Experimental investigation of relaminarizing and transitional channel flows

Abstract

A hot-wire measurement was conducted in a planar channel flow that originated from a strongly disturbed flow in an entrance channel followed by an expansion channel used to reduce the Reynolds number (Re). From ceasing decrease of the streamwise velocity fluctuation energy and the linear extrapolation of the intermittency factor, the lower marginal Re, which is defined as the minimum Re for partial existence of sustainable turbulence, is estimated around 1400 based on the channel width and the bulk velocity. The upper marginal Re at which the intermittency factor reaches one is about 2600. The flow fields passing a turbulent patch were reconstructed with conditional sampling of the streamwise velocity data based on the time of laminar-turbulence interfaces and the reconstructed flow fields indicate a large-scale flow structure across laminar and turbulent parts. This large structure makes it possible for some regions to be at higher Re than the average, so that turbulence can partly survive. The moderate-scale disturbances larger than the turbulent one appear in the non-turbulent parts of the transitional flow, and in these cases the non-turbulent velocity profile is almost identical to the turbulent one. The large-scale fluctuation is observed even over Re = 2600. This leads to the conclusion that a turbulent channel flow close to the upper marginal Re becomes inhomogeneous.