Tripod-supported offshore wind turbines: Modal and coupled analysis and a parametric study using X-SEA and FAST

This paper presents theoretical aspects and an extensive numerical study of the coupled analysis of tripod support structures for offshore wind turbines (OWTs) by using X-SEA and FAST v8 programs. In a number of site conditions such as extreme and longer period waves, fast installation, and lighter foundations, tripod structures are more advantageous than monopile and jacket structures. In the implemented dynamic coupled analysis, the sub-structural module in FAST was replaced by the X-SEA offshore substructure analysis component. The time-histories of the reaction forces and the turbine loads were then calculated. The results obtained from X-SEA and from FAST were in good agreement. The pile-soil-structure interaction (PSSI) was included for reliable evaluation of OWT structural systems. The superelement concept was introduced to reduce the computational time. Modal, coupled and uncoupled analyses of the NREL 5MW OWT-tripod support structure including PSSI were carried out and the discussions on the natural frequencies, mode shapes and resulted displacements are presented. Compared to the uncoupled models, the physical interaction between the tower and the support structure in the coupled models resulted in smaller responses. Compared to the fixed support structures, i.e., when PSSI is not included, the piled-support structure has lower natural frequencies and larger responses attributed to its actual flexibility. The models using pile superelements are computationally efficient and give results that are identical to the common finite element models

Lattice parameters of Scx_{\boldsymbol{\mathsf{x}}}Al1−x_{\boldsymbol{\mathsf{1-x}}}N layers grown on GaN(0001) by plasma-assisted molecular beam epitaxy

An accurate knowledge of the lattice parameters of the new nitride Sc$_\textit{x}$Al$_\textit{1-x}$N is essential for understanding the elastic and piezoelectric properties of this compound as well as for the ability to engineer its strain state in heterostructures. Using high-resolution x-ray diffractometry, we determine the lattice parameters of 100-nm-thick undoped Sc$_\textit{x}$Al$_\textit{1-x}$N layers grown on GaN(0001) templates by plasma-assisted molecular beam epitaxy. The Sc content $\textit{x}$ of the layers is measured independently by both x-ray photoelectron spectroscopy and energy-dispersive x-ray spectroscopy and ranges from 0 to 0.25. The in-plane lattice parameter of the layers linearly increases with increasing $\textit{x}$, while their out-of-plane lattice parameter remains constant. Layers with $\textit{x}$ $\approx$ 0.09 are found to be lattice matched to GaN, resulting in a smooth surface and a structural perfection equivalent to that of the GaN underlayer. In addition, a two-dimensional electron gas is induced at the Sc$_\textit{x}$Al$_\textit{1-x}$N/GaN heterointerface, with the highest sheet electron density and mobility observed for lattice-matched conditions

Control growth orientation of semipolar GaN layers grown on 3C-SiC/(001) Si

Heteroepitaxial growth of GaN buffer layers on 3C-SiC/(001) Si substrates (4°-miscut towards [110]) by metalorganic vapour phase epitaxy has been investigated. High-temperature grown AlxGa1-xN/AlN interlayers were employed to control GaN surface orientations. Semipolar GaN layers with (101¯1), (202¯3) and (101¯2) surface orientations were achieved, as confirmed by X-ray diffraction. Due to the substrate miscut, the growth of (101¯1) layers was twinned along [11¯0]3C-SiC/Si and [1¯10]3C-SiC/Si while the growth of (202¯3) and (101¯2) layers was only along [110]3C-SiC/Si. The (101¯1) layers have rough surface morphology while the (202¯3) and (101¯2) layers have mirror-like smooth surface. For all samples with various surface orientations, different photoluminescence peak emission energies were observed at ∼3.45 eV, 3.78 eV and 3.27 eV at 10 K. These emissions are attributed to the near-band edge of hexagonal GaN, basal-plane stacking faults and partial dislocations, respectively. The dominant luminescence intensity of stacking faults indicates their high density in the GaN layers

A Simultaneous Extraction of Context and Community from pervasive signals using nested Dirichlet process

