  The vortex unsteadiness around a hemisphere-cylinder body at AOAs of 10 to 80
deg was studied using Large Eddy Simulation (LES) and Dynamic Mode
Decomposition (DMD). The Reynolds number (Re) based on the cylinder diameter of
the body is 22000. The results show that vortex oscillations exist over the
forebody at the whole range of AOAs. The oscillation is characterized by
alternate oscillations of a forebody leeward vortex pair up and down and
in-phase swings from side to side. The vortex shedding can be found at the
afterbody as AOAs more than 20o, and the shedding region moves forwards
gradually with AOAs increasing, and accordingly the region of vortex
oscillations contracts and eventually only exists near the nose as AOAs
sufficiently high. The vortex oscillation and shedding all induce fluctuating
side forces along the body, but the ones from vortex oscillations are larger.
The frequencies of vortex oscillations are similar to the ones of vortex
shedding at the AOAs of 10o-40o with St=0.085-0.12, in which the flow fields
over the afterbody are dominated by vortex shedding and forebody-vortex wakes
together; while at AOAs of 50o-80o, the frequencies along the body are
apparently divided into two regimes in which the vortex oscillations over the
forebody have the frequencies of 0.053-0.064, and the vortex shedding over the
afterbody has the frequencies of 0.16-0.2. The DMD analysis shows that except
the mean flows, the most energetic modes correspond to the vortex oscillation
at the forebody and to the vortex shedding at the afterbody respectively, and
the frequencies from DMD are identical to the ones of the side forces obtained
by fast Fourier transform. The time-averaged flow fields for the vortex
oscillations are symmetric, and no apparent asymmetry exists. Therefore, the
vortex pair over the forebody oscillates around a symmetric mean flow field.
