Extreme events of turbulent kinetic energy production and dissipation in turbulent channel flow: particle image velocimetry measurements

It has long been believed that most turbulent kinetic energy (TKE) dissipation occurs in the near-wall region between ejection and sweep events, i.e. beneath near-wall quasi-streamwise (Q-S) vortices, where wall-normal velocity fluctuations are locally zero. Contrary to this view, our experiments, carried out in a fully developed turbulent channel flow at Re-tau = 205 by high-speed planar particle image velocimetry, show that the most intensive dissipative events occurring in the viscous sublayer are associated with very rare sweep events originating in the buffer region rather than with the ejections. We show that these rare events are the result of strong rotational motion of Q-S vortices which cause strong shear flow in the lower region of its downward side. Curiously, the same events are responsible for extreme TKE production taking place in the buffer region where its maximum is observed.