373 research outputs found
Vortex-Induced Vibration of a Neutrally Buoyant Tethered Sphere
Recent preliminary experiments have indicated that a neutrally buoyant tethered sphere develops a large diameter quasi-circular trajectory, unlike the oscillations observed for non-neutrally buoyant tethered spheres. This shows similarities to the path of buoyant bubbles, which may follow zig-zag and/or helical paths depending on the Reynolds number. The current study explores the behaviour using well resolved numerical simulations. The forces like tension, buoyancy and fluid force are considered. It is found that there exist six different flow regimes within the range of the Reynolds number = [50, 800] according to the sphere response. Regime I (Re = [50, 205]) and Regime II (Re = [210, 260]) are characterised by steady axisymmetric flow structure without body movement except the loss of axisymmetry in Regime II. The sphere starts to vibrate from Regime III (Re = [270, 280]). Regime IV (Re = [300, 330]) shows suppressed body oscillation and steep decrease of off-centered distance in the plane normal to streamwise direction (yz plane). In Regime V (Re = [335, 550]), the sphere oscillates around (0, 0) in yz plane. The sphere of Regime VI (Re = [600, 800]) oscillates rather irregularily. The transitions are compared with those for a fixed sphere. In addition, the effect of moving away from neutral buoyancy is examined
Surface-functionalization of PDMS for potential micro-bioreactor and embryonic stem cell culture applications
This study presents a novel and inexpensive method to prepare a disposable micro-bioreactor for stem cell expansion. The micro-bioreactor was fabricated in the form of a fixed bed bioreactor with a microchannel reactor bed. The micro-bioreactor was constructed from polydimethylsiloxane (PDMS), and the microchannel was functionalized to enable cell adhesion and resistance to bovine serum albumin protein adsorption. The PDMS reactor bed surface was activated by oxygen plasma, then aminized with trimethoxysilylpropyl(polyethyleneimine), followed by grafting with carboxylmethyl cellulose (CMC) and gelatin in sequence. The functionalized PDMS surface demonstrated improved hydrophilicity and antifouling properties. The grafting of gelatin promoted cell adhesion. The functionalized surface was found to be biocompatible with MDA-MB-231 and Oct4b2 cells and was demonstrated to facilitate cell proliferation. The expanded Oct4b2 cells retained their proliferation potential, undifferentiated phenotype and pluripotency
Late Neogene tectonically driven crustal exhumation of the Sikkim Himalaya : Insights from inversion of multithermochronologic data
Apatite fission track and zircon (U-Th)/He data are reported for 34 bedrock samples distributed between the foothills and the topographic crest of the Darjeeling-Sikkim Himalaya. The pattern of observed cooling ages does not correlate with topography, rainfall distribution, and the deeply incised high-relief Tista window, indicating that tectonic processes are mainly responsible for their spatial distribution. Inversion of this thermochronometric data set using 3-D thermokinematic modeling constrained by independent geological and geophysical observations was performed to evaluate the contribution of slip partitioning, duplex development, and relief growth on the evolution of the thermal structure of the Himalaya during the last 12Ma. Models involving significant relief growth do not show a substantial influence of topography evolution on the cooling age distribution, while models involving duplex growth demonstrate that tectonic processes exert a dominant influence on their distribution. In concert with equivalent studies in Bhutan, central Nepal, and NW India, our results attest that the lateral variation of the geometry and kinematics of the Himalayan basal decollement locally associated with duplex formation exert a leading influence on lateral variations of middle to upper crustal long-term exhumation rates documented along the strike of the Himalaya.Peer reviewe
Haemodynamical stress in mouse aortic arch with atherosclerotic plaques: Preliminary study of plaque progression
