Transition between quasi 2D and 3D Rayleigh-Bénard convection in a horizontal magnetic field

MHD Rayleigh-Bénard convection was studied experimentally and numerically using a liquid metal inside a box with square horizontal cross section and aspect ratio five. Applying a sufficiently strong horizontal magnetic field converts the convective motion into a flow pattern of quasi-two dimensional rolls arranged parallel to the magnetic field. The aim of this paper is to provide a detailed description of the flow field, which is often considered as quasi 2D. In this paper we focus on the transition from a quasi-two-dimensional state towards a three-dimensional flow occurring with decreasing magnetic field strength. We present systematic flow measurements that were performed by means of ultrasound Doppler velocimetry. The measured data give an insight into the dynamics of the primary convection rolls, the secondary flow induced by Ekman pumping and reveal the existence of small vortices that develop around the convection rolls. The flow measurements are completed by direct numerical simulations. Numerical results and experimental findings show an excellent agreement with respect to the qualitative flow features.