239 research outputs found
Boundary element formulations for the numerical solution of two-dimensional diffusion problems with variable coefficients
This is the post-print version of the final paper published in Computers & Mathematics with Applications. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2012 Elsevier B.V.This paper presents new formulations of the radial integration boundary integral equation (RIBIE) and the radial integration boundary integro-differential equation (RIBIDE) methods for the numerical solution of two-dimensional diffusion problems with variable coefficients. The methods use either a specially constructed parametrix (Levi function) or the standard fundamental solution for the Laplace equation to reduce the boundary-value problem (BVP) to a boundary–domain integral equation (BDIE) or boundary–domain integro-differential equation (BDIDE). The radial integration method (RIM) is then employed to convert the domain integrals arising in both BDIE and BDIDE methods into equivalent boundary integrals. The resulting formulations lead to pure boundary integral and integro-differential equations with no domain integrals. Furthermore, a subdomain decomposition technique (SDBDIE) is proposed, which leads to a sparse system of linear equations, thus avoiding the need to calculate a large number of domain integrals. Numerical examples are presented for several simple problems, for which exact solutions are available, to demonstrate the efficiency of the proposed approaches
Three-Dimensional Spontaneous Flow Transition in a Homeotropic Active Nematic
We study the three-dimensional spontaneous flow transition of an active
nematic in an infinite slab geometry using a combination of numerics and
analytics. We show that it is determined by the interplay of two eigenmodes --
called S- and D-mode -- that are unstable at the same activity threshold and
spontaneously breaks both rotational symmetry and chiral symmetry. The onset of
the unstable modes is described by a non-Hermitian integro-differential
operator, which we determine their exponential growth rates from using
perturbation theory. The S-mode is the fastest growing. After it reaches a
finite amplitude, the growth of the D-mode is anisotropic, being promoted
perpendicular to the S-mode and suppressed parallel to it, forming a steady
state with a full three-dimensional director field and a well-defined
chirality. Lastly, we derive a model of the leading-order time evolution of the
system close to the activity threshold.Comment: 16 pages, 7 figure
Orientational properties of nematic disclinations
Topological defects play a pivotal role in the physics of liquid crystals and
represent one of the most prominent and well studied aspects of mesophases.
While in two-dimensional nematics, disclinations are traditionally treated as
point-like objects, recent experimental studies on active nematics have
suggested that half-strength disclinations might in fact possess a polar
structure. In this article, we provide a precise definition of polarity for
half-strength nematic disclinations, we introduce a simple and robust method to
calculate this quantity from experimental and numerical data and we investigate
how the orientational properties of half-strength disclinations affect their
relaxational dynamics.Comment: 6 pages, 5 figures, supplementary movies at
http://wwwhome.lorentz.leidenuniv.nl/~giomi/sup_mat/20150720
Continuous Mental Effort Evaluation during 3D Object Manipulation Tasks based on Brain and Physiological Signals
Designing 3D User Interfaces (UI) requires adequate evaluation tools to
ensure good usability and user experience. While many evaluation tools are
already available and widely used, existing approaches generally cannot provide
continuous and objective measures of usa-bility qualities during interaction
without interrupting the user. In this paper, we propose to use brain (with
ElectroEncephaloGraphy) and physiological (ElectroCardioGraphy, Galvanic Skin
Response) signals to continuously assess the mental effort made by the user to
perform 3D object manipulation tasks. We first show how this mental effort
(a.k.a., mental workload) can be estimated from such signals, and then measure
it on 8 participants during an actual 3D object manipulation task with an input
device known as the CubTile. Our results suggest that monitoring workload
enables us to continuously assess the 3DUI and/or interaction technique
ease-of-use. Overall, this suggests that this new measure could become a useful
addition to the repertoire of available evaluation tools, enabling a finer
grain assessment of the ergonomic qualities of a given 3D user interface.Comment: Published in INTERACT, Sep 2015, Bamberg, German
Double-diffusive natural convection within a 3D porous enclosure, using the boundary element method
Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.A three-dimensional numerical study based on the boundary
element method (BEM) was performed in order to study the
problem of double-diffusive natural convection within a cubic
enclosure filled with a fluid-saturated porous media, and
subjected to horizontal temperature and concentration
gradients. The fluid-flow within the porous media was modeled
using space-averaged Navier-Stokes equations, coupled with
energy and species equations. The used numerical algorithm is
based on a combination of single domain and sub-domain
BEM, and solves the velocity-vorticity formulation of the
