Initial Wavelength Research Articles

Influences of non-isothermal atmospheric backgrounds on variations of gravity wave parameters

Because of the importance of gravity waves (GWs) in coupling different atmospheric regions, further studies are necessary to investigate the characteristics of GW propagation in a non-isothermal atmosphere. Using a nonlinear numerical model, we simulate the propagation of small amplitude GWs with various wavelengths in different non-isothermal atmospheres. Our results show that the GW vertical wavelength undergoes sharp changes above the stratopause and mesopause region. Specifically, for a GW with an initial vertical wavelength of 5 km, the seasonal background temperature structure difference at 50° latitude can cause the vertical wavelength to vary by ∼2 km in the mesosphere and by as large as ∼4.5 km in the lower thermosphere. In addition, the GW paths exhibit great divergence in the height range of ∼65–110 km. Our results also show that the variations of GW path, vertical wavelength and horizontal phase velocity are not synchronized in a non-isothermal atmosphere as in an isothermal atmosphere. Despite the fact that all GWs change their characteristics as they propagate upward in a non-isothermal atmosphere, the variations relative to the initial parameters at a reference height are similar for different initial vertical wavelengths. Our results indicate that the changing characteristics of a gravity wave in a non-isothermal atmosphere need to be considered when investigating the relationship of GWs at two different heights.

Effect of crystallographic orientation on instability behavior of planar interface in directional solidification

The instability process of planar interface in directional solidification with respect to the crystallographic orientation is studied using a transparent model alloysuccinonitrile-acetone. Three typical crystal grains which have preferred dendrite, tilted dendrite and seaweed patterns at rapid pulling velocity respectively are chosen in our experiment. The experimental results show that the preferred dendrite grain has the shortest incubation time and the smallest initial perturbation wavelength of planar interface instability, the tilted dendrite grain has the largest ones and the seaweed grain has median ones. These results accord qualitatively with previous analytical results and phase-field simulation results. It is also found that the interfacial non-steady-state evolution behaviors of the preferred dendrite grain and the tilted dendrite grain are significantly different from that of the seaweed grain, suggesting that the non-steady-state evolution behavior of planar interface instability is closely related to the crystallographic orientation.

A New Method of Wavelength Calibration for LAMOST by Combining Short- and Long-Exposure Spectral Lines

AbstractIn wavelength calibration using arc lines, the normal approach is to use the strongest unsaturated lines, leaving weak lines unused. A new method is proposed in this paper, which not only utilizes the strong spectral lines, but also makes most use of weak spectral lines. In order to validate the effectiveness of the method we propose, experiments are performed on simulated spectra. Firstly, two kinds of spectra are generated: one with a short exposure and another with a long exposure. Secondly, calibration lines are chosen from the short exposure and long exposure spectra separately according to some rules. Thirdly, the initial wavelength calibration is completed by using the selected short-exposure lines. Fourthly, the approximate centroids of the selected long-exposure lines are obtained by utilizing the result of the initial wavelength calibration. These are then adjusted iteratively to obtain the centroids. Finally, the selected lines from the short- and long-exposures are combined to obtain the final wavelength calibration. Compared with traditional calibration methods which only use short exposures and strong lines, the proposed method is shown to be more accurate.

Spontaneous and deterministic three-dimensional curling of pre-strained elastomeric bi-strips

Three dimensional curls (“hemi-helices”) consisting of multiple, periodic and alternating helical sections of opposite chiralities, separated by perversions, are one of a variety of complex shapes that can be produced by a simple generic process consisting of pre-straining one elastomeric strip, joining it side-by-side to another and then releasing the bi-strip. The initial wavelength of the hemi-helix and the number of perversions are determined by the strip cross-section, the constitutive behavior of the elastomer and the value of the pre-strain. The hemi-helix has no net twist. Topologically, the perversions separate regions of the hemi-helix deforming principally by bending from those where twisting dominates.

