Effects of aftbody vortex shedding on forebody vortex oscillation over hemisphere–cylinder bodies

Previous studies have revealed that the leeward vortices around a hemisphere–cylinder body exhibit low-frequency antisymmetric oscillations at angles of attack (AoAs). The oscillation frequency is around 0.11 at AoAs of 10°–40°, whereas at AoAs of 40°–80°, it drops to 0.06, accompanied by apparent vortex shedding from the aftbody. It remains unclear whether this abrupt frequency drop arises from the interference of the aftbody vortex shedding. In this study, large eddy simulations were employed to evaluate the effects of the vortex shedding on the vortex oscillations over the hemisphere–cylinder bodies at two typical AoAs of 30° and 50° with various fineness ratios. The fluctuation of the sectional side-force coefficient is quantified using the axial distributions of the root mean square (RMS) and Strouhal number St obtained via Fast Fourier Transform. The underlying vortex dynamics are identified using dynamic mode decomposition and mode reconstruction. The results show that even when the aftbody length is reduced to the point where vortex shedding is completely suppressed, the forebody vortex oscillations persist, indicating that the sudden frequency shift at high AoAs is caused by the intrinsic characteristics of the forebody vortices, rather than the aftbody vortex shedding. Furthermore, the amplitude and frequency of the forebody vortex oscillations are almost unaffected by the aftbody length; they slightly decrease when the aftbody is shortened. Similar to the high fineness-ratio model, the RMS increases to a peak over the forebody or mid-body even after the model is shortened; longer bodies exhibit slightly stronger fluctuation intensities. Dominant low-frequency oscillations prevail in the forebody region across all fineness ratios.