315 research outputs found
Prandtl-Blasius temperature and velocity boundary layer profiles in turbulent Rayleigh-B\'{e}nard convection
The shape of velocity and temperature profiles near the horizontal conducting
plates in turbulent Rayleigh-B\'{e}nard convection are studied numerically and
experimentally over the Rayleigh number range and the Prandtl number range . The results show that both the temperature and velocity
profiles well agree with the classical Prandtl-Blasius laminar boundary-layer
profiles, if they are re-sampled in the respective dynamical reference frames
that fluctuate with the instantaneous thermal and velocity boundary-layer
thicknesses.Comment: 10 pages, 6 figure
Horizontal Structures of Velocity and Temperature Boundary Layers in 2D Numerical Turbulent Rayleigh-B\'{e}nard Convection
We investigate the structures of the near-plate velocity and temperature
profiles at different horizontal positions along the conducting bottom (and
top) plate of a Rayleigh-B\'{e}nard convection cell, using two-dimensional (2D)
numerical data obtained at the Rayleigh number Ra=10^8 and the Prandtl number
Pr=4.4 of an Oberbeck-Boussinesq flow with constant material parameters. The
results show that most of the time, and for both velocity and temperature, the
instantaneous profiles scaled by the dynamical frame method [Q. Zhou and K.-Q.
Xia, Phys. Rev. Lett. 104, 104301 (2010) agree well with the classical
Prandtl-Blasius laminar boundary layer (BL) profiles. Therefore, when averaging
in the dynamical reference frames, which fluctuate with the respective
instantaneous kinematic and thermal BL thicknesses, the obtained mean velocity
and temperature profiles are also of Prandtl-Blasius type for nearly all
horizontal positions. We further show that in certain situations the
traditional definitions based on the time-averaged profiles can lead to
unphysical BL thicknesses, while the dynamical method also in such cases can
provide a well-defined BL thickness for both the kinematic and the thermal BLs.Comment: 16 pages, 16 figure
Optimal Taylor-Couette flow: direct numerical simulations
We numerically simulate turbulent Taylor-Couette flow for independently
rotating inner and outer cylinders, focusing on the analogy with turbulent
Rayleigh-B\'enard flow. Reynolds numbers of and
of the inner and outer cylinders, respectively, are
reached, corresponding to Taylor numbers Ta up to . Effective scaling
laws for the torque and other system responses are found. Recent experiments
with the Twente turbulent Taylor-Couette () setup and with a similar
facility in Maryland at very high Reynolds numbers have revealed an optimum
transport at a certain non-zero rotation rate ratio
of about . For large enough in the numerically
accessible range we also find such an optimum transport at non-zero
counter-rotation. The position of this maximum is found to shift with the
driving, reaching a maximum of for . An
explanation for this shift is elucidated, consistent with the experimental
result that becomes approximately independent of the driving strength
for large enough Reynolds numbers. We furthermore numerically calculate the
angular velocity profiles and visualize the different flow structures for the
various regimes. By writing the equations in a frame co-rotating with the outer
cylinder a link is found between the local angular velocity profiles and the
global transport quantities.Comment: Under consideration for publication in JFM, 31 pages, 25 figure
Boundary layer structure in turbulent thermal convection and its consequences for the required numerical resolution
Results on the Prandtl-Blasius type kinetic and thermal boundary layer
thicknesses in turbulent Rayleigh-B\'enard convection in a broad range of
Prandtl numbers are presented. By solving the laminar Prandtl-Blasius boundary
layer equations, we calculate the ratio of the thermal and kinetic boundary
layer thicknesses, which depends on the Prandtl number Pr only. It is
approximated as for and as for
, with . Comparison of the Prandtl--Blasius velocity
boundary layer thickness with that evaluated in the direct numerical
simulations by Stevens, Verzicco, and Lohse (J. Fluid Mech. 643, 495 (2010))
gives very good agreement. Based on the Prandtl--Blasius type considerations,
we derive a lower-bound estimate for the minimum number of the computational
mesh nodes, required to conduct accurate numerical simulations of moderately
high (boundary layer dominated) turbulent Rayleigh-B\'enard convection, in the
thermal and kinetic boundary layers close to bottom and top plates. It is shown
that the number of required nodes within each boundary layer depends on Nu and
Pr and grows with the Rayleigh number Ra not slower than \sim\Ra^{0.15}. This
estimate agrees excellently with empirical results, which were based on the
convergence of the Nusselt number in numerical simulations
Flow reversals in thermally driven turbulence
We analyze the reversals of the large scale flow in Rayleigh-B\'enard
convection both through particle image velocimetry flow visualization and
direct numerical simulations (DNS) of the underlying Boussinesq equations in a
(quasi) two-dimensional, rectangular geometry of aspect ratio 1. For medium
Prandtl number there is a diagonal large scale convection roll and two smaller
secondary rolls in the two remaining corners diagonally opposing each other.
