Some characteristics of low-speed streaks under sheared air-water interfaces

The characteristics of low-speed fluid streaks occurring under sheared air- water interfaces were examined by means of hydrogen bubble visualization te chnique. A critical shear condition under which the streaky structure first appears was determined to be u(tau) approximate to 0.19 cm/s. The mean spa nwise streak spacing increases with distance from the water surface owing t o merging and bursting processes, and a linear relationship describing vari ation of non-dimensional spacing <(<lambda>+)over bar> versus y(+) was foun d essentially independent of shear stress on the interface. Values of <(<la mbda>+)over bar>, however, are remarkably smaller than their counterparts i n the near-wall region of turbulent boundary layers. Though low-speed strea ks occur randomly in time and space, the streak spacing exhibits a lognorma l probability distribution behavior. A tentative explanation concerning the formation of streaky structure is suggested, and the fact that <(<lambda>)over bar> takes rather smaller values than that in wall turbulence is brie fly discussed.