Non-uniform Rate Research Articles

Increased growth rate (1982-2013) in global grasslands biomes

ABSTRACTAcross the Earth’s surface, climate change has been occurring at a non-uniform rate, affecting biomes and directly or indirectly imposing impacts on societies. The productivity of the grassland biome has also been inevitably influenced, which in turn might affect the livestock production. Here, a meaningful indicator – growth rate index (GRI) – is proposed based on plant phenology and quantified from long-term remote sensing data to understand responses of the grassland biome to climate change at a global scale. We found a steady increase in GRI for the global grassland biome from 1982 through 2013, with around 35% of global grasslands exhibiting a significant increase (p < 0.1), and 3.8% of global grasslands experiencing a decrease. Climate warming over the past three decades did not aggravate water stress on worldwide grassland biomes, especially for the typical steppes in the Northern Hemisphere where GRI appeared more sensitive to the variations in precipitation compared to the savanna biome in the Southern Hemisphere. This study suggests that the increases in grassland GRI may have the potential to elevate global livestock production.

Effect of Prandtl number on the average exit temperature of coolant in a heat‐generating vertical parallel plate channel: A conjugate analysis

AbstractA coupled conduction–convection heat transfer analysis is carried out for a two‐dimensional rectangular, vertical parallel plate channel producing volumetric energy (uniform and nonuniform), subjected to laminar forced‐convection incompressible fluid flow under steady‐state conditions. The equations governing the thermal and flow field are solved by using finite difference method, and the resulting algebraic equations are solved by using the tridiagonal matrix algorithm method. Four coolants with their Prandtl numbers (Pr), namely, liquid sodium (Pr = 0.005), sodium‐potassium (Pr = 0.00753), lead (Pr = 0.02252), and helium (Pr = 0.666) are used for the present conjugate analysis. Effects of different thermal and fluid flow parameters such as Reynolds number (ReH) ranging from 500 to 1500, conduction–convection parameter (Ncc), and total heat generation (Qt) on average exit temperature (θae) of coolants are studied. From the obtained results, it is found that the θae of coolant strongly depends on Pr, ReH, Ncc, and Qt when the aspect ratio (Ar) is kept constant. It is also found that with a nonuniform rate of heat generation, the coolant θae is high compared to uniform heat generation rate, whereas with increasing Ncc, the θae decreases, and with increasing Qt, the θae increases irrespective of coolants.

Modified slanted-edge method for camera modulation transfer function measurement using nonuniform fast Fourier transform technique

ISO 12233 slanted-edge method experiences errors using fast Fourier transform (FFT) in the camera modulation transfer function (MTF) measurement due to tilt angle errors in the knife-edge resulting in nonuniform sampling of the edge spread function (ESF). In order to resolve this problem, a modified slanted-edge method using nonuniform fast Fourier transform (NUFFT) for camera MTF measurement is proposed. Theoretical simulations for images with noise at a different nonuniform sampling rate of ESF are performed using the proposed modified slanted-edge method. It is shown that the proposed method successfully eliminates the error due to the nonuniform sampling of the ESF. An experimental setup for camera MTF measurement is established to verify the accuracy of the proposed method. The experiment results show that under different nonuniform sampling rates of ESF, the proposed modified slanted-edge method has improved accuracy for the camera MTF measurement compared to the ISO 12233 slanted-edge method.

Laser direct metal deposition process of thin-walled parts using variable spot by inside-beam powder feeding

PurposeThe purpose of this paper is to improve the forming efficiency and quality of unequal-width parts fabricated by laser direct metal deposition technology, some experiments were designed.Design/methodology/approachA new method by varying laser spot was adopted to fabricate unequal-width single track using one scanning rather than multi-track overlapping in the way of the inside-beam powder feeding, and the thin-walled parts were fabricated layer by layer. The theoretical model among layer thickness of z-axis, height of single track and the section curve order of single track was established.FindingsThe top surface unevenness of the thin-walled parts could be compensated automatically within the laser defocusing ranges from −2.5 to −5 mm and from 0.5 to 2.5 mm. The growth rate with the large width/height ratio was more than the small ratio, while the set height of the single track was uniform. The problem of non-uniform growth rate could be solved based on a stepped single-track method. The thin-walled parts with the smooth top surface was fabricated layer by layer which had a continuously variable width from 1 to 3 mm by splicing the laser defocusing range.Practical implicationsThe shapes of the to-be-fabricated parts affect variable laser spot process in practical applications. For example, it will be difficult to apply variable laser spot process on the parts with the hole features.Originality/valueThis paper provided a guidance for forming unequal-width parts by laser direct metal deposition based on the inside-beam powder feeding.

