1,668 research outputs found
Quantitative characterization of three-dimensional pore structure in hardened cement paste using X-ray microtomography combined with centrifuge driven metal alloy intrusion
In this paper, a centrifuge device is proposed to facilitate the intrusion of a low-melting point metal alloy into the pore space of hardened cement paste. X-ray microtomography is combined with metal centrifugation porosimetry (MCP) to quantitatively investigate 3D pore structure. The low-melting-point metal alloy is melted and introduced into pore space in pastes with water cement ratio of 0.5 and 1.0 at a temperature of 65 °C. 3D pore structure is quantitatively analyzed by X-ray microtomography after the molten metal alloy has been consolidated. A new threshold value segmentation method for pore space was proposed using conversion coefficient on region of interest (ROI). Porosity and pore size distribution are tested by MCP and compared with the results based on mercury intrusion porosimetry (MIP). The results show that the contrast between pore space and solid phase in the X-ray microtomography device image is improved. The total porosity obtained by MCP was found to be consistent with the results obtained by MIP
On the stationarity of linearly forced turbulence in finite domains
A simple scheme of forcing turbulence away from decay was introduced by
Lundgren some time ago, the `linear forcing', which amounts to a force term
linear in the velocity field with a constant coefficient. The evolution of
linearly forced turbulence towards a stationary final state, as indicated by
direct numerical simulations (DNS), is examined from a theoretical point of
view based on symmetry arguments. In order to follow closely the DNS the flow
is assumed to live in a cubic domain with periodic boundary conditions. The
simplicity of the linear forcing scheme allows one to re-write the problem as
one of decaying turbulence with a decreasing viscosity. Scaling symmetry
considerations suggest that the system evolves to a stationary state, evolution
that may be understood as the gradual breaking of a larger approximate symmetry
to a smaller exact symmetry. The same arguments show that the finiteness of the
domain is intimately related to the evolution of the system to a stationary
state at late times, as well as the consistency of this state with a high
degree of isotropy imposed by the symmetries of the domain itself. The
fluctuations observed in the DNS for all quantities in the stationary state can
be associated with deviations from isotropy. Indeed, self-preserving isotropic
turbulence models are used to study evolution from a direct dynamical point of
view, emphasizing the naturalness of the Taylor microscale as a self-similarity
scale in this system. In this context the stationary state emerges as a stable
fixed point. Self-preservation seems to be the reason behind a noted similarity
of the third order structure function between the linearly forced and freely
decaying turbulence, where again the finiteness of the domain plays an
significant role.Comment: 15 pages, 7 figures, changes in the discussion at the end of section
VI, formula (60) correcte
Exact solution of the one-dimensional ballistic aggregation
An exact expression for the mass distribution of the ballistic
aggregation model in one dimension is derived in the long time regime. It is
shown that it obeys scaling with a scaling
function for and for
. Relevance of these results to Burgers turbulence is discussed.Comment: 11 pages, 2 Postscript figure
C02(nu2)-0 Quenching Rate Coefficient Derived from Coincidental Fort Collins Lidar and SABER Measurements
Among the processes governing the energy balance in the mesosphere and lower thermosphere (MLT), the quenching of CO2(V2) vibrational levels in collisions with oxygen atoms plays an important role. However, neither the rate coefficient of this process (k(CO2O)) nor the atomic oxygen concentrations ([O]) in the MLT are well known. The discrepancy between k(CO2O) measured in the lab and retrieved from atmospheric measurements is of about factor of 2.5. At the same time, the discrepancy between [O] in the MLT measured by different instruments is of the same order of magnitude. In this work we used a synergy of a ground based lidar and satellite infrared radiometer to make a further step in understanding of the physics of the region. In this study we apply the night- and daytime temperatures between 80 and 110 km measured by the Colorado State University narrow-band sodium (Na) lidar located at Fort Collins, Colorado for retrieving the product of k(CO2-O) x [O] from the limb radiances in the 15 micron channel measured by the SABER/TIMED instrument for nearly simultaneous common volume measurements of both instruments within +/-1 degree in latitude, +/-2 degrees in longitude and +/-10 minutes in time. We derive k(CO2-O) and its possible variation range from the retrieved product by utilizing the [O] values measured by the SABER and other instruments
Co2(nu2)-o Quenching Rate Coefficient Derived from Coincidental SABER-TIMED and Fort Collins Lidar Observations of the Mesosphere and Lower Thermosphere
