8,690 research outputs found
Implementation, demonstration and validation of a user-defined wall-function for direct precipitation fouling in ANSYS Fluent
In a previous paper (Johnsen et al., 2015) and presentation (Johnsen et al.,
2016), we developed and demonstrated a generic modelling framework for the
modelling of direct precipitation fouling from multi-component fluid mixtures
that become super-saturated at the wall. The modelling concept involves the
1-dimensional transport of the fluid species through the turbulent boundary
layer close to the wall. The governing equations include the Reynolds-averaged
(RANS) advection-diffusion equations for each fluid species, and the axial
momentum and energy equations for the fluid mixture. The driving force for the
diffusive transport is the local gradient in the species' chemical potential.
Adsorption mechanisms are not modelled per se, but the time-scale of adsorption
is reflected in the choice of Dirichlet boundary conditions for the depositing
species, at the fluid-solid interface.
In this paper, the modelling framework is implemented as a user-defined
function (UDF) for the CFD software ANSYS Fluent, to act as a wall boundary
condition for mass-transfer to the wall. The subgrid, 1-dimensional formulation
of the model reduces the computational cost associated with resolving the fine
length-scales at which the boundary-layer mass transfer is determined, and
allows for efficient modelling of industry-scale heat exchangers suffering from
fouling.
The current paper describes the modelling framework, and demonstrates and
validates its applicability in a simplified 2D heat exchanger geometry
(experimental and detailed CFD modelling data by P\"a\"akk\"onen et al. (2012,
2016)). By tuning the diffusivity, only, good agreement with the experimental
data and the detailed CFD model was obtained, in terms of area-averaged
deposition rates.Comment: 12th International Conference on CFD in Oil & Gas, Metallurgical and
Process Industries, SINTEF, Trondheim, NORWAY, May 30th - June 1st, 2017, 9
pages, 9 figure
Nonclassicality in Weak Measurements
We examine weak measurements of arbitrary observables where the object is
prepared in a mixed state and on which measurements with imperfect detectors
are made. The weak value of an observable can be expressed as a conditional
expectation value over an infinite class of different generalized Kirkwood
quasi-probability distributions. "Strange" weak values for which the real part
exceeds the eigenvalue spectrum of the observable can only be found if the
Terletsky-Margenau-Hill distribution is negative, or, equivalently, if the real
part of the weak value of the density operator is negative. We find that a
classical model of a weak measurement exists whenever the
Terletsky-Margenau-Hill representation of the observable equals the classical
representation of the observable and the Terletsky-Margenau-Hill distribution
is nonnegative. Strange weak values alone are not sufficient to obtain a
contradiction with classical models.
We propose feasible weak measurements of photon number of the radiation
field. Negative weak values of energy contradicts all classical stochastic
models, whereas negative weak values of photon number contradict all classical
stochastic models where the energy is bounded from below by the zero-point
energy. We examine coherent states in particular, and find negative weak values
with probabilities of 16% for kinetic energy (or squared field quadrature), 8%
for harmonic oscillator energy and 50% for photon number. These experiments are
robust against detector inefficiency and thermal noise.Comment: 12 pages, 8 figure
The formation of the solar system
The solar system started to form about 4.56 Gyr ago and despite the long
intervening time span, there still exist several clues about its formation. The
three major sources for this information are meteorites, the present solar
system structure and the planet-forming systems around young stars. In this
introduction we give an overview of the current understanding of the solar
system formation from all these different research fields. This includes the
question of the lifetime of the solar protoplanetary disc, the different stages
of planet formation, their duration, and their relative importance. We consider
whether meteorite evidence and observations of protoplanetary discs point in
the same direction. This will tell us whether our solar system had a typical
formation history or an exceptional one. There are also many indications that
the solar system formed as part of a star cluster. Here we examine the types of
cluster the Sun could have formed in, especially whether its stellar density
was at any stage high enough to influence the properties of today's solar
system. The likelihood of identifying siblings of the Sun is discussed.
Finally, the possible dynamical evolution of the solar system since its
formation and its future are considered.Comment: 36 pages, 7 figures, invited review in Physica Script
Weak Measurements with Arbitrary Pointer States
The exact conditions on valid pointer states for weak measurements are
derived. It is demonstrated that weak measurements can be performed with any
pointer state with vanishing probability current density. This condition is
found both for weak measurements of noncommuting observables and for -number
observables. In addition, the interaction between pointer and object must be
sufficiently weak. There is no restriction on the purity of the pointer state.
For example, a thermal pointer state is fully valid.Comment: 4 page
Journal Staff
The aluminum–zinc-vacancy (Al Zn −V Zn ) complex is identified as one of the dominant defects in Al-containing n -type ZnO after electron irradiation at room temperature with energies above 0.8 MeV. The complex is energetically favorable over the isolated V Zn , binding more than 90% of the stable V Zn ’s generated by the irradiation. It acts as a deep acceptor with the (0/− ) energy level located at approximately 1 eV above the valence band. Such a complex is concluded to be a defect of crucial and general importance that limits the n -type doping efficiency by complex formation with donors, thereby literally removing the donors, as well as by charge compensation
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