The evolution and instability of wake structure around an impulsively stopped sphere with a streamwise magnetic field for 600 ≤ Re ≤ 1400

The evolution of wake structure in a flow around an impulsively stopped sphere in an incompressible viscous fluid is investigated under the influence of a streamwise magnetic field at moderate Reynolds numbers. The research parameter range is 600 ≤ Re ≤ 1400 and 0 ≤ N ≤ 10, where Re and N are the Reynolds number and the interaction parameter, respectively. A vortex ring system is developed, which contains the primary and secondary vortex rings after stopping the sphere. The flow will keep axisymmetric before stopping the sphere for the purpose to investigate the instability of shedding vortex rings. Without the magnetic field, an azimuthal instability will develop on the secondary vortex ring when the Reynolds number is large enough, e.g., Re = 1200. It is caused by the centrifugal movement of the secondary vortex ring and this instability can be suppressed by the streamwise magnetic field. Furthermore, an obvious oscillation caused by the motion of the primary vortex ring on the drag force is found after stopping the sphere. The amplitude of such oscillation increases with enlarging the Reynolds number but decreases with enlarging the magnetic field intensity.[Figure not available: see fulltext.].