Spatial Nonuniformity Research Articles

Long‐Term Evolution of Sand Transport Through a River Network: Relative Influences of a Dam Versus Natural Changes in Grain Size From Sand Waves

AbstractTemporal and spatial nonuniformity in supplies of water and sand in a river network leads to sand transport that is in local disequilibrium with the upstream sand supply. In such river networks, sand is transported downstream as elongating waves in which coupled changes in grain size and transport occur. Depending on the magnitude of each sand‐supplying event and the interval between such events, changes in bed‐sand grain size associated with sand‐wave passage may more strongly regulate sand transport than do changes in water discharge. When sand transport is controlled more by episodic resupply of sand than by discharge, upstream dam construction may exacerbate or mitigate sand‐transport disequilibria, thus leading to complicated and difficult‐to‐predict patterns of deposition and erosion. We analyzed all historical sediment‐transport data and embarked on a 4‐year program of continuous sediment‐transport measurements to describe disequilibrium sand transport in a river network. Results indicate that sand transport in long river segments can evolve over ≥50‐year timescales following rare large sand‐supplying events. These natural changes in sand transport in distal downstream river segments can be larger than those caused by an upstream dam. Because there is no way to know a priori whether sand transport in a river has changed in response to changes in the upstream sand supply, contemporary continuous measurements of sand transport are required for accurate sand loads and budgeting. Analysis of only historical sediment‐transport measurements, as is common in the literature, may lead to incorrect conclusions with respect to current or future sediment‐transport conditions.

Spatial Distribution of Pancreatic Stones in Chronic Pancreatitis.

The aim of this study was to establish a standard to describe the spatial distribution of pancreatic stones in chronic pancreatitis (CP). Two hundred forty-seven CP patients with pancreatic stones from June to December 2012 were enrolled. Two-dimensional images from coronal projection of 3-dimensional computed tomography images of pancreatic stones were gained. The number (n) of all stones and the geometric standard deviation (σ) of distances between the centroid of all stones and the centroids of every stone that represented the spatial distribution nonuniformity were calculated by Stone Reconstruction and Identification Programming System. The mean value of n and σ were 13.6 and 22.5; n > 13.6 and σ > 22.5 were determined as "multistones" and "nonuniform," respectively. Compared with alcoholic CP, idiopathic CP was less prone to multistones (odds ratio [OR], 0.310) and more prone to nonuniform (OR, 3.247). Pancreatic pseudocyst (OR, 2.211) in CP course was a risk factor of multistones, whereas diabetes mellitus in first-/second-/third-degree relatives (OR, 0.382) was a protective factor. Age at diagnosis of pancreatic stones (OR, 1.022) was a risk factor of nonuniformity. Compared with idiopathic CP, alcoholic CP patients were prone to more pancreatic stones that distribute more uniformly.

LES Study of Vertical Eddy Diffusivity Estimation in Bottom Boundary Layers

AbstractOsborn’s method is commonly used to obtain diffusion coefficient K from the turbulent dissipation rate ε. This method is the relational expression . The dissipation flux coefficient is often set to a constant value of 0.2, but this study of LES revealed that varies greatly vertically in the bottom boundary layer because of the influence of the seabed. Consequently, the eddy diffusion coefficient is overestimated in the lower part of the bottom boundary, but it is slightly underestimated in the upper part. Therefore, Osborn’s method with constant cannot give the correct diffusivity. Furthermore, even if treating as a function of flux Richardson number , as defined originally by Osborn, the estimation is underestimated by the advection effect because of the influence of spatial nonuniformity. Energy budget analysis revealed that this defect can be improved using the extended flux Richardson number, which can be estimated by multiplying using a constant correction factor. Furthermore, we proposed two alternative estimation methods. For the first method, which estimates the relation between and the gradient Richardson number , can be expressed with instead of with a correction factor. We can estimate the reasonable diffusivity if we have current data supplementary to obtain . For the second method, can be expressed as a similarity function of the height above the bottom normalized by the Ozmidov scale. This method can provide an acceptable estimate of diffusivity without current data for several circumstances.

Short-period fluctuation and spatial distribution of solar irradiance under clouds

Recently, outdoor photovoltaic (PV) module performance characterizations have been carried out mainly for testing installed PV modules. For high-accuracy outdoor measurement, solar irradiance with stability in time and uniformity in space are required. Short-period temporal instability and spatial nonuniformity of solar irradiance are observed using a series of PV module sensors (PVMSs). The speed and direction of cloud shadows moving on PVMSs are evaluated from the fluctuations of irradiance detected by several PVMSs. On the basis of cloud distribution, weather conditions are classified into four types, and the characteristics of solar irradiance fluctuations are examined for each type. The irradiance changes quickly under cumulous clouds. On the other hand, the irradiance becomes weak, but its short-period fluctuations are not evident under layered clouds. The amplitude of irradiance fluctuations reaches 70% of clear-day irradiance at noon in this observation. The spatial nonuniformity evaluated on the basis of the instantaneous difference in irradiance intensity between two PVMSs also reaches 1.8%/m of clear-day irradiance.