Optimal design of current collectors for microfluidic fuel cell with flow-through porous electrodes: Model and experiment

Design optimization of current collectors has been performed to reduce the significant ohmic resistance observed in microfluidic fuel cell (MFC) with flow-through porous electrodes. A three-dimensional computational model is developed to investigate the electron transport characteristics in the porous electrodes, where lateral electron transport is found to encounter high resistance. Influences of different current collector design parameters on the transport resistances are examined and analyzed. The modeling results indicate that current collector position is the most influential factor due to the non-uniform flow rate distribution. Optimal current collector position is located at the high flow rate region instead of the conventional exposed end of the porous electrode. Experimental studies are performed to support the modeling analysis. The experimental results demonstrate that the optimized current collector position can boost the maximum power density by 61%. This study highlights the significance of the current collector design in achieving high performance MFC with flow-through porous electrodes. Based on the results, some general rules have been set for the current collector designs in this energy system, which can provide useful guidance for the future development of MFC.

Geometric disequilibrium of river basins produces long-lived transient landscapes

Although equilibrium has long been considered the attractor state for landscapes, the time required to reach equilibrium or even the possibility of reaching equilibrium is still debated. Using 10Be-based catchment-averaged denudation rates, topographic analysis, and analysis of the basin topology and geometry, including its area-channel length scaling relationship, we show that an ancient postorogenic dome on the North American Craton, the Ozark dome, is not in a state of equilibrium. The persistent state of disequilibrium on the Ozark dome is characterized by nonuniform erosion rates that vary by a factor of three, asymmetric drainage divides, and evidence for drainage rearrangement via stream capture. We find that planform geometric disequilibrium of river basins and drainage area exchange between adjoining basins can hold river networks in a disequilibrium state for potentially hundreds of million years and that, when sustained over time, erosion rate differences associated with drainage area exchange can lead to transient events such as stream capture and production of relief in the form of elevated, low-relief surfaces. Our results suggest that landscapes with slowly moving drainage divides might not reach equilibrium, and that river basin dynamics may contribute to setting the large-scale morphology of old cratonic landscapes.

Vapor-Induced Motion of Liquid Droplets on an Inert Substrate.

Evaporating droplets are known to show complex motion that has conventionally been explained by the Marangoni effect (flow induced by the gradient of surface tension). Here, we show that the droplet motion can be induced even in the absence of the Marangoni effect due to the gradient of the evaporation rate. We derive an equation for the velocity of a droplet subject to the nonuniform evaporation rate and nonuniform surface tension placed on an inert substrate, where the wettability is uniform and unchanged. The equation explains the previously observed attraction-repulsion-chasing behaviors of evaporating droplets.

Numerical investigation of entropy generation to predict irreversibilities in nanofluid flow within a microchannel: Effects of Brownian diffusion, shear rate and viscosity gradient

In the present contribution, irreversibilities caused by heat transfer and friction for the water-TiO2 nanofluid flow in a circular microchannel are investigated by evaluating entropy generation rates. The effects of viscosity gradient, non-uniform shear rate and Brownian diffusion on particle migration are taken into account in order to examine the effect of nanoparticle arrangement on entropy generation rates. The results show that nanoparticle migration alters concentration distribution and consequently, changes entropy generation rates. Nanoparticle migration increases concentration of the particles in central regions, and this migration is more noticeable for higher mean concentrations and larger particles. Thermal entropy generation rate intensifies with increasing wall heat flux and particle size while decreases with increasing concentration. Frictional entropy generation rate increases by concentration increment and decreases by particles enlargement, while it changes trivially by increasing wall heat flux. Frictional entropy generation rate is larger than thermal entropy generation rate in the microchannel under study and therefore, total entropy generation mostly stems from friction. Thus, total entropy generation rate decreases by particles enlargement, which is a positive result according to second law of thermodynamics. Eventually, a model for entropy generation rates is developed using the numerical data by means of Artificial Neural Network (ANN).

