1,055 research outputs found
Recommended from our members
The Characteristics and Applications of Ceramic Laser Fusion and Ceramic Laser Sintering
The aim of present study is to investigate the possible application of the ceramic parts
which are fabricated with the process of Ceramic Laser Fusion or Ceramic Laser Sintering.
The experimental results reveal: (1) CLF can lead to a reduction in the porosity of the
ceramic part but also can induce micro-cracks. Therefore, this process cannot produce a part
with the required strength by a post-process of infiltration; (2) CLS is capable of fabricating a
ceramic part with high porosity. By adjusting the slurry formulation and varying the scanning
energy, the open porosity can be over 90vol% of the total porosity. After a post-process of
infiltration, the density can be increased to 95%; therefore, CLS can apply to produce a part
with high strength. Because the high open porosity leads to a good permeability, the process
of CLS is suitable for the fabrication of ceramic shell mold.Mechanical Engineerin
A dynamic method for magnetic torque measurement
In a magnetic suspension system, accurate force measurement will result in better control performance in the test section, especially when a wider range of operation is required. Although many useful methods were developed to obtain the desired model, however, significant error is inevitable since the magnetic field distribution of the large-gap magnetic suspension system is extremely nonlinear. This paper proposed an easy approach to measure the magnetic torque of a magnetic suspension system using an angular photo encoder. Through the measurement of the velocity change data, the magnetic torque is converted. The proposed idea is described and implemented to obtain the desired data. It is useful to the calculation of a magnetic force in the magnetic suspension system
Diffuse-interface model for rapid phase transformations in nonequilibrium systems
A thermodynamic approach to rapid phase transformations within a diffuse
interface in a binary system is developed. Assuming an extended set of
independent thermodynamic variables formed by the union of the classic set of
slow variables and the space of fast variables, we introduce finiteness of the
heat and solute diffusive propagation at the finite speed of the interface
advancing. To describe the transformation within the diffuse interface, we use
the phase-field model which allows us to follow the steep but smooth change of
phases within the width of diffuse interface. The governing equations of the
phase-field model are derived for the hyperbolic model, model with memory, and
for a model of nonlinear evolution of transformation within the
diffuse-interface. The consistency of the model is proved by the condition of
positive entropy production and by the outcomes of the fluctuation-dissipation
theorem. A comparison with the existing sharp-interface and diffuse-interface
versions of the model is given.Comment: 15 pages, regular article submitted to Physical Review
Attenuation and damping of electromagnetic fields: Influence of inertia and displacement current
New results for attenuation and damping of electromagnetic fields in rigid
conducting media are derived under the conjugate influence of inertia due to
charge carriers and displacement current. Inertial effects are described by a
relaxation time for the current density in the realm of an extended Ohm's law.
The classical notions of poor and good conductors are rediscussed on the basis
of an effective electric conductivity, depending on both wave frequency and
relaxation time. It is found that the attenuation for good conductors at high
frequencies depends solely on the relaxation time. This means that the
penetration depth saturates to a minimum value at sufficiently high
frequencies. It is also shown that the actions of inertia and displacement
current on damping of magnetic fields are opposite to each other. That could
explain why the classical decay time of magnetic fields scales approximately as
the diffusion time. At very small length scales, the decay time could be given
either by the relaxation time or by a fraction of the diffusion time, depending
whether inertia or displacement current, respectively, would prevail on
magnetic diffusion.Comment: 21 pages, 1 figur
Stability of inflationary solutions driven by a changing dissipative fluid
In this paper the second Lyapunov method is used to study the stability of
the de Sitter phase of cosmic expansion when the source of the gravitational
field is a viscous fluid. Different inflationary scenarios related with
reheating and decay of mini-blackholes into radiation are investigated using an
effective fluid described by time--varying thermodynamical quantities.Comment: 17 pages, LaTeX 2.09, 2 figures. To be published in Classical and
Quantum Gravit
Progress Toward Efficient Laminar Flow Analysis and Design
