Adiabatic Propagation Research Articles

Study on the adiabaticity criterion of the thermally-guided very-large-mode-area fiber.

The adiabaticity criterion of the thermally-guided very-large-mode-area (TG VLMA) fiber is presented based on the mode-coupling theory firstly, to the best of our knowledge. The requirement for the adiabatic propagation of fundamental mode is discussed systematically. It is revealed that the pump absorption plays the most important role and the adiabaticity criterion can be met as long as it is small enough. Then, the effects of the configuration parameters of TG VLMA fiber on the up-limitation of pump absorption for the adiabaticity criterion are investigated. It is found that for the straight TG VLMA fiber, reducing the initial refraction index and inner-cladding diameter and utilizing the bi-directional pumping scheme are beneficial to the adiabatic propagation of fundamental mode. The bent TG VLMA fiber is also studied. It is found that the bent fiber is much more difficult to meet the adiabaticity criterion than the straight one. The results show that even with the 100-cm bend radius, the pump absorption should be smaller than 1 dB/m to meet the adiabaticity criterion. It is suggested that enlarging the core-to-cladding ratio can be helpful for loosening the adiabaticity criterion of bent TG VLMA fiber. These pertinent results can provide significant guidance for understanding and designing the TG VLMA fiber and pertinent lasers and amplifiers.

Pressure and temperature influence on propagation indices of n-butane–air gaseous mixtures

Abstract The combustion of n -butane–air in a closed spherical vessel with central ignition was studied at various initial pressures within 0.3–1.3 bar and initial temperatures within 298–430 K, by means of transient pressure-time records. The propagation indices of confined deflagrations in the quiescent stoichiometric mixture are reported: the peak explosion pressure, the explosion time, the maximum rate of pressure rise and the related severity factor. The measured propagation indices are compared with the propagation indices computed under the assumption of an adiabatic propagation. Based on explosion pressure variation with the initial temperature, the heat of combustion of n -butane with air, corrected for the endothermic processes in the burned gas, was determined.

Three-dimensional model benchmarking for cross-slope wedge propagation

Cross-slope wedge propagation is considered for the three-dimensional benchmarking of three underwater acoustic models, one based on normal mode theory and the other two based on ray tracing. To this end, the benchmarking relies on analytic solutions for adiabatic and non-adiabatic propagation, as well as on experimental data from a scale tank experiment (a previous benchmarking with a parabolic equation model is known to provide extremely accurate predictions in all cases). The benchmarking allows to identify the advantages, accuracy and limitations of the considered models.

Acoustic mode travel time variability in the Philippine Sea

During the 2009 NPAL (North Pacific Acoustic Laboratory) PhilSea deep water acoustic propagation experiment, a low frequency source transmitted broadband chirps to a mid-water spanning hydrophone array at a distance of 185 km every three hours for approximately one month. In addition, transmissions took place every five minutes during some time periods. The source and the receiver array contained conductivity and temperature sensors. The motions of the source and receiver moorings were measured using long-baseline acoustic navigation systems. The experiment site was oceanographically dynamic and contained significant internal tide activity. This work builds a model to compare the acoustic and oceanographic variabilities. The acoustic observations are used to estimate the mode travel times, and then related to the internal tide displacements at the source and the receiver arrays. This paper presents the work in three parts. The first part uses matched subspace detectors to estimate the mode travel times. The second uses the environmental observations to build an internal tide model. The third constructs a linear perturbation model to relate the internal tides to the adiabatic mode travel times. The discrepancies between the model and the observations are attributed to factors such as limitations of the perturbation model for the travel times, range variability not accounted for in the adiabatic mode propagation, and signal processing issues.

Influence of self-phase modulation on coherent effects in five-level system

The possibility of formation of a coherent dark state in the five-level system in the entire volume of the medium in an adiabatic pulse propagation is investigated. It is shown that the dark state formation is independent on the value of the first two-photon detuning from the resonance, which may be changed during the propagation in the medium. In the case of the M-type system with the equal strengths of oscillators at the extreme transitions it is shown that the self-phase modulation does not influence the coherent effects while in the ladder-type system it may lead to the destruction of the dark state. The estimation of the length of the medium in which the propagation effects are negligible is obtained.

