The Turbulent Wake of a Flat Plate Near a Free Surface
For several decades it has been realized that the presence of a free surface influences the evolution of mean velocity and turbulence but the mechanisms have not been completely described. Early observations of Nikuradse (1926) showed the flow in straight open channels to be three-dimensional and that the maximum of the streamwise mean velocity occurs below rather than coincident with the free surface. More recently, the studies of Ueda, et al (1977) and Komori, et al (1982) for open channel flows show that the eddy viscosity is significantly attenuated by the presence of the free surface. In the latter paper it is also shown that, near the free surface, velocity fluctuations normal to that surface are diminished while the fluctuations in the plane of the surface are increased, suggesting an overall turbulent kinetic energy preservation in that region. A similar redistribution of the turbulence intensities was observed by Thomas and Hancock (1977). In their work a moving wall experiment was devised such that the wall moved at the velocity of the adjacent turbulent fluid so that no velocity gradients and shear stresses were present at the wall, conditions similar to those at a free surface. Damping of the normal velocity fluctuations and an associated increase of the longitudinal ones is observed. The spanwise fluctuations were only slightly increased. In addition the measurements of Raichlen (1967) and McQuivey and Richardson (1969), also for open channel flows, show that the macroscales of the turbulent eddies decrease considerably near the free surface. It should be pointed out that the latter two experiments do not indicate the promotion of the longitudinal turbulence intensity (the only one measured in the work of Raichlen) and the attenuation of the vertical component as in the experiments of Komori and Thomas and Hancock.
KeywordsFree Surface Eddy Viscosity Reynolds Shear Stress Open Channel Flow Velocity Defect
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