209 research outputs found
Optimal pressure boundary control of steady multiscale fluid-structure interaction shell model derived from koiter equations
Fluid-structure interaction (FSI) problems are of great interest, due to their applicability in science and engineering. However, the coupling between large fluid domains and small moving solid walls presents numerous numerical difficulties and, in some configurations, where the thickness of the solid wall can be neglected, one can consider membrane models, which are derived from the Koiter shell equations with a reduction of the computational cost of the algorithm. With this assumption, the FSI simulation is reduced to the fluid equations on a moving mesh together with a Robin boundary condition that is imposed on the moving solid surface. In this manuscript, we are interested in the study of inverse FSI problems that aim to achieve an objective by changing some design parameters, such as forces, boundary conditions, or geometrical domain shapes. We study the inverse FSI membrane model by using an optimal control approach that is based on Lagrange multipliers and adjoint variables. In particular, we propose a pressure boundary optimal control with the purpose to control the solid deformation by changing the pressure on a fluid boundary. We report the results of some numerical tests for two-dimensional domains to demonstrate the feasibility and robustness of our method
An optimal control method for fluid structure interaction systems via adjoint boundary pressure
In recent year, in spite of the computational complexity, Fluid-structure interaction (FSI) problems have been widely studied due to their applicability in science and engineering. Fluid-structure interaction systems consist of one or more solid structures that deform by interacting with a surrounding fluid flow. FSI simulations evaluate the tensional state of the mechanical component and take into account the effects of the solid deformations on the motion of the interior fluids. The inverse FSI problem can be described as the achievement of a certain objective by changing some design parameters such as forces, boundary conditions and geometrical domain shapes. In this paper we would like to study the inverse FSI problem by using an optimal control approach. In particular we propose a pressure boundary optimal control method based on Lagrangian multipliers and adjoint variables. The objective is the minimization of a solid domain displacement matching functional obtained by finding the optimal pressure on the inlet boundary. The optimality system is derived from the first order necessary conditions by taking the Fréchet derivatives of the Lagrangian with respect to all the variables involved. The optimal solution is then obtained through a standard steepest descent algorithm applied to the optimality system. The approach presented in this work is general and could be used to assess other objective functionals and controls. In order to support the proposed approach we perform a few numerical tests where the fluid pressure on the domain inlet controls the displacement that occurs in a well defined region of the solid domain
Higher Curvature Gravity and the Holographic fluid dual to flat spacetime
Recent works have demonstrated that one can construct a (d+2) dimensional
solution of the vacuum Einstein equations that is dual to a (d+1) dimensional
fluid satisfying the incompressible Navier-Stokes equations. In one important
example, the fluid lives on a fixed timelike surface in the flat Rindler
spacetime associated with an accelerated observer. In this paper, we show that
the shear viscosity to entropy density ratio of the fluid takes the universal
value 1/4\pi in a wide class of higher curvature generalizations to Einstein
gravity. Unlike the fluid dual to asymptotically anti-de Sitter spacetimes,
here the choice of gravitational dynamics only affects the second order
transport coefficients. We explicitly calculate these in five-dimensional
Einstein-Gauss-Bonnet gravity and discuss the implications of our results.Comment: 13 pages; v2: modified abstract, added references; v3: added
clarifying comments, modified discussio
Reversible and Irreversible Spacetime Thermodynamics for General Brans-Dicke Theories
We derive the equations of motion for Palatini F(R) gravity by applying an
entropy balance law T dS= \delta Q+\delta N to the local Rindler wedge that can
be constructed at each point of spacetime. Unlike previous results for metric
F(R), there is no bulk viscosity term in the irreversible flux \delta N. Both
theories are equivalent to particular cases of Brans-Dicke scalar-tensor
gravity. We show that the thermodynamical approach can be used ab initio also
for this class of gravitational theories and it is able to provide both the
metric and scalar equations of motion. In this case, the presence of an
additional scalar degree of freedom and the requirement for it to be dynamical
naturally imply a separate contribution from the scalar field to the heat flux
\delta Q. Therefore, the gravitational flux previously associated to a bulk
viscosity term in metric F(R) turns out to be actually part of the reversible
thermodynamics. Hence we conjecture that only the shear viscosity associated
with Hartle-Hawking dissipation should be associated with irreversible
thermodynamics.Comment: 12 pages, 1 figure; v2: minor editing to clarify Section III, fixed
typos; v3: fixed typo
First record of Temnosewellia minor (Platyhelminthes, Temnocephalidae) in Sicily, with a plea for a re-examination of the identity of the publicly available molecular sequences of the genus
Ectosymbiotic temnocephalan flatworms belonging to the genus Temnosewellia were collected on Cherax destructor in an aquaculture farm in Sicily, Italy. This represents the first record of a temnocephalan species for the fauna of the island. Morphological and molecular identification of the collected specimens proved that they belong to the allochthonous species Temnosewellia minor, which was introduced along with crayfishes bred in aquaculture farms. The phylogenetic analyses carried out for the molecular identification of the Sicilian population highlighted some inconsistencies in the grouping of the Temnosewellia sequences available online, thus stressing the opportunity of a careful re-examination of the voucher samples and their identifications. The risks of its unwary introduction in the wild and the need of monitoring its possible impacts on native biota are briefly discussed