Phase Segregation and Superior Thermoelectric Properties of Mg2Si(1-x)Sb(x) (0 ≤ x ≤ 0.025) Prepared by Ultrafast Self-Propagating High-Temperature Synthesis.

A series of Sb-doped Mg2Si(1-x)Sb(x) compounds with the Sb content x within 0 ≤ x ≤ 0.025 were prepared by self-propagating high-temperature synthesis (SHS) combined with plasma activated sintering (PAS) method in less than 20 min. Thermodynamic parameters of the SHS process, such as adiabatic temperature, ignition temperature, combustion temperature, and propagation speed of the combustion wave, were determined for the first time. Nanoprecipitates were observed for the samples doped with Sb. Thermoelectric properties were characterized in the temperature range of 300-875 K. With the increasing content of Sb, the electrical conductivity σ rises markedly while the Seebeck coefficient α decreases, which is attributed to the increase in carrier concentration. The carrier mobility μ(H) decreases slightly with the increasing carrier concentration but remains larger than the Sb-doped samples prepared by other methods, which is ascribed to the self-purification process associated with the SHS synthesis. In spite of the increasing electrical conductivity with the increasing Sb content x, the overall thermal conductivity κ decreases on account of a significantly falled lattice thermal conductivity κ(L) due to the strong point defect scattering on Sb impurities and possibly enhanced interface scattering on nanoprecipitates. As a result, the sample with x = 0.02 achieves the thermoelectric figure of merit ZT ∼ 0.65 at 873 K, one of the highest values for the Sb-doped binary Mg2Si compounds investigated so far. A subsequent annealing treatment on the sample with x = 0.02 at 773 K for 7 days has resulted in no noticeble changes in the thermoelectric transport properties, indicating an excellent thermal stability of the compounds prepared by the SHS method. Therefore, SHS method can serve as an effective alternative fabrication route to synthesize Mg-Si based themoelectrics and some other functional materials due to the resulting high performance, perfect thermal stability, and feasible production in large scale for commercial application.

Electronically nonadiabatic wave packet propagation using frozen Gaussian scattering.

We present an approach, which allows to employ the adiabatic wave packet propagation technique and semiclassical theory to treat the nonadiabatic processes by using trajectory hopping. The approach developed generates a bunch of hopping trajectories and gives all additional information to incorporate the effect of nonadiabatic coupling into the wave packet dynamics. This provides an interface between a general adiabatic frozen Gaussian wave packet propagation method and the trajectory surface hopping technique. The basic idea suggested in [A. D. Kondorskiy and H. Nakamura, J. Chem. Phys. 120, 8937 (2004)] is revisited and complemented in the present work by the elaboration of efficient numerical algorithms. We combine our approach with the adiabatic Herman-Kluk frozen Gaussian approximation. The efficiency and accuracy of the resulting method is demonstrated by applying it to popular benchmark model systems including three Tully's models and 24D model of pyrazine. It is shown that photoabsorption spectrum is successfully reproduced by using a few hundreds of trajectories. We employ the compact finite difference Hessian update scheme to consider feasibility of the ab initio "on-the-fly" simulations. It is found that this technique allows us to obtain the reliable final results using several Hessian matrix calculations per trajectory.

Nonadiabatic Dynamics for Electrons at Second-Order: Real-Time TDDFT and OSCF2.