Understanding user contexts and group structures plays a central role in pervasive computing. These contexts and community structures are complex to mine from data collected in the wild due to the unprecedented growth of data, noise, uncertainties and complexities. Typical existing approaches would first extract the latent patterns to explain human dynamics or behaviors and then use them as a way to consistently formulate numerical representations for community detection, often via a clustering method. While being able to capture high-order and complex representations, these two steps are performed separately. More importantly, they face a fundamental difficulty in determining the correct number of latent patterns and communities. This paper presents an approach that seamlessly addresses these challenges to simultaneously discover latent patterns and communities in a unified Bayesian nonparametric framework. Our Simultaneous Extraction of Context and Community (SECC) model roots in the nested Dirichlet process theory which allows a nested structure to be built to summarize data at multiple levels. We demonstrate our framework on five datasets where the advantages of the proposed approach are validated

Accounting-based variables as an early warning indicator of financial distress in crisis and non-crisis periods

Financial integration in the Association of Southeast Asian Nations (ASEAN) region is a key focus of the ASEAN Economic Community. Whereas many studies focus on modelling corporate default, this paper identifies early warning indicators of financial distress before a default, using multiple discriminant analysis (MDA) models with a sample of listed and delisted companies in the ASEAN region. The analysis examines 720 companies in 10 different industries across six ASEAN countries from 1997 to 2016. The study constructs individual models for each country as well as an overall model for the entire region, using both in-sample and out-of-sample approaches. This overall model could be useful for an integrated banking system. To ensure robustness, the study also separately examines the predictive performance of the MDA models across different economic crises: the Asian financial crisis (AFC) from 1997 to 2000, the global financial crisis (GFC) from 2007 to 2009 and their pre- and post-crisis periods. We find that profitability ratios are the best indicators of financial distress in the ASEAN region, followed by liquidity and leverage ratios. In addition, our findings reveal common indicators that can be used to predict financial distress across ASEAN countries. The single model performs reasonably well in predicting financial distress 1 year ahead. In addition, the model is extended to incorporate a market-based indicator into the MDA models, the distance to default. However, the inclusion of this indicator does not significantly improve the accuracy of the models in predicting financial distress at listed firms in the ASEAN region

Exciton localization in semipolar ( 112¯2) InGaN multiple quantum wells

The exciton localization in semipolar (112⎯⎯2112¯2) InxGa1−xN (0.13 ≤ x ≤ 0.35) multiple-quantum-well (MQW) structures has been studied by excitation power density and temperature dependent photoluminescence. A strong exciton localization was found in the samples with a linear dependence with In-content and emission energy, consistent with the Stokes-shift values. This strong localization was found to cause a blue-shift of the MQW exciton emission energy at temperature above 100 K, which was found to linearly increase with increasing In-content

Coupled analysis of offshore wind turbine jacket structures with pile-soil-structure interaction using FAST v8 and X-SEA

The coupled analysis between a turbine in operating condition and a complex jacket support structure was formulated in this paper for the reliable evaluation of offshore wind turbine structures including pile-soil-structure interactions (PSSIs). Discussions on the theoretical and simulation aspects of the coupled analysis are presented. The dynamic coupled analysis was implemented in X-SEA program and validated with FAST v8 (fatigue, aerodynamics, structures and turbulence) developed by NREL, USA. By replacing the sub-structural module in the FAST with the component of offshore substructure in the X-SEA, the reaction forces and the turbine loads were calculated in each time step and the results from X-SEA were compared with that from FAST. It showed very good agreement with each other. A case study of a NREL 5MW offshore wind turbine on a jacket support structure was performed. Coupled dynamic analyses of offshore wind turbine and support structures with PSSI were carried out. The results showed that in the coupled analysis, the responses of the structure are significantly less than in the uncoupled analysis. The support structure considering PSSI exhibited decreased natural frequencies and more flexible responses compared to the fixed-support structure. The implemented coupled analysis including PSSI was shown to be more accurate and computationally efficient

