Altering turbulence in compressible base flow using axisymmetric sub-boundary-layer disturbances

The turbulent structures in the highly compressible near-wake region of a cylindrical base, to which an axisymmetric sub-boundary-layer strip disturbance has been applied, are examined in detail using a planar Rayleigh/Mie scattering visualization technique. When the downstream edge of the axisymmetric disturbance is placed approximately 12 momentum thicknesses upstream of the base termination, a base pressure increase of approximately 3% is noted over the no-tab case. The increased base pressure due to the presence of the axisymmetric disturbance is attributed to the transfer of turbulent energy into instability modes that are not supported in the near-wake region. Analysis of the. large-scale turbulent structure visualizations indicates that, near the base, axisymmetric and low-order helical disturbances are present in the developing shear layer. This is indicated by a 35% increase in area-based and 20% increase in centroid-based unsteady motion in the end-view orientation, and a 100% increase in mean end-view structure size at the first imaging position, as compared to the no-tab case. As the shear layer travels downstream, the prominence of these disturbances is quickly diminished, due to the high convective Mach number (1.3) associated with the shear layer near the base. The centroid-based and area-based unsteady motion and end-view structure size all diminish to levels lower than those seen in the no-tab case and remain approximately constant throughout the remaining imaging positions.