1,673 research outputs found
Rayleigh and depinning instabilities of forced liquid ridges on heterogeneous substrates
Depinning of two-dimensional liquid ridges and three-dimensional drops on an
inclined substrate is studied within the lubrication approximation. The
structures are pinned to wetting heterogeneities arising from variations of the
strength of the short-range polar contribution to the disjoining pressure. The
case of a periodic array of hydrophobic stripes transverse to the slope is
studied in detail using a combination of direct numerical simulation and
branch-following techniques. Under appropriate conditions the ridges may either
depin and slide downslope as the slope is increased, or first breakup into
drops via a transverse instability, prior to depinning. The different
transition scenarios are examined together with the stability properties of the
different possible states of the system.Comment: Physics synopsis link:
http://physics.aps.org/synopsis-for/10.1103/PhysRevE.83.01630
Vortices in Thin, Compressible, Unmagnetized Disks
We consider the formation and evolution of vortices in a hydrodynamic
shearing-sheet model. The evolution is done numerically using a version of the
ZEUS code. Consistent with earlier results, an injected vorticity field evolves
into a set of long-lived vortices, each of which has a radial extent comparable
to the local scale height. But we also find that the resulting velocity field
has a positive shear stress, . This effect appears
only at high resolution. The transport, which decays with time as t^-1/2,
arises primarily because the vortices drive compressive motions. This result
suggests a possible mechanism for angular momentum transport in low-ionization
disks, with two important caveats: a mechanism must be found to inject
vorticity into the disk, and the vortices must not decay rapidly due to
three-dimensional instabilities.Comment: 8 pages, 10 figures (high resolution figures available in ApJ
electronic edition
RF input power couplers for high current SRF applications
present day accelerator science. The bERLinPro project is presently being built at HZB to address the challenges involved in high current SRF machines with the goal of generating and accelerating a 100 mA electron beam to 50 MeV in continuous wave cw mode at 1.3 GHz. One of the main challenges in this project is that of handling the high input RF power required for the photo injector as well as booster cavities where there is no energy recovery process. A high power co axial input power coupler is being developed to be used for the photo injector and booster cavities at the nominal beam current. The coupler is based on the KEK cERL design and has been modified to minimise the penetration of the coupler tip in the beam pipe without compromising on beam power coupling Qext 105 . Herein we report on the RF design of the high power 115 kW per coupler, dual couplers per cavity bERLinPro BP coupler along with initial results on thermal calculations. We summarise the RF conditioning of the TTF III couplers modified for cw operation performed in the past at BESSY HZB. A similar conditioning is envisaged in the near future for the low current SRF photo injector and the bERLinPro main linac cryomodule
ggstThe role of tendon microcirculation in Achilles and patellar tendinopathy
Tendinopathy is of distinct interest as it describes a painful tendon disease with local tenderness, swelling and pain associated with sonographic features such as hypoechogenic texture and diameter enlargement. Recent research elucidated microcirculatory changes in tendinopathy using laser Doppler flowmetry and spectrophotometry such as at the Achilles tendon, the patellar tendon as well as at the elbow and the wrist level. Tendon capillary blood flow is increased at the point of pain. Tendon oxygen saturation as well as tendon postcapillary venous filling pressures, determined non-invasively using combined Laser Doppler flowmetry and spectrophotometry, can quantify, in real-time, how tendon microcirculation changes over with pathology or in response to a given therapy. Tendon oxygen saturation can be increased by repetitive, intermittent short-term ice applications in Achilles tendons; this corresponds to 'ischemic preconditioning', a method used to train tissue to sustain ischemic damage. On the other hand, decreasing tendon oxygenation may reflect local acidosis and deteriorating tendon metabolism. Painful eccentric training, a common therapy for Achilles, patellar, supraspinatus and wrist tendinopathy decreases abnormal capillary tendon flow without compromising local tendon oxygenation. Combining an Achilles pneumatic wrap with eccentric training changes tendon microcirculation in a different way than does eccentric training alone; both approaches reduce pain in Achilles tendinopathy. The microcirculatory effects of measures such as extracorporeal shock wave therapy as well as topical nitroglycerine application are to be studied in tendinopathy as well as the critical question of dosage and maintenance. Interestingly it seems that injection therapy using color Doppler for targeting the area of neovascularisation yields to good clinical results with polidocanol sclerosing therapy, but also with a combination of epinephrine and lidocaine
Sources and sinks separating domains of left- and right-traveling waves: Experiment versus amplitude equations
In many pattern forming systems that exhibit traveling waves, sources and
sinks occur which separate patches of oppositely traveling waves. We show that
simple qualitative features of their dynamics can be compared to predictions
from coupled amplitude equations. In heated wire convection experiments, we
find a discrepancy between the observed multiplicity of sources and theoretical
predictions. The expression for the observed motion of sinks is incompatible
with any amplitude equation description.Comment: 4 pages, RevTeX, 3 figur
Influence of through-flow on linear pattern formation properties in binary mixture convection
We investigate how a horizontal plane Poiseuille shear flow changes linear
convection properties in binary fluid layers heated from below. The full linear
field equations are solved with a shooting method for realistic top and bottom
boundary conditions. Through-flow induced changes of the bifurcation thresholds
(stability boundaries) for different types of convective solutions are deter-
mined in the control parameter space spanned by Rayleigh number, Soret coupling
(positive as well as negative), and through-flow Reynolds number. We elucidate
the through-flow induced lifting of the Hopf symmetry degeneracy of left and
right traveling waves in mixtures with negative Soret coupling. Finally we
determine with a saddle point analysis of the complex dispersion relation of
the field equations over the complex wave number plane the borders between
absolute and convective instabilities for different types of perturbations in
comparison with the appropriate Ginzburg-Landau amplitude equation
approximation. PACS:47.20.-k,47.20.Bp, 47.15.-x,47.54.+rComment: 19 pages, 15 Postscript figure
Subharmonic bifurcation cascade of pattern oscillations caused by winding number increasing entrainment
Convection structures in binary fluid mixtures are investigated for positive
Soret coupling in the driving regime where solutal and thermal contributions to
the buoyancy forces compete. Bifurcation properties of stable and unstable
stationary square, roll, and crossroll (CR) structures and the oscillatory
competition between rolls and squares are determined numerically as a function
of fluid parameters. A novel type of subharmonic bifurcation cascade (SC) where
the oscillation period grows in integer steps as is found
and elucidated to be an entrainment process.Comment: 7 pages, 4 figure
Bistability of Slow and Fast Traveling Waves in Fluid Mixtures
The appearence of a new type of fast nonlinear traveling wave states in
binary fluid convection with increasing Soret effect is elucidated and the
parameter range of their bistability with the common slower ones is evaluated
numerically. The bifurcation behavior and the significantly different
spatiotemporal properties of the different wave states - e.g. frequency, flow
structure, and concentration distribution - are determined and related to each
other and to a convenient measure of their nonlinearity. This allows to derive
a limit for the applicability of small amplitude expansions. Additionally an
universal scaling behavior of frequencies and mixing properties is found.
PACS: 47.20.-k, 47.10.+g, 47.20.KyComment: 4 pages including 5 Postscript figure
- …