Decay of turbulence in a duct with transverse magnetic field*

Decay of honeycomb-generated turbulence in a duct with a static transverse magnetic field is studied via high-resolution direct numerical simulations. The work follows the experimental study (Sukoriansky et al, Exp. Fluids 1986), in which the paradoxical observation has been made that high-amplitude velocity fluctuations exist in the far-downstream part of the duct when the honeycomb exit is located in the zone of strong magnetic field.  It is shown in the simulations that such fluctuations are caused by large-scale quasi-two-dimensional structures forming in the flow at the initial stages of the decay and surviving the magnetic suppression. Turbulence properties, in particular the two-point correlation statistics and typical length scales are computed. The study demonstrates that turbulence decay in the presence of a magnetic field is a complex phenomenon critically affected by the state of the flow at the moment the field is introduced.