Free surface effects on the wake of a flat plate

Measurements of the mean velocity and the normal and tangential stresses in the wake of a rough flat plate near a free water surface are presented. The various distributions are compared with similar measurements taken in the wake of the plate when submerged in an effectively infinite fluid. The results reveal striking differences in the manner in which the wake evolves in the two instances. In contrast to the plane wake in an infinite fluid the present experiments show that in the proximity of a free surface the wake center line (defined as the point of maximum velocity defect) migrates toward the free surface. The regions of local maxima and minima typically found in free shear layer turbulence intensity distributions also show this behavior. In addition, both the mean velocity and the turbulent fluctuations decay at a slower rate near the free surface. The implications of the experimental results regarding appropriate free surface boundary conditions for computation are discussed. The equations of continuity and momentum are solved numerically together with equations for the turbulent kinetic energy and isotropic dissipation to complete a turbulence model. The computational model is verified for the infinite wake data. Boundary conditions to simulate the free surface are applied. The numerical results predict the major features which are observed experimentally near the free surface. Originator-supplied keywords include: Hot-film anemometry, Thin shear layer equations, Fluid mechanics, Spectral analysis, and Turbulence model.