These corner flow rolls play a crucial role for the large scale wind reversal:
They grow in kinetic energy and thus also in size thanks to plume detachments
from the boundary layers up to the time that they take over the main, large
scale diagonal flow, thus leading to reversal. Based on this mechanism we
identify a typical time scale for the reversals. We map out the Rayleigh number
vs Prandtl number phase space and find that the occurrence of reversals very
sensitively depends on these parameters.Comment: 4 pages, 4 figure
Thermal boundary layer profiles in turbulent Rayleigh-B\'enard convection in a cylindrical sample
We numerically investigate the structures of the near-plate temperature
profiles close to the bottom and top plates of turbulent Rayleigh-B\'{e}nard
flow in a cylindrical sample at Rayleigh numbers Ra=10^8 to Ra=2\times10^{12}
and Prandtl numbers Pr=6.4 and Pr=0.7 with the dynamical frame method [Q. Zhou
and K.-Q. Xia, Phys. Rev. Lett. 104, 104301 (2010)] thus extending previous
results for quasi-2-dimensional systems to 3D systems for the first time. The
dynamical frame method shows that the measured temperature profiles in the
spatially and temporally local frame are much closer to the temperature profile
of a laminar, zero-pressure gradient boundary layer according to Pohlhausen
than in the fixed reference frame. The deviation between the measured profiles
in the dynamical reference frame and the laminar profiles increases with
decreasing Pr, where the thermal BL is more exposed to the bulk fluctuations
due to the thinner kinetic BL, and increasing Ra, where more plumes are passing
the measurement location.Comment: 5 pages, 2 figure
Biological properties of poly-L-lysine/DNA complexes generated by cooperative binding of the polycation
We have evaluated the effect of NaCl concentration on the mode of binding of poly-L-lysine to DNA and the resulting structural and functional features of the condensed DNA particles using DNA precipitation, DNase I resistance, electron microscopy, and receptor-mediated gene transfer assays. At a high concentration of NaCl and in the presence of excess DNA, poly-L-lysine interacted with DNA cooperatively, fully condensing some of the DNA and leaving the rest of the DNA unbound. At low NaCl concentrations, poly-L-lysine molecules interacted with DNA in a noncooperative fashion, i.e. they bind randomly to the whole population of DNA molecules. Cooperative binding of poly-L-lysine to DNA occurred over a narrow range of NaCl concentrations, and the specific salt concentration depended on the length of the poly-L-lysine. The ability of condensed DNA to withstand digestion by DNase I was correlated with the structural features of the condensed DNA as determined by electron microscopy. Using our condensation procedure, cooperative binding of poly-L-lysine to DNA is a necessary prerequisite for the preparation of condensed DNA having a spherical shape and a diameter of 15-30 nm. Condensed DNA, containing galactosylated poly-L-lysine, was evaluated further for the extent and specificity of receptor-mediated gene transfer into HuH-7 human hepatoma cells via the asialoglycoprotein receptor. Efficient receptor-mediated transfection occurred only when condensed DNA complexes had a spherical shape with a diameter of 15-30 nm; asialofetuin, a natural ligand for the asialoglycoprotein receptor, inhibited this process by up to 90%. Our results support the importance of appropriate DNA condensation for the uptake and ultimate expression of DNA in hepatic cells
Logarithmic temperature profiles in turbulent Rayleigh-B\'enard convection
We report results for the temperature profiles of turbulent Rayleigh-B\'enard
convection (RBC) in the interior of a cylindrical sample of aspect ratio
( and are the diameter and height
respectively). Results from experiment over the Rayleigh number range 4\times
10^{12} \alt Ra \alt 10^{15} for a Prandtl number \Pra \simeq 0.8 and from
direct numerical simulation (DNS) at for \Pra = 0.7
are presented. We find that the temperature varies as where
is the distance from the bottom or top plate. This is the case in the classical
as well as in the ultimate state of RBC. From DNS we find that in the
classical state decreases in the radial direction as the distance from the side
wall increases and becomes small near the sample center
Hydrodynamic stability and mode coupling in Keplerian flows: local strato-rotational analysis
Aims. Qualitative analysis of key (but yet unappreciated) linear phenomena in
stratified hydrodynamic Keplerian flows: (i) the occurrence of a vortex mode,
as a consequence of strato-rotational balance, with its transient dynamics;
(ii) the generation of spiral-density waves (also called inertia-gravity or
waves) by the vortex mode through linear mode coupling in shear
flows. Methods. Non-modal analysis of linearized Boussinesq equations written
in the shearing sheet approximation of accretion disk flows. Results. It is
shown that the combined action of rotation and stratification introduces a new
degree of freedom -- vortex mode perturbation -- which is linearly coupled with
the spiral-density waves. These two modes are jointly able to extract energy
from the background flow and they govern the disk dynamics in the small-scale
range. The transient behavior of these modes is determined by the non-normality
of the Keplerian shear flow. Tightly leading vortex mode perturbations undergo
substantial transient growth, then, becoming trailing, inevitably generate
trailing spiral-density waves by linear mode coupling. This course of events --
transient growth plus coupling -- is particularly pronounced for perturbation
harmonics with comparable azimuthal and vertical scales and it renders the
energy dynamics similar to the 3D unbounded plane Couette flow case.
Conclusions. Our investigation strongly suggests that the so-called bypass
concept of turbulence, which has been recently developed by the hydrodynamic
community for spectrally stable shear flows, can also be applied to Keplerian
disks. This conjecture may be confirmed by appropriate numerical simulations
that take in account the vertical stratification and consequent mode coupling
in the high Reynolds number regime.Comment: A&A (accepted
Metástase, uma visão atualizada
Os autores revisam a Cascata Metastática, nome que designa a seqüência de fenômenos que levam à implantação da célula metastática e sua multiplicação no órgão alvo. Através de uma revisão atualizada da literatura pertinente, expõem as múltiplas teorias e experimentos que tentam esclarecer os complexos mecanismos bioquímicos e enzimáticos que envolvem este fascinante processo, bem como sua implicação terapêutica
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