A multi-fidelity system of computer codes for the analysis and design of vehicles having extensive areas of laminar flow is under development at the NASA Langley Research Center. The overall approach consists of the loose coupling of a flow solver, a transition prediction method and a design module using shell scripts, along with interface modules to prepare the input for each method. This approach allows the user to select the flow solver and transition prediction module, as well as run mode for each code, based on the fidelity most compatible with the problem and available resources. The design module can be any method that designs to a specified target pressure distribution. In addition to the interface modules, two new components have been developed: 1) an efficient, empirical transition prediction module (MATTC) that provides n-factor growth distributions without requiring boundary layer information; and 2) an automated target pressure generation code (ATPG) that develops a target pressure distribution that meets a variety of flow and geometry constraints. The ATPG code also includes empirical estimates of several drag components to allow the optimization of the target pressure distribution. The current system has been developed for the design of subsonic and transonic airfoils and wings, but may be extendable to other speed ranges and components. Several analysis and design examples are included to demonstrate the current capabilities of the system
Magnetic relaxation in the Bianchi-I universe
Extended Einstein-Maxwell model and its application to the problem of
evolution of magnetized Bianchi-I Universe are considered. The evolution of
medium magnetization is governed by a relaxation type extended constitutive
equation. The series of exact solutions to the extended master equations is
obtained and discussed. The anisotropic expansion of the Bianchi-I Universe is
shown to become non-monotonic (accelerated/decelerated) in both principal
directions (along the magnetic field and orthogonal to it). A specific type of
expansion, the so-called evolution with hidden magnetic field, is shown to
appear when the magnetization effectively screens the magnetic field and the
latter disappears from the equations for gravitational field.Comment: 32 page
Thermal Conduction in Systems out of Hydrostatic Equilibrium
We analyse the effects of thermal conduction in a relativistic fluid, just
after its departure from hydrostatic equilibrium, on a time scale of the order
of thermal relaxation time. It is obtained that the resulting evolution will
critically depend on a parameter defined in terms of thermodynamic variables,
which is constrained by causality requirements.Comment: 16 pages, emTex (LaTex 2.09). To appear in Classical and Quantum
Gravit
Steady shear flow thermodynamics based on a canonical distribution approach
A non-equilibrium steady state thermodynamics to describe shear flows is
developed using a canonical distribution approach. We construct a canonical
distribution for shear flow based on the energy in the moving frame using the
Lagrangian formalism of the classical mechanics. From this distribution we
derive the Evans-Hanley shear flow thermodynamics, which is characterized by
the first law of thermodynamics relating infinitesimal
changes in energy , entropy and shear rate with kinetic
temperature . Our central result is that the coefficient is given by
Helfand's moment for viscosity. This approach leads to thermodynamic stability
conditions for shear flow, one of which is equivalent to the positivity of the
correlation function of . We emphasize the role of the external work
required to sustain the steady shear flow in this approach, and show
theoretically that the ensemble average of its power must be
non-negative. A non-equilibrium entropy, increasing in time, is introduced, so
that the amount of heat based on this entropy is equal to the average of
. Numerical results from non-equilibrium molecular dynamics simulation
of two-dimensional many-particle systems with soft-core interactions are
presented which support our interpretation.Comment: 23 pages, 7 figure
Effective dynamics of a nonabelian plasma out of equilibrium
Starting from kinetic theory, we obtain a nonlinear dissipative formalism
describing the nonequilibrium evolution of scalar colored particles coupled
selfconsistently to nonabelian classical gauge fields. The link between the
one-particle distribution function of the kinetic description and the variables
of the effective theory is determined by extremizing the entropy production.
This method does not rely on the usual gradient expansion in fluid dynamic
variables, and therefore the resulting effective theory can handle situations
where these gradients (and hence the momentum-space anisotropies) are expected
to be large. The formalism presented here, being computationally less demanding
than kinetic theory, may be useful as a simplified model of the dynamics of
color fields during the early stages of heavy ion collisions and in phenomena
related to parton energy loss.Comment: 20 two-column pages, 2 figures. v3: minor changes. Accepted for
publication in Phys. Rev.
- …