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

Abstract

The characteristics of low-speed fluid streaks occurring under sheared air-water interfaces were examined by means of hydrogen bubble visualization technique. A critical shear condition under which the streaky structure first appears was determined to beu _τ≈0.19 cm/s. The mean spanwise streak spacing increases with distance from the water surface owing to merging and bursting processes, and a linear relationship describing variation of non-dimensional spacing$\overline {\lambda ^ + }$ versusy ⁺ was found essentially independent of shear stress on the interface. Values of$\overline {\lambda ^ + }$, however, are remarkably smaller than their counterparts in the near-wall region of turbulent boundary layers. Though low-speed streaks occur randomly in time and space, the streak spacing exhibits a lognormal probability distribution behavior. A tentative explanation concerning the formation of streaky structure is suggested, and the fact that$\overline {\lambda ^ + }$ takes rather smaller values than that in wall turbulence is briefly discussed.