15,953 research outputs found
Homogeneous vortex model for liquid slosh in spinning spherical tanks
The problem of forced fluid sloshing in a partially filled spinning spherical tank is solved numerically using the finite element method. The governing equations include Coriolis acceleration, empirical fluid damping and spatially homogeneous vorticity first introduced by Pfeiffer. An exponential instability similar to flutter is detected in the present simulation for fill ratios below 50 percent. This instability appears in the model as a result of the homogeneous vortex assumption since the free slosh equations are neutrally stable in the Liapunov sense
Two-Dimensional Phononic Crystals: Disorder Matters
The design and fabrication of phononic crystals (PnCs) hold the key to
control the propagation of heat and sound at the nanoscale. However, there is a
lack of experimental studies addressing the impact of order/disorder on the
phononic properties of PnCs. Here, we present a comparative investigation of
the influence of disorder on the hypersonic and thermal properties of
two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of
circular holes with equal filling fractions in free-standing Si membranes.
Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman
thermometry based on a novel two-laser approach are used to study the phononic
properties in the gigahertz (GHz) and terahertz (THz) regime, respectively.
Finite element method simulations of the phonon dispersion relation and
three-dimensional displacement fields furthermore enable the unique
identification of the different hypersonic vibrations. The increase of surface
roughness and the introduction of short-range disorder are shown to modify the
phonon dispersion and phonon coherence in the hypersonic (GHz) range without
affecting the room-temperature thermal conductivity. On the basis of these
findings, we suggest a criteria for predicting phonon coherence as a function
of roughness and disorder.Comment: 19 pages, 4 figures, final published version, Nano Letters, 201
Comparison of water balance method and alternative evaporation methods applied to the Aswan High Dam Reservoir
Aswan High Dam Reservoir (AHDR) is a large human-made reservoir situated in southern Egypt and northern Sudan. The reservoir is located in a typical arid zone so that evaporation results in a significant water loss from the reservoir. To quantify these evaporation water losses, different methods can be applied. The water balance method was used to estimate water losses of the AHDR during 43 open-water seasons. Compared to earlier publications, this study used longer time series data and more evaporation approaches. Moreover, we evaluated the deviation between evaporation rates as derived from the water balance method and as calculated using 16 evaporation/evapotranspiration formulas. Five approaches are not well suited for use at the AHDR because they underestimated evaporation rates (e.g. Stephens-Stewart model), or overestimated evaporation rates (e.g. de Bruin model). Annual evaporation rates obtained by the Bowen ratio energy balance method at the three floating stations Raft, Allaqi and Abu Simbel were estimated at 7.9, 6.9 and 6.7 mm d-1, respectively. The monthly water losses of the years 1978 to 1984, a period with reasonable evaporation rates, are used to estimate the evaporation losses. The results of the study show a systematic deviation between the monthly average values determined using the water balance method through the period 1978 to 1984 and the monthly mean values determined by the 16 evaporation calculation approaches at three floating stations. This deviation is particularly clear in the months of May, June and September (primarily lower estimates) as well as in July (primarily higher estimates). The deviation can be attributed to the simplicity of the water balance method as well as to its limited suitability for large reservoirs as the AHDR over short periods like a month. Among the 16 evaporation calculation approaches the mass transfer method provided the most reasonable results under the given site conditions
Altered VEGF splicing isoform balance in tumor endothelium involves activation of splicing factors Srpk1 and Srsf1 by the Wilmsâ tumor suppressor Wt1
Angiogenesis is one hallmark of cancer. Vascular endothelial growth factor (VEGF) is a known inducer of angiogenesis. Many patients benefit from antiangiogenic therapies, which however have limitations. Although VEGF is overexpressed in most tumors, different VEGF isoforms with distinct angiogenic properties are produced through alternative splicing. In podocytes, the Wilmsâ tumor suppressor 1 (WT1) suppresses the Serine/arginine-rich protein-specific splicing factor kinase (SRPK1), and indirectly Serine/arginine-rich splicing factor 1 (Srsf1) activity, and alters VEGF splicing. We analyzed VEGF isoforms, Wt1, Srpk1, and Srsf1 in normal and tumor endothelium. Wt1, Srpk1, Srsf1, and the angiogenic VEGF164a isoform were highly expressed in tumor endothelium compared to normal lung endothelium. Nuclear expression of Srsf1 was detectable in the endothelium of various tumor types, but not in healthy tissues. Inducible conditional vessel-specific knockout of Wt1 reduced Wt1, Srpk1, and Srsf1 expression in endothelial cells and induced a shift towards the antiangiogenic VEGF120 isoform. Wt1(âKTS) directly binds and activates both the promoters of Srpk1 and Srsf1 in endothelial cells. In conclusion, Wt1 activates Srpk1 and Srsf1 and induces expression of angiogenic VEGF isoforms in tumor endothelium
