Experimental lateral wall boundary layer behavior of a differentially rotating split-cylinder flow.

The cylindrical wall boundary layer of a closed cylinder split in two halves at the equator is studied experimentally. When these two parts rotate in exact corotation the internal flow is essentially in solid-body rotation at the angular velocity of both halves. When a slight difference between the rotation frequencies is established a secondary flow is created due to the differential rotation between both sides and restricted to the boundary layer. This behavior of the boundary layer is compared with theoretical and numerical results finding the "sandwich" structure of a Stewartson boundary layer. Time-dependent waves are observed near the cylindrical wall. Their behavior for different values of the control parameters are presented. Finally, a global recirculation mode is also found due to a symmetry-breaking induced between sides that appears because of a slight misalignment of the experimental setup, whose characteristics are compatible with the behavior of a precessing cylinder.

Instabilities triggered in different conducting fluid geometries due to slowly time-dependent magnetic fields.

The main objective of this work is the study and analysis of non-linearities forced through oscillating magnetic fields in a conducting fluid where the instabilities are triggered due to magnetohydrodynamic forces. Different geometries have been studied and different surface patterns that break the symmetries have been observed. First, an InGaSn drop of fluid where the system breaks the azimuthal and radial symmetries depending on the volume is observed. Second, we extend the study to an InGaSn annular configuration where the presence of patterns opens the door to discuss the possibility to extend these results to other configurations as biological systems, where the conducting fluid is an electrolyte. This configuration has an added interest, as it has been proposed that the vertigoes triggered on patients in an MRI test could be generated by the interaction of the magnetic field with the electrolyte present in the inner ear.

Instabilities of conducting fluid layers in weak time-dependent magnetic fields.

We present the experimental analysis of the instabilities generated on a large drop of liquid metal by a time-dependent magnetic field. The study is done exploring the range of tiny values of the control parameter (the ratio between the Lorentz forces and inertia) avoiding nonlinear effects. Two different instabilities break the symmetries generating spatial patterns that appear without a threshold for some specific frequencies (up to the experimental precision) and have been observed for parameter values two orders of magnitude lower than in previously published experiments [J. Fluid Mech. 239, 383 (1992)JFLSA70022-112010.1017/S0022112092004452]. One of the instabilities corresponds to a boundary condition oscillation that generates surface waves and breaks the azimuthal symmetry. The other corresponds to a parametric forcing through a modulation of the Lorentz force. The competition between these two mechanisms produces time-dependent patterns near codimension-2 points.

Stochastic dynamics of particles trapped in turbulent flows.

The long-time dynamics of large particles trapped in two nonhomogeneous turbulent shear flows is studied experimentally. Both flows present a common feature, a shear region that separates two colliding circulations, but with different spatial symmetries and temporal behaviors. Because large particles are less and less sensitive to flow fluctuations as their size increases, we observe the emergence of a slow dynamics corresponding to back-and-forth motions between two attractors, and a super-slow regime synchronized with flow reversals when they exist. Such dynamics is substantially reproduced by a one-dimensional stochastic model of an overdamped particle trapped in a two-well potential, forced by a colored noise. An extended model is also proposed that reproduces observed dynamics and trapping without potential barrier: the key ingredient is the ratio between the time scales of the noise correlation and the particle dynamics. A total agreement with experiments requires the introduction of spatially nonhomogeneous fluctuations and a suited confinement strength.

Experimental dynamics in magnetic field-driven flows compared to thermoconvective convection.

We compare the dynamics obtained in two intermediate aspect ratio (diameter over height) experiments. These systems have rotational symmetry and consist of fluid layers that are destabilized using two different methods. The first one is a classical Bénard-Marangoni experiment, where the destabilizing forces, buoyancy and surface tension, are created by temperature gradients. The second system consists of a large drop of liquid metal destabilized using oscillating magnetic fields. In this configuration, the instability is generated by a radial Lorentz force acting on the conducting fluid. Although there are many important differences between the two configurations, the dynamics are quite similar: the patterns break the rotational symmetry, and different azimuthal and radial wavenumbers appear depending on the experimental control parameters. These patterns in most cases are stationary, but for some parameters they exhibit different dynamical behaviours: rotations, transitions between different solutions or cyclic connections between different patterns.

Experimentally observed route to spatiotemporal chaos in an extended one-dimensional array of convective oscillators.

We report experimental evidence of the route to spatiotemporal chaos in a large one-dimensional array of hotspots in a thermoconvective system. As the driving force is increased, a stationary cellular pattern becomes unstable toward a mixed pattern of irregular clusters which consist of time-dependent localized patterns of variable spatiotemporal coherence. These irregular clusters coexist with the basic cellular pattern. The Fourier spectra corresponding to this synchronization transition reveal the weak coupling of a resonant triad. This pattern saturates with the formation of a unique domain of high spatiotemporal coherence. As we further increase the driving force, a supercritical bifurcation to a spatiotemporal beating regime takes place. The new pattern is characterized by the presence of two stationary clusters with a characteristic zig-zag geometry. The Fourier analysis reveals a stronger coupling than the previous mixed pattern and enables us to find out that this beating phenomenon is produced by the splitting of the fundamental spatiotemporal frequencies in a narrow band. Both secondary instabilities are phaselike synchronization transitions with global and absolute character. Far beyond this threshold, a new instability takes place when the system is not able to sustain the spatial frequency splitting, although the temporal beating remains inside these domains. These experimental results may support the understanding of other systems in nature undergoing similar clustering processes.