Threshold wavelength on filaments of complex fluids

The breakup into drops of complex-fluid filaments – usually jets of polymeric and surfactant solutions – plays a central role in many engineering applications ranging from crop spraying to food processing and propulsion systems. To assess the influence of interfacial perturbations in the breakup process, we followed the dynamics of the initial wavelength of surface instability on complex-fluid filaments using direct numerical simulation. We found a threshold wavelength at low Reynolds numbers corresponding to a change on the filament's configuration near breakup from large primary drops connected by slender liquid threads for wavelengths below the threshold to clearly defined satellite drops in between the primary drops for wavelengths larger than the threshold. We also found that shear-thinning effects, by reducing the viscous resistance to the Marangoni stress responsible for the formation of the satellites, cause the threshold to appear at shorter wavelengths.

Instability wave models for the near-field fluctuations of turbulent jets

AbstractPrevious work has shown that aspects of the evolution of large-scale structures, particularly in forced and transitional mixing layers and jets, can be described by linear and nonlinear stability theories. However, questions persist as to the choice of the basic (steady) flow field to perturb, and the extent to which disturbances in natural (unforced), initially turbulent jets may be modelled with the theory. For unforced jets, identification is made difficult by the lack of a phase reference that would permit a portion of the signal associated with the instability wave to be isolated from other, uncorrelated fluctuations. In this paper, we investigate the extent to which pressure and velocity fluctuations in subsonic, turbulent round jets can be described aslinearperturbations to the mean flow field. The disturbances are expanded about the experimentally measured jet mean flow field, and evolved using linear parabolized stability equations (PSE) that account, in an approximate way, for the weakly non-parallel jet mean flow field. We utilize data from an extensive microphone array that measures pressure fluctuations just outside the jet shear layer to show that, up to an unknown initial disturbance spectrum, the phase, wavelength, and amplitude envelope of convecting wavepackets agree well with PSE solutions at frequencies and azimuthal wavenumbers that can be accurately measured with the array. We next apply the proper orthogonal decomposition to near-field velocity fluctuations measured with particle image velocimetry, and show that the structure of the most energetic modes is also similar to eigenfunctions from the linear theory. Importantly, the amplitudes of the modes inferred from the velocity fluctuations are in reasonable agreement with those identified from the microphone array. The results therefore suggest that, to predict, with reasonable accuracy, the evolution of the largest-scale structures that comprise the most energetic portion of the turbulent spectrum of natural jets, nonlinear effects need only be indirectly accounted for by considering perturbations to the mean turbulent flow field, while neglecting any non-zero frequency disturbance interactions.

Photoluminescence of diamondoid crystals

The photoluminescence of diamondoids in the solid state is examined. All of the diamondoids are found to photoluminesce readily, with initial excitation wavelengths ranging from 233 nm to 240 nm (5.3 eV). These excitation energies are more than 1 eV lower than any previously studied saturated hydrocarbon material. The emission is found to be heavily shifted from the absorption, with emission wavelengths of roughly 295 nm (4.2 eV) in all cases. In the dissolved state, however, no fluorescence is observed for excitation wavelengths as short as 200 nm. We also discuss predictions and measurements of the quantum yield. Our predictions indicate that the maximum yield may be as high as 25%. Our measurement of one species, diamantane, gives a yield of 11%, the highest ever reported for a saturated hydrocarbon, even though it was likely not at the optimal excitation wavelength.

LOSA-M2 aerosol Raman lidar

The scanning LOSA-M2 aerosol Raman lidar, which is aimed at probing atmosphere at wavelengths of 532 and 1064 nm, is described. The backscattered light is received simultaneously in two regimes: analogue and photon-counting. Along with the signals of elastic light scattering at the initial wavelengths, a 607-nm Raman signal from molecular nitrogen is also recorded. It is shown that the height range of atmosphere probing can be expanded from the near-Earth layer to stratosphere using two (near- and far-field) receiving telescopes, and analogue and photon-counting lidar signals can be combined into one signal. Examples of natural measurements of aerosol stratification in atmosphere along vertical and horizontalpaths during the expeditions to the Gobi Desert (Mongolia) and Lake Baikal areas are presented.

What happens to the initial planar instability when the thermal gradient is increased during directional solidification?

The positive thermal gradient is one of the most important parameters during directional solidification. The increase of the thermal gradient usually stabilizes the planar interface in the steady state analysis. However, in the initial transient range of planar instability, the thermal gradient presents complicated effects. Time-dependent analysis shows that the increase of the thermal gradient can enhance both the stabilizing effects and the destabilizing effects on a planar interface. The incubation time first decreases and then increases with the increase of the thermal gradient. Moreover, the initial average wavelength always increases with the thermal gradient increasing, contrary to the effect of the thermal gradient on the steady cellular/dendritic spacing. This reveals the types of spacing adjustment after planar instability.

