Vortex oscillations around a hemisphere-cylinder body with a high fineness ratio

Vortex unsteadiness around a hemisphere cylinder at angles of attack of 10-80 deg was studied using large-eddy simulation and dynamic mode decomposition. The cylinder body has a fineness ratio of 24.5 (length-to-diameter ratio). The results revealed that oscillations of leeward vortices exist over a forebody at the whole range of angles of attack in addition to vortex shedding at an afterbody. The vortex oscillations consist of antisymmetric modes (alternate oscillation) and symmetric modes (in-phase oscillation). The alternate vortex oscillations correspond to the most energetic modes and are responsible for fluctuating sectional side forces at the forebody that are greater than the ones from vortex shedding at the afterbody. The Strouhal numbers St of both alternate vortex oscillations and shedding at angles of attack of 10-40 deg are similar with St = 0.085-0.12, whereas at angles of attack of 50-80 deg, the Strouhal numbers are apparently divided into two regimes: the vortex oscillations with St = 0.053-0.064 at the forebody, and the vortex shedding with St = 0.16-0.2 at the afterbody. The vortex shedding regions progressively move forward with increasing angles of attack, and accordingly the regions of vortex oscillations contract and eventually only exist near the nose at sufficiently high angles of attack. The computed frequencies are in good agreement with the measured values of hot-wire probes in previous experiments.