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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 λuτ/ν 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

The spacing of streaks in unsteady turbulent wall-bounded flow