ALBERT

Description: Experimental work to investigate plane Couette flow has been performed in the Reynolds number range of 750 ≤ Re (=hUb/(2v)) ≤ 5000 or 50 ≤ Re* (=hu*/v) ≤ 253 where Ub, u* and h are moving wall speed, friction velocity and channel half-height, respectively. The low-Reynolds-number effect on the wall friction coefficient Cf, mean velocity profile and statistical turbulence quantities is discussed in relation to the turbulent Poiseuille flow properties. Since the shear stress is constant in Couette flow, the flow is free from the effect of shear stress gradient and the Reynolds number effect therefore can be seen explicitly, uncontaminated by this effect. A flow region diagram is given to show how the low-Reynolds-number effect penetrates into the wall region. The area of the buffer region is contracted by the low-Reynolds-number effect when Re* ≤ 150, so that the additive constant B of the log law decrease as Re* decreases. Also, Cf has a larger value than in Poiseuille flow in the low Re* range. The log-law area in Couette flow is 2-3 times as wide as that in Poiseuille flow. The defect law is Re*-dependent and the non-dimensional velocity gradient at the core, Rs = (dU1/ dx2(h/u*), increases from 3 to 4.2 as Re* increases from 50 to 253. The peak value of streamwise turbulence intensity u1p + has a constant value of 2.88 but decreases sharply as Re* reduces below 150. The large longitudinal vortices extending the entire height of the channel are shown to be sustained in Couette flow that is oscillating around their average position. This causes a slow fluctuation with large amplitude in the streamwise velocity component. These vortices make the Couette flow three-dimensional and the skin friction coefficient varies 20% sinuously in the spanwise direction, for example. Also, the zero-crossing time separation of streamwise velocity auto-correlation R11(τ) becomes longer as τ =40h/Ub, which is 3 times as long as that in Poiseuille flow. © 2005 Cambridge University Press.