Thermomagnetic instability of a ferrofluid Couette flow under a magnetic field in Rayleigh-stable regimes

  • HIREMATH, Anupam M (Normandy Le Havre University)
  • MEYER, Antoine (Brandenburg Technology University)
  • MUTABAZI, Innocent (Normandy Le Havre University)

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Ferrofluids are colloidal water- (or oil)-based suspensions of magnetic nanoparticles. They have many technological applications including vacuum rotary seals, moving coil loudspeakers, exclusion seal for hard disk drive, stepper motor, cooling of transformers and heat transfer. The magnetic particles under the magnetic field pertain a ponderomotive force known as Kelvin force which contains a non-conservative term that plays the role of a centripetal buoyancy and can destabilize the flow. This magnetic centripetal buoyancy can be associated with an effective magnetic gravity. The linear stability of a ferrofluid flow in a cylindrical annulus with an outward heating and imposed magnetic field is investigated in the case of stationary annulus and in Rayleigh-stable regimes of the circular Couete flows (Keplerian regime, solid-body rotation, sole rotation of the outer cylinder) in which the centrifugal force plays a stabilizing role. In the case of a stationary cylindrical annulus, it has been shown that thermomagnetic convection appears in form of helical stationary vortices when the magnetic Rayleigh number exceeds a critical value which depends on the curvature. The effects of rotation and of the Earth gravity are investigated. Solid-body rotation delays the onset of thermoconvection while Earth gravity dominates the thermal convection for weak magnetic field while magnetic gravity overwhelms for large intensities of the magnetic field.