Phonon modes and topological phonon properties in (GaN)x/(AlN)x and (AlGaN)x/(GaN)x superlattices

To effectively regulate thermal transport for the near-junction thermal management of GaN electronics, it is imperative to gain an understanding of the phonon characteristics of GaN nanostructures, particularly the topological phonon properties connected to low-dissipation surface phonon states. In this work, a comprehensive study on phonon modes and topological phonon properties is performed from first principles in (GaN)x/(AlN)x and (AlGaN)x/(GaN)x (x=1,2,3) superlattices. Phonon modes, including the dispersion relation, density of states, and participation ratio, were calculated for six GaN superlattices. The participation ratio results did not reveal the localized phonon mode. In topological phonon analyses, it is found that Weyl phonons with a Chern number of 1(-1) are present in all six GaN superlattices, consisting of trivial (GaN) and nontrivial (AlN and AlGaN) combinations. These phonons are located on either side of the kz = 0 plane symmetrically in the Brillouin zone. With the increase in the number of phonon branches in superlattices, the number of Weyl phonon points also increases from dozens to hundreds. One Weyl phonon with significant and clean surface states is selected and analyzed for each GaN superlattice. Among them, the Weyl phonon in (GaN)2/(AlN)2 superlattice mainly results from the lattice vibrations of Al and Ga atoms, while the Weyl phonons in other superlattices mainly result from the lattice vibrations of N atoms. The Weyl phonons at opposite kz planes form pairs in (GaN)2/(AlN)2, AlGaN/GaN, and (AlGaN)2/(GaN)2. Effects of strain including biaxial and uniaxial strain on Weyl phonons in GaN/AlN and AlGaN/GaN superlattices are investigated. Results indicate that Weyl phonons persist in large strain states, however, no monoclinic trend is observed due to the accidental degeneracy of these superlattices.Comment: 44 pages,15 figures,accepted by Physica Script

Investigation and utilization of lactic acid bacteria for cider maturation processes.

Investigation and utilization of Lactic Acid Bacteria for cider maturation processes Cider maturation involves a number of transformations mediated by Lactic Acid Bacteria (LAB). A comparison of two heterofermentative lactic acid bacteria (Oenococcus oeni 11648 and Lactobacillus brevis X2), capable of malolactic fermentation, was made to assess the relative merits of these two organisms in cider maturation processes. Initial work studied LAB propagation on safe (free of animal extracts) simple media. Optimum growth conditions were determined. The two strains were shown to be highly adapted to the cider environment being able to tolerate ethanol, up to 10% (v/v). O. oeni was shown to be more tolerant to ethanol and low pH condition compared to L. brevis. The growth of both strains was significantly stimulated by low levels of glucose (~ 1mM) in the presence of organic acids (5g/l malate) such that the specific growth rate and cell yield were stimulated by 167% and 260% respectively. The growth of the organisms was studied in batch, continuous and membrane bioreactor (MBR) systems with pH control. The highest maximum specific growth rates of O. oeni with 0.066h-1 was obtained at pH 4.5 when the mixture of 5g/l glucose-5g/l fructose was added to the culture medium, while L. brevis with 0.110h-1 was obtained at pH 5.5 in the 10g/1 glucose medium. Both strains were grown in continuous cultures at a variety of dilution rates and the precise growth parameters were determined and these data show that L. brevis gave higher biomass yield (Yx/smax), faster volumetric cell productivity (Qxmax) higher ATP yield (YATPmax) than O. oeni. In the MBR culture system, the growth of two LAB showed that high cell concentrations were possible, O. oeni cultures to 12 g/1 of dry weight, this is over 32 times higher than batch (0.357 g/1) and continuous culture (0.37 g/1). The volumetric productivities of the three systems were 7.9 mg/l/h, 17.6 mg/l/h and 180mg/l/h in batch, continuous culture and MBR respectively. Similar results were obtained for the growth of L. brevis. Having produced high cell concentrations of O. oeni and L. brevis in the MBR, the system was employed for rapid, continuous malolactic transformation of synthetic green cider medium. The levels of malate removed were a function of the residence time and the alcohol concentration. Starting with 2 g/1 malate, over 85% could be removed using a 6 hour residence time and compared very favourably with conventional batch fermentations that take typically 20 - 40 days. The MBR system was run successfully for a period of over 9 days and with correct management could be run for much longer periods. In these conditions, O. oeni showed greater stability and viability than L. brevis in cider environment as it is more alcohol and acid tolerant. These studies provide a basis for developing strategies for rapid maturation of cider using an MBR system. Future work for further optimization of the MBR operation with real cider is discussed. The economic viability of MBR in cider maturation needs to be assessed