Atherosclerotic plaques develop at particular sites in the arterial tree, and this regional localisation depends largely on haemodynamic parameters (such as wall shear stress; WSS) as described in the literature. Plaque rupture can result in heart attack or stroke and hence understanding the development and vulnerability of atherosclerotic plaques is critically important. The purpose of this study is to characterise the haemodynamics of blood flow in the mouse aortic arch using numerical modelling. The geometries are digitalised from synchrotron imaging and realistic pulsatile blood flow is considered under rigid wall assumptions. Two cases are considered; arteries with and without plaque. Mice that are fed under fat diet present plaques in the aortic arch whose size is dependent on the number of weeks under the diet. The plaque distribution in the region is however relatively constant through the different samples. This result underlines the influence of the geometry and consequently of the wall shear stresses for plaque formation with plaques growing in region of relative low shear stresses. A discussion of the flow field in real geometry in the presence and absence of plaques is conducted. The presence of plaques was shown to alter the blood flow and hence WSS distribution, with regions of localised high WSS, mainly on the wall of the brachiocephalic artery where luminal narrowing is most pronounced. In addition, arch plaques are shown to induce recirculation in the blood flow, a phenomenon with potential influence on the progression of the plaques. The oscillatory shear index and the relative residence time have been calculated on the geometry with plaques to show the presence of this recirculation in the arch, an approach that may be useful for future studies on plaque progression
Three-dimensional numerical simulation of blood flow in mouse aortic arch around atherosclerotic plaques
Atherosclerosis is a progressive disease, involving the build-up of lipid streaks in artery walls, leading to plaques. Understanding the development of atherosclerosis and plaque vulnerability is critically important since plaque rupture can result in heart attack or stroke. Plaques can be divided into two distinct types: those likely to rupture (vulnerable) or less likely to rupture (stable). In the last decade, researchers have been interested in studying the influence of the mechanical effects (blood shear stress, pressure forces and structural stress) on the plaque formation, progression and rupture processes but no general agreement has been found. The purpose of the present work is to include more realistic conditions for the numerical calculations of the blood flow by implementing real geometries with plaques in the numerical model. Hemodynamical parameters are studied in both diseased and healthy configurations. The healthy configuration is obtained by removing numerically the plaques from three dimensional geometries obtained by micro-computed tomography. A new hemodynamical parameter is also introduced to relate the location of plaques to the characteristics of the flow in the healthy configuration
Three-dimensional Calculations of High and Low-mass Planets Embedded in Protoplanetary Discs
We analyse the non-linear, three-dimensional response of a gaseous, viscous
protoplanetary disc to the presence of a planet of mass ranging from one Earth
mass (1 M) to one Jupiter mass (1 M) by using the ZEUS hydrodynamics
code. We determine the gas flow pattern, and the accretion and migration rates
of the planet. The planet is assumed to be in a fixed circular orbit about the
central star. It is also assumed to be able to accrete gas without expansion on
the scale of its Roche radius. Only planets with masses M \gsim 0.1 M
produce significant perturbations in the disc's surface density. The flow
within the Roche lobe of the planet is fully three-dimensional. Gas streams
generally enter the Roche lobe close to the disc midplane, but produce much
weaker shocks than the streams in two-dimensional models. The streams supply
material to a circumplanetary disc that rotates in the same sense as the
planet's orbit. Much of the mass supply to the circumplanetary disc comes from
non-coplanar flow. The accretion rate peaks with a planet mass of approximately
0.1 M and is highly efficient, occurring at the local viscous rate. The
migration timescales for planets of mass less than 0.1 M, based on torques
from disc material outside the planets' Roche lobes, are in excellent agreement
with the linear theory of Type I (non-gap) migration for three-dimensional
discs. The transition from Type I to Type II (gap) migration is smooth, with
changes in migration times of about a factor of 2. Starting with a core which
can undergo runaway growth, a planet can gain up to a few M with little
migration. Planets with final masses of order 10 M would undergo large
migration, which makes formation and survival difficult.Comment: Accepted by MNRAS, 18 pages, 13 figures (6 degraded resolution).
