Experimental investigation of two-dimensional MHD turbulence

Magnetohydrodynamics (MHD) offers a unique opportunity to study the behavior of two-dimensional turbulent flows. It is generally known that a strong external magnetic field B perpendicular to the direction of the mean flow will suppress any velocity fluctuations in the direction of B. In this case, the equations of fluid motion can be integrated analytically along B in the limit of a high Hartmann number M, leading to the so-called ''quasi two-dimensional'' (Q2D) approximation. An experimental configuration was developed which meets these requirements. It consists of a closed loop channel with electrically insulating walls and a rectangular electric potential probe array located in the top Hartmann wall. This probe contains 6 x 11 measurement pins which can be sampled simultaneously at high frequencies, allowing visualisation and statistical analysis of time-dependent flow structures over the area of the probe array. The present work investigates the flow around a circular cylinder whose symmetry axis is aligned with B. It will be seen that the Q2D method can suppress any three-dimensional instabilities that appear in ordinary hydrodynamics. The following results are presented: Determination of the critical Reynolds number Rc for the onset of time-dependent vortex patterns in the cylinder wake depending on M. For comparison with existing theoretical calculations, this experiment is additionally done for a shear flow in a rectangular channel with an inhomogeneous electrical wall conductivity. Both cases show a general Rc(M)-dependence which is recognized as a typical sign of Q2D stability limits., Visualisation of time-dependent vortex structures using still images and time series at different positions in the cylinder wake. For R > 5000, the periodic vortex street is observed to break up as single vortices and vortex groups are leaving the regular pattern in the spanwise direction., The energy equation is deduced for quasi two-dimensional flows and analyzed experimentally to point out the energy transport mechanisms. The results for the production, convection and dissipation of turbulent kinetic energy explain the development and the decay of the vortex street as well as the observed break-up at R > 5000. Special emphasis is placed on the analysis of different dissipation mechanisms in quasi two-dimensional flows. (orig.)