4,250 research outputs found
Effect of a polymer additive on heat transport in turbulent Rayleigh-B\'enard convection
Measurements of heat transport, as expressed by the Nusselt number , are
reported for turbulent Rayleigh-B\'enard convection of water containing up to
120 ppm by weight of poly-[ethylene oxide] with a molecular weight of
g/mole. Over the Rayleigh number range 5\times 10^9 \alt Ra
\alt 7 \times 10^{10} is smaller than it is for pure water by up to 10%.Comment: 3 pages, 2 figure
Absense of slow transients, and the effect of imperfect vertical alignment, in turbulent Rayleigh-Benard convection
We report experimental results for the influence of a tilt angle beta
relative to gravity on turbulent Rayleigh-Benard convection of cylindrical
samples. The measurements were made at Rayleigh numbers R up to 10^11 with two
samples of height L equal to the diameter D (aspect ratio Gamma = D/L = 1). The
fluid was water with a Prandtl number sigma = 4.38. In contrast to the
experiences reported by Chilla et. al. (2004) for a similar sample but with
Gamma = 0.5 (D = 0.5 and L = 1.0 m), we found no long relaxation times. For R =
9.4 times 10^10 we measured the Nusselt number N as a function of tilt angle
beta and obtained a small beta dependence about a factor of 50 smaller than the
result found by Chilla et. al. (2004) for their Gamma = 0.5 sample. We measured
side-wall temperatures at eight equally spaced azimuthal locations on the
horizontal mid-plane of the sample and used their cross-correlation functions
to find the turn-over time of the large-scale circulation (LSC). The resulting
Reynolds numbers R_e^cc were found to increase with beta. An important
conclusion is that the increase of R_e^cc with beta of the LSC does not
significantly influence the heat transport. Over the range 10^9 < R < 10^11 the
enhancement of R_e^cc at constant beta due to the tilt could be described by a
power law of R with an exponent of -1/6, consistent with a simple model that
balances the additional buoyancy due to the tilt angle by the shear stress
across the boundary layers. Even a small tilt angle dramatically suppressed the
azimuthal meandering and the sudden reorientations characteristic of the LSC in
a sample with beta = 0. The azimuthal mean of the temperature at the horizontal
mid-plane within our resolution was independent of beta.Comment: 32 pages, 17 figures. Under consideration for publication in J. Fluid
Mec
Heat transport by turbulent Rayleigh-B'enard Convection in cylindrical cells with aspect ratio one and less
We present high-precision measurements of the Nusselt number N as a function
of the Rayleigh number R for cylindrical samples of water (Prandtl number sigma
= 4.4) with a diameter D of 49.7 cm and heights L = 116.3, 74.6, and 50.6 cm,
as well as for D = 24.8 cm and L = 90.2 cm. For each aspect ratio Gamma = D/L =
0.28, 0.43, 0.67, and 0.98 the data cover a range of a little over a decade of
R. The maximum R ~= 10^12 and Nusselt number N ~= 600 were reached for Gamma =
0.43 and D = 49.7. The data were corrected for the influence of the finite
conductivity of the top and bottom plates on the heat transport in the fluid to
obtain estimates of N_infty for plates with infinite conductivity. The results
for N_infty and Gamma >= 0.43 are nearly independent of Gamma. For Gamma =
0.275 N_infty falls about 2.5 % below the other data. For R ~<= 10^11, the
effective exponent gamma_eff of N_infty = N_0 R^gamma_eff is about 0.321,
larger than those of the Grossmann-Lohse model with its current parameters by
about 0.01. For the largest Rayleigh numbers covered for Gamma = 0.98, 0.67,
and 0.43, gamma_eff saturates at the asymptotic value gamma = 1/3 of the
Grossmann-Lohse model. The data do not reveal any crossover to a Kraichnan
regime with gamma_eff > 1/3.Comment: 10 pages, 5 figure
Heat transport by turbulent Rayleigh-Benard Convection in cylindrical samples with aspect ratio one and larger
We present high-precision measurements of the Nusselt number N as a function
of the Rayleigh number R for cylindrical samples of water (Prandtl number sigma
= 4.38) with diameters D = 49.7, 24.8, and 9.2 cm, all with aspect ratio Gamma
= D/L = 1 (L is the sample height). In addition, we present data for D = 49.7
and Gamma = 1.5, 2, 3, and 6. For each sample the data cover a range of a
little over a decade of R. For Gamma = 1 they jointly span the range 10^7 < R <
10^11. Where needed, the data were corrected for the influence of the finite
conductivity of the top and bottom plates and of the side walls on the heat
transport in the fluid to obtain estimates of N_infinity for plates with
infinite conductivity and sidewalls of zero conductivity. For Gamma = 1 the
effective exponent gamma_eff of N_infinity = N_0 R^gamma_eff ranges from 0.28
near R = 10^8 to 0.333 near R = 7 times10^10. For R < 10^10 the results are
consistent with the Grossmann-Lohse model. For larger R, where the data
indicate that N_infinity(R) = R^1/3, the theory has a smaller gamma_eff than
1/3 and falls below the data. The data for Gamma > 1 are only a few percent
smaller than the Gamma = 1 results.Comment: 10 pages, 5 figures. Under consideration for publication in J. Fluid
Mec
Heat transport in turbulent Rayleigh-Benard convection: Effect of finite top- and bottom-plate conductivity
We describe three apparatus, known as the large, medium, and small apparatus,
used for high-precision measurements of the Nusselt number N as a function of
