92 research outputs found
Precession-driven flows in non-axisymmetric ellipsoids
We study the flow forced by precession in rigid non-axisymmetric ellipsoidal
containers. To do so, we revisit the inviscid and viscous analytical models
that have been previously developed for the spheroidal geometry by,
respectively, Poincar\'e (Bull. Astronomique, vol. XXVIII, 1910, pp. 1-36) and
Busse (J. Fluid Mech., vol. 33, 1968, pp. 739-751), and we report the first
numerical simulations of flows in such a geometry. In strong contrast with
axisymmetric spheroids, where the forced flow is systematically stationary in
the precessing frame, we show that the forced flow is unsteady and periodic.
Comparisons of the numerical simulations with the proposed theoretical model
show excellent agreement for both axisymmetric and non-axisymmetric containers.
Finally, since the studied configuration corresponds to a tidally locked
celestial body such as the Earth's Moon, we use our model to investigate the
challenging but planetary-relevant limit of very small Ekman numbers and the
particular case of our Moon
Tidally driven dynamos in a rotating sphere
Large-scale planetary or stellar magnetic fields generated by a dynamo effect
are mostly attributed to flows forced by buoyancy forces in electrically
conducting fluid layers. However, these large-scale fields may also be
controlled by tides, as previously suggested for the star -boo, Mars or
the Early Moon. By simulating a small local patch of a rotating fluid,
\cite{Barker2014} have recently shown that tides can drive small-scale dynamos
by exciting a hydrodynamic instability, the so-called elliptical (or tidal)
instability. By performing global magnetohydrodynamic simulations of a rotating
spherical fluid body, we investigate if this instability can also drive the
observed large-scale magnetic fields. We are thus interested by the dynamo
threshold and the generated magnetic field in order to test if such a mechanism
is relevant for planets and stars. Rather than solving the problem in a
geometry deformed by tides, we consider a spherical fluid body and add a body
force to mimic the tidal deformation in the bulk of the fluid. This allows us
to use an efficient spectral code to solve the magnetohydrodynamic problem. We
first compare the hydrodynamic results with theoretical asymptotic results, and
numerical results obtained in a truely deformed ellipsoid, which confirms the
presence of the elliptical instability. We then perform magnetohydrodynamic
simulations, and investigate the dynamo capability of the flow. Kinematic and
self-consistent dynamos are finally simulated, showing that the elliptical
instability is capable of generating dipole dominated large-scale magnetic
field in global simulations of a fluid rotating sphere.Comment: Astrophysical Journal Letters In press, (accepted) (2014) (accepted
Spontaneous generation of inertial waves from boundary turbulence in a librating sphere
In this work, we report the excitation of inertial waves in a librating
sphere even for libration frequencies where these waves are not directly
forced. This spontaneous generation comes from the localized turbulence induced
by the centrifugal instabilities in the Ekman boundary layer near the equator
and does not depend on the libration frequency. We characterize the key
features of these inertial waves in analogy with previous studies of the
generation of internal waves in stratified flows from localized turbulent
patterns. In particular, the temporal spectrum exhibits preferred values of
excited frequency. This first-order phenomenon is generic to any rotating flow
in the presence of localized turbulence and is fully relevant for planetary
applications
Precession-driven flows in non-axisymmetric ellipsoids
International audienceWe study the flow forced by precession in rigid non-axisymmetric ellipsoidal containers. To do so, we revisit the inviscid and viscous analytical models that have been previously developed for the spheroidal geometry by, respectively, Poincaré (Bull. Astronomique, vol. XXVIII, 1910, pp. 1-36) and Busse (J. Fluid Mech., vol. 33, 1968, pp. 739-751), and we report the first numerical simulations of flows in such a geometry. In strong contrast with axisymmetric spheroids, where the forced flow is systematically stationary in the precessing frame, we show that the forced flow is unsteady and periodic. Comparisons of the numerical simulations with the proposed theoretical model show excellent agreement for both axisymmetric and non-axisymmetric containers. Finally, since the studied configuration corresponds to a tidally locked celestial body such as the Earth's Moon, we use our model to investigate the challenging but planetary-relevant limit of very small Ekman numbers and the particular case of our Moon
Toward a 2D multiphysic code with solid-solid & fluid interactions for industrial related problems
In the present study, applications of the SPH method to industrial related issues are considered by starting from an existing open source 2D SPH code, namely the SPHYSICS code, which offers an effective ground for numerical developments, which are performed in order to bring an answer to industrial problems, such as simulations of solid/fluid coupling in a free surface flow context. The purpose of the present paper is therefore to expose the numerical developments which yield an enhanced version (referred to as "SPHYSIC2") of the initial code. Firstly, the different features added to obtain the operational code needed for engineering applications are described, and so are the problems raised on this way, offering a kind of review of SPH methods for engineers. Secondly, the validation of the proposed code is partially presented with two well known but difficult test cases, namely the classical "dam break" and "wedge entry"problems. Thirdly, principles of a method to solve solid/solid contacts, frequently present in realistic configuration, are exposed and applied to achieve more complex simulations. Finally, perspectives for new features of the SPHYSIC2 code are exposed and discussed
Wetting morphologies on an array of fibers of different radii
We investigate the equilibrium morphology of a finite volume of liquid placed
on two parallel rigid fibers of different radii. As observed for identical
radii fibers, the liquid is either in a column morphology or adopts a drop
shape depending on the inter-fiber distance. However the cross-sectional area
and the critical inter-fiber distance at which the transition occurs are both
modified by the polydispersity of the fibers. Using energy considerations, we
analytically predict the critical inter-fiber distance corresponding to the
transition between the column and the drop morphologies occurs. This distance
depends both on the radii of the fibers and on the contact angle of the liquid.
