Experimental Generation of Large-Scale Motion for Boundary Layer Control

Adding kinetic energy to the near-wall region of turbulent boundary layer (TBL) flow is necessary for technologically relevant situations such as boundary layer separation control. It is well known that turbulent flows, despite apparent randomness, are dominated by motions with temporal and spatial coherence, commonly termed “coherent structures”, Large-Scale or Very Large-Scale Motions (LSM/VLSMs), which contain a large fraction of the TKE which enhance mixing. This work explores novel flow control using the LSMs/VLSMs present in the turbulent boundary layer to increase the near wall momentum of the boundary layer for separation control. These large-scale motions are modelled as a train of hairpin vortices in a flat plate laminar boundary layer. The structures are generated using a proportional solenoid valve which generate short pulses of compressed air released in a crossflow via a circular orifice. These hairpin vortices are captured by stereoscopic PIV to assess if the structures dissipate or form turbulent spots downstream. These structures will be used in future experiments with active flow control to entrain them into the near wall boundary layer region for momentum addition.