3,881 research outputs found
Identification of Demand through Statistical Distribution Modeling for Improved Demand Forecasting
Demand functions for goods are generally cyclical in nature with
characteristics such as trend or stochasticity. Most existing demand
forecasting techniques in literature are designed to manage and forecast this
type of demand functions. However, if the demand function is lumpy in nature,
then the general demand forecasting techniques may fail given the unusual
characteristics of the function. Proper identification of the underlying demand
function and using the most appropriate forecasting technique becomes critical.
In this paper, we will attempt to explore the key characteristics of the
different types of demand function and relate them to known statistical
distributions. By fitting statistical distributions to actual past demand data,
we are then able to identify the correct demand functions, so that the the most
appropriate forecasting technique can be applied to obtain improved forecasting
results. We applied the methodology to a real case study to show the reduction
in forecasting errors obtained
Array-based iterative measurements of SmKS travel times and their constraints on outermost core structure
Vigorous convection in Earth's outer core led to the suggestion that it is chemically homogeneous. However, there is increasing seismic evidence for structural complexities close to the outer core's upper and lower boundaries. Both body waves and normal mode data have been used to estimate a P wave velocity, V_p, at the top of the outer core (the Eā layer), which is lower than that in the Preliminary Reference Earth Model. However, these low V_p models do not agree on the form of this velocity anomaly. One reason for this is the difficulty in retrieving and measuring SmKS arrival times. To address this issue, we propose a novel approach using data from seismic arrays to iteratively measure SmKS-SKKS-differential travel times. This approach extracts individual SmKS signal from mixed waveforms of the SmKS series, allowing us to reliably measure differential travel times. We successfully use this method to measure SmKS time delays from earthquakes in the FijiāTonga and Vanuatu subduction zones. SmKS time delays are measured by waveform cross correlation between SmKS and SKKS, and the crossācorrelation coefficient allows us to access measurement quality. We also apply this iterative scheme to synthetic SmKS seismograms to investigate the 3āD mantle structure's effects. The mantle structure corrections are not negligible for our data, and neglecting them could bias the V_p estimation of uppermost outer core. After mantle structure corrections, we can still see substantial time delays of S3KS, S4KS, and S5KS, supporting a low V_p at the top of Earth's outer core
Dynamic analysis of flexible rotor-bearing systems using a modal approach
The generalized dynamic equations of motion were obtained by the direct stiffness method for multimass flexible rotor-bearing systems. The direct solution of the equations of motion is illustrated on a simple 3-mass system. For complex rotor-bearing systems, the direct solution of the equations becomes very difficult. The transformation of the equations of motion into modal coordinates can greatly simplify the computation for the solution. The use of undamped and damped system mode shapes in the transformation are discussed. A set of undamped critical speed modes is used to transform the equations of motion into a set of coupled modal equations of motion. A rapid procedure for computing stability, steady state unbalance response, and transient response of the rotor-bearing system is presented. Examples of the application of this modal approach are presented. The dynamics of the system is further investigated with frequency spectrum analysis of the transient response
Doped Mott insulators are insulators: hole localization in the cuprates
We demonstrate that a Mott insulator lightly doped with holes is still an
insulator at low temperature even without disorder. Hole localization obtains
because the chemical potential lies in a pseudogap which has a vanishing
density of states at zero temperature. The energy scale for the pseudogap is
set by the nearest-neighbour singlet-triplet splitting. As this energy scale
vanishes if transitions, virtual or otherwise, to the upper Hubbard band are
not permitted, the fundamental length scale in the pseudogap regime is the
average distance between doubly occupied sites. Consequently, the pseudogap is
tied to the non-commutativity of the two limits ( the on-site
Coulomb repulsion) and (the system size).Comment: 4 pages, 3 .eps file
The Van der Waals interaction of the hydrogen molecule - an exact local energy density functional
We verify that the van der Waals interaction and hence all dispersion
interactions for the hydrogen molecule given by: W"= -{A/R^6}-{B/R^8}-{C/R^10}-
..., in which R is the internuclear separation, are exactly soluble. The
constants A=6.4990267..., B=124.3990835 ... and C=1135.2140398... (in Hartree
units) first obtained approximately by Pauling and Beach (PB) [1] using a
linear variational method, can be shown to be obtainable to any desired
accuracy via our exact solution. In addition we shall show that a local energy
density functional can be obtained, whose variational solution rederives the
exact solution for this problem. This demonstrates explicitly that a static
local density functional theory exists for this system. We conclude with
remarks about generalising the method to other hydrogenic systems and also to
helium.Comment: 11 pages, 13 figures and 28 reference
The role of thermal and lubricant boundary layers in the transient thermal analysis of spur gears
An improved convection heat-transfer model has been developed for the prediction of the transient tooth surface temperature of spur gears. The dissipative quality of the lubricating fluid is shown to be limited to the capacity extent of the thermal boundary layer. This phenomenon can be of significance in the determination of the thermal limit of gears accelerating to the point where gear scoring occurs. Steady-state temperature prediction is improved considerably through the use of a variable integration time step that substantially reduces computer time. Computer-generated plots of temperature contours enable the user to animate the propagation of the thermal wave as the gears come into and out of contact, thus contributing to better understanding of this complex problem. This model has a much better capability at predicting gear-tooth temperatures than previous models
Novel Nanostructured SiO2/ZrO2 Based Electrodes with Enhanced Electrochemical Performance for Lithium-ion Batteries
In this article, a novel anode material with high electrochemical performance, made of elements abundant on the Earth, is reported for use in lithium ion batteries. A chemically synthesised material (SiO2/ZrO2) containing Si-O-Zr bonds, exhibits as much as 2.1 times better electrochemical performance at the 10th cycle than a physically mixed material (SiO2 + ZrO2) of the same elements. When compared to synthesized SiO2 or conventional graphite-based electrodes, the SiO2/ZrO2 anode shows superior capability and cycling performance. This superior performance is ascribed to the effect of ternary compounds, which contributes not only to increasing the packing density, but also to creating the Si-O-Zr bond that makes additional reactions between SiO2/ZrO2 and lithium ions possible. The Si-O-Zr bond also contributes to improved conductivity for SSZ and provides facile paths for charge transfer at the electrode/electrolyte interface. Therefore, the overall internal resistance in a battery would be decreased and better performance could thus be obtained, with this type of anode. In every result, the positive influence of the Si-O-Zr bonds in the anode of a lithium ion battery was confirmed
The synthesis of thermochemically stable single phase lanthanum titanium aluminium oxide
Lanthanum titanium aluminium oxide (LaTi2Al9O19, LTA) synthesized by solid state reaction has been proven to be a promising thermal barrier material. However, LTA synthesized via solid state reaction requires a high processing temperature of at least 1500 Ā°C for 24 h. In this paper, single phase LTA was synthesized by solāgel at a lower temperature (1350 Ā°C) and the process parameters, phase composition, and relative thermal properties were investigated. Two-step calcination was used to obtain fine LTA powders. According to X-ray diffraction, the best calcination temperature of solāgel synthesized LTA is 1350 Ā°C. XRD results also showed the thermochemical stability of solāgel synthesized LTA, which does not react with Al2O3 up to 1500 Ā°C, to be excellent. Compared to LTA synthesized by solid state reaction, solāgel synthesized LTA has higher coefficients of thermal expansion (CTEs) which are circa 10.2Ć10ā6 Ā°Cā1 at 950 Ā°C, related to the size dependent characteristic of CTEs. Therefore, solāgel synthesized LTA can be a promising candidate as a thermal barrier material on Ni-based superalloys
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