Digital implementation of the cellular sensor-computers
Two different kinds of cellular sensor-processor architectures are used nowadays in various
applications. The first is the traditional sensor-processor architecture, where the sensor and the
processor arrays are mapped into each other. The second is the foveal architecture, in which a
small active fovea is navigating in a large sensor array. This second architecture is introduced
and compared here. Both of these architectures can be implemented with analog and digital
processor arrays. The efficiency of the different implementation types, depending on the used
CMOS technology, is analyzed. It turned out, that the finer the technology is, the better to use
digital implementation rather than analog
Helium bubble formation in ultrafine and nanocrystalline tungsten under different extreme conditions
We have investigated the effects of helium ion irradiation energy and sample temperature on the performance of grain boundaries as helium sinks in ultrafine grained and nanocrystalline tungsten. Irradiations were performed at displacement and non-displacement energies and at temperatures above and below that required for vacancy migration. Microstructural investigations were performed using Transmission Electron Microscopy (TEM) combined with either in-situ or ex-situ ion irradiation. Under helium irradiation at an energy which does not cause atomic displacements in tungsten (70 eV), regardless of temperature and thus vacancy migration conditions, bubbles were uniformly distributed with no preferential bubble formation on grain boundaries. At energies that can cause displacements, bubbles were observed to be preferentially formed on the grain boundaries only at high temperatures where vacancy migration occurs. Under these conditions, the decoration of grain boundaries with large facetted bubbles occurred on nanocrystalline grains with dimensions less than 60 nm. We discuss the importance of vacancy supply and the formation and migration of radiation-induced defects on the performance of grain boundaries as helium sinks and the resulting irradiation tolerance of ultrafine grained and nanocrystalline tungsten to bubble formatio
Evaluating Maintainability Prejudices with a Large-Scale Study of Open-Source Projects
Exaggeration or context changes can render maintainability experience into
prejudice. For example, JavaScript is often seen as least elegant language and
hence of lowest maintainability. Such prejudice should not guide decisions
without prior empirical validation. We formulated 10 hypotheses about
maintainability based on prejudices and test them in a large set of open-source
projects (6,897 GitHub repositories, 402 million lines, 5 programming
languages). We operationalize maintainability with five static analysis
metrics. We found that JavaScript code is not worse than other code, Java code
shows higher maintainability than C# code and C code has longer methods than
other code. The quality of interface documentation is better in Java code than
in other code. Code developed by teams is not of higher and large code bases
not of lower maintainability. Projects with high maintainability are not more
popular or more often forked. Overall, most hypotheses are not supported by
open-source data.Comment: 20 page
On the Fitness Landscapes of Interdependency Models in the Travelling Thief Problem
Since its inception in 2013, the Travelling Thief Problem (TTP) has been
widely studied as an example of problems with multiple interconnected
sub-problems. The dependency in this model arises when tying the travelling
time of the "thief" to the weight of the knapsack. However, other forms of
dependency as well as combinations of dependencies should be considered for
investigation, as they are often found in complex real-world problems. Our goal
is to study the impact of different forms of dependency in the TTP using a
simple local search algorithm. To achieve this, we use Local Optima Networks, a
technique for analysing the fitness landscape
Efficiency of the foreign currency options market
This paper provides a new test of the efficiency of the currency option markets for four major currencies-British Pound, Euro, Swiss Frank and Japanese Yen vis-a-vis the U.S. dollar. The approach is to simulate trading strategies to see if the well-accepted no-arbitrage condition of putâcall parity (PCP) holds in a trading environment. Augmented DickeyâFuller and PhilipsâPerron tests are used to check for the presence of unit roots in the data, followed by a formal econometric analysis. The results indicate that the most currency option prices do not violate the PCP conditions, when transaction costs are allowed for
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