67,091 research outputs found
Fluid mechanics of nodal flow due to embryonic primary cilia
Breaking of leftâright symmetry is crucial in vertebrate development. The role of cilia-driven flow has been the subject of many recent publications, but the underlying mechanisms remain controversial. At approximately 8 days post-fertilization, after the establishment of the dorsalâventral and anteriorâposterior axes, a depressed structure is found on the ventral side of mouse embryos, termed the ventral node. Within the node, âwhirlingâ primary cilia, tilted towards the posterior, drive a flow implicated in the initial leftâright signalling asymmetry. However, the underlying fluid mechanics have not been fully and correctly explained until recently and accurate characterization is required in determining how the flow triggers the downstream signalling cascades. Using the approximation of resistive force theory, we show how the flow is produced and calculate the optimal configuration to cause maximum flow, showing excellent agreement with in vitro measurements and numerical simulation, and paralleling recent analogue experiments. By calculating numerical solutions of the slender body theory equations, we present time-dependent physically based fluid dynamics simulations of particle pathlines in flows generated by large arrays of beating cilia, showing the far-field radial streamlines predicted by the theory
Quasicontinuum simulation of fracture at the atomic scale
We study the problem of atomic scale fracture using the recently developed quasicontinuum method in which there is a systematic thinning of the atomic-level degrees of freedom in regions where they are not needed. Fracture is considered in two distinct settings. First, a study is made of cracks in single crystals, and second, we consider a crack advancing towards a grain boundary (GB) in its path. In the investigation of single crystal fracture, we evaluate the competition between simple cleavage and crack-tip dislocation emission. In addition, we examine the ability of analytic models to correctly predict fracture behaviour, and find that the existing analytical treatments are too restrictive in their treatment of nonlinearity near the crack tip. In the study of GB-crack interactions, we have found a number of interesting deformation mechanisms which attend the advance of the crack. These include the migration of the GB, the emission of dislocations from the GB, and deflection of the crack front along the GB itself. In each case, these mechanisms are rationalized on the basis of continuum mechanics arguments
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Impact and spreading of microdrops on homo- and heterogeneous solids: Modelling and benchmark simulations
This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.The finite element framework developed for the high accuracy computation of dynamic wetting phenomena in Sprittles & Shikhmurzaev, Int. J. Num. Meth. Fluids 2011 is used to develop a code for the simulation of unsteady flows such as microdrop impact and spreading. The accuracy of the code for
describing free-surface flows is tested by comparing its results to those obtained in previous numerical studies for the large amplitude oscillations of free liquid drops in zero gravity. The capability of our code
to produce high resolution benchmark calculations for dynamic wetting flows, using either conventional modelling or the more sophisticated interface formation model, is demonstrated by simulating microdrop impact and spreading on surfaces of greatly differing wettability. The simulations allow one to see features of the drop shape which are beyond the resolution of experiments. Directions of our research programme that follows the presented study are outlined
On acoustic propagation in three-dimensional rectangular ducts with flexible walls and porous linings
This is the post-print version of the Article. The official published version can be accessed from the links below - Copyright @ 2012 Acoustical Society of AmericaThe focus of this article is toward the development of hybrid analytic-numerical mode-matching methods for model problems involving three-dimensional ducts of rectangular cross-section and with flexible walls. Such methods require first closed form analytic expressions for the natural fluid-structure coupled waveforms that propagate in each duct section and second the corresponding orthogonality relations. It is demonstrated how recent theory [Lawrie, Proc. R. Soc. London, Ser. A 465, 2347â2367 (2009)] may be extended to a wide class of three-dimensional ducts, for example, those with a flexible wall and a porous lining (modeled as an equivalent fluid) or those with a flexible internal structure, such as a membrane (the âdrum-likeâ silencer). Two equivalent expressions for the eigenmodes of a given duct can be formulated. For the ducts considered herein, the first ansatz is dependent on the eigenvalues/eigenfunctions appropriate for wave propagation in the corresponding two-dimensional flexible-walled duct, whereas the second takes the form of a Fourier series. The latter offers two advantages: no âroot-findingâ is involved and the method is appropriate for ducts in which the flexible wall is orthotropic. The first ansatz, however, provides important information about the orthogonality properties of the three-dimensional eigenmodes
KOI 1224, a Fourth Bloated Hot White Dwarf Companion Found With Kepler
We present an analysis and interpretation of the Kepler binary system KOI
1224. This is the fourth binary found with Kepler that consists of a thermally
bloated, hot white dwarf in a close orbit with a more or less normal star of
spectral class A or F. As we show, KOI 1224 contains a white dwarf with Teff =
14400 +/- 1100 K, mass = 0.20 +/- 0.02 Msun, and radius = 0.103 +/- 0.004 Rsun,
and an F-star companion of mass = 1.59 +/- 0.07 Msun that is somewhat beyond
its terminal-age main sequence. The orbital period is quite short at 2.69802
days. The ingredients that are used in the analysis are the Kepler binary light
curve, including the detection of the Doppler boosting effect; the NUV and FUV
fluxes from the Galex images of this object; an estimate of the spectral type
of the F-star companion; and evolutionary models of the companion designed to
match its effective temperature and mean density. The light curve is modelled
with a new code named Icarus which we describe in detail. Its features include
the full treatment of orbital phase-resolved spectroscopy, Doppler boosting,
irradiation effects and transits/eclipses, which are particularly suited to
irradiated eclipsing binaries. We interpret the KOI 1224 system in terms of its
likely evolutionary history. We infer that this type of system, containing a
bloated hot white dwarf, is the direct descendant of an Algol-type binary. In
spite of this basic understanding of the origin of KOI 1224, we discuss a
number of problems associated with producing this type of system with this
short of an short orbital period.Comment: 14 pages, 8 figures, 2 tables, submitted to Ap
Viscous flows in corner regions: Singularities and hidden eigensolutions
Numerical issues arising in computations of viscous flows in corners formed
by a liquid-fluid free surface and a solid boundary are considered. It is shown
that on the solid a Dirichlet boundary condition, which removes multivaluedness
of velocity in the `moving contact-line problem' and gives rise to a
logarithmic singularity of pressure, requires a certain modification of the
standard finite-element method. This modification appears to be insufficient
above a certain critical value of the corner angle where the numerical solution
becomes mesh-dependent. As shown, this is due to an eigensolution, which exists
for all angles and becomes dominant for the supercritical ones. A method of
incorporating the eigensolution into the numerical method is described that
makes numerical results mesh-independent again. Some implications of the
unavoidable finiteness of the mesh size in practical applications of the finite
element method in the context of the present problem are discussed.Comment: Submitted to the International Journal for Numerical Methods in
Fluid
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