Development of pre-set counter-rotating streamwise vortices in wavy channel

•Pre-set counter-rotating streamwise vortices have been successfully quantified.•Various size and shape of vortices as Re, channel gap, and amplitude changes.•Disappearance of the mushroom-like structures near the second peak. Development of counter-rotating streamwise vortices in a rectangular channel with one-sided wavy surface has been experimentally quantified using hot-wire anemometry. The wavy surface has fixed amplitude of 3.75mm. The counter-rotating vortices are pre-set by means of a sawtooth pattern cut at the leading edge of the wavy surface. Variations of the central streamwise velocity Uc with a channel gap H=35mm and 50mm (corresponding to a Reynolds number from 1600 to 4400) change the instability of the flow which can be distinguished from the velocity contours at a certain spanwise plane. The streamwise velocity contours and turbulence intensity for Reynolds number Re=3100 and H=35mm show the disappearance of the mushroom-like vortices prior to turbulence near the second peak of the wavy surface, while for higher Re, this phenomenon occurs earlier. Under certain conditions, for example, for Re=4400 and H=50mm, the splitting of the vortices can also be observed.