64 research outputs found
Intercage dynamics of C-60 in doped crystals
No full textThe low-frequency regions of the Inelastic Neutron Scattering spectra of K3C60, Rb3C60, and Rb6C60 are modelled by atomistic Molecular Dynamics simulations followed by Fourier Transform. The use of a standard force field for the interatomic interactions, together with the simple hypothesis that the charges are homogeneously distributed over the cage, obtains a reasonable agreement with the experiments for the spectra of K3C60 and Rb6C60, but systematically underestimates the location of some bands, especially for Rb3C60. Analysis of the results suggests that the inaccuracy is caused by the atom-atom van der Waals interactions
Guest dynamics in endohedrally doped fullerenes
No full textWe use molecular dynamics to calculate the vibrational density of states of several endohedral monometallofullerenes and compare the results with the experimental values. The vibrational patterns depend weakly on the charge transfer from the metal to the fullerene cage, with the largest variations of similar to 2 cm(-1) observed upon switching off completely the Coulomb interactions, and more strongly on the cage isomer, with variations of up to similar to 13 cm(-1). Analysis of the metal motions inside the cage shows that they can be chaotic. The origin of such chaotic behavior is discussed
A Woodward-Hoffmann approach to the C-60 cluster opening leading to homo[6]1fullerenes
No full textThe electrocyclic valence isomerization interconverting bismethano[60]fullerenes 1 and 2 with 1,2,3,6-(type I) and 1,2,3,4-addition patterns (type II) into the corresponding cluster-opened bismethanohomo[60]fullerenes was investigated computationally with respect to the energy profile and to the retention of molecular orbital symmetry. Whereas for 1 the closed form is unstable and spontaneously isomerizes to the cluster-open adduct, the energy difference between the open and closed isomers of 2 is comparatively small. The B3LYP/6-31G* energy of open 2 is only 1.2 kcal mol(-1) higher than that of the closed form. Analysis of the frontier orbital symmetries of 1, 2, and the [5,6]-bridged methano-homo[60]fullerene 3 showed that during these electrocylic valence isomerizations the orbital symmetry is retained and, as a consequence, a Woodward-Hoffmann-type selection principle is valid
A 3.3-GS/s 6-b Fully Dynamic Pipelined ADC With Linearized Dynamic Amplifier
No full textManuscript received April 27, 2021; revised June 29, 2021; accepted July 2, 2021. This article was approved by Associate Editor Jeffrey Gealow. This work was supported in part by the Science and Technology Development Fund, Macau, under Grant 0003/2019/AFJ and Grant SKL-AMSV (UM)-2020-2022; and in part by the Research Grants of the University of Macau under Grant MYRG2019-00021-AMSV. (Corresponding author: Chi-Hang Chan.
Intra cage dynamics of molecular hydrogen confined in cages of two different dimensions of clathrate hydrates
No full textIn porous materials the molecular confinement is often realized by means of weak Van der Waals interactions between the molecule and the pore surface. The understanding of the mechanism of such interactions is important for a number of applications. In order to establish the role of the confinement size we have studied the microscopic dynamics of molecular hydrogen stored in the nanocages of clathrate hydrates of two different dimensions. We have found that by varying the size of the pore the diffusive mobility of confined hydrogen can be modified in both directions, i.e. reduced or enhanced compared to that in the bulk solid at the same temperatures. In the small cages with a mean crystallographic radius of 3.95 the confinement reduces diffusive mobility by orders of magnitude. In contrast, in large cages with a mean radius of 4.75 hydrogen molecules displays diffusive jump motion between different equilibrium sites inside the cages, visible at temperatures where bulk H2 is solid. The localization of H2 molecules observed in small cages can promote improved functional properties valuable for hydrogen storage application
Solar Radiation in Large-Eddy Simulations Coupled to a General Circulation Model
