ON THE STREAK SPACING AND VORTEX ROLL SIZE IN A TURBULENT CHANNEL FLOW

Streamwise high vorticity rolls and streaks in the turbulent channel f lows have been the subject of many studies due to their important role in turbulence production, as a result of sweeping, ejection, and burs ting of these structures. Understanding the physics of these streamwis e structures is important in controlling drag producing events. Invest igations of the average streak spacing of the low-speed streaks have r esulted in the generally accepted range of lambda(+) = lambda u(tau)/v = 100 +/- 20, where lambda is the mean spanwise spacing between strea ks, normalized to the viscous length nu/u(tau). It is also reported,(1 ,2) for y(+) less than or equal to 30, that the streak spacing grows n early linearly with distance from the wall. The previous studies mostl y have focused on distances close to the wall. Here we report on corre lation measurements extended into the log layer, which show that the l inear growth of the vortex diameter and the streak spacing extends wel l in the log layer. Arguments are presented to distinguish these two m easures. (C) 1995 American Institute of Physics.