88 research outputs found
Numerical simulation of helical-vortex effects in Rayleigh-Bénard convection
International audienceA numerical approach is substantiated for searching for the large-scale alpha-like instability in thermoconvective turbulence. The main idea of the search strategy is the application of a forcing function which can have a physical interpretation. The forcing simulates the influence of small-scale helical turbulence generated in a rotating fluid with internal heat sources and is applied to naturally induced fully developed convective flows. The strategy is tested using the Rayleigh-Bénard convection in an extended horizontal layer of incompressible fluid heated from below. The most important finding is an enlargement of the typical horizontal scale of the forming helical convective structures accompanied by a cells merging, an essential increase in the kinetic energy of flows and intensification of heat transfer. The results of modeling allow explaining how the helical feedback can work providing the non-zero mean helicity generation and the mutual intensification of horizontal and vertical circulation, and demonstrate how the energy of the additional helical source can be effectively converted into the energy of intensive large-scale vortex flow
Interplay between dipole and quadrupole modes of field influence in liquid-crystalline suspensions of ferromagnetic particles
In the framework of continuum theory we study orientational transitions
induced by electric and magnetic fields in ferronematics, i.e., in
liquid-crystalline suspensions of ferromagnetic particles. We have shown that
in a certain electric field range the magnetic field can induce a sequence of
re-entrant orientational transitions in ferronematic layer: nonuniform phase
--- uniform phase --- nonuniform phase. This phenomenon is caused by the
interplay between the dipole (ferromagnetic) and quadrupole (dielectric and
diamagnetic) mechanisms of the field influence on a ferronematic structure. We
have found that these re-entrant Freedericksz transitions exhibit tricritical
behavior, i.e., they can be of the first or the second order. The character of
the transitions depends on a degree of redistribution of magnetic admixture in
the sample exposed to uniform magnetic field (magnetic segregation). We
demonstrate how electric and magnetic fields can change the order of
orientational transitions in ferronematics. We show that electric Freedericksz
transitions in ferronematics subjected to magnetic field have no re-entrant
nature. Tricritical segregation parameters for the transitions induced by
electric or magnetic fields are obtained analytically. We demonstrate the
re-entrant behavior of ferronematic by numerical simulations of the
magnetization and optical phase lag.Comment: 12 pages, 9 figures, to be published in Soft Matte
Effective triplet interactions in nematic colloids
Three-body effective interactions emerging between parallel cylindrical rods
immersed in a nematic liquid crystals are calculated within the Landau-de
Gennes free energy description. Collinear, equilateral and midplane
configurations of the three colloidal particles are considered. In the last two
cases the effective triplet interaction is of the same magnitude and range as
the pair one
Implementation of an electrical network monitoring system based on a non-contact temperature sensor and a current sensor based on the Hall effect
The safety of people and the environment is an important issue at any enterprise. One of the main threats in the production environment is the occurrence of fire and the failure of electrical equipment. Fire alarm systems provide immediate evacuation of people and fight the source of fire. The article describes implementation of an electrical circuit system based on a non-contact temperature sensor and a Hall effect current sensor, which is necessary for fire prevention. The experiment showed that the data obtained is sufficient for timely change of current level in the conductors and prevention of emergency situation. The article consists of an introduction, theoretical information about the temperature sensor and current sensor based on the Hall effect, description of Bluetooth Mesh network topology, development of a wireless network diagram for the electrical network monitoring system, conclusion
Forces between elongated particles in a nematic colloid
Using molecular dynamics simulations we study the interactions between elongated colloidal particles (length to breath ratio ≫1) in a nematic host. The simulation results are compared to the results of a Landau–de Gennes elastic free energy. We find that depletion forces dominate for the sizes of the colloidal particles studied. The tangential component of the force, however, allows us to resolve the elastic contribution to the total interaction. We find that this contribution differs from the quadrupolar interaction predicted at large separations. The difference is due to the presence of nonlinear effects, namely, the change in the positions and structure of the defects and their annihilation at small separations
Defect structures and torque on an elongated colloidal particle immersed in a liquid crystal host
Combining molecular dynamics and Monte Carlo simulation we study defect
structures around an elongated colloidal particle embedded in a nematic liquid
crystal host. By studying nematic ordering near the particle and the
disclination core region we are able to examine the defect core structure and
the difference between two simulation techniques. In addition, we also study
the torque on a particle tilted with respect to the director, and modification
of this torque when the particle is close to the cell wall
Inhomogeneous States in a Small Magnetic Disk with Single-Ion Surface Anisotropy
We investigate analytically and numerically the ground and metastable states
for easy-plane Heisenberg magnets with single-ion surface anisotropy and disk
geometry. The configurations with two half-vortices at the opposite points of
the border are shown to be preferable for strong anisotropy. We propose a
simple analytical description of the spin configurations for all values of a
surface anisotropy. The effects of lattice pinning leads to appearance of a set
of metastable configurations.Comment: 10 pages, 7 figures; submitted to Phys. Rev.
Principal molecular axis and transition dipole moment orientations in liquid crystal systems: an assessment based on studies of guest anthraquinone dyes in a nematic host
An assessment of five different definitions of the principal molecular axis along which molecules align in a nematic liquid crystal system has been made by analysing fully atomistic molecular dynamics (MD) simulations of a set of anthraquinone dyes in the cyanobiphenyl-based nematic host mixture E7. Principal molecular axes of the dyes defined by minimum moment of inertia, minimum circumference, minimum area, maximum aspect ratio, and surface tensor models were tested, and the surface tensor model was found to give the best description. Analyses of MD simulations of E7 alone showed that the surface tensor model also gave a good description of the principal molecular axes of the host molecules, suggesting that this model may be applicable more generally. Calculated dichroic order parameters of the guest-host systems were obtained by combining the surface tensor analysis with fixed transition dipole moment (TDM) orientations from time-dependent density functional theory (TD-DFT) calculations on optimised structures of the dyes, and the trend between the dyes generally matched the trend in the experimental values. Additional analyses of the guest-host simulations identified the range of conformers explored by the flexible chromophores within the dyes, and TD-DFT calculations on corresponding model structures showed that this flexibility has a significant effect on the TDM orientations within the molecular frames. Calculated dichroic order parameters that included the effects of this flexibility gave a significantly improved match with the experimental values for the more flexible dyes. Overall, the surface tensor model has been shown to provide a rationale for the experimental alignment trends that is based on molecular shape, and molecular flexibility within the chromophores has been shown to be significant for the guest-host systems: The computational approaches reported here may be used as a general aid in the predictive design of dyes with appropriate molecular shapes and flexibilities for guest-host applications
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