Optical Density Measurements and Analysis for Single-Mode Initial-Condition Buoyancy-Driven Mixing

The Texas A&M water channel experiment is modified to examine the effect of single-mode initial conditions on the development of buoyancy-driven mixing (Rayleigh-Taylor) with small density differences (low-Atwood number). Two separated stratified streams of ~5°C difference are convected and unified at the end of a splitter plate outfitted with a servo-controlled flapper. The top (cold) stream is dyed with Nigrosine and density is measured optically through the Beer-Lambert law. Quantification of the subtle differences between different initial conditions required the optical measurement uncertainties to be significantly reduced. Modifications include a near-uniform backlighting provided through quality, repeatable, professional studio flashes impinging on a white-diffusive surface. Also, a black, absorptive shroud isolates the experiment and the optical path from reflections. Furthermore, only the red channel is used in the Nikon D90 CCD camera where Nigrosine optical scatterring is lower. This new optical setup results in less than 1% uncertainty in density measurements, and 2.5% uncertainty in convective velocity. With the Atwood uncertainty reduced to 4% using a densitometer, the overall mixing height and time uncertainty was reduced to 5% and 3.5%, respectively. Initial single-mode wavelengths of 2, 3, 4, 6, and 8 cm were examined as well as the baseline case where no perturbations were imposed. All non-baseline cases commence with a constant velocity that then slows, eventually approaching the baseline case. Larger wavelengths grow faster, as well as homogenize the flow at a faster rate. The mixing width growth rates were shown to be dependent on initial conditions, slightly outside of experimental uncertainty.

Performance analysis of a fiber Bragg grating filter-based strain/temperature sensing system based on a modified Gaussian function approximation method

This paper analyzes the performance of a fiber Bragg grating (FBG) filter-based strain and/or temperature sensing system based on a modified Gaussian function (MGF) approximation method. Instead of using a conventional Gaussian function, we propose the MGF, which can capture the characteristics of the sidelobes of the reflected spectrum, to model the FBG sensor and filter. We experimentally demonstrate that, by considering the contributions of the sidelobes with the MGF approximation method, behaviors of the FBG filter-based FBG displacement and/or temperature sensing system can be predicted more accurately. The predicted behaviors include the saturation, the sensitivity, the sensing range, and the optimal initial Bragg wavelengths of the FBG sensing system.

Propagation of Gaussian–Schell beam in turbulent atmosphere of three-layer altitude model

We propose a method that is used to derive the moment radius of intensity distribution in a turbulent atmosphere. From this study, we have found that the second moment radius is affected only by the first-order expansion coefficient of the wave structure function. If our attention is directed to a higher moment radius, a higher order approximation of the expansion needs to be used. As an example, the propagation of a Gaussian-Schell beam in a slant path has been studied based on the turbulent atmosphere of a three-layer model. The variation of some beam properties, such as the relative waist width, angular spread, and kurtosis parameter with the initial waist width, wavelength, and zenith angle, has been analyzed and discussed in detail. The study shows that there is little difference between the three-layer model and the Kolmogorov model in studying uplink propagation, and the difference is large for downlink propagation. The intensity profile of the Gaussian beam in turbulence does not keep a Gaussian shape unless the beam spreading due to turbulence is very large or very small.

A tale of three taxes: photo-gyro-gravitactic bioconvection

The term bioconvection encapsulates the intricate patterns in concentration, due to hydrodynamic instabilities, that may arise in suspensions of non-neutrally buoyant, biased swimming microorganisms. The directional bias may be due to light (phototaxis), gravity (gravitaxis), a combination of viscous and gravitational torques (gyrotaxis) or other taxes. The aim of this study is to quantify experimentally the wavelength of the initial pattern to form from an initially well-mixed suspension of unicellular, swimming green algae as a function of concentration and illumination. As this is the first such study, it is necessary to develop a robust and meticulous methodology to achieve this end. The phototactic, gyrotactic and gravitactic alga Chlamydomonas augustae was employed, with various red or white light intensities from above or below, as the three not altogether separable taxes were probed. Whilst bioconvection was found to be unresponsive to changes in red light, intriguing trends were found for pattern wavelength as a function of white light intensity, depending critically on the orientation of the illumination. These trends are explored to help unravel the mechanisms. Furthermore, comparisons are made with theoretical predictions of initial wavelengths from a recent model of photo-gyrotaxis, encouragingly revealing good qualitative agreement.

