Quantitative measurement of the lifetime of localized turbulence in pipe flow

Transition to turbulence in a pipe is characterized by the increase of the characteristic lifetimes of localized turbulent spots ('puffs') with Increasing Reynolds number (Re). Previous experiments are based on visualization or indirect measurements of the lifetime probability. Here we report quantitative direct measurements of the lifetimes based on accurate pressure measurements combined with laser Doppler anemometry (LDA). The characteristic lifetime is determined directly from the lifetime probability. It is shown that the characteristic lifetime does not diverge at finite Re, and follows an exponential scaling for the observed range 1725 <= Re <= 1955. Over this small Re range the lifetime increases over four orders of magnitude. The results show that the puff velocity is not constant, and the rapid disintegration of puffs occurs within 20-70 pipe diameters.