Reynolds stress and shear flow generation

The so-called Reynolds stress may give a measure of the self-consistent flo w generation in turbulent fluids and plasmas by the small-scale turbulent f luctuations. A measurement of the Reynolds stress can thus help to predict flows, e.g. shear flows in plasmas. This may assist the understanding of im proved confinement scenarios such as H-mode. confinement regimes. However, the determination of the Reynolds stress requires measurements of the plasm a potential, a task that is difficult in general and nearly impossible in h ot plasmas in large devices. In this work we investigate an alternative method, based on density measure ments, to estimate the Reynolds stress, and demonstrate the validity range of this quantity, which we term the pseudo-Reynolds stress. The advantage o f such a quantity is that accurate measurements of density fluctuations are much easier to obtain experimentally. Prior to the treatment of the pseudo -Reynolds stress, we present analytical and numerical results which demonst rate that the Reynolds stress in a plasma, indeed, generates a poloidal she ar flow. The numerical simulations are performed both in a drift wave turbu lence regime and a resistive interchange turbulence regime. Finally, the im plications of misaligned probe arrays on the determination of Reynolds stre sses are investigated, and alignment is found to be important but not sever e.