4,580 research outputs found
Clustering of floaters on the free surface of a turbulent flow: an experimental study
We present an experimental study of the statistical properties of
millimeter-size spheres floating on the surface of a turbulent flow. The flow
is generated in a layer of liquid metal by an electromagnetic forcing. By using
two magnet arrays, we are able to create one highly fluctuating flow and
another, more stationary flow. In both cases, we follow the motion of hundreds
of particles floating at the deformed interface of the liquid metal. We
evidence the clustering of floaters by a statistical study of the local
concentration of particles. Some dynamical properties of clusters are exposed.
We perform spatial correlations between particle concentration and
hydrodynamical quantities linked with inertial effects; with vortical motion,
and with horizontal divergence (corresponding to compressibility in the
surface). From comparing these correlations, we propose the so-called surface
compressibility as the main clustering mechanism in our system. Hence, although
floaters are not passive scalar and move on a deformed surface, the scenario is
similar to the one reported for passive scalar on an almost flat free surface
of a turbulent flow.Comment: 11 pages, 7 figures. Accepted to publication in European Journal of
Mechanics B/Fluid
Dynamic OLS estimation of the U.S. import demand for Mexican crude oil
This paper estimates the U.S. import demand for crude oil from Mexico. The analysis is based on time series from January 1990 to December 2010. Time series properties of the processes that generate the data are assessed in order to specify the order of integration for each series. According to results from unit root tests, all the series under study are unit root non-stationary. The paper then estimates the cointegrating import demand regression using Dynamic OLS procedure. Residuals from the DOLS cointegrating regression are tested and found to be stationary; thus, the cointegrating regression is not spurious. According to estimation results, U.S. import demand for Mexican crude oil is income inelastic, perfect price inelastic, and responsive to changes in both U.S. stock of oil (excluding SPR) and unemployment rate in the U.S. Also, this paper points to the estimate bias from omitting relevant variables as it is common in the mainstream literature on crude oil import demand.Crude Oil Demand, Unit Root, Dynamic OLS.
Using quality models in software package selection
The growing importance of commercial off-the-shelf software packages requires adapting some software engineering practices, such as requirements elicitation and testing, to this emergent framework. Also, some specific new activities arise, among which selection of software packages plays a prominent role. All the methodologies that have been proposed recently for choosing software packages compare user requirements with the packages' capabilities. There are different types of requirements, such as managerial, political, and, of course, quality requirements. Quality requirements are often difficult to check. This is partly due to their nature, but there is another reason that can be mitigated, namely the lack of structured and widespread descriptions of package domains (that is, categories of software packages such as ERP systems, graphical or data structure libraries, and so on). This absence hampers the accurate description of software packages and the precise statement of quality requirements, and consequently overall package selection and confidence in the result of the process. Our methodology for building structured quality models helps solve this drawback.Peer ReviewedPostprint (published version
The convex hull of a finite set
We study -separately convex hulls of finite
sets of points in , as introduced in
\cite{KirchheimMullerSverak2003}. When is considered as a
certain subset of matrices, this notion of convexity corresponds
to rank-one convex convexity . If is identified instead
with a subset of matrices, it actually agrees with the quasiconvex
hull, due to a recent result \cite{HarrisKirchheimLin18}.
We introduce " complexes", which generalize constructions. For a
finite set , a " -complex" is a complex whose extremal points
belong to . The "-complex convex hull of ", , is the union
of all -complexes. We prove that is contained in the
convex hull .
We also consider outer approximations to convexity based in the
locality theorem \cite[4.7]{Kirchheim2003}. Starting with a crude outer
approximation we iteratively chop off "-prisms". For the examples in
\cite{KirchheimMullerSverak2003}, and many others, this procedure reaches a
" -complex" in a finite number of steps, and thus computes the
convex hull.
We show examples of finite sets for which this procedure does not reach the
convex hull in finite time, but we show that a sequence of outer
approximations built with -prisms converges to a -complex. We
conclude that is always a " -complex", which has interesting
consequences
Moral hazard in a voluntary deposit insurance system: Revisited
This paper extends Wheelock and Kumbhakar’s (1995) test for moral hazard in the Kansas deposit insurance system (1910-1920). This paper tests and finds evidence of omitted bank-specific effects. Estimates in Wheelock and Kumbhakar (1995), as a result, are biased. This paper introduces unobserved individual heterogeneity to the test for moral hazard, corrects their estimates, and finds more evidence of moral hazard in the Kansas deposit insurance system
Non-radial pulsations in the Be/X binaries 4U0115+63 and SAXJ2103.5+4545
The discovery of non-radial pulsations (NRP) in the Be/X binaries of the
Magellanic Clouds (MC, eg. Fabrycky 2005, Coe et al. 2005, Schmidtke & Cowley
2005) provided a new approach to understand these complex systems, and, at the
same time, favoured the synergy between two different fields: stellar
pulsations and X-ray binaries. This breakthrough was possible thanks to the
MACHO and OGLE surveys. However, in our Galaxy, only two Be/X have been
reported to show NRP: GROJ2058+42 (Kiziloglu et al. 2007) and LSI+61 235 (Sarty
et al. 2009). Our objective is to study the short-term variability of Galactic
Be/X binaries, compare them to the Be/X of the MC and to the isolated Galactic
Be observed with CoRoT and Kepler. We present preliminary results of two Be/X
stars, namely 4U0115+63 and SAXJ2103.5+4545 showing multiperiodicity and
periodicity respectively, most probably produced by non-radial pulsations.Comment: 2 pages, 2 figures, IAUS 272 "Active OB stars: structure, evolution,
mass loss and critical limits" conference, Paris, July 2010, submitte
The Impact of Infrastructure on Mexican Manufacturing Growth
This paper analyses the impact of infrastructure on the growth rate of the Mexican manufacturing sector. For such purpose, two measures of infrastructure are used: highways and electricity. Further, we also estimate the degree of returns to scale and the markup. We pooled two digit industries to obtain the estimates of the whole manufacturing sector. For the entire manufacturing sector, our results do not show evidence of increasing returns but the existence of market power cannot be rejected. We find that both types of public infrastructure have a significant effect on manufacturing growth and its inclusion reduces the estimated values of returns to scale and market power. Once we use sectoral data, we obtain mixed results: public infrastructure affects significantly only some sectors.
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