Experimental study of coherent structures in turbulent pipe flow
Coherent structures seem to play an important role in turbulent shear flows. In the last years, unstable solutions with the form of traveling waves have been identified in numerical simulations in plane Couette and pipe flow. It is believed that these solutions form an strange saddle in the phase space, so that the flow state transiently visits these unstable solutions before eventually returning to the laminar state and indeed, these structures were observed in experiments in the form of transients . Furthermore, coherent structures which live between the laminar and the turbulent state have been numerically found, suggesting the existence of a separatrix in the phase space between the laminar and turbulent profile . The main aim of our study is to extend the previous observations of coherent structures, characterize and quantify the occurrence of such periodic states and to rationalize their role in turbulent puffs and slugs.