183 research outputs found
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
Conformally Exact Black Hole Perturbed by a Marginal Operator
We have examined effective theory induced by gauged WZW models, in which the
tachyon field is added as a marginal operator. Due to this operator added, we
must further add the higher order corrections, which modifies the original
configuration, to make the theory full-conformally invariant. It has been found
that 2d is a critical dimension in the sense that the metric obtained from
gauged WZW is modified by the tachyon condensation for , but not for .Comment: 11 pages Latex, FIT-HE-94-8
XASH-3, a novel Xenopus achaete-scute homolog, provides an early marker of planar neural induction and position along the mediolateral axis of the neural plate
We have isolated a novel Xenopus homolog of the Drosophila achaete-scute genes, called XASH-3. XASH-3 expression is neural specific and is detected as early as stage 11 1/2, making it one of the earliest markers of neural induction so far described. Moreover, XASH-3 expression within the neural plate is regionally restricted. Transverse bands of XASH-3 mRNA mark discrete positions along the anteroposterior axis, while longitudinal bands mark a discrete position along the mediolateral axis. This latter site of XASH-3 expression appears to demarcate the prospective sulcus limitans, a boundary zone that later separates the functionally distinct dorsal (alar) and ventral (basal) regions of the spinal cord. In sandwich explants lacking any underlying mesoderm, XASH-3 is expressed in longitudinal stripes located lateral to the midline. This provides the first indication that planar or midline-derived inductive signals are sufficient to establish at least some aspects of positional identity along the mediolateral axis of the neural plate. By contrast, the transverse stripes of XASH-3 expression are not detected, suggesting that this aspect of anteroposterior neural pattern is lost or delayed in the absence of vertically passed signals. The restricted mediolateral expression of XASH-3 suggests that mediolateral patterning of the neural plate is an early event, and that this regionalization can be achieved in the absence of inducing signals derived from underlying mesoderm
Semiclassical vs. Exact Solutions of Charged Black Hole in Four Dimensions and Exact O(d,d) Duality
We derive a charged black hole solution in four dimensions described by
WZW coset model. Using the algebraic
Hamiltonian method we calculate the corresponding solution that is exact to all
orders in . It is shown that unlike the 2D black hole, the
singularity remains also in the exact solution, and moreover, in some range of
the gauge parameter the space-time does not fulfil the cosmic censor
conjecture, we find a naked singularity outside the black hole. Exact
dual models are derived as well, one of them describes a 4D space-time with a
naked singularity. Using the algebraic Hamiltonian approach we also find the
exact to all orders transformation of the metric and the dilaton field
for general WZW coset models and show the correction with respect to the
transformations in one loop order.Comment: 42 pages, (typographical errors in pages 33 and 35
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
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
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
Fluid flows in a librating cylinder
International audienceThe flow in a cylinder driven by time harmonic oscillations of the rotation rate, called longitudinal librations, is investigated. Using a theoretical approach and axisymmetric numerical simulations, we study two distinct phenomena appearing in this librating flow. First, we investigate the occurrence of a centrifugal instability near the oscillating boundary, leading to the so-called Taylor-Görtler vortices. A viscous stability criterion is derived and compared to numerical results obtained for various libration frequencies and Ekman numbers. The strongly nonlinear regime well above the instability threshold is also documented. We show that a new mechanism of spontaneous generation of inertial waves in the bulk could exist when the sidewall boundary layer becomes turbulent. Then, we analyse the librating flow below the instability threshold and characterize the mean zonal flow correction induced by the nonlinear interaction of the boundary layer flow with itself. In the frequency regime where inertial modes are not excited, we show that the mean flow correction in the bulk is a uniform rotation, independent of the Ekman number and cylinder aspect ratio, in perfect agreement with the analytical results of Wang [J. Fluid. Mech., 41, pp. 581 - 592, 1970]. When inertial modes are resonantly excited, the mean flow correction is found to have a more complex structure. Its amplitude still scales as the square of the libration amplitude but now depends on the Ekman number
Experimental study of libration-driven zonal flows in non-axisymmetric containers
International audienceOrbital dynamics that lead to longitudinal libration of celestial bodies also result in an elliptically deformed equatorial core-mantle boundary. The non-axisymmetry of the boundary leads to a topographic coupling between the assumed rigidmantle and the underlying low viscosity fluid.The present experimental study investigates theeffect of non axisymmetric boundaries on the zonal flow driven by longitudinal libration. For large enough equatorial ellipticity, we report intermittent space-filling turbulence in particular bands of resonant frequency correlated with larger amplitude zonal flow. The mechanism underlying the intermittent turbulence has yet to be unambiguously determined. Nevertheless, recent numerical simulations in triaxial and biaxial ellipsoids suggest that it may be associated with the growth and collapse of an elliptical instability (Cebron et al., 2012). Outside of the band of resonance, we find that the background flow is laminar and the zonal flow becomes independent of the geometry at first order, in agreement with a non linear mechanism in the Ekman boundary layer (e.g. Calkins et al.; 2010, Sauret and Le Dizes, 2012b)
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