The regeneration of long streaks in wall-bounded flows

A substantial fraction of the kinetic energy of wall-bounded turbulence is in long streaky structures of the streamwise velocity. They have been proposed as necessary parts of the turbulence regeneration cycle, but Jimenez (JFM 2022) showed that turbulence survives when streaks are artificially shortened to lengths comparable to those of the `quadrants' of the tangential Reynolds stress. In this view, the long streaks of natural flows are not part of the cycle, but one of its by-products, opening the question of why are they so long. We study the lengthening of initially short streaks by suddenly removing the shortening torque from manipulated flows. The elongation starts near the wall and proceeds upwards as dy/dt∼1.5 u_τ. It is fast, with elongation rates O(100 u_τ) near the wall. This is much faster than any velocity in the flow, suggesting that the initial lengthening is a short-range continuity-driven rearrangement of pre-existing streak fragments. On the other hand, it is inconsistent with models of streaks as wakes of other structures. The effect is insensitive to the Reynolds number, and extends into the logarithmic layer.