Bottom Density Research Articles

Effect of magnesium doping on band gap and optical properties of SrZrO3 perovskite: A first-principles study

In this paper we present the First-principles calculations, established on the density functional theory (DFT) by using generalized gradient approximation (GGA) and ultra-soft pseudo-potential (USP), study to investigate Magnesium (Mg) doping outcome on the different properties of SrZrO3. The impacts on electronic structure and hence optical properties from Mg-doping have been explored and are reviewed in detail by implementing the conception of TDOS and PDOS. Replacement of Strontium (Sr) atom by Mg tunes the electronic band structure quite significantly with the emergence of new states at Gamma point. Due to this SrZrO3 indirect band gap is transformed to the direct one for Mg-doped SrZrO3. The density of states for Mg-doped in SrZrO3 relocate themselves at slightly lower energies and there is strong interaction between Mg-atom and its surrounding atoms is observed. Furthermore, at the conduction band bottom and partial density of states of SrZrO3 change appreciably and hence we conclude that the electronic band structure is affected by Mg-doping. The optical properties of the un-doped and doped SrZrO3 are investigated and are correlated with electronic structure. The enormous variation in optical properties and band gap (i.e indirect to direct) by doping of Mg declares this material as attractive contender for optoelectronic materials.

Infrared Light Management Using a Nanocrystalline Silicon Oxide Interlayer in Monolithic Perovskite/Silicon Heterojunction Tandem Solar Cells with Efficiency above 25%

AbstractPerovskite/silicon tandem solar cells are attractive for their potential for boosting cell efficiency beyond the crystalline silicon (Si) single‐junction limit. However, the relatively large optical refractive index of Si, in comparison to that of transparent conducting oxides and perovskite absorber layers, results in significant reflection losses at the internal junction between the cells in monolithic (two‐terminal) devices. Therefore, light management is crucial to improve photocurrent absorption in the Si bottom cell. Here it is shown that the infrared reflection losses in tandem cells processed on a flat silicon substrate can be significantly reduced by using an optical interlayer consisting of nanocrystalline silicon oxide. It is demonstrated that 110 nm thick interlayers with a refractive index of 2.6 (at 800 nm) result in 1.4 mA cm−² current gain in the silicon bottom cell. Under AM1.5G irradiation, the champion 1 cm2 perovskite/silicon monolithic tandem cell exhibits a top cell + bottom cell total current density of 38.7 mA cm−2 and a certified stabilized power conversion efficiency of 25.2%.

Prediction of water channeling along cement-aquifuge interface in CBM well: Model development and experimental verification

Controlling water channeling (e.g. water inrush) is vital for coalbed methane (CBM) development. In this work, the prediction models for bottom water channeling along cement-aquifuge interface (CAI) in CBM well were deduced to describe the time variation of water channeling at different stages during production. The prediction is focused on the evolution process characteristics of the bottom water channeling along CAI. The models indicate that the water channeling time is related to the differential pressure of both ends of channeling pathway, and the tortuosity of the channeling pathway, the height of bottom water channeling, fracture size of CAI, bottom water density, bottom water viscosity, bottom water surface tension, contact angle, bottom water dissolution time, permeability of mud cake before and after bottom water dissolution, and hole diameter. In order to verify the models, the simulation device was established independently. The experimental devices and methods are constructed to verify the prediction models. The results illustrate that the predicted results of the models are in good agreement with the experimental data. This study will provide a new insight into the solution of water channeling problem, which is common in the exploitation of CBM reservoirs.

Comparison of two Bayesian-point-estimation methods in multiple-source localization

