Oscillatory boundary layer over fixed rough beds
The accurate study of characteristics of the flow under gravity waves has become of prime importance due to the growing demand for structural engineering designs in the coastal environment. Although many investigations have been carried out, the progress of fundamental research was slow due to the lack of an adequate velocity measuring instrument. However in recent years, the development of the Laser Doppler Velocimeter has made it possible to observe the orbital velocity very close to a bed without disturbing the flow. This technique was used in this investigation, in which observations of the oscillatory flow under gravity waves were carried out above smooth, two-dimensional and three-dimensional rough beds. For the smooth bed case it was found that the velocity profile throughout the depth was well presented by the Stokes second order shear wave equation, except that the theoretical predictions underestimated the observed results, and a linear relationship was obtained for the velocity coefficients between the two sets of values. As for mean velocity the profile was in close agreement with the Longuet- Higgins conduction solution, and it was found to have a negative value (in opposite direction to wave progression) in the bulk of fluid and always positive values within the boundary layer. The rough beds made little change to the flow in the bulk of fluid. As for inside the boundary layer, the laminar boundary layer was eliminated due to the large size of the rough bed, but for a small size rough bed the flow became laminar at the edge of the boundary layer, and a perturbed laminar boundary layer velocity profile was traceable. However, the two rough beds had similar influences on the flow except for the roughness size and Reynolds number values. Inside the roughness elements of the rough beds vortex formation was clearly observed and the comprehensive range of measurements of these formations are analysed and discussed.