Flow visualizations and measurements of turbulent structures in drag-altered axisymmetric compressible base flows

The effects that drag-altering mechanisms, including afterbody boattailing, sub-boundary layer disturbances (i.e., tabs), and base bleed, have on the turbulent structures present in the near wake of an axisymmetric, supersonic base flow are examined via passive scalar Mie scattering, product formation Mie scattering, and acetone planar laser-induced fluorescence visualizations. Knowledge of the mean turbulent structure size, shape, and orientation at key locations in the flowfield was ascertained by applying a spatial correlation analysis technique, and the bulk motion and convolution of the shear layer were also measured.; A 5° afterbody boattail leads to 20% more (but less active) turbulent structures in the end-view, and side-view structures that are larger and more inclined toward the local flow direction, indicating that less mixing occurs in the developing shear layer than for the blunt base. End-view shear layer motion (sloshing) was less prominent in the recompression region and developing wake due to the weakened activity of the turbulent structures compared to the blunt base case.; Sub-boundary layer disturbances on the afterbody significantly alter the mixing in the near-wake region. Delta-shaped disturbances increase mixing and reduce base pressure in the near wake due to the generation of streamwise turbulent structures. Axisymmetric strip disturbances, conversely, decrease mixing and increase base pressure, since they transfer energy into axisymmetric modes that are not amplified in the near wake due to the highly compressible conditions experienced there.; Base bleed alters the turbulent structures in the near wake by altering the base region topography. The ejection of bleed fluid into the outer shear layer leads to increased shear layer growth and unsteadiness. The wake-core region expands with increasing bleed rates, and ‘extra’ strain rate effects become less prominent in the evolution of the turbulent structures.; The complex interactions present in the recompression and reattachment regions of the flowfield are shown to de-stabilize the turbulent structure organization. The mean structure statistics of all drag-altered flowfields demonstrate that the dominant organization present upstream is significantly weakened or lost due to passage through these regions. When the base bleed rate is sufficient to inhibit the formation of the primary recirculation region, the structures evident upstream survive into the developing wake region.