Environmental uncertainty represents the limiting factor in matched-field localization. Within a Bayesian framework, both the environmental parameters, and the source parameters are considered to be unknown variables. However, including environmental parameters in multiple-source localization greatly increases the complexity and computational demands of the inverse problem. In the paper, the closed-form maximumlikelihood expressions for source strengths and noise variance at each frequency allow these parameters to be sampled implicitly, substantially reducing the dimensionality and difficulty of the inversion. This paper compares two Bayesian-point-estimation methods: the maximum a posteriori (MAP) approach and the marginal posterior probability density (PPD) approach to source localization. The MAP approach determines the sources locations by maximizing the PPD over all source and environmental parameters. The marginal PPD approach integrates the PPD over the unknowns to obtain a sequence of marginal probability distribution over source range or depth. Monte Carlo analysis of the two approaches for a test case involving both geoacoustic and water-column uncertainties indicates that: (1) For sensitive parameters such as source range, water depth and water sound speed, the MAP solution is better than the marginal PPD solution. (2) For the less sensitive parameters, such as, bottom sound speed, bottom density, bottom attenuation and water sound speed, when the SNR is low, the marginal PPD solution can better smooth the noise, which leads to better performance than the MAP solution. Since the source range and depth are sensitive parameters, the research shows that the MAP approach provides a slightly more reliable method to locate multiple sources in an unknown environment.

A Data Assimilated Regional Ionosphere Model Using the Total Electron Content from the Korean GPS Network

In this study, we develop a time-dependent three-dimensional regional ionosphere model over the Korean peninsula (32° N - 42° N, 122° E - 132° E, and 100 - 1000 km in altitude) by using data assimilation technique. Slant total electron content (STEC) data from 80 global positioning system (GPS) receiver stations on the Korean peninsula are assimilated into International Reference Ionosphere (IRI) 2016 as a background model. The assimilated results are validated with independent sources of electron density information: 1) GPS STEC not used in the assimilation and 2) f o F2 and bottom side electron density profiles from two ionosondes. Results show that data assimilation estimates the observations with a 3 - 41% improvement over the climatic IRI 2016 model.

Experimental investigation on pool boiling mechanism of two-level gradient metal foams in deionized water, aqueous surfactant solutions and polymeric additive solutions

In the present study, metal skeleton temperature, bubble growth, pool boiling curves, and the solid-liquid static contact angle of two-level gradient foams have been experimentally investigated in deionized water, aqueous surfactant solutions and polymeric additive solutions under atmospheric pressure. The surfactant is Triton X-100 and the polymeric additive is polyvinyl pyrrolidone. A two-level gradient foam sample is made by a uniform copper foam layer, a uniform nickel foam layer and a copper plate with the thickness of 2 mm. The pore densities of uniform foams are 5 PPI, 20 PPI and 40 PPI. The foam porosity is fixed as 0.98. The experimental results show that, foam layer position reversal has a great effect on pool boiling heat transfer of the gradient foam. The pool boiling heat transfer rate of the gradient nickel-copper foam increases with increasing bottom foam pore density. The temperature of corner skeleton on the upper surface of the gradient foam is higher than that of inner skeleton because of the heat flow from escaping bubbles by collision, cutting, stagnation, etc. The metal skeleton temperature is heavily affected by the additives of Triton X-100 and polyvinyl pyrrolidone due to the variations of the solid-liquid contact angles.

Leakage current analysis of polymeric insulators under uniform and non‐uniform pollution conditions

This study presents two different methods under uniform and non-uniform pollution layer in order to measure and calculate the leakage current (LC) of silicone rubber insulators. Experimental test for evaluating the LC analysis of polluted insulator have been done in a laboratory clean fog chamber. The electric field and potential distributions were obtained from finite element method software for 3D models. The mathematical background and circuit theory are described in details by a section of insulator and using the extended form factor formula. The surface conductivity used in the calculations was extracted from the measured LC after wetting rate. LC characteristics under 1 : 1, 1 : 2, 1 : 5 and 1 : 10 ratios of top to bottom surface salt deposit density on polymeric insulators are studied. To verify the proposed models of this study, the results of experimental data and two other approaches are compared with together before dry-band formation. Moreover, a dynamic LC model under uniform pollution layer has been introduced and extended in order to calculate the LC when the formation of dry-bands along the insulator surface occurs. The dynamic model is drawn from experimental data and measured surface conductivity.

