596 research outputs found
Self-similar decay of high Reynolds number Taylor-Couette turbulence
We study the decay of high-Reynolds number Taylor-Couette turbulence, i.e.
the turbulent flow between two coaxial rotating cylinders. To do so, the
rotation of the inner cylinder (Re, the outer cylinder is at
rest) is stopped within 12 s, thus fully removing the energy input to the
system. Using a combination of laser Doppler anemometry and particle image
velocimetry measurements, six decay decades of the kinetic energy could be
captured. First, in the absence of cylinder rotation, the flow-velocity during
the decay does not develop any height dependence in contrast to the well-known
Taylor vortex state. Second, the radial profile of the azimuthal velocity is
found to be self-similar. Nonetheless, the decay of this wall-bounded
inhomogeneous turbulent flow does not follow a strict power law as for decaying
turbulent homogeneous isotropic flows, but it is faster, due to the strong
viscous drag applied by the bounding walls. We theoretically describe the decay
in a quantitative way by taking the effects of additional friction at the walls
into account.Comment: 7 pages, 6 figure
Azimuthal velocity profiles in Rayleigh-stable Taylor-Couette flow and implied axial angular momentum transport
We present azimuthal velocity profiles measured in a Taylor-Couette
apparatus, which has been used as a model of stellar and planetary accretion
disks. The apparatus has a cylinder radius ratio of , an
aspect-ratio of , and the plates closing the cylinders in the
axial direction are attached to the outer cylinder. We investigate angular
momentum transport and Ekman pumping in the Rayleigh-stable regime. The regime
is linearly stable and is characterized by radially increasing specific angular
momentum. We present several Rayleigh-stable profiles for shear Reynolds
numbers , both for
(quasi-Keplerian regime) and (sub-rotating regime)
where is the inner/outer cylinder rotation rate. None of the
velocity profiles matches the non-vortical laminar Taylor-Couette profile. The
deviation from that profile increased as solid-body rotation is approached at
fixed . Flow super-rotation, an angular velocity greater than that of
both cylinders, is observed in the sub-rotating regime. The velocity profiles
give lower bounds for the torques required to rotate the inner cylinder that
were larger than the torques for the case of laminar Taylor-Couette flow. The
quasi-Keplerian profiles are composed of a well mixed inner region, having
approximately constant angular momentum, connected to an outer region in
solid-body rotation with the outer cylinder and attached axial boundaries.
These regions suggest that the angular momentum is transported axially to the
axial boundaries. Therefore, Taylor-Couette flow with closing plates attached
to the outer cylinder is an imperfect model for accretion disk flows,
especially with regard to their stability.Comment: 22 pages, 10 figures, 2 tables, under consideration for publication
in Journal of Fluid Mechanics (JFM
Computational Screening of Tip and Stalk Cell Behavior Proposes a Role for Apelin Signaling in Sprout Progression
Angiogenesis involves the formation of new blood vessels by sprouting or
splitting of existing blood vessels. During sprouting, a highly motile type of
endothelial cell, called the tip cell, migrates from the blood vessels followed
by stalk cells, an endothelial cell type that forms the body of the sprout. To
get more insight into how tip cells contribute to angiogenesis, we extended an
existing computational model of vascular network formation based on the
cellular Potts model with tip and stalk differentiation, without making a
priori assumptions about the differences between tip cells and stalk cells. To
predict potential differences, we looked for parameter values that make tip
cells (a) move to the sprout tip, and (b) change the morphology of the
angiogenic networks. The screening predicted that if tip cells respond less
effectively to an endothelial chemoattractant than stalk cells, they move to
the tips of the sprouts, which impacts the morphology of the networks. A
comparison of this model prediction with genes expressed differentially in tip
and stalk cells revealed that the endothelial chemoattractant Apelin and its
receptor APJ may match the model prediction. To test the model prediction we
inhibited Apelin signaling in our model and in an \emph{in vitro} model of
angiogenic sprouting, and found that in both cases inhibition of Apelin or of
its receptor APJ reduces sprouting. Based on the prediction of the
computational model, we propose that the differential expression of Apelin and
APJ yields a "self-generated" gradient mechanisms that accelerates the
extension of the sprout.Comment: 48 pages, 10 figures, 8 supplementary figures. Accepted for
publication in PLoS ON
Mechanical cell-matrix feedback explains pairwise and collective endothelial cell behavior in vitro
In vitro cultures of endothelial cells are a widely used model system of the
collective behavior of endothelial cells during vasculogenesis and
angiogenesis. When seeded in an extracellular matrix, endothelial cells can
form blood vessel-like structures, including vascular networks and sprouts.
