Abstract

In 1883 Osborne Reynolds discovered a peculiar state for flow of water in a pipe [1]. This state corresponds to the transition from the quiescent, laminar state at low Re (Reynolds number, a dimensionless measure of the flow velocity) to the roiling, turbulent state at high Re. The laminar state is devoid of fluctuations; the turbulent state is inundated by a broad range of fluctuations. The transitional state is distinct from the laminar and turbulent states, and consists of localized patches-- "flashes"--of fluctuations. Later work unveiled many features of these flashes: they can proliferate or fade away, maintain their shape or continually expand [2,3,4]. The nature of the fluctuations in the flashes, however, remained unknown. We conduct experiments of flow of water in a pipe and focus on the fluctuations in the flashes. Our results suggest a startling conclusion: the fluctuations in the flashes partake in the well-known Richardson-Kolmogorov energy cascade, the signature of high Re turbulent state. To wit, transition is not a distinct state: the flashes are turbulent [5].