Well-posedness analysis and numerical implementation of a linearized two-dimensional bottom sediment transport problem

A two-dimensional linearized model of coastal sediment transport due to the action of waves is studied. Up till now, one-dimensional sediment transport models have been used. The model under study makes allowance for complicated bottom relief, the porosity of the bottom sediment, the size and density of sediment particles, gravity, wave-generated shear stress, and other factors. For the corresponding initial–boundary value problem the uniqueness of a solution is proved, and an a priori estimate for the solution norm is obtained depending on integral estimates of the right-hand side, boundary conditions, and the norm of the initial condition. A conservative difference scheme with weights is constructed that approximates the continuous initial–boundary value problem. Sufficient conditions for the stability of the scheme, which impose constraints on its time step, are given. Numerical experiments for test problems of bottom sediment transport and bottom relief transformation are performed. The numerical results agree with actual physical experiments.

Validation of deep-sea coral and sponge distribution models in the Aleutian Islands, Alaska

Abstract Spatial management of vulnerable benthic ecosystem components such as deep-sea corals and sponges requires adequate maps of their distribution. These maps are often based on statistical models of survey data. The objective of this project was to validate the predictions of existing presence or absence and abundance models of deep-sea corals and sponges in the Aleutian Islands that were based on bottom trawl survey data. Model validation was conducted by comparing bottom trawl survey model predictions to the observations of an in situ camera survey conducted at randomly selected locations. The measures of goodness of fit (area-under-the-receiver-operator-curve, AUC) for the bottom trawl survey model predictions of camera survey observations ranged from 0.59 to 0.77 (for sponges and coral, respectively) and indicated that the bottom trawl survey models predicted the probability of presence for corals accurately across the Aleutian Islands. The bottom trawl survey models explained as little as 3% of the variability in Stylasteridae density and up to 17% of the variability in coral density. These results indicate that models of deep-sea coral distributions based on presence and absence data from bottom trawl surveys can be accurate and can provide useful information for spatial management of these vulnerable taxa. However, for some other taxa, such as sponges, care should be taken interpreting the results of bottom trawl survey models. An interesting finding of this study was that the residuals from the bottom trawl survey model-camera density relationships were negative in areas that remained open to fishing after 2005, possibly indicating an effect of continued bottom trawling on the abundance of corals in these areas. This study highlights the importance of validating models of species distribution using independent surveys, so that the results can be used with confidence to support decision-making processes.

Drop test simulation and surrogate-based optimization of a dishwasher mechanical structure and its packaging module

A drop test simulation of the mechanical structure of a redesigned dishwasher is performed by using a detailed finite element (FE) model. The nonlinear explicit FE code LS-DYNA® is used for the drop impact simulations. The FE model is validated through real tests of two drop scenarios (vertical and inclined to the side). An optimization study is performed in order to determine the optimum design variables for better crash performance. The effects of geometric parameters and material properties on the weights of certain components (ie, dogleg plate and bottom foam) are investigated. A surrogate-based optimization approach is used to find optimum values for the dogleg plate thickness, bottom foam density and increment of the bottom foam height to minimize the weights of both components. Two different surrogate models are used to predict optimization problem constraints that have a crucial role in the crash performance of the dishwasher mechanical structure and packaging module: the polynomial response surface and radial basis functions. The results showed that the dogleg plate mass can be slightly reduced and the bottom foam mass can be significantly reduced in order to obtain the optimum dishwasher configuration and better crashworthiness. The weights of the dogleg plate and bottom foam could be lowered by as much as 5.95 and 24.8 %, respectively. Finally, multi-objective optimization is performed by minimizing a composite objective function that provides a compromise between the weights of both components. The results showed that weight reductions of 2.3 and 21.5 % could be obtained for the dogleg plate and bottom foam, respectively.

Geoacoustic inversion base on modeling ocean-bottom reflection wave

Ocean-bottom parameters surveying experiment was conducted at two sites in Chinese South Sea. A vertical array of four elements was used to measure reflection waves from ocean-bottom with the explosive charges. The ocean-bottom reflection waves are processed by analyzing the arriving time to identify the sediment layers, and then, a geoacoustic model with several numbers of homogeneous fluid sediment layers overlying a homogenous substrate half space is constructed. Using the direct arriving wave, an approach that matches the measured and modeled ocean-bottom reflection wave is to invert the bottom velocity and density profile. A low velocity layer is obviously observed at one site and a complex seabed profile at another site is established. The modeled wave form is compared with the measured one and shows a relative agreement.

