Near wake of an impulsively started disk

The near wake of an impulsively started disk was studied computationally by a finite element code with a Smagorinsky turbulence model. The shear layer separating from the disk lip rolled up into a symmetric starting vortex ring at first. As time evolved, the vortex stretched in the downstream direction and flow instabilities caused the vortex ring to become wavy eventually leading to the breakup of the ring. The complete breakup and shedding of the starting vortex ring took a time of approximately 14D/U, where D is the disk diameter and U is the freestream velocity. The starting vortex ring circulation attained a plateau of ~2.6UD at a time of about 4D/U, in good agreement with the experimental findings by Balligand(2000). The radial circulation profiles are Gaussian during the symmetric phase and collapse together at a time of 4D/U. Beyond this time, the vortex ring celerity is constant and vorticity extends to the symmetry axis. The base pressure coefficient becomes positive as the vortex ring moves away from the disk and remains positive until the ring is completely shed. The breakup of the starting vortex ring is concurrent with the appearance of an azimuthal pressure gradient and core flow. Following the breakup of the starting vortex ring, the flow became three-dimensional and settled into an open wake. The mean drag and base pressure coefficients were nearly constant after a time of approximately 30D/U and matched very well against experimental data.