agreement with the composite values. A strong cyclonic shear zone is developed in the midtroposphere preceding the depression. The necessary condition for barotropic (baroclinic) instability is satisfied in the midtroposphere (boundary layer). Strong northward transport of momentum by the depression against the southward shear is found. The strong growth of the MD in the lower troposphere is due to downward transfer of excess energy gained in the midtroposphere from the barotropic energy conversion and east–west direct thermal circulation as the vertical energy flux. The baroclinic interaction contributes to the maintenance of the cold core in the lower troposphere. The diabatic heating rate is computed and its role in the genesis and growth of MD is investigated.

Influence of tropical convective enhancement in Pacific on the trend of stratospheric sudden warmings in Northern Hemisphere

The exploration of the trend in stratospheric sudden warmings (SSWs) is conducive to predict SSWs in the future. Utilizing the National Centre for Environmental Prediction Reanalysis (NCEP) (1948–2020) and Japanese 55-year Reanalysis (JRA55) (1958–2020), we investigated the duration and strength of SSWs in the Northern Hemisphere occurred in the boreal winter (December–February). We found the duration of SSWs tends to increase and the strength of SSWs tends to strengthen from 1948 to 2003. After 2003, these trends did not continue. We utilized the observed cloudiness from the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) to find that the convective activities in the tropical Central Pacific were enhanced during 1948–2003, and the enhancement of the convective activities did not continue after 2003. The circulation anomalies caused by the enhanced convective activities propagate to the high latitudes through wave trains. The anomalies of circulation and the climatological circulation at high latitudes interfere with each other and superimpose, which has a significant impact on planetary wave 1 (PW1). As a result, the PW1 also showed an increasing trend from 1948 to 2003 and a decreasing trend after 2003. After the stratosphere filters out the planetary wave with a large wavenumber, PW1 accounts for more proportion of planetary waves, which causes the trend in SSWs to change.

Efficient seismic risk analysis of gravity dams via screening of intensity measures and simulated non-parametric fragility curves

The computational cost of nonlinear numerical simulation of gravity dams brings resistance to the popularization of seismic risk analysis under the framework of Performance-based Earthquake Engineering. Insufficient seismic records and inappropriate parametric fragility models will mislead the assessment of structural performance and total risk. A computationally efficient methodology suitable for a large number of seismic waves is proposed based on screening for intensity measures and a surrogate model to obtain the non-parametric fragility curves. It specifically integrates the rigorous numerical technique of seismic damage to the dam-reservoir-foundation system, the comprehensive comparison of intensity measures, the surrogate model for classification of limit states, and the seismic risk analysis of gravity dams. A typical high gravity dam and a classical artificial neural network model are employed to illustrate the effectiveness of the proposed methodology through a step-by-step scheme. The result shows that the simulated non-parametric fragility curves are more competent to consider the uncertainty of ground motions. The obtained structural fragility and risk is more accurate than the conventional method.

On propagation of waves in pressurized fiber-reinforced hyperelastic tubes based on a reduced model

A refined dynamic finite-strain shell theory for incompressible hyperelastic materials was developed by the authors recently. In this paper, we first derive the associated linearized incremental theory, and then use it to investigate wave propagation in a fiber-reinforced hyperelastic tube that is subjected to an axial pre-stretch and internal pressure. We obtain the dispersion relations for both axisymmetric and non-axisymmetric waves and discuss their accuracy by comparing them with the exact dispersion relations. The bending effect is also examined by comparing the dispersion curves based on the present theory and membrane theory. It is shown that the present theory is more accurate than the membrane theory in studying wave propagation and the bending effect plays an important role in some wave modes for relatively large wavenumbers. The effects of the pressure, axial pre-stretch and fiber angle on the dispersion relations are displayed. These results provide the necessary validation for using the refined dynamic finite-strain shell theory and wave propagation to determine arterial properties.

Instability of co-flow in a Hele-Shaw cell with cross-flow varying thickness

We analyse the stability of the interface between two immiscible fluids both flowing in the horizontal direction in a thin cell with vertically varying gap width. The dispersion relation for the growth rate of each mode is derived. The stability is approximately determined by the sign of a simple expression, which incorporates the density difference between the fluids and the effect of surface tension in the along- and cross-cell directions. The latter arises from the varying channel width: if the non-wetting fluid is in the thinner part of the channel, the interface is unstable as it will preferentially migrate into the thicker part. The density difference may suppress or complement this effect. The system is always stable to sufficiently large wavenumbers owing to the along-flow component of surface tension.