Swimming of a ludion in a stratified sea
Patrice LE GAL
CNRS/Aix-Marseille Université
We describe and model experimental results on the dynamics of a “ludion” — a neutrally buoyant
body — immersed in a layer of stably stratified salt water. By oscillating a piston inside a cylinder com-
municating with a vessel containing the stably stratified layer of salt water, it is easy to periodically vary
the hydrostatic pressure of the fluid. The ludion or Cartesian diver, initially positioned at its equilibrium
height and free to move horizontally, can then oscillate vertically when forced by the pressure oscillations.
Depending on the ratio of the forcing frequency to the Brunt–Väisälä frequency of the stratified fluid, the
ludion can emit its own internal gravity waves that we measure by a classical Particle Image Velocimetry
technique. Our experimental results describe first the resonance of the vertical motions of the ludion when
excited at different frequencies. A theoretical oscillator model is then derived taking into account added
mass and added friction coefficients and its predictions are compared to the experimental data. Then, for
the larger oscillation amplitudes, we observe and describe a bifurcation towards free horizontal motions
(see figure). Although the internal gravity wave frequencies are affected by the Doppler shift induced by
the horizontal displacement velocities, it seems that, contrary to surface waves associated with Couder
walkers [1] they are not the cause of the horizontal swimming. This does not however, exclude possible
interactions between the ludion and internal gravity waves and possible hydrodynamic quantum analogies
to be explored in the future.
IMAGES
Chaîne de gouttes marcheuses (plus de détails...)
CONFÉRENCES
19e Journées de l'Hydrodynamique, Ecole Centrale de Nantes, 26 Novembre 2024
New Challenges in Turbulence Research VII, École de Physique des Houches, 10 Février 2025
New Challenges in Turbulence Research VII, École de Physique des Houches, 10 Février 2025