Characterizing, modelling and understanding the climate variability of the deep water formation in the North-Western Mediterranean Sea

Observing, modelling and understanding the climate-scale variability of the deep water formation (DWF) in the North-Western Mediterranean Sea remains today very challenging. In this study, we first characterize the interannual variability of this phenomenon by a thorough reanalysis of observations in order to establish reference time series. These quantitative indicators include 31 observed years for the yearly maximum mixed layer depth over the period 1980–2013 and a detailed multi-indicator description of the period 2007–2013. Then a 1980–2013 hindcast simulation is performed with a fully-coupled regional climate system model including the high-resolution representation of the regional atmosphere, ocean, land-surface and rivers. The simulation reproduces quantitatively well the mean behaviour and the large interannual variability of the DWF phenomenon. The model shows convection deeper than 1000 m in 2/3 of the modelled winters, a mean DWF rate equal to 0.35 Sv with maximum values of 1.7 (resp. 1.6) Sv in 2013 (resp. 2005). Using the model results, the winter-integrated buoyancy loss over the Gulf of Lions is identified as the primary driving factor of the DWF interannual variability and explains, alone, around 50 % of its variance. It is itself explained by the occurrence of few stormy days during winter. At daily scale, the Atlantic ridge weather regime is identified as favourable to strong buoyancy losses and therefore DWF, whereas the positive phase of the North Atlantic oscillation is unfavourable. The driving role of the vertical stratification in autumn, a measure of the water column inhibition to mixing, has also been analyzed. Combining both driving factors allows to explain more than 70 % of the interannual variance of the phenomenon and in particular the occurrence of the five strongest convective years of the model (1981, 1999, 2005, 2009, 2013). The model simulates qualitatively well the trends in the deep waters (warming, saltening, increase in the dense water volume, increase in the bottom water density) despite an underestimation of the salinity and density trends. These deep trends come from a heat and salt accumulation during the 1980s and the 1990s in the surface and intermediate layers of the Gulf of Lions before being transferred stepwise towards the deep layers when very convective years occur in 1999 and later. The salinity increase in the near Atlantic Ocean surface layers seems to be the external forcing that finally leads to these deep trends. In the future, our results may allow to better understand the behaviour of the DWF phenomenon in Mediterranean Sea simulations in hindcast, forecast, reanalysis or future climate change scenario modes. The robustness of the obtained results must be however confirmed in multi-model studies.

Variations of the ionospheric parameters and vertical electron density distribution at the northern edge of the EIA from 2010 to 2015 along 95°E and comparison with the IRI-2012

The vertical electron density profiles over Dibrugarh (27.5°N, 95°E, 43° dip) a low mid latitude station normally located at the northern edge of the EIA for the period of July 2010 till October 2015 are constructed from the measured bottom side profiles and ionosonde-GPS TEC assisted Topside Sounder Model (TSM) topside profiles. The bottom side density profiles are obtained by using POLAN on the manually scaled ionograms. The topside is constructed by the modified ionosonde assisted TSM model (TaP-TSM assisted by POLAN) which is integrated with POLAN for the first time. The reconstructed vertical profile is compared with the IRI predicted density profile and the electron density profile obtained from the COSMIC/FORMOSAT radio occultation measurements over Dibrugarh. The bottom side density profiles are fitted to the IRI bottom side function to obtain best-fit bottom side thickness parameter B0 and shape parameter B1. The temporal and solar activity variation of the B-parameters over Dibrugarh are investigated and compared to those predicted by IRI-2012 model with ABT-2009 option. The bottom side thickness parameter B0 predicted by the IRI model is found to be similar to the B0 measured over Dibrugarh in the night time and the forenoon hours. Differences are observed in the early morning and the afternoon period. The IRI doesn’t reproduce the morning collapse of B0 and overestimates the B0 over Dibrugarh in the afternoon period, particularly in summer and equinox. The IRI model predictions are closest to the measured B0 in the winter of low solar activity. The B0 over Dibrugarh is found to increase by about 15% with solar activity during the period of study encompassing almost the first half of solar cycle 24 but solar activity effect was not observed in the B1 parameter. The topside profile obtained from TaP profiler is thicker than the IRI topside in equinox from afternoon to sunrise period but is similar to the IRI in summer daytime. The differences in the bottom side may be attributed to the non-inclusion of ground measurements from 90°E to 100°E longitude in the ABT-2009 model while differences in the topside could be due the non-uniform longitudinal distribution of topside sounder profiles data and the stronger fountain effect in this longitude.

