Coherent structures in a turbulent boundary layer. Part 1: generation of 'artificial' bursts.

An attempt was made to produce 'artificial' burst in an effort to better understand the physics and structure of coherent events in a turbulent boundary layer. These were generated in a turbulent boundary layer developed on a flat plate towed in an 18 m water channel and with negligible freestream turbulence. The burstlike events were produced by either withdrawing nearwall fluid from two minute holes separated in the spanwise direction or by pitching a miniature delta wing that was flush mounted to the wall. Either of these two actions generates a hairpin-like vortex and low speed streak that resembles naturally occurring structures. The resulting sequence of events that occur at a given location can be uniquely controlled, allowing detailed examination via phase locked measurements and flow visualization. The proof of concept of artificial burst generation was established by comparing the artificial events with natural, random bursts using flow visualization and hot film signals. (from authors' abstract)