Probing the outer-layer-inner-layer dynamics of a turbulent boundary layer using synthetic jets

The fraction of the turbulent kinetic energy associated with large-scale velocity structures in the logarithmic region of the boundary layer has been shown to increase with increasing Reynolds number, highlighting their importance for flows of engineering interest. This increase in energy is associated with an increase in the interaction with smaller-scale turbulence nearer to the wall through amplitude and frequency modulation. In this combined experimental/simulation study, we use synthetic jets to interact predominantly with large-scale structures in a turbulent boundary layer. Spectral analysis and conditional averaging is performed to ascertain the impact of the synthetic jets on these structures. The indirect influence on smaller-scale structures is determined by measurements of the wall shear stress. The results show that, in most cases, the synthetic jets reduce the energy of the large-scale structures and their coherence with the wall, with a corresponding increase in local wall shear stress. For a small range of forcing conditions, coherence is actually increased with the wall, with a corresponding decrease in local wall shear stress. This suggests that the larger-scale structures work to suppress the drag-increasing aspect of the inner layer.