Effects of Non-uniform Pollution on the AC Flashover Performance of Suspension Insulators

The non-uniform distribution of contamination on insulator surface has appreciable effects on flashover voltage, and corresponding researches are valuable for the better selection of outdoor insulation. In this paper, two typical types of porcelain and glass insulators which are widely used in ac lines were taken as the research subjects, and their corrections of AC flashover voltage under nonuniform pollution were studied. Besides, their flashover characteristics under different ratio (T/B) of top to bottom surface salt deposit density (SDD) were investigated, including the analysis of flashover voltage, surface pollution layer conductivity and critical leakage current. Test results gave the modified formulas for predicting flashover voltage of the two samples, which can be directly applied in the transmission line design. Also, the analysis delivered that, the basic reason why the flashover voltage increases with the decrease of T/B, is due to the decrease of equivalent surface conductivity of the whole surface and the decrease of critical leakage current. This research will be of certain value in providing references for outdoor insulation selection, as well as in proposing more information for revealing pollution flashover mechanism.

Understanding flow dynamics and density currents in a river-reservoir system under upstream reservoir releases

ABSTRACTA three-dimensional flow and temperature model was applied for a 124 km river-reservoir system from Lewis Smith Dam tailrace to Bankhead Lock & Dam, Alabama. The model was calibrated against measured water levels, temperatures, velocities and flow rates from 4 May to 3 September 2011 under small constant release (2.83 m3/s) and large intermittent releases (~140 m3/s) from an upstream reservoir. Distributions of simulated flow and temperatures and particle tracking at various locations were analyzed which revealed the complex interactions of density currents, dynamic surface waves and solar heating. Flows in the surface and bottom layers moved in both upstream and downstream directions. If there was small constant release only from Smith Dam, simulated bottom temperatures at Cordova were on average 4.8°C higher than temperatures under actual releases. The momentum generated from large releases pushed bottom density currents downstream, but the released water took several days to reach Cordova.Editor D. Koutsoyiannis; Associate editor B. Dewals

Bottom sediments affect Sonneratia mangrove forests in the prograding Mekong delta, Vietnam

Mangrove forests exert a strong influence on tropical deltas by trapping sediments discharged by rivers and by stabilizing the substrate with roots. Understanding the dynamics of sediments and morphology in and around mangrove forests is critical in order to assess the resilience of coastlines in a period of accelerated sea level rise. In this research, sediment samples, mangrove forest characteristics, and remote sensing data are used to investigate the relationship between mangroves and sediment substrate in the Mekong Delta, Vietnam. Our data show a significant correlation between percent of sand in bottom sediments and density of Sonneratia caseolaris forest. We ascribe this result to higher sediment disturbance in muddy areas that prevents seedling establishment. This correlation potentially allows the determination of substrate characteristics from vegetation attributes detected by remote sensing, despite the impenetrability of the forest canopy. The results presented herein suggest that a supply of sand from the river and hydrodynamic processes moving the sand ashore control the density of the Sonneratia mangrove forests at this location, promoting tidal flat colonization and canopy expansion.

Snow density and ground permittivity retrieved from L-band radiometry: Application to experimental data

