Low-frequency unsteadiness of vortices over an ogive-cylinder body was studied using numerical simulation and Dynamic Mode Decomposition (DMD). The results revealed that the vortices over the fore-body of the slender body can exhibit vortex oscillation with a non-dimensional frequency of 0.054 at 70° angle-of-attack, but the flow pattern over the aft-body downstream is Kármán vortex shedding. The vortex oscillation induces larger sectional side-forces on the body than vortex shedding. The DMD analysis for the sectional flow fields at various sections shows that the most energetic mode corresponds to the vortex oscillation at the fore-body and the vortex shedding at the aft-body except the mode for a mean flow, and the associated frequencies are identical to the ones of the sectional side-forces obtained by Fourier transformation.
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