Visualization of counter-rotating streamwise vortices in a rectangular channel with one-sided wavy surface

Counter-rotating streamwise vortices in a rectangular cross-section channel with a wavy surface on one side were visualized by smoke-wire flow visualization technique. The use of six equilateral triangle patterns cut at the leading edge of the wavy surface results in relatively uniform spanwise wavelengths of these vortices. Visualization results show that these vortex structures remained visible until the second peak of surface waviness in the channel for Re = 3350, while for Re = 5300 more engulfment occurs in the spanwise structures of the vortices at the first peak of the surface waviness. The more engulfment could increase the mixing which results in early breakdown of the streamwise vortices prior to turbulence. It is also revealed that the evolution of these counter-rotating streamwise vortices is confined within the boundary layer. It causes the spanwise variation of the boundary layer thickness indicated by the spanwise wavy pattern at the edge of the boundary layer.