Filtering method of detecting solar irradiance conditions for photovoltaic module performance characterization under unstable and nonuniform irradiance

The temporal instability and spatial nonuniformity of solar irradiance, and their relationship are analyzed by field observations, and a method of detecting the events in which irradiance is stable and uniform is proposed for outdoor photovoltaic (PV) module performance characterizations. The relationships of instability and nonuniformity of irradiance with measurement conditions are discussed, e.g., a long measurement time worsens instability. When events are observed with repeated measurements under certain conditions and selected under the condition that the instability is low afterwards, the nonuniformity of the selected events is low. This means that for measuring and detecting uniform events well, two sensors are required, but they can also be detected with a single sensor by repeated measurements and selecting with instability. This suggests that repeated measurements may be a filtering method for finding stable and uniform events, which is required for high-accuracy outdoor PV module performance characterizations.

Precise calibration of spatial phase response nonuniformity arising in liquid crystal on silicon.

In order to calibrate the spatial phase response nonuniformity of liquid crystal on silicon (LCoS), we propose to use a Twyman-Green interferometer to characterize the wavefront distortion, due to the inherent curvature of the device. During the characterization, both the residual carrier frequency introduced by the Fourier transform evaluation method and the lens aberration are error sources. For the tilted phase error introduced by residual carrier frequency, the least mean square fitting method is used to obtain the tilted phase error. Meanwhile, we use Zernike polynomials fitting based on plane mirror calibration to mitigate the lens aberration. For a typical LCoS with 1×12,288 pixels after calibration, the peak-to-valley value of the inherent wavefront distortion is approximately 0.25λ at 1550nm, leading to a half-suppression of wavefront distortion. All efforts can suppress the root mean squares value of the inherent wavefront distortion to approximately λ/34.

Strong terahertz radiation generation by cosh-Gaussian laser beams in axially magnetized collisional plasma under non-relativistic ponderomotive regime

AbstractWe propose a scheme for terahertz (THz) radiation generation by non-linear mixing of two cosh-Gaussian laser beams in axially magnetized plasma with spatially periodic density ripple where electron-neutral collisions have been taken into account. The laser beams exert a non-linear ponderomotive force due to spatial non-uniformity in the intensity. The plasma electrons acquire non-linear oscillatory velocity under the influence of ponderomotive force. This oscillatory velocity couples with preformed density ripples (n′ = n0αeiαz) to generate a strong transient non-linear current that resonantly derives THz radiation of frequency ~ωh (upper hybrid frequency). Laser frequencies (ω1 and ω2) are chosen such that the beat frequency (ω) lies in the THz region. The periodicity of density ripple provides phase-matching conditions (ω = ω1 − ω2 and $\vec k = \vec k_1 - \vec k_2 + {\rm \vec \alpha} $) to transfer maximum momentum from laser to THz radiation. The axially applied external magnetic field can be utilized to enhance the non-linear coupling and control various parameters of generated THz wave. The effects of decentered parameters (b), collisional frequency (νen), and magnetic field strength (B0 = ωcm/e) are analyzed for strong THz radiation generation. Analytical results show that the amplitude of THz wave enhances with decentered parameters as well as with the magnitude of axially applied magnetic field. The THz amplitude is found to be highly sensitive to collision frequency.

Present status of numerical modeling of hydrogen negative ion source plasmas and its comparison with experiments: Japanese activities and their collaboration with experimental groups

The present status of kinetic modeling of particle dynamics in hydrogen negative ion (H−) source plasmas and their comparisons with experiments are reviewed and discussed with some new results. The main focus is placed on the following topics, which are important for the research and development of H− sources for intense and high-quality H− ion beams: (i) effects of non-equilibrium features of electron energy distribution function on volume and surface H− production, (ii) the origin of the spatial non-uniformity in giant multi-cusp arc-discharge H− sources, (iii) capacitive to inductive (E to H) mode transition in radio frequency-inductively coupled plasma H− sources and (iv) extraction physics of H− ions and beam optics, especially the present understanding of the meniscus formation in strongly electronegative plasmas (so-called ion–ion plasmas) and its effect on beam optics. For these topics, mainly Japanese modeling activities, and their domestic and international collaborations with experimental studies, are introduced with some examples showing how models have been improved and to what extent the modeling studies can presently contribute to improving the source performance. Close collaboration between experimental and modeling activities is indispensable for the validation/improvement of the modeling and its contribution to the source design/development.