Offline Diagnostics of Skin Sea Surface Temperature from a Prognostic Scheme and Its Application in Typhoon Forecasting Using the CMA-TRAMS Model over South China

In the Tropical Regional Atmospherical Model System of South China of the China Meteorological Administration (CMA-TRAMS), the skin sea surface temperature (Ts) remains fixed during the forecast time. This limits the model’s performance in describing interactions between air and sea. The offline diagnostics and online analysis coupled with the CMA-TRAMS of Ts prognostic scheme were discussed. The results of the offline diagnostics showed that the profile shape parameter, ν, and initial temperature, Tb, were sensitive to the forecasted Ts. Based on our observations, when ν was set to 0.2 and Tb was the averaged Ts without obvious diurnal variation, the forecasted Ts was relatively reasonable. The forecasted Ts of CMA-TRAMS after coupling with the Ts scheme had diurnal variations during the overall forecast time, which was different from the fixed Ts from the uncoupled model. There existed a certain difference of forecasted Ts between uncoupled and coupled models in those days influenced by typhoons. The biases and Root Mean Square Errors (RMSEs) for the temperature and moisture in the lower layer and those for the wind speed in most layers were reduced and, therefore, the accuracy of environmental field forecasting was improved from the coupled model. The typhoon track errors after 36-h decreased due to the improvement of steering flow on the west side of subtropical high from the coupled model. However, the difference of typhoon intensity errors was insignificant, which might mean that the differences of forecasted Ts and heat flux between uncoupled and coupled model are small. The reasons for the small difference need to be further investigated

Characteristics of Spring Sea Surface Currents near the Pearl River Estuary Observed by a Three-Station High-Frequency Surface Wave Radar System

The processes of ocean dynamics are complex near the Pearl River Estuary and are not clear due to a lack of abundant observations. The spatial characteristics of the spring sea surface currents in the adjacent waters of the Pearl River Estuary were analyzed using the current data observed by a three-station high-frequency surface wave radar system (HFSWRS). Compared with the two-station HFSWRS, the deviation of current velocity and direction observed by the three-station HFSWRS from the underway measurements decreased by 42.86% and 38.30%, respectively. The analyzed results show that the M2 tidal current is the dominant current among all the tidal constituents, followed by K1, with angles of inclination ranging from 130° to 150°. The tidal flow is dominated by northwest–southeast back-and-forth flow. In the southern part of the observed area, which is far from the coastline, the tidal current ellipses exhibit a circular pattern. The prevalent tidal current type in this region is irregularly semi-diurnal, and the shallow water constituents also have a significant effect. The tidal energy in the adjacent waters of the Pearl River Estuary is affected by potential energy flux and kinetic energy flux. As the water depth and currents velocity increase in the southeast direction, the tidal energy flux increases. In the nearshore zone, the direction of tidal energy flux varies along the coastline. The changes in the residual current within the observed area are correlated with the sea surface wind field. Based on the high-precision sea surface current observed by the three-station HFSWRS, the characteristics of the ocean dynamic processes near the Pearl River Estuary are analyzed comprehensively, which provides important reference and confidence for the application of the developing new radar observing network with about 10 radar stations near the Pearl River Estuary

The greater roles of indigenous microorganisms in removing nitrobenzene from sediment compared with the exogenous Phragmites australis and strain JS45

