237 research outputs found
Radiation-cooled Dew Water Condensers Studied by Computational Fluid Dynamic (CFD)
Harvesting condensed atmospheric vapour as dew water can be an alternative or
complementary potable water resource in specific arid or insular areas. Such
radiation-cooled condensing devices use already existing flat surfaces (roofs)
or innovative structures with more complex shapes to enhance the dew yield. The
Computational Fluid Dynamic - CFD - software PHOENICS has been programmed and
applied to such radiation cooled condensers. For this purpose, the sky
radiation is previously integrated and averaged for each structure. The
radiative balance is then included in the CFD simulation tool to compare the
efficiency of the different structures under various meteorological parameters,
for complex or simple shapes and at various scales. It has been used to precise
different structures before construction. (1) a 7.32 m^2 funnel shape was
studied; a 30 degree tilted angle (60 degree cone half-angle) was computed to
be the best compromise for funnel cooling. Compared to a 1 m^2 flat condenser,
the cooling efficiency was expected to be improved by 40%. Seventeen months
measurements in outdoor tests presented a 138 % increased dew yield as compared
to the 1 m^2 flat condenser. (2) The simulation results for 5 various condenser
shapes were also compared with experimental measurement on corresponding pilots
systems: 0.16 m^2 flat planar condenser, 1 m^2 and 30 degree tilted planar
condenser, 30 m^2 and 30 degree tilted planar condenser, 255 m^2 multi ridges,
a preliminary construction of a large scale dew plant being implemented in the
Kutch area (Gujarat, India)
Dew frequency across the US from a network of in situ radiometers
Dew formation is a ubiquitous process, but its importance to energy budgets or
ecosystem health is difficult to constrain. This uncertainty arises largely
because of a lack of continuous quantitative measurements on dew across
ecosystems with varying climate states and surface characteristics. This
study analyzes dew frequency from the National Ecological Observatory Network
(NEON), which includes 11 grasslands and 19 forest sites from 2015 to 2017.
Dew formation is determined at 30 min intervals using in situ radiometric
surface temperatures from multiple heights within the canopy along with
meteorological measurements. Dew frequency in the grasslands ranges from
15 % to 95 % of the nights with a strong linear
dependency on the nighttime relative humidity (RH), while dew frequency in
the forests is less frequent and more homogeneous (25±14 %, 1
standard deviation – SD). Dew mostly forms at the top of the canopy for the
grasslands due to more effective radiative cooling and within the canopy for the
forests because of higher within the canopy RH. The high temporal resolution of
our data showed that dew duration reaches maximum values
(∼6–15 h) for RH∼96 % and for a
wind speed of ∼0.5ms-1, independent of the ecosystem type.
While dew duration can be inferred from the observations, dew yield needs to
be estimated based on the Monin–Obukhov similarity theory. We find yields of
0.14±0.12mmnight-1 (1 SD from nine grasslands) similar to
previous studies, and dew yield and duration are related by a quadratic
relationship. The latent heat flux released by dew formation is estimated to
be non-negligible (∼10Wm-2), associated with a Bowen ratio
of ∼3. The radiometers used here provide canopy-averaged surface
temperatures, which may underestimate dew frequency because of localized cold
points in the canopy that fall below the dew point. A statistical model is
used to test this effect and shows that dew frequency can increase by an
additional ∼5 % for both ecosystems by considering a reasonable
distribution around the mean canopy temperature. The mean dew duration is
almost unaffected by this sensitivity analysis. In situ radiometric surface
temperatures provide a continuous, non-invasive and robust tool for studying
dew frequency and duration on a fine temporal scale.</p
Capillary wave turbulence on a spherical fluid surface in low gravity
We report the observation of capillary wave turbulence on the surface of a
fluid layer in a low-gravity environment. In such conditions, the fluid covers
all the internal surface of the spherical container which is submitted to
random forcing. The surface wave amplitude displays power-law spectrum over two
decades in frequency, corresponding to wavelength from to a few . This
spectrum is found in roughly good agreement with wave turbulence theory. Such a
large scale observation without gravity waves has never been reached during
ground experiments. When the forcing is periodic, two-dimensional spherical
patterns are observed on the fluid surface such as subharmonic stripes or
hexagons with wavelength satisfying the capillary wave dispersion relation
Casimir Forces at Tricritical Points: Theory and Possible Experiments
Using field-theoretical methods and exploiting conformal invariance, we study
Casimir forces at tricritical points exerted by long-range fluctuations of the
order-parameter field. Special attention is paid to the situation where the
symmetry is broken by the boundary conditions (extraordinary transition).
Besides the parallel-plate configuration, we also discuss the geometries of two
separate spheres and a single sphere near a planar wall, which may serve as a
model for colloidal particles immersed in a fluid. In the concrete case of
ternary mixtures a quantitative comparison with critical Casimir and van der
Waals forces shows that, especially with symmetry-breaking boundaries, the
tricritical Casimir force is considerably stronger than the critical one and
dominates also the competing van der Waals force.Comment: 18 pages, Latex, 3 postscript figures, uses Elsevier style file
Fog and Dew Collection Projects in Croatia
The present paper discusses the fog and dew water collection in Croatia.