We develop a new model to simulate nonradiative relaxation and dephasing by combining real-time Hartree-Fock and density functional theory (DFT) with our recent open-systems theory of electronic dynamics. The approach has some key advantages: it has been systematically derived and properly relaxes noninteracting electrons to a Fermi-Dirac distribution. This paper combines the new dissipation theory with an atomistic, all-electron quantum chemistry code and an atom-centered model of the thermal environment. The environment is represented nonempirically and is dependent on molecular structure in a nonlocal way. A production quality, O(N(3)) closed-shell implementation of our theory applicable to realistic molecular systems is presented, including timing information. This scaling implies that the added cost of our nonadiabatic relaxation model, time-dependent open self-consistent field at second order (OSCF2), is computationally inexpensive, relative to adiabatic propagation of real-time time-dependent Hartree-Fock (TDHF) or time-dependent density functional theory (TDDFT). Details of the implementation and numerical algorithm, including factorization and efficiency, are discussed. We demonstrate that OSCF2 approaches the stationary self-consistent field (SCF) ground state when the gap is large relative to k(b)T. The code is used to calculate linear-response spectra including the effects of bath dynamics. Finally, we show how our theory of finite-temperature relaxation can be used to correct ground-state DFT calculations.

Nanofocusing of UV light in aluminum V-grooves

Using the finite difference time domain method, this paper investigates electromagnetic nanofocusing of ultraviolet light incident upon Al V-groove structures. A parametric study of the field enhancement around the V-groove tip area has been conducted through the change of groove depth, width, and the tip angle. A short-wavelength threshold of about 180 nm, lying well above the plasma wavelength of Al, has been identified, below which the electric field enhancement at the tip rapidly decreases. Through simplification of the V-groove model into a metal-dielectric-metal waveguide with gap width corresponding to depth-dependent V-groove width, modal solutions were obtained for the complex propagation constant, from which the adiabatic parameter and mode propagation length were determined. These results suggest that the decrease in propagation length with decreasing wavelength is the underlying cause of the short-wavelength threshold.

Analysis of ASB assisted failure in a high strength steel under high loading rate

Abstract In the present work, the engineering, high strength ARMOX500T steel was submitted to a Kalthoff and Winkler type impact test in view of evaluating its crack arrest capability under dynamic loading. From an impact velocity of the order of 150 m/s, the crack propagation is seen to be preceded by adiabatic shear banding (ASB) leading to a premature plate failure. The whole chronology of the plate failure mechanisms (weak localization, strong localization in the form of ASB then cracking) was observed thanks to the use of an ultra-high speed camera. Further digital image analysis allows for establishing displacement fields describing the kinematics induced by both adiabatic shear banding and crack propagation, in the perspective of being implemented into an embedded band/crack based model in the context of dynamic plasticity and fracture.

Asymptotic solution for the problem of sound propagation in a sea with an underwater canyon.

The problem of the sound propagation in shallow-water waveguide with a seabottom featuring canyon-type inhomogeneity of a specific form is considered. The sound pressure in such waveguide is represented in a form of modal expansion and the equations for modal coefficients are derived. In case of a single-mode adiabatic propagation, it is possible to neglect the mode interaction and omit the coupling terms in these equations. The uncoupled equation for the mode amplitude admits an explicit analytical solution via the separation of variables.

Predicting range-frequency interference patterns from broadband sources in a range-dependent, continental shelf environment

The 2007 CALOPS dataset contains horizontal line array received signals from numerous surface vessels transiting an area off the coast of southeast Florida. Parabolic equation propagation models using measured bathymetric, sound velocity profile, and sediment parameter data suggest that substantial mode coupling should be occurring for several down-slope propagation scenarios in the dataset. However, received acoustical data for these scenarios, in the form of range-frequency interference patterns, are best described by adiabatic propagation. This discrepancy has implications that affect waveguide invariant parameter estimation, which is necessary for invariant-based passive ranging. The causes of the incongruous model output will be discussed, as well as their effects on estimating waveguide invariant distributions for these scenarios. [This research was supported by the Applied Research Laboratory, at the Pennsylvania State University through the Eric Walker Graduate Assistantship Program.]

Adiabatic pulse propagation in a dispersion-increasing fiber for spectral compression exceeding the fiber dispersion ratio limitation

Adiabatic soliton spectral compression in a dispersion-increasing fiber (DIF) with a linear dispersion ramp is studied both numerically and experimentally. The anticipated maximum spectral compression ratio (SCR) would be limited by the ratio of the DIF output to the input dispersion values. However, our numerical analyses indicate that SCR greater than the DIF dispersion ratio is feasible, provided the input pulse duration is shorter than a threshold value along with adequate pulse energy control. Experimentally, a SCR of 28.6 is achieved in a 1 km DIF with a dispersion ratio of 22.5.