Abstract The potential of retrieving the bottom layer snow density and soil permittivity under dry snow cover conditions from L-band passive microwave observations was analyzed using multi-angular brightness temperatures measured at horizontal and vertical polarization over two test sites in northern Finland. The near-continuous time series of L-band brightness temperatures covers a total of six winter seasons, over both dry mineral soil in a forest clearing, and organic soil over a wetland site. Detailed measurements of snow and soil conditions are available from both sites. Complementing a previous theoretical study, we show that dry snow cover influences the observed L-band brightness temperatures as a result of both impedance matching and changes in the refraction angle at the snow–soil interface. Exploiting these effects, we demonstrate the retrieval of the bottom layer snow density and the influence of dry snow cover on simultaneously retrieved soil permittivity — a consideration which is currently not accounted for in Soil Moisture and Ocean Salinity (SMOS) retrievals of soil permittivity in the presence of dry snow. Depending on season, the mean bias error between retrieved and in situ snow density measured in the lower snow layers was between − 6 kg m − 3 and 15 kg m − 3 for the forest clearing site, and between 37 kg m − 3 and 90 kg m − 3 for the wetland site, demonstrating the feasibility of the retrieval approach at the plot scale. In winter conditions, the ground permittivity retrieved without considering the impact of dry snow on L-band emission was, on average, 35% lower for both test sites, which indicates possible errors in current SMOS ground permittivity retrievals under dry snow conditions. The application of SMOS data to simultaneously retrieve dry snow density and ground permittivity is a complex task due to heterogeneous land cover and snow/soil conditions within SMOS pixels (≈ 45 km resolution). An approach that considers land cover variations and the spatial variability of snow cover is required to fully determine the feasibility of the methodology to aid e.g. improving estimates snow water equivalent from other sensors, and to take into account effects of dry snow in SMOS-based retrievals of ground permittivities. The results should also be applicable to other L-band sensors in space, such as the recently launched NASA Soil Moisture Active Passive (SMAP) mission.

Geoacoustic Inversion for Bottom Parameters in the Deep-Water Area of the South China Sea**Supported by the National Natural Science Foundation of China under Grant Nos 11434012, 41561144006, 11174312 and 11404366.

Bottom acoustic parameters play an important role in sound field prediction. Acoustic parameters in deep water are not well understood. Bottom acoustic parameters are sensitive to the transmission-loss (TL) data in the shadow zone of deep water. We propose a multiple-step TL inversion method to invert sound speed, density and attenuation in deep water. Based on a uniform liquid half-space bottom model, sound speed of the bottom is inverted by using the long range TL at low frequency obtained in an acoustic propagation experiment conducted in the South China Sea (SCS) in summer 2014. Meanwhile, bottom density is estimated combining with the Hamilton sediment empirical relationship. Attenuation coefficients at different frequencies are then estimated from the TL data in the shadow zones by using the known sound speed and density as a constraint condition. The nonlinear relationship between attenuation coefficient and frequency is given in the end. The inverted bottom parameters can be used to forecast the transmission loss in the deep water area of SCS very well.

Expansion vessel for supercritical hydrogen in a spallation neutron source moderator circuit

High-energy neutrons are being decelerated by passing through supercritical parahydrogen circulated by pumps in a closed loop. Fluctuations in neutron heat load cause changes of the circuits’ local and average temperature and hence significant pressure variations caused by the almost incompressible behavior of hydrogen. Solutions by adding a variable volume in form of a helium gas-backed metal bellow to mitigate pressure deviations are already in use. This paper presents an alternative approach by introducing a vertical storage vessel for supercritical hydrogen in a side branch of the moderator loop, with cold incompressible high density hydrogen at the bottom and warmer compressible lower density hydrogen at the top.

I-V CHARACTERISTICS AND TRANSCONDUCTANCE MODELING FOR DUAL CHANNEL ALGAN/GAN MODFETS

In this paper we present 2-D analytical model for I-V characteristics and transconductance of dual channel AlGaN/GaN Modulation Doped Field Effect Transistor (DC-MODFET) to demonstrate the current-voltage as well as transfer characteristics of the device structure under different bias conditions. The model analyses sheet charge density and finaly current density in each of the top and bottom channels using effective device threshold expressions based on spontaneous and piezoelectric polarization dependent two dimensional electron gas (2-DEG). An aluminum mole fraction graded doping profile in bottom barrier layer has been used for bottom channel sheet carrier density. Trapping/detrapping surface states are also considered to determine device drive current and trans-conductance expressions. The presence of double channels in the device minimize the current collapse problems, is accurately explained in our proposed model. The current collapse that occurs more in the top channel nearer to the gate, is subsidized by the current in bottom channel, hence minimizing overall current collapse in the device. Finally, the numerical predictions from our model are compared with the published experimental results and found to be in close agreement.