governing equations. The influences of the main controlling
parameters, such as the porous Rayleigh number, Darcy
number, Lewis number, and the buoyancy coefficient were
investigated, by focusing on those situations, where the flowfield
becomes 3D. The results for overall heat and solutetransfer
through the porous enclosure are presented in terms of
Nusselt and Sherwood numbers as functions of the governing
parameters, and then compared to the numerical benchmarks
published in literature.mp201
Three-dimensional spontaneous flow transition in a homeotropic active nematic
Active nematics are driven, non-equilibrium systems relevant to biological processes including tissue mechanics and morphogenesis, and to active metamaterials in general. We study the three-dimensional spontaneous flow transition of an active nematic in an infinite slab geometry using a combination of numerics and analytics. We show that it is determined by the interplay of two eigenmodes – called S- and D-mode – that are unstable at the same activity threshold and spontaneously breaks both rotational symmetry and chiral symmetry. The onset of the unstable modes is described by a non-Hermitian integro-differential operator, which we determine their exponential growth rates from using perturbation theory. The S-mode is the fastest growing. After it reaches a finite amplitude, the growth of the D-mode is anisotropic, being promoted perpendicular to the S-mode and suppressed parallel to it, forming a steady state with a full three-dimensional director field and a well-defined chirality. Lastly, we derive a model of the leading-order time evolution of the system close to the activity threshold
Defect structures in nematic liquid crystals around charged particles
We numerically study the orientation deformations in nematic liquid crystals
around charged particles. We set up a Ginzburg-Landau theory with inhomogeneous
electric field. If the dielectric anisotropy varepsilon_1 is positive, Saturn
ring defects are formed around the particles. For varepsilon_1<0, novel "ansa"
defects appear, which are disclination lines with their ends on the particle
surface. We find unique defect structures around two charged particles. To
lower the free energy, oppositely charged particle pairs tend to be aligned in
the parallel direction for varepsilon_1>0 and in the perpendicular plane for
varepsilon_1<0 with respect to the background director . For identically
charged pairs the preferred directions for varepsilon_1>0 and varepsilon_1<0
are exchanged. We also examie competition between the charge-induced anchoring
and the short-range anchoring. If the short-range anchoring is sufficiently
strong, it can be effective in the vicinity of the surface, while the director
orientation is governed by the long-range electrostatic interaction far from
the surface.Comment: 10 papes, 12 figures, to appear in European Physical Journal
Improving long term driving comfort by taking breaks - how break activity affects effectiveness
During long duration journeys, drivers are encouraged to take regular breaks. The benefits of breaks have been documented for safety; breaks may also be beneficial for comfort. The activity undertaken during a break may influence its effectiveness. Volunteers completed 3 journeys on a driving simulator. Each 130 min journey included a 10 min break after the first hour. During the break volunteers either stayed seated, left the simulator and sat in an adjacent room, or took a walk on a treadmill. The results show a reduction in driver discomfort during the break for all 3 conditions, but the effectiveness of the break was dependent on activity undertaken. Remaining seated in the vehicle provided some improvement in comfort, but more was experienced after leaving the simulator and sitting in an adjacent room. The most effective break occurred when the driver walked for 10 min on a treadmill. The benefits from taking a break continued until the end of the study (after a further hour of driving), such that comfort remained the best after taking a walk and worst for those who remained seated. It is concluded that taking a break and taking a walk is an effective method for relieving driving discomfort
Self-assembly of colloid-cholesteric composites provides a possible route to switchable optical materials
Colloidal particles dispersed in liquid crystals can form new materials with
tunable elastic and electro-optic properties. In a periodic `blue phase' host,
particles should template into colloidal crystals with potential uses in
photonics, metamaterials, and transformational optics. Here we show by computer
simulation that colloid/cholesteric mixtures can give rise to regular crystals,
glasses, percolating gels, isolated clusters, twisted rings and undulating
colloidal ropes. This structure can be tuned via particle concentration, and by
varying the surface interactions of the cholesteric host with both the
particles and confining walls. Many of these new materials are metastable: two
or more structures can arise under identical thermodynamic conditions. The
observed structure depends not only on the formulation protocol, but also on
the history of an applied electric field. This new class of soft materials
should thus be relevant to design of switchable, multistable devices for
optical technologies such as smart glass and e-paper.Comment: Manuscript with 3 figures plus supporting text and figure
- …