THE EFFECT OF BOUNDARY-CONDITIONS AND SHEAR RATE ON STREAK FORMATION AND BREAKDOWN IN TURBULENT CHANNEL FLOWS

Experiments in a horizontal channel indicate that low-speed/high-speed streaks form in the region very close to the interface if shear is imposed on the liquid surface by the motion of a gas. They also form at the wall. Even though the boundary conditions at the wall and at the interface are different (no slip versus nearly free slip with regard to fluctuations), the main characteristics of the streaks appear similar. The spanwise spacing of the streaks when nondimensionalized using the local shear stress and kinematic viscosity is about 100 units whether the streaks are at the wall or at the liquid surface. The ejection and breakdown of the streaks also show qualitatively similar features. For example, the burst frequency appears to scale on local inner variables and the numerical values in nondimensionalized frequency units are about the same whether the bursts originate near the interface or the wall. A number of experiments have been done varying the shear rate at the interface to determine its role in streak formation and breakdown. From the present experiments it is concluded that the shear rate has the main influence on the phenomena and the effect of the boundary conditions is much less important. © 1990 American Institute of Physics.