Cone Beam Computed Tomography Image Quality Improvement Using a Deep Convolutional Neural Network.

IntroductionCone beam computed tomography (CBCT) plays an important role in image-guided radiation therapy (IGRT), while having disadvantages of severe shading artifact caused by the reconstruction using scatter contaminated and truncated projections. The purpose of this study is to develop a deep convolutional neural network (DCNN) method for improving CBCT image quality.MethodsCBCT and planning computed tomography (pCT) image pairs from 20 prostate cancer patients were selected. Subsequently, each pCT volume was pre-aligned to the corresponding CBCT volume by image registration, thereby leading to registered pCT data (pCTr). Next, a 39-layer DCNN model was trained to learn a direct mapping from the CBCT to the corresponding pCTr images. The trained model was applied to a new CBCT data set to obtain improved CBCT (i-CBCT) images. The resulting i-CBCT images were compared to pCTr using the spatial non-uniformity (SNU), the peak-signal-to-noise ratio (PSNR) and the structural similarity index measure (SSIM).ResultsThe image quality of the i-CBCT has shown a substantial improvement on spatial uniformity compared to that of the original CBCT, and a significant improvement on the PSNR and the SSIM compared to that of the original CBCT and the enhanced CBCT by the existing pCT-based correction method.ConclusionWe have developed a DCNN method for improving CBCT image quality. The proposed method may be directly applicable to CBCT images acquired by any commercial CBCT scanner.

Wideband infrared trap detector based upon doped silicon photocurrent devices.

We have designed, fabricated, and measured infrared trap detectors made from arsenic-doped silicon (Si:As) blocked impurity band (BIB) photodetectors. These trap detectors are composed of two detectors in a wedge geometry, with an entrance aperture diameter of either 1 or 3mm. The detectors were calibrated for quantum efficiency against a pyroelectric reference detector using a Fourier transform spectral comparator system, and etalon effects and spatial uniformity of the traps were also quantified. Measurements of the traps at a temperature of 10K show that nearly ideal external quantum efficiency (>90%) can be attained over much of the range from 4 to 24μm, with significant responsivity from 2 to 30μm. The traps exhibited maximum etalon oscillations of only 2%, which is about 10 times smaller amplitude than those of the single Si:As BIB detectors measured under similar conditions. Spatial nonuniformity across the entrance apertures of the traps was about 1%. The combination of high detectivity, wideband wavelength coverage, spectral flatness, and spatial uniformity make these trap detectors an excellent reference detector for spectrally resolved measurements and radiometric calibrations over the near- to far-infrared wavelength range.

Multimode scanning near-field photoluminescence spectroscopy and its application for studies of InGaN epitaxial layers and quantum wells

The paper reviews our recent achievements in developing a multimode scanning near-field optical microscopy (SNOM) technique. The multimode SNOM apparatus allows us to simultaneously measure spatial variations of photoluminescence spectra in the illumination and illumination-collection modes, their transients and sample surface morphology. The potential of this technique has been demonstrated on a polar InGaN epitaxial layer and nonpolar InGaN/GaN quantum wells. SNOM measurements have allowed revealing a number of phenomena, such as the band potential fluctuations and their correlation to the surface morphology, spatial nonuniformity of the radiative and nonradiative lifetimes, as well as the extended band nature of localized states. The combination of different modes enabled measurements of the ambipolar carrier diffusion and its anisotropy.

Non-uniformity of gas/liquid flow in a riser and impact of operation and pipe configuration on slugging characteristics

A pipeline-riser system is a typical complex pipe configuration in which the non-uniformity of gas/liquid flow is observed to be significant under certain flow rates. This article interprets slugging flow, the most common flow regime in a pipeline-riser system, in the context of non-uniformity, which differs from the approaches used to characterise slugging cycles in previous studies. By comparing data collected by several transmitters positioned locally on the riser and comparing flow in a simple vertical pipe, both the temporal and spatial non-uniformity of the gas/liquid distribution in the riser is presented and interpreted; the relationship between them is also illustrated. The non-uniformity of the flow provides an effective means to quantitatively evaluate the severity of slugging in a pipeline-riser system under different operating conditions. In addition to flow uniformity, specific flow characteristics can be affected by operating conditions as well as pipe configuration. Several factors closely related to the design of experimental facilities and procedures are investigated by a case study of three experimental loops with different layouts and dimensions, and several general findings are summarised. The results may contribute to a better and more specific understanding of gas/liquid flow mechanisms in a pipeline-riser system in addition to serving as a means to evaluate the features of laboratory pipeline-riser systems and the applicability of laboratory results in industrial cases.