Radio structure of the blazar 1156 + 295 with sub-pc resolution

1156+295 is a flat-spectrum quasar which is loud at radio and gamma-ray. Previous observations of the source revealed a radio morphology on pc to kpc scales consistent with a helical jet model. In our present research, this source was observed with the VLBA at 86, 43 and 15 GHz on four epochs from 10 May 2003 to 13 March 2005 aiming at studying the structure of the innermost jet in order to understand the relation between the helical structure and the astrophysical processes in the central engine. A core-jet structure with six jet components is identified. The apparent transverse velocities of the six jet components derived from proper motion measurements are in the range between 3.6 c and 11.6 c. The overall jet shape shows oscillatory morphology with multiple curvatures on pc scales which might be indicative of a helical pattern. Models of helical jet are discussed on the basis of both Kelvin-Helmholtz (K-H) instability and jet precession. The K-H instability model shows better agreement with the observed data. The overall radio structure on the scale from sub-pc to kpc appears to be fitted with a hydrodynamic model with the fundamental helical mode in Kelvin-Helmholtz (K-H) instability. This helical mode with an initial characteristic wavelength of 0.2 pc is excited at the base of the jet on the scale of 0.005 pc (or 1000R_s, the typical size of the broad line region for a super massive black hole of $4.3\times10^8M_{\odot}$). A presessing jet model can also fit the observed jet structure on the scale between 10 pc and 300 pc. However, additional astrophysical processes may be required for the presessing jet model in order to explain the bendings on the inner jet structure (1 to 10 pc) and re-collimation of the large scale jet outflow (>300 pc).

Sensitivity of guided mode resonance filter-based biosensor in visible and near infrared ranges

The sensitivity of guided mode resonance filter (GMRF) biosensors was analyzed when the initial peak wavelength (IPW) changed from 400 to 1600 nm. The sensitivity of the GMRF to the sample refractive index and thickness was analyzed using rigorous coupled wave analysis method. Results indicate that for a certain IPW, the sensitivity of the GMRF did not change when the sample refractive index varied, but drastically decreased and achieved constant sensitivity with increased sample thickness. When the IPW increased, the sensitivity improved in relation to both sample refractive index and sample thickness. Furthermore, the capability of sample thickness improved with longer IPW. These results are helpful in obtaining an optimal GMRF biosensor.

Numerical Study of Horizontal Shear Instability Waves along Narrow Cold Frontal Rainbands

Abstract The effects of variations in low-level ambient vertical shear and horizontal shear on the alongfront variability of narrow cold frontal rainbands (NCFRs) that propagate into neutral and slightly unstable environments are investigated through a series of idealized cloud-resolving simulations. In cases initialized with slightly unstable sounding and weak ambient cross-frontal vertical shears, core-gap structures of precipitation along NCFRs occur that are associated with wavelike disturbances that derive their kinetic energy mainly from the mean local vertical shear and buoyancy. However, over a wide range of environmental conditions, core-gap structures of precipitation occur because of the development of a horizontal shear instability (HSI) wave along the NCFRs. The growth rate and amplitude of the HSI wave decrease significantly as the vertical shear of the ambient cross-front wind is reduced. These decreases are a consequence of the enhancement of the low-level local vertical shear immediately behind the leading edge. The strong local vertical shear acts to damp the vorticity edge wave on the cold air side of the shear zone, thereby suppressing the growth of the HSI wave through the interaction of the two vorticity edge waves. It is also noted that the initial wavelength of the HSI wave increases markedly with increasing horizontal shear. The local vertical shear around the leading edge is shown to damp long HSI waves more strongly than short waves, and the horizontal shear dependency of the wavelength is explained by the decrease in the magnitude of the vertical shear relative to that of the horizontal shear.

