Delayed detached eddy simulation for wake flow analysis of axisymmetric boattail models under low-speed conditions

This study investigates the steady and unsteady behaviors of near-wake flow for axisymmetric models acquired with conical boattails with different slant angles under low-speed conditions. Delayed detached eddy simulation (DDES) was applied for the numerical processes. The Proper Orthogonal Decomposition (POD) was then used to reconstruct the dominant modes of the pressure at the base and velocity fields on the symmetry plane. Both near-wake and far-wake flows were considered in this study to obtain a full pattern of the wake. It was shown that the DDES allows for obtaining a highly accurate wake flow field. The large-scale structure of the base flow with two dominant modes of vortex shedding at St(D) = 0.19-0.24 and bubble pumping at St(D) = 0.06-0.08 occurs for the axisymmetric blunt-based model. At boattail angles up to 16 degrees, those frequencies are clearly distinguished from the time series of POD modes of the velocity fluctuations. However, for higher angles, the motions of the "bubble pumping" and vortex shedding combine to generate frequencies in the range of St(D) = 0.07-0.15. The fully separated flow disturbs a ratio of one-third for the bubble pumping and vortex shedding frequencies, which was shown widely in previous studies. The reduction of frequency band and large-scale features for boattail angles of 16 degrees explains the low drag generated for that configuration.