The feasibility of using Phragmites australis-JS45 system in removing nitrobenzene from sediments was conducted. However, it was observed that nitrobenzene degraded rapidly and was removed completely within 20 days in native sediments, raising the possibility that indigenous microorganisms may play important roles in nitrobenzene degradation. Consequently, this study aimed to verify this possibility and investigate the potential nitrobenzene degraders among indigenous microorganisms in sediments. The abundance of inoculated strain JS45 and indigenous bacteria in sediments was quantified using real-time polymerase chain reaction. Furthermore, community structure of the indigenous bacteria was analyzed through high throughput sequencing based on Illumina MiSeq platform. The results showed that indigenous bacteria in native sediments were abundant, approximately 10(14) CFU/g dry weight, which is about six orders of magnitude higher than that in fertile soils. In addition, the levels of indigenous Proteobacteria (Acinetobacter, Comamonadaceae_uncultured, Pseudomonas, and Thauera) and Firmicutes (Clostridium, Sporacetigenium, Fusibacter, Youngiibacter, and Trichococcus) increased significantly during nitrobenzene removal. Their quantities sharply decreased after nitrobenzene was removed completely, except for Pseudomonas and Thauera. Based on the results, it can be concluded that indigenous microorganisms including Proteobacteria and Firmicutes can have great potential for removing nitrobenzene from sediments. Although P. australis-JS45 system was set up in an attempt to eliminate nitrobenzene from sediments, and the system did not meet the expectation. The findings still provide valuable information on enhancing nitrobenzene removal by optimizing the sediment conditions for better growth of indigenous Proteobacteria and Firmicutes. (C) Higher Education Press and Springer-Verlag GmbH Germany 2018

Estimation of the Manning’s n coefficient in multi-constituent tidal models by assimilating satellite observations with the adjoint data assimilation

The bottom friction is critical for the dissipation of the global tidal energy. The bottom friction coefficient is traditionally determined using the Manning’s n formulation in tidal models. The Manning’s n coefficient in the Manning’s n formulation is vital for the accurate simulation and prediction of the tide in coastal shallow waters, but it cannot be directly measured and contains large amounts of uncertainties. Based on a two-dimensional multi-constituent tidal model with the adjoint data assimilation, the estimation of the Manning’s n coefficient is investigated by assimilating satellite observations in the Bohai, Yellow and East China Seas with the simulation of four principal tidal constituents M2, S2, K1 and O1. In the twin experiments, the Manning’s n coefficient is assumed to be constant, and it is estimated by assimilating the synthetic observations at the spatial locations of the satellite tracks. Regardless the inclusion of artificial random observational errors associated with synthetic observations, the model performance is improved as evaluated by the independent synthetic observations. The prescribed ‘real’ Manning’s n coefficient is reasonably estimated, indicating that the adjoint data assimilation is an effective method to estimate the Manning’s n coefficient in multi-constituent tidal models. In the practical experiments, the errors between the independent observations at the tidal gauge stations and the corresponding simulated results of the four principal tidal constituents are substantially decreased under both scenarios of the constant and spatially-temporally varying Manning’s n coefficient estimated by assimilating the satellite observations with the adjoint data assimilation. In addition, the estimated spatial and temporal variation trend is robust and not affected by the model settings. The spatially-temporally varying Manning’s n coefficient is negatively correlated with the current speed and shows significant spatial variation in the shallow water areas. This study demonstrates that the Manning’s n coefficient can be reasonably estimated by the adjoint data assimilation, which allows significant improvement in accurate simulation of the ocean tide

Improve the Accuracy in Numerical Modeling of Suspended Sediment Concentrations in the Hangzhou Bay by Assimilating Remote Sensing Data Utilizing Combined Techniques of Adjoint Data Assimilation and the Penalty Function Method

