An experimental observation of intermittent extreme events in boundary layer transition

One of the fundamental characteristics of an extreme event, which has been widely studied in diverse branches of physics, is the non-Gaussian nature of the probability distribution function. Specifically events which have amplitudes that are twice the significant wave height, which is a measure of the background state, are characterized as extreme events. Here we find a parallel of this event during the laminar–turbulent transition in a flat-plate boundary layer subjected to freestream turbulence. During the transition process spontaneous intermittent turbulent fluctuations are observed with amplitudes twice the significant wave height of the background state. The probability distribution function of events during the transition process is quantified using hotwire anemometry revealing a long-tail non-Gaussian nature as a whole, which is a fundamental characteristic of extreme events. The spatial structure of extreme fluctuation shows that they are large amplitude spatially localized structures which were obtained from simultaneous measurements made using hot-wire and time-resolved particle image velocimetry techniques. The probability of occurrence of such high intensity spatially localized turbulent fluctuations increases with the freestream velocity. Though the phenomenon of intermittency during boundary layer transition has been extensively studied, the objective of the present work is to establish its similarities to extreme events predicted in other fields of physics.