Acoustic mode coupling due to subaqueous sand dunes in the South China Sea: Extension of the adiabatic criterion to waveguides with bedforms

The large subaqueous sand dunes on the upper continental slope of the South China Sea (SCS) create a range-dependent ocean acoustic waveguide within which acoustic energy is expected to couple between propagating normal modes. Here, the criterion of adiabatic invariance is extended to the case of a waveguide possessing bedforms. The morphological features of the bedforms modeled in this theoretical and numerical investigation are based on echosounder observations of the SCS sand dune field during a research cruise in the spring of 2012 on the Taiwanese R/V Ocean Researcher 2 (OR2). Using the extended criterion for adiabatic invariance to examine mode propagation over these bedforms, results demonstrate that bedforms increase mode coupling strength such that the criterion for adiabatic propagation is exceeded for waveguides with small bedform amplitude to water depth ratios; increasing bedform amplitude enhances mode coupling. Physically, initially bottom-trapped mode 1 energy abruptly couples to higher adjacent modes, with most of the energy preferentially settling into a few select modes. The scattered energy fills the water column to near-surface depths downrange of the bedforms. Numerical simulations confirm the extended criterion parameterization. [This work was supported by National Science Council of Taiwan.]

Corotation resonances for gravity waves and their impact on angular momentum transport in stellar interiors

AbstractGravity waves, which propagate in radiation zones, can extract or deposit angular momentum by radiative and viscous damping. Another process, poorly explored in stellar physics, concerns their direct interaction with the differential rotation and the related turbulence. In this work, we thus study their corotation resonances, also called critical layers, that occur where the Doppler-shifted frequency of the wave approaches zero. First, we study the adiabatic and non-adiabatic propagation of gravity waves near critical layers. Next, we derive the induced transport of angular momentum. Finally, we use the dynamical stellar evolution code STAREVOL to apply the results to the case of a solar-like star. The results depend on the value of the Richardson number at the critical layer. In the first stable case, the wave is damped. In the other unstable and turbulent case, the wave can be reflected and transmitted by the critical layer with a coefficient larger than one: the critical layer acts as a secondary source of excitation for gravity waves. These new results can have a strong impact on our understanding of angular momentum transport processes in stellar interiors along stellar evolution where strong gradients of angular velocity can develop.

Acoustic mode coupling due to subaqueous sand dunes in the South China Sea

The large subaqueous sand dunes on the upper continental slope of the South China Sea are expected to couple acoustic propagating normal modes. In this letter, the criterion of adiabatic invariance is extended to the case of a waveguide possessing bedforms. Using the extended criterion to examine mode propagation over the bedforms observed in the sand dune field in 2012, results demonstrate that bedforms increase mode coupling strength such that the criterion for adiabatic propagation is exceeded for waveguides with small bedform amplitude to water depth ratios; increasing bedform amplitude enhances mode coupling. Numerical simulations confirm the extended criterion parameterization.

Surface hopping dynamics using a locally diabatic formalism: Charge transfer in the ethylene dimer cation and excited state dynamics in the 2-pyridone dimer

In this work, the advantages of a locally diabatic propagation of the electronic wave function in surface hopping dynamics proceeding on adiabatic surfaces are presented providing very stable results even in challenging cases of highly peaked nonadiabatic interactions. The method was applied to the simulation of transport phenomena in the stacked ethylene dimer radical cation and the hydrogen bonded 2-pyridone dimer. Systematic tests showed the reliability of the method, in situations where standard methods relying on an adiabatic propagation of the wave function and explicit calculation of the nonadiabatic coupling terms exhibited significant numerical instabilities. Investigations of the ethylene dimer radical cation with an intermolecular distance of 7.0 Å provided a quantitative description of diabatic charge trapping. For the 2-pyidone dimer, a complex dynamics was obtained: a very fast (<10 fs) initial S(2)∕S(1) internal conversion; subsequent excitation energy transfers with a characteristic time of 207 fs; and the occurrence of proton coupled electron transfer (PCET) in 26% of the trajectories. The computed characteristic excitation energy transfer time of 207 fs is in satisfactory agreement with the experimental value of 318 fs derived from the vibronic exciton splittings in a monodeuterated 2-pyridone dimer complex. The importance of nonadiabatic coupling for the PCET related to the electron transfer was demonstrated by the dynamics simulations.

