3,138 research outputs found
The creeping motion of immiscible drops through a converging/diverging tube
Results of experiments on the low-Reynolds-number flow of liquid drops through a horizontal circular tube with a diameter that varies sinusoidally with axial position are reported. Measurements of the contribution of the drop to the local pressure gradient and the relative velocity of the drop are correlated with the time-dependent drop shape. Both Newtonian and viscoelastic suspending fluids are considered. The viscosity ratio, volumetric flow rate and drop size are varied in the experiment, and both neutrally buoyant and non-neutrally buoyant drops are studied. Comparison with previous results for a straight-wall tube shows that the influence of the tube boundary geometry on the drop shape is substantial, but the qualitative effect of the tube shape depends strongly on the relative importance of viscous forces compared to interfacial tension for the particular experiment. For Newtonian fluids, two modes of drop breakup, which are distinguished by the magnitude of the viscosity ratio, are observed. When the suspending fluid is viscoelastic, both shear-thinning and time-dependent rheological effects are present
Magnetic fields around evolved stars: further observations of HO maser polarization
We aim to detect the magnetic field and infer its properties around four AGB
stars using HO maser observations. The sample we observed consists of the
following sources: the semi-regular variable RT Vir and the Mira variables AP
Lyn, IK Tau, and IRC+60370. We observed the 6 HO maser
rotational transition, in full-polarization mode, to determine its linear and
circular polarization. Based on the Zeeman effect, one can infer the properties
of the magnetic field from the maser polarization analysis. We detected a total
of 238 maser features, in three of the four observed sources. No masers were
found toward AP Lyn. The observed masers are all located between 2.4 and 53.0
AU from the stars. Linear and circular polarization was found in 18 and 11
maser features, respectively. We more than doubled the number of AGB stars in
which magnetic field has been detected from HO maser polarization, as our
results confirm the presence of fields around IK Tau, RT Vir and IRC+60370. The
strength of the field along the line of sight is found to be between 47 and 331
mG in the HO maser region. Extrapolating this result to the surface of the
stars, assuming a toroidal field ( r), we find magnetic fields
of 0.3-6.9 G on the stellar surfaces. If, instead of a toroidal field, we
assume a poloidal field ( r), then the extrapolated magnetic
field strength on the stellar surfaces are in the range between 2.2 and
115 G. Finally, if a dipole field ( r) is assumed, the
field strength on the surface of the star is found to be between 15.8 and
1945 G. The magnetic energy of our sources is higher than the thermal and
kinetic energy in the HO maser region of this class of objects. This leads
us to conclude that, indeed, magnetic fields probably play an important role in
shaping the outflows of evolved stars. (abridged)Comment: 15 pages, 5 figures, 7 tables. Accepted for publication in A&
Formation of Nanopillar Arrays in Ultrathin Viscous Films: The Critical Role of Thermocapillary Stresses
Experiments by several groups during the past decade have shown that a molten
polymer nanofilm subject to a large transverse thermal gradient undergoes
spontaneous formation of periodic nanopillar arrays. The prevailing explanation
is that coherent reflections of acoustic phonons within the film cause a
periodic modulation of the radiation pressure which enhances pillar growth. By
exploring a deformational instability of particular relevance to nanofilms, we
demonstrate that thermocapillary forces play a crucial role in the formation
process. Analytic and numerical predictions show good agreement with the pillar
spacings obtained in experiment. Simulations of the interface equation further
determine the rate of pillar growth of importance to technological
applications.Comment: 5 pages, 4 figure
Evaluating the engagement of universities in capacity building for sustainable development in local communities
Universities have the potential to play a leading role in enabling communities to develop more sustainable ways of living and working however, sustainable communities may only emerge with facilitation, community learning and continual efforts to build their capacities. Elements of programme planning and evaluation on the one hand, and capacity building on the other, are needed. The latter entails approaches and processes that may contribute to community empowerment; universities may either lead such approaches, or be key partners in an endeavour to empower communities to address the challenges posed by the need for sustainable development. Although capacity building and the promotion of sustainable development locally, are on the agenda for universities who take seriously regional engagement, very little is published that illustrates or describes the various forms of activities that take place. Further, there is a paucity of studies that have evaluated the work performed by universities in building capacity for sustainable development at the local level. This paper is an attempt to address this need, and entails an empirical study based on a sample of universities in the United Kingdom, Germany, Portugal and Brazil. The paper examines the extent to which capacity building for sustainable development is being undertaken, suggests the forms that this might take and evaluates some of the benefits for local communities. The paper concludes by reinforcing that universities have a critical role to play in community development; that role has to prioritise the sustainability agenda
Orientation dynamics of weakly Brownian particles in periodic viscous flows
Evolution equations for the orientation distribution of axisymmetric
particles in periodic flows are derived in the regime of small but non-zero
Brownian rotations. The equations are based on a multiple time scale approach
that allows fast computation of the relaxation processes leading to statistical
equilibrium. The approach has been applied to the calculation of the effective
viscosity of a thin disk suspension in gravity waves.Comment: 16 pages, 7 eps figures include
Soft lubrication: the elastohydrodynamics of non-conforming and conforming contacts
We study the lubrication of fluid-immersed soft interfaces and show that
elastic deformation couples tangential and normal forces and thus generates
lift. We consider materials that deform easily, due to either geometry (e.g. a
shell) or constitutive properties (e.g. a gel or a rubber), so that the effects
of pressure and temperature on the fluid properties may be neglected. Four
different system geometries are considered: a rigid cylinder moving parallel to
a soft layer coating a rigid substrate; a soft cylinder moving parallel to a
rigid substrate; a cylindrical shell moving parallel to a rigid substrate; and
finally a cylindrical conforming journal bearing coated with a thin soft layer.
In addition, for the particular case of a soft layer coating a rigid substrate
we consider both elastic and poroelastic material responses. For all these
cases we find the same generic behavior: there is an optimal combination of
geometric and material parameters that maximizes the dimensionless normal force
as a function of the softness parameter = hydrodynamic pressure/elastic
stiffness = surface deflection/gap thickness which characterizes the
fluid-induced deformation of the interface. The corresponding cases for a
spherical slider are treated using scaling concepts.Comment: 61 pages, 20 figures, 2 tables, submitted to Physics of Fluid
The scenario of two-dimensional instabilities of the cylinder wake under EHD forcing: A linear stability analysis
We propose to study the stability properties of an air flow wake forced by a dielectric barrier discharge (DBD) actuator, which is a type of electrohydrodynamic (EHD) actuator. These actuators add momentum to the flow around a cylinder in regions close to the wall and, in our case, are symmetrically disposed near the boundary layer separation point.
Since the forcing frequencies, typical of DBD, are much higher than the natural shedding frequency of the flow, we will be considering the forcing actuation as stationary.
In the first part, the flow around a circular cylinder modified by EHD actuators will be experimentally studied by means of particle image velocimetry (PIV). In the second part, the EHD actuators have been numerically implemented as a boundary condition on the cylinder surface. Using this boundary condition, the computationally obtained base flow is then compared with the experimental one in order to relate the control parameters from both methodologies.
After validating the obtained agreement, we study the Hopf bifurcation that appears once the flow starts the vortex shedding through experimental and computational approaches. For the base flow derived from experimentally obtained snapshots, we monitor the evolution of the velocity amplitude oscillations. As to the computationally obtained base flow, its stability is analyzed by solving a global eigenvalue problem obtained from the linearized Navier–Stokes equations. Finally, the critical parameters obtained from both approaches are compared
Estimativas de parâmetros genéticos na variedade de milho BR 5028 - São Francisco no nordeste brasileiro.
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