Suspended sediment dynamics play an important role in controlling nearshore and estuarine geomorphology and the associated ecological environments. Modeling the transport of suspended sediment is a complicated and challenging research topic. The goal of this study is to improve the accuracy of modeling the suspended sediment concentrations (SSCs) with newly developed techniques. Based on a three-dimensional suspended cohesive sediment transport model, the transport of suspended sediment and SSCs are simulated by assimilating SSCs retrieved from the Geostationary Ocean Color Imager (GOCI) with the adjoint data assimilation in the Hangzhou Bay, a typical strong tidal estuary along the coast of the East China Sea. To improve the effect of the data assimilation, the penalty function method, in which the reasonable constraints of the estimated model parameters are added to the cost function as penalty terms, will be introduced for the first time into the adjoint data assimilation in the SSCs modeling. In twin experiments, the prescribed spatially varying settling velocity is estimated by assimilating the synthetic SSC observations, and the results show that the penalty function method can further improve the effect of data assimilation and parameter estimation, regardless of synthetic SSC observations being contaminated by random artificial errors. In practical experiments, the spatially varying settling velocity is firstly estimated by assimilating the actual GOCI-retrieved SSCs. The results demonstrate that the simulated results can be improved by the adjoint data assimilation, and the penalty function method can additionally reduce the mean absolute error (MAE) between the independent check observations and the corresponding simulated SSCs from 1.44 × 10−1 kg/m3 to 1.30 × 10−1 kg/m3. To pursue greater simulation accuracy, the spatially varying settling velocity, resuspension rate, critical shear stress and initial conditions are simultaneously estimated by assimilating the actual GOCI-retrieved SSCs to simulate the SSCs in the Hangzhou Bay. When the adjoint data assimilation and the penalty function method are simultaneously used, the MAE between the independent check observations and the corresponding simulated SSCs is just 9.90 × 10−2 kg/m3, which is substantially less than that when only the settling velocity is estimated. The MAE is also considerably less than that when the four model parameters are estimated to be without using the penalty function method. This study indicates that the adjoint data assimilation can effectively improve the SSC simulation accuracy, and the penalty function method can limit the variation range of the estimated model parameters to further improve the effect of data assimilation and parameter estimation

Variability of Chlorophyll-a and Secchi Disk Depth (1997–2019) in the Bohai Sea Based on Monthly Cloud-Free Satellite Data Reconstructions

Ocean colour data are crucial for monitoring and assessing marine ecosystems. In this study, the Data Interpolating Empirical Orthogonal Functions (DINEOF) approach was applied to the Ocean Colour Climate Change Initiative (OC-CCI), chlorophyll-a (Chl-a) and Secchi disk depth (Zsd) to completely reconstruct the missing pixels in the Bohai Sea during 1997–2019. The results of cross-validation demonstrate that the DINEOF reconstructed data have a good agreement with the satellite-measured data. Based on monthly cloud-free satellite data reconstructions, the Zsd series showed high negative correlation with log10 (Chl-a). The Zsd as a function of log10 (Chl-a) can be well fitted by the cubic polynomial in the offshore waters. The Chl-a in the entire Bohai Sea showed a significant decreasing trend (−0.013 mg/m3/year), while the Zsd exhibited a significant increasing trend (0.0065 m/year), and both had regional-seasonal variations. In addition, the ensemble empirical mode decomposition (EEMD) results reveal highly nonlinear trends of Chl-a and Zsd. The linear and nonlinear trends of Chl-a and Zsd suggest the deterioration of water quality in the Bohai Sea was not continued over the past two decades. This study presents the first simultaneous investigation of Chl-a and Zsd using the 23 years of cloud-free reconstructions in the Bohai Sea

The development of coral concretes and their upgrading technologies: a critical review

The utilization of locally available raw materials is extremely necessary for the offshore island construction in ocean. Coral can be used as aggregate but is usually light and porous, having rough surface, weak adherence to the attachments and high concentration of sea salts. These characteristics affect the workability, mechanical properties, volume stability and durability of resulting concretes. This review paper discussed the instability of Portland cement hydration products under the ocean environment, the low strength and stiffness of coral aggregates, their large connected porosity and the weak interface microstructure between the cement matrix and coral aggregates. To overcome these problems, such as low grade of strengths, inadequate corrosion resistance and high brittleness, this paper proposed some perspective techniques: modification of coral aggregate, development of new cementitious materials, fiber reinforcement and mix proportion design