No full textThe present study aims to assess and advance the prediction skill of solar radiation in high resolution weather forecasts by large-eddy simulations (LES). The GPU-Resident Atmospheric Simulation Platform (GRASP) was used to simulate the atmosphere around the Cabauw experimental site for atmospheric research (CESAR) in the Netherlands. Large-scale boundary conditions were provided by coupling the simulation to a general circulation model (GCM). Radiative tendencies were calculated using two di_erent implementations of the Rapid Radiative Transfer Method for GCMs (RRTM-G): one runs in advance of the simulation using pre-calculated atmospheric _elds, the other employs dynamically updated _elds during the simulation. Both con_gurations generated simulations of every day in 2016, which were compared to each other and validated using observations from the Baseline Surface Radiation Network (BSRN). This study revealed that the implementation of interactive radiation altered cloud representation in the simulations, in most cases causing clouds to rise. This process correlated with an increase in turbulence kinetic energy of up to 2 m2/s2 locally. The clouds that were raised tended to break up more often between 5 and 7 km altitude, leading to a decrease in average cloud fraction and increase in short-wave down-welling radiation. The results suggest that the implementation of interactive radiation enabled the development of cloud top entrainment instabilities, which could be responsible for the cloud breakup in these cases. Regardless of the chosen implementation of the radiative transfer method, large errors are made in the prediction of surface solar radiation. GRASP produced root mean squared errors (RMSE) of 122.4 W/m2 and 115.7 W/m2 using prescribed and interactive radiation, respectively, while the large-scale model used to provide the initial and boundary conditions to the simulation produced an RMSE of 87.3 W/m2. The large error in GRASP's prediction of surface solar radiation can partly be attributed to conversion errors made during GRASP's initialization of the thermodynamic state, which lead to erroneous diagnoses of the liquid water content in the atmosphere.Applied Physic
Wing in ground vehicles operating as high-speed passenger ferries: A feasibility and profitability study
No full textWing-in-ground (hereafter referred to as WIG) vehicles operate a few meters above water. The vehicles make use of the ground-effect (an effect where the drag is reduced, and the wings of the vehicles generate an increased lift). The unique speed and distance range of a WIG vehicle — a vehicle that, in functionality and appearance, sits somewhere between a ship and an aircraft — could be an ideal option for high-speed passenger transport on routes with ‘medium’ distances. That said, WIG vehicles have hardly ever been seen in civilian applications, such as high-speed passenger transport. This report answers the question: can WIG vehicles operating as high-speed passenger ferries, be feasible and profitable? This feasibility is assessed on the basis of literature research and real examples of WIG performance, functionality and profitability. Suitable routes and vehicles are selected based on criteria analyses. Using financial estimations, the profitability of two suitable WIG vehicles on two separate routes is investigated.The specifications of recently built WIG vehicles and their design drivers are analyzed by performing a literature research. This information reveals that WIG vehicles have one main limitation: a low seaworthiness. It is then likely that this limits the seakeeping operability for a number of vehicles. To deal with this limitation, suitable vehicles are selected using a criteria analysis. The literature research shows that the WSH-500 and the Aron M80 both have a relatively good seaworthiness. The vehicles differentiate in size (48p & 6p). The costs of the vehicles are estimated by using general aviation cost estimation theory. A criteria analysis based on seakeeping operability and number of passengers and distance, shows two suitable routes: ‘Buenos Aires – Montevideo’ and ‘Helsinki – Tallinn’. The potential demand for WIG transport on these routes is estimated by creating demand curves of transport. The strategy to market WIG transport on these routes is estimated by using a market research to gage the potential demand. A suitable strategy is to target the top niche of customers on these routes. The profitability is estimated by combining the suitable vehicles, cost of vehicles, routes and strategy. The net present value and internal rate of return is calculated for a number of cases that are explained in this paper. The cases vary mainly on the following aspects: vehicle, total investment and route. In conclusion, WIG vehicles operating as high-speed passenger ferries can be feasible and profitable. The technology behind WIG vehicles has a technological readiness level of 8 out of 9. There are multiple, full-scale prototypes made and tested. Suitable vessels and routes have been selected to deal with the primary limitation that comes with WIG vehicles; they have low seaworthiness capabilities. The seakeeping operability rates of the selected vehicles and routes range between 91% and 100%. The net present value (NPV) and internal rate of return (IRR) of investments made in a WIG transport company can be positive in a number of cases
Conflict Free R Tree: A CRDT-based index for P2P systems