Generation of coherent X-ray radiation through modulation compression

In this paper, we propose a scheme to generate tunable coherent X-ray radiation for future light source applications. This scheme uses an energy chirped electron beam, a laser modulator, a laser chirper and two bunch compressors to generate a prebunched kilo-Ampere current electron beam from a few tens Ampere electron beam out of a linac. The initial modulation energy wavelength can be compressed by a factor of 1 + h b R 56 a in phase space, where h b is the energy bunch length chirp introduced by the laser chirper, R 56 a is the momentum compaction factor of the first bunch compressor. As an illustration, we present an example to generate more than 400 MW, 170 attoseconds pulse, 1 nm coherent X-ray radiation using a 60 A electron beam out of the linac and 200 nm laser seed. Both the final wavelength and the radiation pulse length in the proposed scheme are tunable by adjusting the compression factor and the laser parameters.

SHORELINE SAND WAVES AND BEACH NOURISHMENTS

The effects of the feedback between the changing coastal morphology and the wavefield on the generation and propagation of large scale (O(1-10 km)) shoreline sand waves is examined with a quasi-2D morphodynamic model. Traditional shoreline change models do not include this feedback and are only able to describe diffusion of shoreline sand waves and furthermore they are unable to describe migration. It is found with the present model that if there is a dominant littoral drift, the feedback causes downdrift migration of coastline features no matter if they grow or decay. Consistently with previous studies, simulations show that a rectilinear coastline becomes unstable and sand waves tend to grow spontaneously from random perturbations, if the wave incidence angle is larger then about 42o (θc) at the depth of closure (high angle wave instability). The initial wavelengths at which the sand waves develop are 2-3 km and this is similar to previous linear stability analysis. The implications of high angle wave instability for beach nourishments are investigated. The nourished shoreline retreats initially due to cross-shore transport because the nourished profile is steeper than the equilibrium profile. When a dominant littoral drift is present, the nourishment also migrates downdrift. If the wave angle at the depth of closure is below θc the alongshore transport contributes to the diffusion of the nourishment. However, if the angle is above θc (constant high angle wave conditions) the diffusion is reversed and the nourishment can trigger the formation of a shoreline sand wave train. Numerical experiments changing the proportion of 'high angle waves' and 'low angle waves' in the wave climate show that relatively small proportions of low angle waves slow down the growth of sand waves. These simulations with more realistic wave climates show shoreline sand waves that migrate downdrift maintaining more or less the same amplitude for years.

Heating of nonequilibrium electrons by laser radiation in solid transparent dielectrics

A computer simulation of the heating of nonequilibrium electrons by an intense high-frequency electromagnetic field leading to the bulk damage of solid transparent dielectrics under single irradiation has been carried out. The dependences of the avalanche ionization rate on threshold field strength have been derived. Using the Fokker-Planck equation with a flux-doubling boundary condition is shown to lead to noticeable errors even at a ratio of the photon energy to the band gap ∼0.1. The series of dependences of the critical fields on pulse duration have been constructed for various initial lattice temperatures and laser wavelengths, which allow the electron avalanche to be identified as a limiting breakdown mechanism. The ratio of the energy stored in the electron subsystem to the excess (with respect to the equilibrium state) energy of the phonon subsystem by the end of laser pulse action has been calculated both with and without allowance for phonon heating. The influence of phonon heating on the impact avalanche ionization rate is analyzed.

Phase mask coronagraphy at JPL and Palomar

For the imaging of faint companions, phase mask coronagraphy has the dual advantages of a small inner working angle and high throughput. This paper summarizes our recent work in developing phase masks and in demonstrating their capabilities at JPL. Four-quadrant phase masks have been manufactured at JPL by means of both evaporation and etching, and we have been developing liquid crystal vortex phase masks in partnership with a commercial vendor. Both types of mask have been used with our extreme adaptive optics well-corrected subaperture at Palomar to detect known brown dwarf companions as close as ~ 2.5 λ/D to stars. Moreover, our recent vortex masks perform very well in laboratory tests, with a demonstrated infrared contrast of about 10^(−6) at 3 λ/D, and contrasts of a few 10^(−7) with an initial optical wavelength device. The demonstrated performance already meets the needs of ground-based extreme adaptive optics coronagraphy, and further planned improvements are aimed at reaching the 10^(−10) contrast needed for terrestrial exoplanet detection with a space-based coronagraph.