'Synthetic' Large Scale Motions Organize Small Scale Motions in the Turbulent Boundary Layer

The relationship between large- and small-scale motions in a non-equilibrium turbulent boundary layer was studied experimentally. A zero-pressure-gradient flat plate turbulent boundary layer was perturbed by a short array of two-dimensional roughness elements, both statically and under dynamic actuation. The dynamic forcing generated a `synthetic' very-large-scale motion (VLSM) within the flow which was observed by phase-locked flow measurements. The phase-relationship between both synthetic and natural VLSMs and the small scale motions within the boundary layer was studied by cross-correlation and cospectral techniques, to reveal an organizing effect of the artificial VLSM on smaller scales