Multi-Modal Forcing of the Turbulent Separated Shear Flow Past a Rib

Experiments have been conducted to study the development of flow behind a surface mounted rib under different phase controlled excitation. Single mode excitation and multi-mode excitation with different relative phases are studied. The results presented include the coherent and random components of the turbulent energy and shear stresses, the energy exchange with the mean flow and between the modes, and the phase decorrelation of the coherent components. The fundamental-subharmonic excitation does not provide any significant improvements in the shear layer growth over the fundamental excitation. The shear layer growth correlates with the subharmonic mode development. The large scale structures are significant even after the reattachment region as evident from the magnitude of the coherent components of the turbulent energy and the shear stress. The binary exchange terms are significant in the near-field region whose sign is phase dependent, i.e., it reverses its sign based on the phase difference between the fundamental and 1st subharmonic mode. The location of the fundamental and subharmonic peaks are different from the peak location of their respective energy exchange with the mean flow; this is attributed to the significance of the binary energy exchange between the fundamental and the subharmonic mode in this region. The excitation regularizes the flow leading to low phase jitter in the near field region. The origin and development of phase decorrelation is attributed primarily to the subharmonic instability.