P. Le Gal

Instabilities of the flow between a rotating and a stationary disk

This experimental study is devoted to the description of the different patterns resulting from instabilities which appear in the flow between a rotating and a stationary disk enclosed by a stationary sidewall. With the help of visualizations we describe the different flow regimes as functions of two control parameters: the Reynolds number and the aspect ratio of the gap separating the disks, which are varied over large continuous ranges. Moreover, visualizations and ultrasonic anemometry lead to the description of the different instabilities and to the construction of a transition diagram that summarizes the domains of existence of the various patterns. Two different scenarios of transition are mainly followed by the flow. When the gap between the two disks is more than the thickness of the two disk boundary layers, circular and spiral waves destabilize the stationary disk boundary layer. Transition occurs in this case by the mixing of these waves. On the other hand, when the two boundary layers are merged, finite-size turbulent structures can appear. They consist of turbulent spots or turbulent spirals which invade the laminar domains as the Reynolds number of the flow is increased.

A systematic numerical study of the tidal instability in a rotating triaxial ellipsoid

The full non-linear evolution of the tidal instability is studied numerically in an ellipsoidal fluid domain relevant for planetary cores applications. Our numerical model, based on a finite element method, is first validated by reproducing some known analytical results. This model is then used to address open questions that were up to now inaccessible using theoretical and experimental approaches. Growth rates and mode selection of the instability are systematically studied as a function of the aspect ratio of the ellipsoid and as a function of the inclination of the rotation axis compared to the deformation plane. We also quantify the saturation amplitude of the flow driven by the instability and calculate the viscous dissipation that it causes. This tidal dissipation can be of major importance for some geophysical situations and we thus derive general scaling laws which are applied to typical planetary cores.

Elliptical instability in terrestrial planets and moons

The presence of celestial companions means that any planet may be subject to three kinds of harmonic mechanical forcing: tides, precession/nutation, and libration. These forcings can generate flows in internal fluid layers, such as fluid cores and subsurface oceans, whose dynamics then significantly differ from solid body rotation. In particular, tides in non-synchronized bodies and libration in synchronized ones are known to be capable of exciting the so-called elliptical instability, i.e. a generic instability corresponding to the destabilization of two-dimensional flows with elliptical streamlines, leading to three-dimensional turbulence. We aim here at confirming the relevance of such an elliptical instability in terrestrial bodies by determining its growth rate, as well as its consequences on energy dissipation, on magnetic field induction, and on heat flux fluctuations on planetary scales. Previous studies and theoretical results for the elliptical instability are re-evaluated and extended to cope with an astrophysical context. In particular, generic analytical expressions of the elliptical instability growth rate are obtained using a local WKB approach, simultaneously considering for the first time (i) a local temperature gradient due to an imposed temperature contrast across the considered layer or to the presence of a volumic heat source and (ii) an imposed magnetic field along the rotation axis, coming from an external source. The theoretical results are applied to the telluric planets and moons of the solar system as well as to three Super-Earths: 55 CnC e, CoRoT-7b, and GJ 1214b. For the tide-driven elliptical instability in non-synchronized bodies, only the Early Earth core is shown to be clearly unstable. For the libration-driven elliptical instability in synchronized bodies, the core of Io is shown to be stable, contrary to previously thoughts, whereas Europa, 55 CnC e, CoRoT-7b and GJ 1214b cores can be unstable. The subsurface ocean of Europa is slightly unstable}. However, these present states do not preclude more unstable situations in the past.

Experimental analysis of the stratorotational instability in a cylindrical Couette flow.

This study is devoted to the experimental analysis of the stratorotational instability (SRI). This instability affects the classical cylindrical Couette flow when the fluid is stably stratified in the axial direction. In agreement with recent theoretical and numerical analyses, we describe for the first time in detail the destabilization of the stratified flow below the Rayleigh line (i.e., the stability threshold without stratification). We confirm that the unstable modes of the SRI are nonaxisymmetric, oscillatory, and take place as soon as the azimuthal linear velocity decreases along the radial direction. This new instability is relevant for accretion disks.

On the effects of an imposed magnetic field on the elliptical instability in rotating spheroids

The effects of an imposed magnetic field on the development of the elliptical instability in a rotating spheroid filled with a conducting fluid are considered. Theoretical and experimental studies of the spin-over mode, as well as a more general short-wavelength Lagrangian approach, demonstrate that the linear growth rate of the instability and the square amplitude of the induced magnetic field fall down linearly with the square of the imposed magnetic field. Application of the results to the Galilean moon Io confirms the fundamental role played by the elliptical instability at the planetary scale.