Influence of nanoparticle concentrations on flow boiling heat transfer coefficients of Al 2 O 3 /R141b in micro heat exchanger by direct metal laser sintering

Abstract Al2O3/R141b + Span-80 nanorefrigerant for 0.05 wt.% to 0.4 wt.% is prepared by ultrasonic vibration to investigate the influence of nanoparticle concentrations on flow boiling heat transfer of Al2O3/R141b + Span-80 in micro heat exchanger by direct metal laser sintering. Experimental results show that nanoparticle concentrations have significantly impact on heat transfer coefficients by homogeneity test of variances according to mathematical statistics. The heat transfer performance of Al2O3/R141b + Span-80 nanorefrigerant is enhanced after adding nanoparticles in the pure refrigerant R141b. The heat transfer coefficients of 0.05 wt.%, 0.1 wt.%, 0.2 wt.%, 0.3 wt.% and 0.4 wt.% Al2O3/R141b + Span-80 nanorefrigerant respectively increase by 55.0%, 72.0%, 53.0%, 42.3% and 39.9% compared with the pure refrigerant R141b. The particle fluxes from viscosity gradient, non-uniform shear rate and Brownian motion cause particles to migrate in fluid especially in the process of flow boiling. This migration motion enhances heat transfer between nanoparticles and fluid. Therefore, the heat transfer performance of nanofluid is enhanced. It is important to note that the heat transfer coefficients nonlinearly increase with nanoparticle concentrations increasing. The heat transfer coefficients reach its maximum value at the mass concentration of 0.1% and then it decreases slightly. There exists an optimal mass concentration corresponding to the best heat transfer enhancement. The reason for the above phenomenon is attributed to nanoparticles deposition on the minichannel wall by Scanning Electron Microscopy observation. The channel surface wettability increases during the flow boiling experiment in the mass concentration range from 0.2 wt.% to 0.4 wt.%. The channel surface with wettability increasing needs more energy to produce a bubble. Therefore, the heat transfer coefficients decrease with nanoparticle concentrations in the range from 0.2 wt.% to 0.4 wt.%. In addition, a new correlation has been proposed by fitting the experimental data considering the influence of mass concentrations on the heat transfer performance. The new correlation can effectively predict the heat transfer coefficient.

Design Principles of Current Collectors in Microfluidic Fuel Cell with Flow-through Porous Electrodes

Abstract Computational and experimental studies have been performed to investigate the effects of current collector design on the performance of microfluidic fuel cell (MFC) with flow-through porous electrodes. Characteristics of electron transport in MFC with flow-through porous electrodes are investigated based on a three-dimensional computational model. The lateral electron transport in the porous electrode is found to encounter high resistance. To improve the cell performance, influences of different current collector design parameters on the transport resistances are examined. Physical origins for the influences of different design parameters are also discussed. The results demonstrate that current collector position is the most influential factor due to the non-uniform flow rate distribution. In the experimental study, cell performances revealed maximum power density when current collectors were located at the high flow rate region. An increase of 61% was observed when the current collectors moved from the conventional exposed ends of the electrodes to the high flow rate regions in the electrode active area. Based on the results, some general rules are set for the current collector designs of MFCs.

Performance of Non-Uniform Duty-Cycled ContikiMAC in Wireless Sensor Networks

&lt;span lang="EN-GB"&gt;Wireless Sensor Network (WSN) is a promising technology in Internet of Things (IoTs) because it can be implemented in many applications. However, a main drawback of WSN is it has limited energy because each sensor node is powered using batteries. Therefore, duty-cycle mechanisms are introduced to reduce power consumption of the sensor nodes by ensuring the sensor nodes in the sleep mode almost of the time in order to prolong the network lifetime. One of the de-facto standard of duty-cycle mechanism in WSN is ContikiMAC, which is the default duty-cycle mechanism in Contiki OS. ContikiMAC ensures nodes can participate in network communication yet keep it in sleep mode for roughly 99\% of the time. However, it is found that the ContikiMAC does not perform well in dynamic network conditions. In a bursty network, ContikiMAC provides a poor performance in term of packet delivery ratio, which is caused by congestion. One possible solution is ContikiMAC should increase its duty-cycle rate in order to support the bursty traffic. This solution creates a non-uniform duty-cycle rates among the sensor nodes in the network. This work aims to investigate the effect of non-uniform duty-cycle rates on the performance on ContikiMAC. Cooja simulator is selected as the simulation tool. Three different simulation scenarios are considered depending on the Clear Channel Assessment Rate (CCR) configurations: a low uniform CCR value (Low-CCR), a high uniform CCR value (High-CCR) and non-uniform CCR values (Non-uniform-CCR). The simulation results show that the Low-CCR scenario provides the worst performance of PDR. On the other hand, the High-CCR scenario provides the best performance of PDR. The Non-uniform-CCR provides PDR in between of Low-CCR and High-CCR.&lt;/span&gt;

Magnetic excursions in the late Matuyama Chron (Olduvai to Matuyama‐Brunhes boundary) from North Atlantic IODP sites

