Manipulation of a turbulent boundary layer using a large-eddy break-up device

Large eddy break-up (LEBU) device has been used in the past as means to reduce skin friction drag associated with a turbulent boundary layer (TBL). In this work we report wind-tunnel measurements on a flat-plate TBL perturbed by a LEBU placed at the edge of the logarithmic layer. Particle image velocimetry (PIV) measurements show that the LEBU alters the wall-normal velocity gradient and Reynolds shear stress not only near its location but also close to the wall, thereby directly affecting the near-wall turbulence cycle; this is also seen from the hotwire measurements downstream of LEBU. An internal layer formed over the surface of the LEBU interacts with the oncoming eddies and introduces synthetic scales inside the TBL (typically three times the boundary-layer thickness) whose signature can be found in the perturbed boundary layer. Proper orthogonal decomposition is carried out on PIV frames to understand the dominant modes with and without the presence of LEBU. These results show that a LEBU can affect the structure of the oncoming TBL in a complex manner and act as an effective scale manipulator for the TBL. More results on the effect of LEBU will be presented during the talk.