Subcritical instabilities in a convective fluid layer under a quasi-one-dimensional heating.

The study and characterization of the diversity of spatiotemporal patterns generated when a rectangular layer of fluid is locally heated beneath its free surface is presented. We focus on the instability of a stationary cellular pattern of wave number ks which undergoes a globally subcritical transition to traveling waves by parity-breaking symmetry. The experimental results show how the emerging traveling mode (2ks/3) switches on a resonant triad (ks, ks/2, 2ks/3) within the cellular pattern yielding a "mixed" pattern. The nature of this transition is described quantitatively in terms of the evolution of the fundamental modes by complex demodulation techniques. The Bénard-Marangoni convection accounts for the different dynamics depending on the depth of the fluid layer and on the vertical temperature difference. The existence of a hysteresis cycle has been evaluated quantitatively. When the bifurcation to traveling waves is measured in the vicinity of the codimension-2 bifurcation point, we measure a decrease of the subcritical interval in which the traveling mode becomes unstable. From the traveling wave state the system undergoes a global secondary bifurcation to an alternating pattern which doubles the wavelength (ks/2) of the primary cellular pattern; this result compares well with theoretical predictions [P. Coullet and G. Iooss, Phys. Rev. Lett. 64, 866 (1990)]. In this cascade of bifurcations towards a defect dynamics, bistability due to the subcritical behavior of our system is the reason for the coexistence of two different modulated patterns connected by a front. These fronts are stationary for a finite interval of the control parameters.

Slow dynamics in a turbulent von Kármán swirling flow.

An experimental study of a turbulent von Kármán flow in a cylinder is presented. The mean flow is stationary up to a Reynolds number Re=10(4) where a bifurcation takes place. The new regime breaks some symmetries of the problem and becomes time dependent because of equatorial vortices moving with a precession movement. In the exact counterrotating case, a bistable regime appears and spontaneous reversals of the azimuthal velocity are registered. A three-well potential model with additive noise reproduces this dynamic. A regime of periodic response is observed when a very weak forcing is applied.

Nonlinear magnetic induction by helical motion in a liquid sodium turbulent flow.

We report an experimental study of the magnetic field B--> induced by a turbulent swirling flow of liquid sodium submitted to a transverse magnetic field B-->(0). We show that the induced field can behave nonlinearly as a function of the magnetic Reynolds number, R(m). At low R(m), the induced mean field along the axis of the flow, , and the one parallel to B-->(0), , first behave like R(2)(m), whereas the third component, , is linear in R(m). The sign of is determined by the flow helicity. At higher R(m), B--> strongly depends on the local geometry of the mean flow: decreases to zero in the core of the swirling flow but remains finite outside. We compare the experimental results with the computed magnetic induction due to the mean flow alone.

Local marangoni number at the onset of hydrothermal waves

We report the results of an experiment in which a layer of fluid, placed in a container open to the air, is subjected to a horizontal temperature difference DeltaT. The fluid height h is kept constant and both the horizontal temperature difference and the horizontal dimensions of the fluid layer are changed. In this configuration, when DeltaT goes beyond a certain threshold, waves propagating from the cold to the hot side appear (hydrothermal waves) with a determined group velocity. We study the influence of the container geometry on these waves. Close to the threshold, they are spatially localized near the hot side and a local Marangoni number is defined to describe this behavior. If DeltaT is further increased, the waves fill the whole fluid layer. We also find an agreement between our experimental results and theoretical works and simulations.

[Exchange of chloride and bicarbonate at terminal ileum in rabbit: the process active character and physiological sense (author's transl)]. / Intercambio de cloruro y bicarbonato en íleon terminal de conejo: carácter activo del proceso y sentido fisiológico del mismo.

The process of chloride and bicarbonate exchange at the terminal ileum in rabbit and the way it is affected by the alkalinization or acidification of the caecum have been studied. Chloride absorption and bicarbonate secretion takes place at the terminal ileum in rabbit. Administration of DNP (5 X 10(-4) M) in the solution together with an i.v. injection of the same substance (0.11 mg/100 g weight) decreased chloride absorption and, quite markedly, bicarbonate secretion in the ileum. Introduction of an alkaline solution into the caecum reduces bicarbonate secretion in the ileum and simultaneously decreases chloride absorption. Introduction of an acid solution into the caecum produces an increase in ileal bicarbonate secretion, while it reduces chloride absorption.