AbstractThe sedimentary record of late Matuyama magnetic excursions, between the end of the Olduvai Subchron and the Matuyama‐Brunhes boundary, has relied on North Atlantic Ocean Drilling Program (ODP) Sites 983/984, and individual recordings at ODP Site 1021 from the California Margin, and Core MD972143 from the west Philippine Sea. North Atlantic IODP Sites U1305, U1306, U1307 (Eirik Drift, off SE Greenland), and Site U1308 (central Atlantic) provide new records from u‐channel samples and discrete (8 cm3) cubic samples, collected from multiple holes drilled at each site. The results confirm the existence of the Intra‐Jaramillo, Punaruu, Bjorn, Gardar, and Gilsa excursions and possibly the Kamikatsura excursion. These excursions are not recorded at all sampled sites, although they do correspond to excursions recorded at ODP Sites 983/984 and to relative paleointensity minima recorded at all studied sites. For some records, including that of the Bjorn excursion (1251 ka), antipodal magnetization directions yield virtual geomagnetic poles that reach high northern latitudes, implying that the excursion involved reversal of the main axial dipole field. On the other hand, excursions are often partially and inadequately recorded, attributable to a combination of the brevity of geomagnetic excursions, bioturbation, and filtering of the magnetization signal by the detrital remanent magnetization acquisition process, nonuniform sedimentation rates at millennial/centennial scales, and possibly unrecognized drilling/sampling disturbance.

ROTATION PERIODS OF WIDE BINARIES IN THE KEPLER FIELD

ABSTRACT In a search of proper motion catalogs for common proper motion stars in the field of the Kepler spacecraft I identified 93 likely binary systems. A comparison of their rotation periods is a test of the gyrochronology concept. To find their periods I calculated the autocorrelation function (ACF) of the Kepler mission photometry for each star. In most systems for which good periods can be found, the cooler star has a longer period than the hotter component, in general agreement with models. However, there is a wide range in the gradients of lines connecting binary pairs in a period–color diagram. Furthermore, near the solar color, only a few stars have longer periods than the Sun, suggesting that they, and their cooler companions, are not much older than the Sun. In addition, there is an apparent gap at intermediate periods in the period distribution of the late K and early M stars. Either star formation in this direction has been variable, or stars evolve in period at a non-uniform rate, or some stars evolve more rapidly than others at the same mass. Finally, using the ACF as a measure of the activity level, I found that while the F, G, and early K stars become less active as their periods increase, there is no correlation between period and activity for the mid K to early M stars.

Multiscale Structure Simulation Using Adaptive Mesh in DGTD Method

This paper presents an adaptive mesh methodology to construct simulation source which has unlimited spectrum coverage for the discontinuous Galerkin time-domain (DGTD) method. The limitation of grid size (i.e., λ/20) for the construction of time-domain signal is relaxed by using a nonuniform sampling technique. The excitation source is modeled by using a standard gauss pulse with a nonuniform sampling rate. The modeling is tested for broadband spectrum coverage of up to hypothetical frequencies (e.g., X-ray: $\text{10}^{\text{18}}$ Hz and Gamma-ray: $\text{10}^{\text{20}}$ Hz). The problem of memory for dealing with such a high frequency during fast Fourier transform (FFT) analysis is resolved by reducing the number of points used for calculating the FFT. Then, multiscale structures composed of contrasting thicknesses (small thickness and ultrasmall thickness with wide thickness) are analyzed for the input reflection coefficients up to 10 GHz. The improvement in computational resources of the adaptive mesh techniques in terms of execution time is compared with that of the uniform mesh technique. The adaptive mesh technique uses an optimal number of finite elements for the discretization of the computational domain, hence, saving plenty of time while finding the numerical solutions. The whole analysis is performed by using the DGTD method in one dimension and the truncation of the boundary for the normally incident wave is achieved by employing the first-order Silver-Muller absorbing boundary condition (SM-ABC).

The stability of serpentization due to the water flow in a kimberlite pipe

Abstract A linear analysis of the stability of the course of serpentization, that is, of the exothermic hydration reaction, due to the flow of water in a kimberlite pipe is carried out, taking both the heat conduction and the convective heat transfer by the fluid saturating the pipe rocks into account. It is shown that two different serpentization processes exist: a homogeneous process and an inhomogeneous process associated with a loss of stability by the homogeneous process and a non-uniform reaction rate distribution. Dimensionless similarity parameters that determine the course of the reaction are proposed. It is shown that convective heat transfer promotes a stabilization of the flow and the formation of a homogeneous serpentinite distribution. Other conditions being equal, an increase in the convective heat flux leads to an increase in the wavelengths of the unstable perturbations and to a decrease in their amplitude. A critical value of the flow rate exists, and, when this is exceeded, instability does not develop and serpentinization takes place under homogeneous conditions.

