Letter: The link between the Reynolds shear stress and the large structures of turbulent Couette-Poiseuille flow

Abstract

The length and width of the long and wide structures appearing in turbulent Couette flows are studied by means of a new dataset of direct numerical simulation covering a stepped transition from pure Couette flow to pure Poiseuille one, at Reτ 130, based on the stationary wall. The existence of these structures is linked to the averaged Reynolds stress, uv: as soon as in any part of the channel uv changes its sign, the structures disappear. The length and width of the rolls are found to be, approximately, 50h and 2.5h, respectively. For this Reynolds number, simulations with a domain shorter than 100h cannot properly describe the behaviour of the longest structures of the flow.