Spatially localized particle energization by Landau damping in current sheets produced by strong Alfvén wave collisions

Understanding the removal of energy from turbulent fluctuations in a magnetized plasma and the consequent energization of the constituent plasma particles is a major goal of heliophysics and astrophysics. Previous work has shown that nonlinear interactions among counterpropagating Alfvén waves – or Alfvén wave collisions – are the fundamental building block of astrophysical plasma turbulence and naturally generate current sheets in the strongly nonlinear limit. A nonlinear gyrokinetic simulation of a strong Alfvén wave collision is used to examine the damping of the electromagnetic fluctuations and the associated energization of particles that occurs in self-consistently generated current sheets. A simple model explains the flow of energy due to the collisionless damping and the associated particle energization, as well as the subsequent thermalization of the particle energy by collisions. The net particle energization by the parallel electric field is shown to be spatially localized, and the nonlinear evolution is essential in enabling spatial non-uniformity. Using the recently developed field–particle correlation technique, we show that particles resonant with the Alfvén waves in the simulation dominate the energy transfer, demonstrating conclusively that Landau damping plays a key role in the spatially localized damping of the electromagnetic fluctuations and consequent energization of the particles in this strongly nonlinear simulation.

Ионное сопротивление наноструктурированного электрохимического электрода в неравновесных условиях

AbstractThe ionic resistance of the membrane electrode assembly (MEA) of O_2/H_2 fuel cells is studied in situ in a broad range of MEA compositions by using electrochemical impedance spectroscopy and polarization measurements. The MEAs are composed of platinum on carbon, carbon nanotubes, and Nafion, a proton- conducting polymer. We investigate atypical growth of the MEA resistance with the increasing Nafion content and spatial nonuniformity in the MEA ionic resistivity. A mechanism underlying this phenomenon is proposed.

An alternative derivation of second law results to better relate derivation to practical exergy analysis

A more general and physically intuitive alternative to the classical macroscopic derivation of second law results is proposed. Instead of using imaginary reversible processes occurring within heat engines that operate between infinite temperature reservoirs, the new derivation is applicable to any arbitrary control volume across which heat and/or work interactions occur. The arbitrary control volume is discretised into infinitesimally small elements. So-called 'Interface Equations' are developed at the interfaces of these elements, utilising the second law statement that heat transfer occurs from higher to lower temperatures. Terms from the interface equations are then rearranged at each element to show that dS ≥ dQ/T; all other second-law formulation follow from this result. The derivation allows reversible processes to be mathematically defined, which in turn, allows irreversibilities and entropy generation to be understood in terms of spatial non-uniformity of temperature distribution.

Effects of Steam Ingestion on Under Fuselage Inlet Performance During a Catapult-Assisted Takeoff Process

A naval aircraft has the potential to experience inlet performance decline when taking off from the carrier deck with the steam-driven catapult assistance. The steam ingested into inlet may cause time-dependent rise and spatial distortion of the total temperature on the inlet–exit, which would decrease the compressor stall margin and then lower the performance of the turbine engine. In this paper, these temporal and spatial temperature nonuniformities are numerically studied using the dual-time-step transient method with a real aircraft/inlet model taken into account. The flowfield characteristics of a designed baseline case are first analyzed, indicating that the engine’s suction effect and the wind velocity relative to the aircraft are two key factors affecting the steam ingestion. The former is dominant at the beginning of takeoff since the aircraft's velocity is low, while the latter is increasingly significant as the aircraft accelerates. Next, parametric studies show that the greater the wind speed is, the less significantly the flowfield of the inlet–exit would be influenced by the steam. The effects are also studied among various steam leakage profiles—two are constant in time histories of the steam leakage rate, whereas the other two are nonlinear with the maximum value at different instants. It is found that the temperature rise rate of the inlet–exit would increase apparently if the steam leakage rate reaches the maximum earlier.

Using two-chamber photoplasma for creating photovoltaic converter

Two-chamber photoplasma is developed for study photovoltaic converters. 2D simulation of conical geometry (similar to one of a solar collector) photoplasma parameters for Na-Ar gas mixture was fulfilled. The present study shows strong spatial non-uniformity of electron density and the electric potential distributions in the two-chamber cell that provides notable value of photo-emf.

