Development of subharmonic disturbance in spanwise-periodic low-speed streaks

Development of a subharmonic sinuous mode in spanwise-periodic low-speed streaks is studied experimentally. Periodic low-speed streaks are generated in a laminar boundary layer by using small pieces of screen set normal to the wall with an equal interval in the spanwise direction. Well-controlled disturbances are introduced to excite the subharmonic instablity mode whose spanwise wavelength is twice the streak spacing. The instability mode develops similar to that in a single low-speed streak in the linear instability stage. Beyond the nonlinear saturation stage of subharmonic instability, however, quasi-streamwise vortices developing along the neighboring low-speed streaks strongly interact with each other in each cycle, causing horseshoe vortices to develop in a staggered array. At the latest stage of subharmonic streak breakdown, near-wall low-speed streaks with a lateral spacing of 80 wall units in terms of the most probable value newly develop, leading to wall turbulence.