Elliptic instability of a stratified fluid in a rotating cylinder

In this paper, we analyse the characteristics of the elliptic instability in a finite cylinder in the presence of both background rotation and axial stratification. A general formula for the linear growth rate of the stationary sinuous modes is derived including viscous and detuning effects in the limit of small eccentricity. This formula is discussed and compared to experimental results which are obtained in a cylinder filled with salted water for two different eccentricities by varying the stratification, the background rotation and the cylinder rotation. A good agreement with the theory concerning the domain of instability of the sinuous modes is demonstrated. Other elliptic instability modes, oscillating at the cylinder angular frequency are also evidenced together with a new type of instability mode, which could be connected to a centrifugal instability occurring during the experimental phase of spin-up. The nonlinear regime of the elliptic instability is also documented. In contrast with the homogeneous case, no cycle involving growth, breakdown and re-laminarization is observed in the presence of strong stratification. The elliptic instability in a stratified fluid seems to yield either a persistent turbulent state or a weakly nonlinear regime.

Magnetohydrodynamic simulations of the elliptical instability in triaxial ellipsoids

The elliptical instability can take place in planetary cores and stars elliptically deformed by gravitational effects, where it generates large-scale three-dimensional flows assumed to be dynamo capable. In this work, we present the first magneto-hydrodynamic numerical simulations of such flows, using a finite-element method. We first validate our numerical approach by comparing with kinematic and dynamic dynamos benchmarks of the literature. We then systematically study the magnetic field induced by various modes of the elliptical instability from an imposed external field in a triaxial ellipsoidal geometry, relevant in a geo- and astrophysical context. Finally, in tidal induction cases, the external magnetic field is suddenly shut down and the decay rates of the magnetic field are systematically reported.

Law of spreading of the crest of a breaking wave

In a wide range of conditions, ocean waves break. This can be seen as the manifestation of a singularity in the dynamics of the fluid surface, moving under the effect of the fluid motion underneath. We show that, at the onset of breaking, the wave crest expands in the spanwise direction as the square root of time. This is first derived from a theoretical analysis and then compared with experimental findings. The agreement is excellent.

Elliptical instability in rotating cylinders : liquid metal experiments under imposed magnetic field

In this paper, we present new theoretical and experimental results on the elliptical instability in a liquid metal contained in a rotating deformable cylinder in the presence of an imposed magnetic field. The imposed field, which is aligned with the rotating axis, has a double interest. On the one hand, it permits an analysis of the inertial waves excited by the elliptical instability by measuring their induced magnetic fields. On the other hand, it permits the control of the instability by acting on the Joule damping. In this paper, firstly an analytical calculation of the magnetic field induced by the flow and its associated Joule damping is presented. Also, the linear and weakly nonlinear theories of the elliptical instability are extended to include magnetic field e!ects. Then, the description of the experiments starts by the presentation of the e!ect of the imposed magnetic field strength. Close to the instability threshold, both superand subcritical bifurcations are identified. When the imposed field is decreased, we observe a transition towards complex nonlinear evolutions that we describe with the help of two-dimensional phase diagrams. In a second set of experiments, we vary the eccentricity of the elliptic deformation over a large range in order to demonstrate that far from the instability threshold, the mean inertial wave amplitude is uncorrelated to the eccentricity. We show that, for a given eccentricity, this mean amplitude decreases when the rotation rate increases. In a last series of experiments, we focus on the description of the nonlinear evolution of an oscillatory eigenmode which is di!erent from the principal stationary mode.

Effects of wall compliance on the laminar–turbulent transition of torsional Couette flow

Torsional Couette flow between a rotating disk and a stationary wall is studied experimentally. The surface of the disk is either rigid or covered with a compliant coating. The influence of wall compliance on characteristic flow instabilities and on the laminar–turbulent flow transition is investigated. Data obtained from analysing flow visualizations are discussed. It is found that wall compliance favours two of the three characteristic wave patterns associated with the transition process and broadens the parameter regime in which these patterns are observed. The results for the effects of wall compliance on the third pattern are inconclusive. However, the experiments indicate that the third pattern is not a primary constituent of the laminar–turbulent transition process of torsional Couette flow.