Evolution of wake structure around an impulsively stopped sphere with a streamwise magnetic field at 60≤Re≤300

With a streamwise magnetic field, the evolution of wake structure in a flow around an impulsively stopped sphere in an incompressible viscous fluid is investigated. The research parameter range is 60≤Re≤300 and 0≤N≤10, where Re and N are the Reynolds number and the interaction parameter, respectively. For present cases, the flow is fully developed before the sphere stopped and its symmetrical feature of the wake will be preserved after the sphere stopped. A complicated vortex structure system including a primary vortex ring, a fragmented secondary vortex, and an accompanying vortex is formed, which is summarized in a {N,Re} phase diagram. A scaling law of the drag force is found after the sphere stopped. It decays as t∗-2/3 at a small timescale and t∗-1 or t∗-7/6 for different Reynolds numbers at a large timescale in the absence of a magnetic field, where t∗ is the dimensionless time after abruptly stopping the sphere. When the magnetic field is applied, the decay rate of drag force is slower at a small timescale but faster at a large timescale. Meanwhile, peak azimuthal vorticity at the primary vortex ring core is shown to decay as t∗-4/3 at a large timescale when the flow is axisymmetric. It will decay faster under the influence of a streamwise magnetic field.