1,096 research outputs found
Heat exchanger and method of making
A heat exchanger of increased effectiveness is disclosed. A porous metal matrix is disposed in a metal chamber or between walls through which a heat-transfer fluid is directed. The porous metal matrix has internal bonds and is bonded to the chamber in order to remove all thermal contact resistance within the composite structure. Utilization of the invention in a rocket chamber is disclosed as a specific use. Also disclosed is a method of constructing the heat exchanger
Heat exchanger and method of making
A heat exchange of increased effectiveness is disclosed. A porous metal matrix is disposed in a metal chamber or between walls through which a heat-transfer fluid is directed. The porous metal matrix has internal bonds and is bonded to the chamber in order to remove all thermal contact resistance within the composite structure. Utilization of the invention in a rocket chamber is disclosed as a specific use. Also disclosed is a method of constructing the heat exchanger
Clustering and gelation of hard spheres induced by the Pickering effect
A mixture of hard-sphere particles and model emulsion droplets is studied
with a Brownian dynamics simulation. We find that the addition of nonwetting
emulsion droplets to a suspension of pure hard spheres can lead to both
gas-liquid and fluid-solid phase separations. Furthermore, we find a stable
fluid of hard-sphere clusters. The stability is due to the saturation of the
attraction that occurs when the surface of the droplets is completely covered
with colloidal particles. At larger emulsion droplet densities a percolation
transition is observed. The resulting networks of colloidal particles show
dynamical and mechanical properties typical of a colloidal gel. The results of
the model are in good qualitative agreement with recent experimental findings
[E. Koos and N. Willenbacher, Science 331, 897 (2011)] in a mixture of
colloidal particles and two immiscible fluids.Comment: 5 figures, 5 page
Computer simulations of colloidal transport on a patterned magnetic substrate
We study the transport of paramagnetic colloidal particles on a patterned
magnetic substrate with kinetic Monte Carlo and Brownian dynamics computer
simulations. The planar substrate is decorated with point dipoles in either
parallel or zigzag stripe arrangements and exposed to an additional external
magnetic field that oscillates in time. For the case of parallel stripes we
find that the magnitude and direction of the particle current is controlled by
the tilt angle of the external magnetic field. The effect is reliably obtained
in a wide range of ratios between temperature and magnetic permeability.
Particle transport is achieved only when the period of oscillation of the
external field is greater than a critical value. For the case of zigzag stripes
a current is obtained using an oscillating external field normal to the
substrate. In this case, transport is only possible in the vertex of the
zigzag, giving rise to a narrow stream of particles. The magnitude and
direction of the particle current are found to be controlled by a combination
of the zigzag angle and the distance of the colloids from the substrate.
Metropolis Monte Carlo and Brownian dynamics simulations predict results that
are in good agreement with each other. Using kinetic Monte Carlo we find that
at high density the particle transport is hindered by jamming.Comment: 8 pages, 9 figure
Crystallization and gelation in colloidal systems with short-ranged attractive interactions
We systematically study the relationship between equilibrium and
non-equilibrium phase diagrams of a system of short-ranged attractive colloids.
Using Monte Carlo and Brownian dynamics simulations we find a window of
enhanced crystallization that is limited at high interaction strength by a
slowing down of the dynamics and at low interaction strength by the high
nucleation barrier. We find that the crystallization is enhanced by the
metastable gas-liquid binodal by means of a two-stage crystallization process.
First, the formation of a dense liquid is observed and second the crystal
nucleates within the dense fluid. In addition, we find at low colloid packing
fractions a fluid of clusters, and at higher colloid packing fractions a
percolating network due to an arrested gas-liquid phase separation that we
identify with gelation. We find that this arrest is due to crystallization at
low interaction energy and it is caused by a slowing down of the dynamics at
high interaction strength. Likewise, we observe that the clusters which are
formed at low colloid packing fractions are crystalline at low interaction
energy, but glassy at high interaction energy. The clusters coalesce upon
encounter.Comment: 8 pages, 8 figure
Electromagnetic waves in gravitational wave spacetimes
We have considered the propagation of electromagnetic waves in a space-time representing an exact gravitational plane wave and calculated the induced changes on the four-potential field Aμ of a plane electromagnetic wave. By choosing a suitable photon round-trip in a Michelson interferometer, we have been able to identify the physical effects of the exact gravitational wave on the electromagnetic field, i.e. phase shift, change of the polarization vector, angular deflection and delay. These results have been exploited to study the response of an interferometric gravitational wave detector beyond the linear approximation of the general theory of relativity. A much more detailled examination of this problem can be found in our paper recently published in Classical and Quantum Gravity (28 (2011) 235007)
A comparison between matter wave and light wave interferometers for the detection of gravitational waves
We calculate and compare the response of light wave interferometers and
matter wave interferometers to gravitational waves. We find that metric matter
wave interferometers will not challenge kilometric light wave interferometers
such as Virgo or LIGO, but could be a good candidate for the detection of very
low frequency gravitational waves
Structural Transitions in A Crystalline Bilayer : The Case of Lennard Jones and Gaussian Core Models
We study structural transitions in a system of interacting particles arranged
as a crystalline bilayer, as a function of the density and the distance
between the layers. As is decreased a sequence of transitions involving
triangular, rhombic, square and centered rectangular lattices is observed. The
sequence of phases and the order of transitions depends on the nature of
interactions.Comment: 11 pages,6 figure
Effects of microstructure and casting defects on the fatigue behavior of the high-pressure die-cast AlSi9Cu3(Fe) alloy
Abstract High-pressure die-cast (HPDC) components are being increasingly used due to good flexibility and high productivity. These aspects make HPDC suitable to produce several mass components, especially for the automotive sector. Due to the rapid filling of the die and high cooling rate, the process generally leads to the formation of a wide variety of defects, such as porosity and oxide films. Such defects might act as starting points for fatigue cracks and thus deteriorating the fatigue behavior of the casting. To this respect, the fatigue behavior of die cast aluminum alloys is an important aspect to consider when assessing the performance of complex castings for automotive applications. In the light of these aspects, the goal of this work is to describe how the microstructure affects the fatigue crack initiation and propagation. Die cast AlSi9Cu3(Fe) specimens were produced by means of a specifically designed die and the microstructure was preliminary characterized. Uniaxial fatigue tests were performed at load control with a stress ratio of R = 0.1 and at a single level of stress amplitude. After the fatigue tests, the samples were investigated to assess the propagation of the fatigue cracks; the starting points of cracks were specifically identified and the obtained data suggested how defects strongly influence the damage mechanism of the material
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