3D numerical analysis of influence of the non-uniform deposition rate on the hillock density at HVPE-GaN surface

In this study, hydride vapor phase epitaxy (HVPE), one of the feasible methods to produce the GaN thin film, has been used to conduct experiments under different temperatures. In order to study the factors affecting the distribution of the density of the hillocks along radial direction, we have conducted a 3D calculation to observe the fluid flow, mass transfer and deposition rate distribution using the CFD-ACE program. The numerical results have shown that the wafers have experienced high and low growth rate alternately. The growth rate fluctuations at different distances from the inlets are compared by standard deviation analysis. These standard deviations of deposition rates along the azimuthal direction increase from the center to the periphery, which might explain why the density of the hillocks increases from the center to the periphery in the experiments. Moreover, it is found that the non-uniform deposition rates are the result of low speed rotation of the susceptor. Increasing the rotation speed of the susceptor increases the uniformity of the gas flow pattern and deposition rate, which means that the high rotation speed can decrease the standard deviation of the deposition rate along azimuthal direction. Consequently, the density of the hillocks can be decreased.

Impact of thermophoresis on nanoparticle distribution in nanofluids

This research attempts to study the effects of particle migration on concentration distribution of the water-TiO2 nanofluid inside a circular tube. The scale analysis shows that thermophoresis can have an essential role on particle migration and consequently, on concentration distribution. Therefore, the concentration distribution of particles is obtained by considering the effects of thermophoresis, non-uniform shear rate, Brownian diffusion, and viscosity gradient. The results reveal that as the particles become larger, the concentration distribution becomes more non-uniform. Meanwhile, thermophoresis intensifies non-uniformity of concentration distribution and its effect is more noticeable at higher mean concentrations.

Evaluation of some effective parameters in performance of a helical two-sided manure distributor

Organic manures play an important role in improvement of soil fertility, increasing of crop yield and quality. The beneficial advantages of manure depend firstly on the method of spreading. Usually current manure spreaders cannot broadcast manures with different moisture contents and have non-uniform application rate. So the main objective of this research is performance evaluation of a new helical two-sided manure distributor that would be capable to spread manure at different moisture contents with long distribution width in uniform patterns. Experimental evaluation of the machine performance at two moisture content of manure in three forward velocities and three rotational speeds of drum showed that the moisture content of manure and rotational speed of drums had significantly effects (p = 0.01) on the distribution width and uniformity of the distribution pattern. The maximum distribution width of the machine at moisture content of 23% w.b. and 54% w.b. and in 1000 rpm rotational speed of drum were 19.3 m and 23.3 m respectively. Results indicated that the manure distribution uniformity increased by increasing the manure moisture and the distribution pattern of machine has an acceptable correlation with oval distribution pattern (p = 0.05).

Second-gradient modelling of orientation development and rheology of dilute confined suspensions

We address the extension of Jeffery’s model, governing the orientation of rods immersed in a Newtonian fluid, to confined regimes occurring when the thickness of the flow domain is narrower than the rod length. The main modelling ingredients concern: (i) the consideration of the rod interactions with one or both gap walls and their effects on the rod orientation kinematics; and (ii) the consideration of non-uniform strain rates at the scale of the rod, requiring higher-order descriptions. Such scenarios are very close to those encountered in real composites forming processes and have never been appropriately addressed from a microstructural point of view. We also show that confinement conditions affect the rheology of the suspension.

Un microsimulador para México (Mextax) y su aplicación a las reformas impositivas de 2010

We develop a tax micro-simulator model (MEXTAX) that can quantify the revenue and distributional impact of tax reforms in Mexico using micro-level data. We use MEXTAX to assess revenue-raising reforms to Mexico’s direct and indirect tax systems of 2010. Initial proposals by the Executive Power included the introduction of a uniform expenditure tax covering traditionally untaxed necessities (such as food). The reform approved by the Congress replaced this with an increase in the standard (non-uniform) rate of VAT, to avoid regressive impacts. Both reform packages included other minor changes to income tax and excise duties. We argue that given that indirect taxes were changed the most in both reforms, expenditure should be used to measured living standards and proportional progressivity. We find that both the reform package proposed and the reform package approved are progressive if expenditure is used as a measure of living standards, although this is not the case for the proposed reform if income is used. However, the proposed reform would have raised more revenues than the approved reform and we find that these foregone revenues due to the amendments could have been used to target poorer households more effectively using more direct instruments for redistribution. We also find that using alternative assumptions about missing income or labor supply response affect quantitatively, but not qualitatively, results. The model can be extended to incorporate further behavioral margins and to other countries with similar tax structures.