We perform experiments using a perfectly wetting liquid on two parallel nylon
fibers: the results are in good agreement with our analytical model. The
morphology of the capillary bridges between fibers of different radii is
relevant to the modeling of large arrays of polydisperse fibers
Spontaneous generation of inertial waves from boundary turbulence in a librating sphere
In this work, we report the excitation of inertial waves in a librating sphere even for libration frequencies where these waves are not directly forced. This spontaneous generation comes from the localized turbulence induced by the centrifugal instabilities in the Ekman boundary layer near the equator and does not depend on the libration frequency. We characterize the key features of these inertial waves in analogy with previous studies of the generation of internal waves in stratified flows from localized turbulent patterns. In particular, the temporal spectrum exhibits preferred values of excited frequency. This first-order phenomenon is generic to any rotating flow in the presence of localized turbulence and is fully relevant for planetary application
Elliptical instability in hot Jupiter systems
Several studies have already considered the influence of tides on the
evolution of systems composed of a star and a close-in companion to tentatively
explain different observations such as the spin-up of some stars with hot
Jupiters, the radius anomaly of short orbital period planets and the
synchronization or quasi-synchronization of the stellar spin in some extreme
cases. However, the nature of the mechanism responsible for the tidal
dissipation in such systems remains uncertain. In this paper, we claim that the
so-called elliptical instability may play a major role in these systems,
explaining some systematic features present in the observations. This
hydrodynamic instability, arising in rotating flows with elliptical
streamlines, is suspected to be present in both planet and star of such
systems, which are elliptically deformed by tides. The presence and the
influence of the elliptical instability in gaseous bodies, such as stars or hot
Jupiters, are most of the time neglected. In this paper, using numerical
simulations and theoretical arguments, we consider several features associated
to the elliptical instability in hot-Jupiter systems. In particular, the use of
ad hoc boundary conditions makes it possible to estimate the amplitude of the
elliptical instability in gaseous bodies. We also consider the influence of
compressibility on the elliptical instability, and compare the results to the
incompressible case. We demonstrate the ability for the elliptical instability
to grow in the presence of differential rotation, with a possible synchronized
latitude, provided that the tidal deformation and/or the rotation rate of the
fluid are large enough. Moreover, the amplitude of the instability for a
centrally-condensed mass of fluid is of the same order of magnitude as for an
incompressible fluid for a given distance to the threshold of the instability.
Finally, we show that the assumption of the elliptical instability being the
main tidal dissipation process in eccentric inflated hot Jupiters and
misaligned stars is consistent with current data.Comment: Icarus (2013) http://dx.doi.org/10.1016/j.icarus.2012.12.01
Evolution of water waves generated by subaerial solid landslide
International audienceWaves generated by aerial and subaerial landslides are studied experimentally, theoretically and numerically. A set of experiments are done in a wave tank of 18 m long, 0.65 m wide and 1.5 m deep. Numerical simulations are in good agreement with the experiments. Basedon numerical and experimental results, we derive different scaling laws which show a good agreement with the experiments and the simulations. These scaling laws allow thus to predict the time evolution of the maximum amplitude wave generated by an aerial solid landslide, which is a relevant quantity for wave forecast
Experimental and numerical study of mean zonal flows generated by librations of a rotating spherical cavity
International audienceWe study both experimentally and numerically the steady zonal flow generated by longitudinal librations of a spherical rotating container. This study follows the recent weakly nonlinear analysis of Busse (2010), developed in the limit of small libration frequency - rotation rate ratio, and large libration frequency - spin-up time product. Using PIV measurements as well as results from axisymmetric numerical simulations, we confirm quantitatively the main features of Busse's analytical solution: the zonal flow takes the form of a retrograde solid body rotation in the fluid interior, which does not depend on the libration frequency nor on the Ekman number, and which varies as the square of the amplitude of excitation. We also report the presence of an unpredicted prograde flow at the equator near the outer wall
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