Zavizan, the highest meteorological station in Croatia(1594m) is chosen for
collecting of fog water with a standard fog collector (SFC). The highest daily
collection rate was 27.8 L / m2. The highest daily collection rate in days
without rain was 19.1 l/m2. Dew is also a noticeable source of water,
especially during the drier summer season. Dew condensers in Croatia have been
installed on the Adriatic coast (Zadar) and islands Vis and Bisevo. We report
and discuss the data collected since 2003. In the small Bisevo island, a
special roof has been designed to improve the formation and collection of dew
on a house. Data from April 2005 will be presented and discussed.Comment: accessible sur
http://balwois.mpl.ird.fr/balwois/administration/full_paper/ffp-587.pd
Analytic Solution of Emden-Fowler Equation and Critical Adsorption in Spherical Geometry
In the framework of mean-field theory the equation for the order-parameter
profile in a spherically-symmetric geometry at the bulk critical point reduces
to an Emden-Fowler problem. We obtain analytic solutions for the surface
universality class of extraordinary transitions in for a spherical shell,
which may serve as a starting point for a pertubative calculation. It is
demonstrated that the solution correctly reproduces the Fisher-de Gennes effect
in the limit of the parallel-plate geometry.Comment: (to be published in Z. Phys. B), 7 pages, 1 figure, uuencoded
postscript file, 8-9
Critical Casimir effect in classical binary liquid mixtures
If a fluctuating medium is confined, the ensuing perturbation of its
fluctuation spectrum generates Casimir-like effective forces acting on its
confining surfaces. Near a continuous phase transition of such a medium the
corresponding order parameter fluctuations occur on all length scales and
therefore close to the critical point this effect acquires a universal
character, i.e., to a large extent it is independent of the microscopic details
of the actual system. Accordingly it can be calculated theoretically by
studying suitable representative model systems.
We report on the direct measurement of critical Casimir forces by total
internal reflection microscopy (TIRM), with femto-Newton resolution. The
corresponding potentials are determined for individual colloidal particles
floating above a substrate under the action of the critical thermal noise in
the solvent medium, constituted by a binary liquid mixture of water and
2,6-lutidine near its lower consolute point. Depending on the relative
adsorption preferences of the colloid and substrate surfaces with respect to
the two components of the binary liquid mixture, we observe that, upon
approaching the critical point of the solvent, attractive or repulsive forces
emerge and supersede those prevailing away from it. Based on the knowledge of
the critical Casimir forces acting in film geometries within the Ising
universality class and with equal or opposing boundary conditions, we provide
the corresponding theoretical predictions for the sphere-planar wall geometry
of the experiment. The experimental data for the effective potential can be
interpreted consistently in terms of these predictions and a remarkable
quantitative agreement is observed.Comment: 30 pages, 17 figure
Casimir Forces between Spherical Particles in a Critical Fluid and Conformal Invariance
Mesoscopic particles immersed in a critical fluid experience long-range
Casimir forces due to critical fluctuations. Using field theoretical methods,
we investigate the Casimir interaction between two spherical particles and
between a single particle and a planar boundary of the fluid. We exploit the
conformal symmetry at the critical point to map both cases onto a highly
symmetric geometry where the fluid is bounded by two concentric spheres with
radii R_- and R_+. In this geometry the singular part of the free energy F only
depends upon the ratio R_-/R_+, and the stress tensor, which we use to
calculate F, has a particularly simple form. Different boundary conditions
(surface universality classes) are considered, which either break or preserve
the order-parameter symmetry. We also consider profiles of thermodynamic
densities in the presence of two spheres. Explicit results are presented for an
ordinary critical point to leading order in epsilon=4-d and, in the case of
preserved symmetry, for the Gaussian model in arbitrary spatial dimension d.
Fundamental short-distance properties, such as profile behavior near a surface
or the behavior if a sphere has a `small' radius, are discussed and verified.
The relevance for colloidal solutions is pointed out.Comment: 37 pages, 2 postscript figures, REVTEX 3.0, published in Phys. Rev. B
51, 13717 (1995
Effects of Turbulent Mixing on the Critical Behavior
Effects of strongly anisotropic turbulent mixing on the critical behavior are
studied by means of the renormalization group. Two models are considered: the
equilibrium model A, which describes purely relaxational dynamics of a
nonconserved scalar order parameter, and the Gribov model, which describes the
nonequilibrium phase transition between the absorbing and fluctuating states in
a reaction-diffusion system. The velocity is modelled by the d-dimensional
generalization of the random shear flow introduced by Avellaneda and Majda
within the context of passive scalar advection. Existence of new nonequilibrium
types of critical regimes (universality classes) is established.Comment: Talk given in the International Bogolyubov Conference "Problems of
Theoretical and Mathematical Physics" (Moscow-Dubna, 21-27 August 2009
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