Endothelial morphogenesis depends on a large number of chemical and mechanical
factors, including the compliancy of the extracellular matrix, the available
growth factors, the adhesion of cells to the extracellular matrix, cell-cell
signaling, etc. Although various computational models have been proposed to
explain the role of each of these biochemical and biomechanical effects, the
understanding of the mechanisms underlying in vitro angiogenesis is still
incomplete. Most explanations focus on predicting the whole vascular network or
sprout from the underlying cell behavior, and do not check if the same model
also correctly captures the intermediate scale: the pairwise cell-cell
interactions or single cell responses to ECM mechanics. Here we show, using a
hybrid cellular Potts and finite element computational model, that a single set
of biologically plausible rules describing (a) the contractile forces that
endothelial cells exert on the ECM, (b) the resulting strains in the
extracellular matrix, and (c) the cellular response to the strains, suffices
for reproducing the behavior of individual endothelial cells and the
interactions of endothelial cell pairs in compliant matrices. With the same set
of rules, the model also reproduces network formation from scattered cells, and
sprouting from endothelial spheroids. Combining the present mechanical model
with aspects of previously proposed mechanical and chemical models may lead to
a more complete understanding of in vitro angiogenesis.Comment: 25 pages, 6 figures, accepted for publication in PLoS Computational
Biolog
Taylor-Couette turbulence at radius ratio : scaling, flow structures and plumes
Using high-resolution particle image velocimetry we measure velocity
profiles, the wind Reynolds number and characteristics of turbulent plumes in
Taylor-Couette flow for a radius ratio of 0.5 and Taylor number of up to
. The extracted angular velocity profiles follow a log-law more
closely than the azimuthal velocity profiles due to the strong curvature of
this setup. The scaling of the wind Reynolds number with the Taylor
number agrees with the theoretically predicted 3/7-scaling for the classical
turbulent regime, which is much more pronounced than for the well-explored
case, for which the ultimate regime sets in at much lower Ta. By
measuring at varying axial positions, roll structures are found for
counter-rotation while no clear coherent structures are seen for pure inner
cylinder rotation. In addition, turbulent plumes coming from the inner and
outer cylinder are investigated. For pure inner cylinder rotation, the plumes
in the radial velocity move away from the inner cylinder, while the plumes in
the azimuthal velocity mainly move away from the outer cylinder. For
counter-rotation, the mean radial flow in the roll structures strongly affects
the direction and intensity of the turbulent plumes. Furthermore, it is
experimentally confirmed that in regions where plumes are emitted, boundary
layer profiles with a logarithmic signature are created
Exploring the phase space of multiple states in highly turbulent Taylor-Couette flow
We investigate the existence of multiple turbulent states in highly turbulent
Taylor-Couette flow in the range of to ,
by measuring the global torques and the local velocities while probing the
phase space spanned by the rotation rates of the inner and outer cylinder. The
multiple states are found to be very robust and are expected to persist beyond
. The rotation ratio is the parameter that most strongly
controls the transitions between the flow states; the transitional values only
weakly depend on the Taylor number. However, complex paths in the phase space
are necessary to unlock the full region of multiple states. Lastly, by mapping
the flow structures for various rotation ratios in a Taylor-Couette setup with
an equal radius ratio but a larger aspect ratio than before, multiple states
were again observed. Here, they are characterized by even richer roll structure
phenomena, including, for the first time observed in highly turbulent TC flow,
an antisymmetrical roll state.Comment: 9 pages, 7 figure
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