168 research outputs found
Toward a continuum model for particle-induced velocity fluctuations in suspension flow through a stenosed geometry
Non-particulate continuum descriptions allow for computationally efficient modeling of suspension flows at scales that are inaccessible to more detailed particulate approaches. It is well known that the presence of particles influences the effective viscosity of a suspension and that this effect has thus to be accounted for in macroscopic continuum models. The present paper aims at developing a non-particulate model that reproduces not only the rheology but also the cell-induced velocity fluctuations, responsible for enhanced diffusivity. The results are obtained from a coarse-grained blood model based on the lattice Boltzmann method. The benchmark system comprises a flow between two parallel plates with one of them featuring a smooth obstacle imitating a stenosis. Appropriate boundary conditions are developed for the particulate model to generate equilibrated cell configurations mimicking an infinite channel in front of the stenosis. The averaged flow field in the bulk of the channel can be described well by a non-particulate simulation with a matched viscosity. We show that our proposed phenomenological model is capable to reproduce many features of the velocity fluctuations
A simplified particulate model for coarse-grained hemodynamics simulations
Human blood flow is a multi-scale problem: in first approximation, blood is a
dense suspension of plasma and deformable red cells. Physiological vessel
diameters range from about one to thousands of cell radii. Current
computational models either involve a homogeneous fluid and cannot track
particulate effects or describe a relatively small number of cells with high
resolution, but are incapable to reach relevant time and length scales. Our
approach is to simplify much further than existing particulate models. We
combine well established methods from other areas of physics in order to find
the essential ingredients for a minimalist description that still recovers
hemorheology. These ingredients are a lattice Boltzmann method describing rigid
particle suspensions to account for hydrodynamic long range interactions
and---in order to describe the more complex short-range behavior of
cells---anisotropic model potentials known from molecular dynamics simulations.
Paying detailedness, we achieve an efficient and scalable implementation which
is crucial for our ultimate goal: establishing a link between the collective
behavior of millions of cells and the macroscopic properties of blood in
realistic flow situations. In this paper we present our model and demonstrate
its applicability to conditions typical for the microvasculature.Comment: 12 pages, 11 figure
Low temperature magnetic structure of CeRhIn by neutron diffraction on absorption-optimized samples
Two aspects of the ambient pressure magnetic structure of heavy fermion
material CeRhIn have remained under some debate since its discovery:
whether the structure is indeed an incommensurate helix or a spin density wave,
and what is the precise magnitude of the ordered magnetic moment. By using a
single crystal sample optimized for hot neutrons to minimize neutron absorption
by Rh and In, here we report an ordered moment of . In
addition, by using spherical neutron polarimetry measurements on a similar
single crystal sample, we have confirmed the helical nature of the magnetic
structure, and identified a single chiral domain
Helimagnon Bands as Universal Spin Excitations of Chiral Magnets
MnSi is a cubic compound with small magnetic anisotropy, which stabilizes a
helimagnetic spin spiral that reduces to a ferromagnetic and antiferromagnetic
state in the long- and short-wavelength limit, respectively. We report a
comprehensive inelastic neutron scattering study of the collective magnetic
excitations in the helimagnetic state of MnSi. In our study we observe a rich
variety of seemingly anomalous excitation spectra, as measured in well over
twenty different locations in reciprocal space. Using a model based on only
three parameters, namely the measured pitch of the helix, the measured
ferromagnetic spin wave stiffness and the amplitude of the signal, as the only
free variable, we can simultaneously account for \textit{all} of the measured
spectra in excellent quantitative agreement with experiment. Our study
identifies the formation of intense, strongly coupled bands of helimagnons as a
universal characteristic of systems with weak chiral interactions.Comment: 8 pages, 4 figures, references updated, introduction updated,
reformatte
Acetylene, Vinylidene, and the Vinyl Cation in Ground and Excited States
Ab initio calculations using the improved virtual orbital formalism
are reported for acetylene, vinylidene, and the vinyl cation,
C2H,+, in classical and bridged geometries. Electronic transition
energies and equilibrium geometries for ground and lower lying
electronically excited states have been calculated. A modified
Walsh diagram for acetylene and simple molecular orbital considerations
explain excited state structures and energy orderings.
While acetylene in the ground state is much more stable than
vinylidene, the energies of several corresponding excited states are
comparable. The stabilities of bridged and classical structures of
the vinyl cation are very similar in the ground state, but in the
various excited states either strcture can predominate. The proton
affinity of acetylene in the ground state should be appreciably
lower than in excited states
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