Travelling waves in a cylindrical magnetohydrodynamically forced flow

We present an experimental study of a liquid metal flow electromagnetically forced in a large aspect ratio coaxial cylindrical geometry. An azimuthal Lorentz force is applied on the liquid metal gap, through a radial current and an axial magnetic field. Using ultrasonic velocity measurements, we focus on the effect of these two parameters on the flow properties. We show that, depending on the strength of the magnetic field and not only on the applied Lorentz force, different dynamical states exist. We first observe a stationary structure at low forcing. Then, two other regimes of different travelling waves are exhibited at higher forcing. We characterize them by their different frequencies and speeds. Higher magnetic fields clearly promote the faster waves. Connections with other magnetohydrodynamics instabilities are discussed. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3694804]