Paper with high-resolution figures available at
http://www.astro.ex.ac.uk/people/mbate
Repair of Impaired Pulmonary Function Is Possible in Very-Long-Term Allogeneic Stem Cell Transplantation Survivors
AbstractBoth early- and late-onset noninfectious pulmonary injury are important contributors to the nonrelapse mortality seen after allogeneic stem cell transplantation (allo-SCT), particularly in subjects conditioned with high-dose total body irradiation (TBI). To characterize the kinetics of recovery from pulmonary injury in long-term survivors, we collected data on 138 subjects who survived > 3 years (median survival, 10.2 years) after predominantly TBI-based allo-SCT from their HLA-matched siblings. Baseline pulmonary function tests served as the reference for subsequent measurements at 3, 5, 10, and 15 years for each survivor. The only parameter showing a clinically and statistically significant decline post-transplant was adjusted diffusion capacity of lung for carbon monoxide (DLCO), which reached a nadir at 5 years but surprisingly normalized at the 10-year mark. Multivariable modeling identified chronic graft-versus-host disease (P < .02) and abnormal baseline-adjusted DLCO (P < .03) as the only significant factors associated with the decline in adjusted DLCO at 5 years but excluded smoking, conditioning intensity, baseline C-reactive protein level, TBI dose to the lungs, disease, and demographic variables. In conclusion, pulmonary injury as monitored by the adjusted DLCO continues to deteriorate in the first 5 years after allo-SCT but recovers at 10 years
Have proto-planetary discs formed planets?
It has recently been noted that many discs around T Tauri stars appear to
comprise only a few Jupiter-masses of gas and dust. Using millimetre surveys of
discs within six local star-formation regions, we confirm this result, and find
that only a few percent of young stars have enough circumstellar material to
build gas giant planets, in standard core accretion models. Since the frequency
of observed exo-planets is greater than this, there is a `missing mass'
problem. As alternatives to simply adjusting the conversion of dust-flux to
disc mass, we investigate three other classes of solution. Migration of planets
could hypothetically sweep up the disc mass reservoir more efficiently, but
trends in multi-planet systems do not support such a model, and theoretical
models suggest that the gas accretion timescale is too short for migration to
sweep the disc. Enhanced inner-disc mass reservoirs are possible, agreeing with
predictions of disc evolution through self-gravity, but not adding to
millimetre dust-flux as the inner disc is optically thick. Finally, the
incidence of massive discs is shown to be higher at the {\it proto}stellar
stages, Classes 0 and I, where discs substantial enough to form planets via
core accretion are abundant enough to match the frequency of exo-planets.
Gravitational instability may also operate in the Class 0 epoch, where half the
objects have potentially unstable discs of \ga30 % of the stellar mass.
However, recent calculations indicate that forming gas giants inside 50 AU by
instability is unlikely, even in such massive discs. Overall, the results
presented suggest that the canonically 'proto-planetary' discs of Class II T
Tauri stars {\bf have globally low masses in dust observable at millimetre
wavelengths, and conversion to larger bodies (anywhere from small rocks up to
planetary cores) must already have occurred.}Comment: Accepted for publication in MNRAS (main journal
A systematic approach towards the identification and protection of vulnerable marine ecosystems
Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Marine Policy 49 (2014):146-154, doi:10.1016/j.marpol.2013.11.017.The United Nations General Assembly in 2006 and 2009 adopted resolutions that call for the identification and protection of vulnerable marine ecosystems (VMEs) from significant adverse impacts of bottom fishing. While general criteria have been produced, there are no guidelines or protocols that elaborate on the process from initial identification through to the protection of VMEs. Here, based upon an expert review of existing practices, a 10-step framework is proposed: 1) Comparatively assess potential VME indicator taxa and habitats in a region; 2) determine VME thresholds; 3) consider areas already known for their ecological importance; 4) compile information on the distributions of likely VME taxa and habitats, as well as related environmental data; 5) develop predictive distribution models for VME indicator taxa and habitats; 6) compile known or likely fishing impacts; 7) produce a predicted VME naturalness distribution (areas of low cumulative impacts); 8) identify areas of higher value to user groups; 9) conduct management strategy evaluations to produce trade-off scenarios; 10) review and re-iterate, until spatial management scenarios are developed that fulfil international obligations and regional conservation and management objectives. To date, regional progress has been piecemeal and incremental. The proposed 10-step framework combines these various experiences into a systematic approach.The New Zealand Ministry of Science and Innovation (now known as the Ministry of Business, Innovation and Employment) provided funding for the worksho
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