the Rayleigh number R for cylindrical samples of fluid and present results
illustrating the influence of the finite conductivity of the top and bottom
plates on the heat transport in the fluid. We used water samples at a mean
temperature of 40 degrees C (Prandtl number sigma = 4.4). The samples in the
large apparatus had a diameter D of 49.69 cm and heights L = 116.33, 74.42,
50.61, and 16.52 cm. For the medium apparatus we had D = 24.81 cm, and L =
90.20 and 24.76 cm. The small apparatus contained a sample with D = 9.21 cm,
and L = 9.52 cm. For each aspect ratio Gamma = D/L the data covered a range of
a little over a decade of R. The maximum R = 10^12 with Nusselt numbers N = 600
was reached for Gamma = 0.43. Measurements were made with both Aluminum and
Copper top and bottom plates of nominally identical size and shape. For the
large and medium apparatus the results with Aluminum plates fall below those
obtained with Copper plates, thus confirming qualitatively the prediction by
Verzicco that plates of finite conductivity diminish the heat transport in the
fluid. The Nusselt number N_infinity for plates with infinite conductivity was
estimated by fitting simultaneously Aluminum- and Copper-plate data sets to an
effective powerlaw for N_infinity multiplied by a correction factor f(X) = 1 -
exp[-(aX)^b] that depends on the ratio X of the thermal resistance of the fluid
to that of the plates as suggested by Verzicco. Within their uncertainties the
parameters a and b were independent of Gamma for the large apparatus and showed
a small Gamma-dependence for the medium apparatus. The correction was larger
for the large, smaller for the medium, and negligible for the small apparatus.Comment: 35 pages, 11 figures. Under consideration for publication in Phys. of
Fluid
Modulating light with light via giant nano-opto-mechanical nonlinearity of plasmonic metamaterial
From the demonstration of saturable absorption by Vavilow and Levshin in
1926, and with invention of the laser, unavailability of strongly nonlinear
materials was a key obstacle for developing optical signal processing, in
particular in transparent telecommunication networks. Today, most advanced
photonic switching materials exploit gain dynamics and near-band and excitonic
effects in semiconductors, nonlinearities in organic media with
weakly-localized electrons and nonlinearities enhanced by hybridization with
metamaterials. Here we report on a new type of artificial nonlinearity that is
nano-opto-mechanical in nature. It was observed in an artificial metamaterial
array of plasmonic meta-molecules supported by a flexible nano-membrane. Here
nonlinearity is underpinned by the reversible reconfiguration of its structure
induced by light. In a film of only 100 nanometres thickness we demonstrated
modulation of light with light using milliwatt power telecom diode lasers.Comment: 6 pages, 5 figure
Syndrome mermaid - severe syndrome of caudal regression
Congenital limb malformations rank behind congenital heart disease as the most common birth defects observed in infants. The term «sirenomelia» («mermaid syndrome») is derived from the physical similarity of the affected fetus to mythical creatures mermaids - charming women with the lower part of the body in the form of a fish tail, where there is a fusion of the lower extremities and partial or complete fusion of the feet
Safety of gas pipelines and the problem of environmental protection
The paper focuses on the problems of environmental protection that seem to be among the most urgent ones around the world. There is no such a sphere of productive activity of an individual which would not influence the environment without changing its condition, which is likely to lead to destruction of its dynamic balance. Recent studies show that the main oil and gas pipeline service is regarded as a potential source of environmental pollution by oil products. Accidents on main gas pipelines cause enormous damage, leading to massive pollution of water reservoirs and onshore. The key reasons for it are pipeline long run life, depreciation and, as a result, a steady tendency of increase in the number of accidents with serious social, ecological and economic consequences [4]. The scientific and methodological base of quantitative risk emergency assessment is necessary for the choice of adequate measures aimed at prevention of catastrophic crashes and accidents. In case of the successful solution to this problem it seems vital that interests of environment protection be integrated into the area of adoption of the engineering decisions making it possible to minimize the consequences of negative impacts in the most rational (least costly) ways. In these conditions the analysis of environmental risks of main gas pipelines becomes especially urgent
Topological superconductivity in proximity to type-II superconductors
One-dimensional systems proximity coupled to a superconductor can be driven into a topological superconducting phase by an external magnetic field. Here, we investigate the effect of vortices created by the magnetic field in a type-II superconductor providing the proximity effect. We identify different ways in which the topological protection of Majorana modes can be compromised and discuss strategies to circumvent these detrimental effects. Our findings are also relevant to topological phases of proximitized quantum Hall edge states
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