The present study has carried out highly accurate measurements of secondary currents in fully-developed straight open channel flows by making use of a powerful two-color Laser Doppler Anemometer system with a direct digital signal processing. The primary and secondary velocity distributions, the maximum-velocity-dip phenomena, the bed and side-wall shear stress distributions and also the turbulence characteristics were made clear in channels of the aspect ratio of 2. The effects of free surface on three-dimensional structure were then examined by comparison with the data of closed duct flow. Most noticeable feature is that a strong free-surface vortex is produced due to high an-isotropy of turbulence which is caused by the existence of free surface. This vortex produces a velocity dip and also restrains the development of the bottom vortex.
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NEZU et al. (1986) studied this question.
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