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.

Description: Turbulence quantities have been measured for a low-Reynolds-number fully developed two-dimensional channel flow subjected to system rotation. Turbulence intensities, Reynolds shear stress, correlation coefficient, skewness and flatness factors, four-quadrant analysis, autocorrelation coefficient and power spectra are investigated. According to the dimensional analysis, the relevant parameters of this flow are the Reynolds number Re* = u* D/v and the Coriolis parameter Rc = Ω v/u*2 for the wall region, and Re* and Ω D/u* for the turbulent core-region. The existence of a Coriolis region where turbulence intensities are defined by a new variable yc* = y/δc has been clarified on the pressure side in the rotating channel flow. The amount of turbulent kinetic energy transported by the Coriolis term is extremely small compared to the production term in the transport equation of Reynolds normal stress. However, the Coriolis term makes a large contribution to Reynolds shear stress transport on the pressure side of the channel. It is caused by the strong ejection which occurs periodically on the pressure side even though the ejection frequency is low. The strong ejection is conjectured to be caused by a large-scale longitudinal structure like a roll cell on the pressure side of the channel. © 2005 Cambridge University Press.