Assessment of seasonal winter temperature forecast errors in the regcm model over northern Vietnam

This study verified the seasonal six-month forecasts for winter temperatures for northern Vietnam in 1998–2018 using a regional climate model (RegCM4) with the boundary conditions of the climate forecast system Version 2 (CFSv2) from the National Centers for Environmental Prediction (NCEP). First, different physical schemes (land-surface process, cumulus, and radiation parameterizations) in RegCM4 were applied to generate 12 single forecasts. Second, the simple ensemble forecasts were generated through the combinations of those different physical formulations. Three subclimate regions (R1, R2, R3) of northern Vietnam were separately tested with surface observations and a reanalysis dataset (Japanese 55-year reanalysis (JRA55)). The highest sensitivity to the mean monthly temperature forecasts was shown by the land-surface parameterizations (the biosphere−atmosphere transfer scheme (BATS) and community land model version 4.5 (CLM)). The BATS forecast groups tended to provide forecasts with lower temperatures than the actual observations, while the CLM forecast groups tended to overestimate the temperatures. The forecast errors from single forecasts could be clearly reduced with ensemble mean forecasts, but ensemble spreads were less than those root-mean-square errors (RMSEs). This indicated that the ensemble forecast was underdispersed and that the direct forecast from RegCM4 needed more postprocessing

Significant contribution from impurity-band transport to the room temperature conductivity of silicon-doped AlGaN

Silicon-doped n-type (0 0 0 1) AlGaN materials with 60% and 85% AlN content were studied close to the doping condition that gives the lowest resistivity (Si/III ratios in the ranges 2.8–34  ×  10−5 and 1.3–6.6  ×  10−5, respectively). Temperature-dependent conductivity and Hall-effect measurements showed that, apart from the diffusion-like transport in the conduction band, a significant amount of the conductivity was due to phonon-assisted hopping among localized states in the impurity band, which became almost completely degenerate in the most doped sample of the Al0.6Ga0.4N series. In the doping range explored, impurity-band transport was not only dominant at low temperature, but also significant at room-temperature, with contributions to the total conductivity up to 46% for the most conductive sample. We show that, as a consequence of this fact, the measurements of Hall carrier concentration and Hall mobility using the usual single-channel approach are not reliable, even at high temperatures. We propose a simple method to separate the contributions of the two channels. Our model, although only approximate, can be used to gain insight into the doping mechanism: particularly it shows that the room-temperature free-electron concentration in the conduction band of the Al0.6Ga0.4N material reaches its maximum at about 1.6  ×  1018 cm−3, well below the value that would have been obtained with the standard single-channel analysis of the data. This maximum is already achieved at dopant concentrations lower than the one that gives the best conductivity. However, further increase of the doping levels are required to enhance the impurity-band channel, with concentrations of the carriers participating in this type of transport that increase from 2.1  ×  1018 cm−3 up to 4.3  ×  1018 cm−3. For the Al0.85Ga0.15N, even though it was not possible to estimate the actual carrier concentrations, our measurements suggest that a significant impurity-band channel is present also in this material

Size-dependent bandwidth of semipolar (1122) light-emitting-diodes

The limited modulation bandwidth of commercial light-emitting diodes (LEDs) is one of the critical bottlenecks for visible light communications. Possible approaches to increase the bandwidth include the use of micron sized LEDs, which can withstand higher current densities, as well as the use of LED structures that are grown on different crystal planes to the conventional polar c-plane. We compare c-plane InGaN/GaN LEDs with semipolar ( 112¯¯¯2 ) LEDs containing a 4- and 8-nm single quantum well. The modulation bandwidth of semipolar LEDs with active areas varying from 200×200 to 30×30μm2 is shown to be governed by both current density and size. A small signal bandwidth of over 800 MHz for a relatively low applied current density of 385 A/cm2 is reported for 30×30μm2 LEDs with 8-nm thick quantum well. An optical link using an easy non-return-to-zero ON–OFF keying modulation scheme with a data rate of 1.5 Gb/s is demonstrated