No full textCRDTs are data structures that allow conflict free replication and modification. In theory, CRDTs seem like a natural fit for open P2P networks, however there are obstacles to overcome. Firstly, many proposed CRDTs are grow-only because (i) CRDTs may track deletions in permanent tombstone values or (ii) they may gather permanent information on every peer in the system. As such, CRDTs are not well adapted to open P2P networks. Many peers may come and go over time and persistent accurate information about all peers will grow too large eventually. Secondly, some types of CRDT (mainly op-based CmRDT) require causal message delivery which is hard in open P2P networks. Thirdly, CRDTs are typically built with the assumption that all peers need all data in a replica and thus all data is fully replicated, even though a client may only be interested in a small subset.To address these issues a new state-based CvRDT is proposed: BloomCRDT, which is a variation on the OR-set that replaces its 푅 grow-only set with Bloom filters. It does not need knowledge of other peers or their state and avoids tombstones. This makes BloomCRDT well suited for use in open P2P networks. The grow-only aspect is vastly reduced compared to the standard OR-set. However, the BloomCRDT itself is indivisible and can grow to be too large if it stores many elements. This limit is not high enough to accommodate demanding applications. Scalability can be dramatically improved by combining multiple BloomCRDTs to form a Conflict Free R-Tree (CFRT). Each node of the R-Tree is represented by a BloomCRDT. Concurrent modifications are allowed and, due to the characteristics of the R-tree, this results in a consistent data structure with overlapping ranges. Since an R-Tree’s efficiency is reduced when ranges overlap, a periodic optimization algorithm can be used to eliminate the overlapping ranges.A proof-of-concept implementation was made of BloomCRDT and CFRT using Python and PyIpv8. Experiments where performed using the Gumby experimental framework and the DAS5 cluster. The experiments show that BloomCRDT performs as designed: it keeps a smaller state than other solutions while being invariant to the number of identities that have interacted with a BloomCRDT instance. The CFRT is shown to be able to balance key/value entries over multiple BloomCRDT instances using favorable messaging metrics. Moreover the CFRT, and by extension BloomCRDT, tolerate network faults and can work with up to 90% message loss. A final experiment uses real world data to compare the CFRT to Tribler’s channels implementation, and shows that even at scale a CFRT can outperform the current channels implementation by orders of magnitude in terms of user responsiveness
Automatic Failure Diagnosis for Flow Control Valves
No full textThe increased implementation of digitalisation all over the world has led to an exponential growth of available data across various industries. Consequently, there is a large growth of Machine Learning (ML) techniques being applied to process data. Predictive maintenance is a digital strategy using condition-based monitoring techniques to track the performance of equipment to detect possible defects in advance. In oil refineries, various types of process equipment are used, of which flow control valves are essential to regulate the throughput of heavy, possibly dangerous material. Control valve failures can lead to production loss and increase maintenance costs. This paper addresses the use of time-series data of the valve controller for automated failure diagnosis of flow control valves. Statistical features are extracted from the time series and the significant predictors for the output are adopted in the model using the ANOVA test. The classification and prediction of the failure behaviour are performed using Random Forest (RF) classification. The performance of the diagnosis is measured using the indicators: accuracy and log loss. Findings show that five failure behaviour categories can be predicted, using new and unseen data for the model, with high accuracy(81%).Mechanical Engineerin
Monohull versus Semi-submersible for offshore heavy lift crane operations
No full textIn this thesis the differences between the monohull and semi-submersible are investigated. For a general heavy lift crane operation and its operational profile, the difference is performance and total cost are calculated. These are used to determine the turning point, the point where the semi-submersible is more favourable for a heavy lift crane operation. The monohull has relatively low capital and operational costs, but also a lower workability. There is thus a balance that has to be investigated and compared between the two vessel types. This is done for the North Sea and West Africa, which have significantly different sea conditions.Marine Technology | Ship Design, Production and Operation
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