THE SPACING OF STREAKS IN UNSTEADY TURBULENT WALL-BOUNDED FLOW

The spacing of streaks of low-speed fluid has been studied experimentally in a wall-bounded turbulent flow in which sinusoidal unsteadiness was superposed on an otherwise steady mainstream over a range of frequencies. The modulation of phase-conditioned streak spacing about its mean value does not follow the steady wall-flow relation of lambda u(tau)/v equal to a constant. Instead, it is quite accurately described by a local length scale that models the momentary value of the total shear distortion of large eddies of the flow. This single shear-distortion length scale also correlates well with the streak spacing measured in steady wall-bounded flows and in unbounded homogeneous turbulent flow at high shear rates. The apparent generality of these results implies that the streak-spacing selection mechanism depends strongly on the strain history of large-scale coherent motions and so should be investigated in the context of the coupled straining processes of turbulence production.