The universal viscosity to entropy density ratio from entanglement
We present evidence that the universal Kovtun-Son-Starinets shear viscosity
to entropy density ratio of 1/4\pi can be associated with a Rindler causal
horizon in flat spacetime. Since there is no known holographic (gauge/gravity)
duality for this spacetime, a natural microscopic explanation for this
viscosity is in the peculiar properties of quantum entanglement. In particular,
it is well-known that the Minkowski vacuum state is a thermal state and carries
an area entanglement entropy density in the Rindler spacetime. Based on the
fluctuation-dissipation theorem, we expect a similar notion of viscosity
arising from vacuum fluctuations. Therefore, we propose a holographic Kubo
formula in terms of a two-point function of the stress tensor of matter fields
in the bulk. We calculate this viscosity assuming a minimally coupled scalar
field theory and find that the ratio with respect to the entanglement entropy
density is exactly 1/4\pi in four dimensions. The issues that arise in
extending this result to non-minimally coupled scalar fields, higher spins, and
higher dimensions provide interesting hints about the relationship between
entanglement entropy and black hole entropy.Comment: 30 pages; v2: footnote added, minor editin
Gravity from Quantum Information
It is suggested that the Einstein equation can be derived from Landauer's
principle applied to an information erasing process at a local Rindler horizon
and Jacobson's idea linking the Einstein equation with thermodynamics. When
matter crosses the horizon, the information of the matter disappears and the
horizon entanglement entropy increases to compensate the entropy reduction. The
Einstein equation describes an information-energy relation during this process,
which implies that entropic gravity is related to the quantum entanglement of
the vacuum and has a quantum information theoretic origin.Comment: 7 pages, revtex4-1, 2 figures, recent supporting results adde
Sicilian byzantine icons through the use of non-invasive imaging techniques and optical spectroscopy: The case of the madonna dell’elemosina
The iconographic heritage is one of the treasures of Byzantine art that have enriched the south of Italy, and Sicily in particular, since the early 16th century. In this work, the investigations of a Sicilian Icon of Greek-Byzantine origin, the Madonna dell’Elemosina, is reported for the first time. The study was carried out using mainly non-invasive imaging techniques (photography in reflectance and grazing visible light, UV fluorescence, infrared reflectography, radiography, and computed tomography) and spectroscopic techniques (X-ray fluorescence and infrared spectroscopy). The identification of the constituent materials provides a decisive contribution to the correct historical and artistic placement of the Icon, a treasure of the Eastern European historical community in Sicily. Some hidden details have also been highlighted. Most importantly, the information obtained enables us to define its conservation state, the presence of foreign materials, and to direct its protection and restoration
Local Entropy Current in Higher Curvature Gravity and Rindler Hydrodynamics
In the hydrodynamic regime of field theories the entropy is upgraded to a
local entropy current. The entropy current is constructed phenomenologically
order by order in the derivative expansion by requiring that its divergence is
non-negative. In the framework of the fluid/gravity correspondence, the entropy
current of the fluid is mapped to a vector density associated with the event
horizon of the dual geometry. In this work we consider the local horizon
entropy current for higher-curvature gravitational theories proposed in
arXiv:1202.2469, whose flux for stationary solutions is the Wald entropy. In
non-stationary cases this definition contains ambiguities, associated with
absence of a preferred timelike Killing vector. We argue that these ambiguities
can be eliminated in general by choosing the vector that generates the subset
of diffeomorphisms preserving a natural gauge condition on the bulk metric. We
study a dynamical, perturbed Rindler horizon in Einstein-Gauss-Bonnet gravity
setting and compute the bulk dual solution to second order in fluid gradients.
We show that the corresponding unambiguous entropy current at second order has
a manifestly non-negative divergence.Comment: 28 pages, 2 appendices; v2: added references, fixed typos, one
clarifying commen
Conservative entropic forces
Entropic forces have recently attracted considerable attention as ways to
reformulate, retrodict, and perhaps even "explain'" classical Newtonian gravity
from a rather specific thermodynamic perspective. In this article I point out
that if one wishes to reformulate classical Newtonian gravity in terms of an
entropic force, then the fact that Newtonian gravity is described by a
conservative force places significant constraints on the form of the entropy
and temperature functions. (These constraints also apply to entropic
reinterpretations of electromagnetism, and indeed to any conservative force
derivable from a potential.)
The constraints I will establish are sufficient to present real and
significant problems for any reasonable variant of Verlinde's entropic gravity
proposal, though for technical reasons the constraints established herein do
not directly impact on either Jacobson's or Padmanabhan's versions of entropic
gravity. In an attempt to resolve these issues, I will extend the usual notion
of entropic force to multiple heat baths with multiple "temperatures'" and
multiple "entropies".Comment: V1: 21 pages; no figures. V2: now 24 pages. Two new sections (reduced
mass formulation, decoherence). Many small clarifying comments added
throughout the text. Several references added. V3: Three more references
added. V4: now 25 pages. Some extra discussion on the relation between
Verlinde's scenario and the Jacobson and Padmanabhan scenarios. This version
accepted for publication in JHE
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