Among the processes governing the energy balance in the mesosphere and lower thermosphere (MLT), the quenching of CO2(nu2) vibrational levels by collisions with O atoms plays an important role. However, there is a factor of 3-4 discrepancy between the laboratory measurements of the CO2-O quenching rate coefficient, k(sub VT),and its value estimated from the atmospheric observations. In this study, we retrieve k(sub VT) in the altitude region85-105 km from the coincident SABER/TIMED and Fort Collins sodium lidar observations by minimizing the difference between measured and simulated broadband limb 15 micron radiation. The averaged k(sub VT) value obtained in this work is 6.5 +/- 1.5 X 10(exp -12) cubic cm/s that is close to other estimates of this coefficient from the atmospheric observations.However, the retrieved k(sub VT) also shows altitude dependence and varies from 5.5 1 +/-1 10(exp -12) cubic cm/s at 90 km to 7.9 +/- 1.2 10(exp -12) cubic cm/s at 105 km. Obtained results demonstrate the deficiency in current non-LTE modeling of the atmospheric 15 micron radiation, based on the application of the CO2-O quenching and excitation rates, which are linked by the detailed balance relation. We discuss the possible model improvements, among them accounting for the interaction of the non-thermal oxygen atoms with CO2 molecules
Circulation Statistics in Three-Dimensional Turbulent Flows
We study the large limit of the loop-dependent characteristic
functional , related
to the probability density function (PDF) of the circulation around a closed
contour . The analysis is carried out in the framework of the
Martin-Siggia-Rose field theory formulation of the turbulence problem, by means
of the saddle-point technique. Axisymmetric instantons, labelled by the
component of the strain field -- a partially annealed variable in
our formalism -- are obtained for a circular loop in the plane, with
radius defined in the inertial range. Fluctuations of the velocity field around
the saddle-point solutions are relevant, leading to the lorentzian asymptotic
behavior . The
subleading correction and the asymmetry between right and left PDF tails due to
parity breaking mechanisms are also investigated.Comment: Computations are discussed in a more detailed way; accepted for
publication in Physical Review
Supersonic turbulence and structure of interstellar molecular clouds
The interstellar medium (ISM) provides a unique laboratory for highly
supersonic, driven hydrodynamics turbulence. We present a theory of such
turbulence, confirm it by numerical simulations, and use the results to explain
observational properties of interstellar molecular clouds, the regions where
stars are born.Comment: 5 pages, 3 figures include
Large Amplitude Perturbations in Mesospheric OH Meinel and 87-km Na Lidar Temperatures Around the Autumnal Equinox
Two high‐precision CEDAR instruments, an OH Mesospheric Temperature Mapper (MTM) and a Na Temperature Lidar, have been used to investigate seasonal variability in the mid‐latitude temperature at ∼87 km altitude over the western USA. Here we report the observation of a large perturbation in mesospheric temperature that occurs shortly after the autumnal equinox in close association with the penetration of planetary‐wave energy from the troposphere into the mesosphere. This perturbation has been observed on three occasions and exhibits a departure of up to ∼25–30 K from the nominal seasonal trend during a disturbed period of ∼2 weeks. Such behavior represents a dramatic transient departure from the seasonal trend expected on the basis of current empirical models. These novel results coupled with a recent TIME‐GCM modeling study [Liu et al., 2000] provide important insight into the role of planetary waves in mesospheric variability during the equinox periods
Dispersive stabilization of the inverse cascade for the Kolmogorov flow
It is shown by perturbation techniques and numerical simulations that the
inverse cascade of kink-antikink annihilations, characteristic of the
Kolmogorov flow in the slightly supercritical Reynolds number regime, is halted
by the dispersive action of Rossby waves in the beta-plane approximation. For
beta tending to zero, the largest excited scale is proportional to the
logarithm of one over beta and differs strongly from what is predicted by
standard dimensional phenomenology which ignores depletion of nonlinearity.Comment: 4 pages, LATEX, 3 figures. v3: revised version with minor correction
An Intersecting Loop Model as a Solvable Super Spin Chain
In this paper we investigate an integrable loop model and its connection with
a supersymmetric spin chain. The Bethe Ansatz solution allows us to study some
properties of the ground state. When the loop fugacity lies in the physical
regime, we conjecture that the central charge is for integer .
Low-lying excitations are examined, supporting a superdiffusive behavior for
. We argue that these systems are interesting examples of integrable
lattice models realizing conformal field theories.Comment: latex file, 7 page
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