Behavior of preheated premixed flames at rich conditions

Over the past decade, a series of studies has demonstrated non-catalytic hydrogen production via partial oxidation of fuel-rich mixtures that lie well beyond the conventional rich flammability limits. In all cases, heat recirculation from the post-reaction zone to the unburned mixture leads to significant increases of combustion temperatures and mass burning flux. Although the feasibility of self-sustained combustion at extremely rich conditions is well established, fundamental questions on this combustion regime have not been addressed. As a first step, the present study relates the inherently non-adiabatic operation of heat-recirculating combustors to adiabatic combustion with highly preheated fuel/air mixtures. Standard numerical and experimental techniques are used to illuminate fundamental aspects of rich combustion for a broad range of inlet temperatures (Tin=200–1000K) and equivalence ratios (ϕ=1.33–2.7). Numerical results show conventional flame structures at equivalence ratios in excess of ϕ=2.0 for CH4/air combustion, which is corroborated by experiments with a flat flame burner and highly preheated fuel/air mixtures. A detailed numerical analysis reveals that adiabatic flame propagation at rich preheated conditions is governed by characteristic temperatures T★ within the flame structure. These temperatures are shown to be primarily dependent on the laminar burning flux, whereas the equivalence ratio plays a subordinate role.

Comparison of hybrid three-dimensional modeling with measurements on the continental shelf

During the CALOPS 2007 experiment, off the coast of Fort Lauderdale, Florida, three-dimensional (3D) multipath was observed using a bottom mounted horizontal line array during source tows along the 200 m isobath [Kevin D. Heaney and James J. Murray, J. Acoust. Soc. Am. 125(4), 1394-1402 (2008)]. In this paper a hybrid modeling approach is presented to model the 3D sound on the Florida shelf, nearly shaped like the canonical wedge. The hybrid approach combines vertical acoustic normal modes with the parabolic equation solution (in range/cross-range). The approach is shown to satisfy the 3D Cartesian-coordinate wave equation in the limit of adiabatic mode propagation. In the adiabatic mode parabolic equation (AMPE) approach modal phase speeds vs position are used as the input to the parabolic equation computation with dimensions of easting (km) and northing (km). Vertical adiabatic modes and horizontal rays are also computed to illustrate the 3D multipath arrival. The AMPE field is computed for all the modes for each element of the horizontal array. Beamforming vs source range is then conducted and excellent agreement with data is achieved.

Horizontal coherence of low-frequency fixed-path sound in a continental shelf region with internal-wave activity

Sound at 85 to 450 Hz propagating in approximately 80-m depth water from fixed sources to a joint horizontal/vertical line array (HLA/VLA) is analyzed. The data are from a continental shelf area east of Delaware Bay (USA) populated with tidally generated long- and short-wavelength internal waves. Sound paths are 19 km in the along-shore (along internal-wave crest) direction and 30 km in the cross-shore direction. Spatial statistics of HLA arrivals are computed as functions of beam steering angle and time. These include array gain, horizontally lagged spatial correlation function, and coherent beam power. These quantities vary widely in magnitude, and vary over a broad range of time scales. For example, correlation scale can change rapidly from forty to five wavelengths, and correlation-scale behavior is anisotropic. In addition, the vertical array can be used to predict correlation expected for adiabatic propagation with cylindrical symmetry, forming a benchmark. Observed variations are in concert with internal-wave activity. Temporal variations of three coherence measures, horizontal correlation length, array gain, and ratio of actual correlation length to predicted adiabatic-mode correlation length, are very strong, varying by almost a factor of ten as internal waves pass.