River banks and channel axis curvature: Effects on the longitudinal dispersion in alluvial rivers

Abstract The fate and transport of soluble contaminants released in natural streams are strongly dependent on the spatial variations of the flow field and of the bed topography. These variations are essentially related to the presence of the channel banks and to the planform configuration of the channel. Large velocity gradients arise near to the channel banks, where the flow depth decreases to zero. Moreover, single thread alluvial rivers are seldom straight, and usually exhibit meandering planforms and a bed topography that deviates from the plane configuration. Channel axis curvature and movable bed deformations drive secondary helical currents which enhance both cross sectional velocity gradients and transverse mixing, thus crucially influencing longitudinal dispersion. The present contribution sets up a rational framework which, assuming mild sloping banks and taking advantage of the weakly meandering character often exhibited by natural streams, leads to an analytical estimate of the contribution to longitudinal dispersion associated with spatial non-uniformities of the flow field. The resulting relationship stems from a physics-based modeling of the flow in natural rivers, and expresses the bend averaged longitudinal dispersion coefficient as a function of the relevant hydraulic and morphologic parameters. The treatment of the problem is river specific, since it relies on an explicit spatial description, although linearized, of the flow field that establishes in the investigated river. Comparison with field data available from tracer tests supports the robustness of the proposed framework, given also the complexity of the processes that affect dispersion dynamics in real streams.

Experimental and Numerical Investigation of Fuel-Lean H2/CO/Air and H2/CH4/Air Catalytic Microreactors

ABSTRACTThe catalytic combustion of fuel-lean H2/CO/air and H2/CH4/air mixtures (equivalence ratios φ = 0.3–0.5) was investigated experimentally and numerically in a 30 × 30 × 4 mm3 microreactor made of SiC and equipped with six 1.5-mm internal diameter platinum tubes. The goal was to demonstrate high surface temperatures (>1200 K) with good spatial uniformity, for power generation applications in conjunction with thermophotovoltaic devices. Surface temperatures were measured with an infrared camera while exhaust gas compositions were assessed with a micro gas chromatograph. Three-dimensional simulations with detailed hetero-/homogeneous chemistry, conjugate heat transfer in the solid, and external heat losses complemented the measurements. The diverse transport (Lewis number), kinetic (catalytic reactivity), and thermodynamic (volumetric heat release rate) properties of the H2, CO, and CH4 fuels gave rise to rich combustion phenomena. Optimization of the channel flow directions mitigated the high spatial non-uniformities of temperature, which were induced by the low Lewis number of H2. Measured surface temperature distributions had mean values as high as 1261 K, with standard deviations as low as 10.6 K. Syngas or biogas (H2/CO mixtures) yielded lower wall temperatures compared to undiluted H2, even for small volumetric CO:H2 ratios (1:9 and 2:8). Although CO had a high catalytic reactivity when combusting in H2/CO mixtures, its larger than unity Lewis number did not allow for the attainment of high surface temperatures. Mixtures of H2/CH4 (such as fuels produced by natural gas decarbonization) were the least attractive due to the substantially lower catalytic reactivity of CH4.

Development of laser-induced fluorescence to quantify in-cylinder fuel wall films

Laser-induced fluorescence of a fuel tracer is a very sensitive technique to image in-cylinder liquid fuel films, but quantification of the measured film thickness has proven difficult so far. This article describes improvements in the quantification procedure and presents an example application in a motored, optically accessible spark-ignition engine with direct injection. We designed a calibration tool that could be pressurized and heated, allowing investigation of the laser-induced fluorescence intensities at temperatures exceeding the liquid’s standard-pressure boiling point. The fluorescence intensity of liquid toluene and 3-Pentanone dissolved in isooctane upon excitation with a pulsed laser at 266 nm was investigated as a function of temperature and pressure. Consistent with the literature results on gas-phase laser-induced fluorescence, the signal from toluene was much stronger than from 3-Pentanone, about two orders of magnitude for films thinner than 50 μm. Laser-induced fluorescence from both tracers decreased with increasing temperature but that of toluene significantly more. The response to pressure was less pronounced. For imaging across a large field of view, the spatial non-uniformity of laser excitation and detection efficiency was taken into account using a solid fluorescing substrate, an inexpensive Schott-glass WG280 filter. Isooctane with 0.5 vol.% toluene was used for application in the motored engine, imaging the liquid film on the piston-top window after direct injection from a central multi-hole injector. Air as a bulk gas was found to be advantageous over nitrogen in that gas-phase fluorescence was quenched by oxygen. The imaged film distributions and thicknesses and the derived total fuel film mass were physically plausible. Consistent with the recent literature results from a constant pressure vessel, increasing injection pressure from 50 to 100 bar did not decrease wall wetting but further increase to 200 bar did.