Experimental study of particle motion on a smooth and rough bed under shoaling waves using particle image velocimetry.

摘要

In the first phase of this research, motion of discrete spherical particles of 1.58 mm and specific weight 2.5 gm/cc on a 2% and 3% plane slope were studied in a laboratory wave flume under shoaling wave conditions. These experimental measurements were then used to calculate the individual component forces and to check the balance of the momentum equation of motions for bed load conditions under waves.;It was found that sediment velocities and fluid velocities were nearly in phase. All the components of modeled forces of equation of motions were one scale smaller than the drag force. The drag force was the dominant force in the equation of motion. The magnitude of drag force varied with the use of model for coefficient of drag to compute the drag force. The model for coefficient of drag by Carty produced a greater drag force and the model for coefficient of drag by Stokes (24/Res) produced the least drag force respectively. Experimental data suggest that the friction force was greatly underestimated by using the coefficient of sliding or rolling friction which leads to an imbalance in the equation of motion. The imbalance in the equation of motions could not be explained by experimental uncertainties alone. It was suspected that a thin viscous fluid layer was trapped between the sediment and the smooth bed during their motions. The equation of motions was approximately balanced by replacing solid state frictions model and neglecting lift force with the viscous friction force model based on viscous shear stress. The study also evaluated several commonly used formulae for the coefficient of drag and lift force using experimental fluid shear rate. The use of Saffman shear lift force did not reduce the bottom friction force as the fluid shear rate over bed was negligible.;In the second phase of this research, motion of discrete spherical particle (1.58 mm diameter; specific weight 2.5 gm/cc) on a closely glued single sediment layer bed was studied in a laboratory wave flume inclined at 2% slope under shoaling wave conditions. The closely glued single sediment layer (rough bed) was prepared from glass beads of diameter 1.2 to 1.85 mm. The range of wave-height-to-water-depth ratio was between 0.366 and 0.521. Motion of loose discrete spherical (sediment) particle and the associated fluid velocity field were measured simultaneously using particle image velocimetry with the cameras viewing the flow in an oblique direction. The measurement plane was parallel to the bed and located at an elevation of ½ particle diameter over rough bed. Morphological image processing techniques were used to separate the tracer (fluid) phase and sediment phases from the same two-phase particle image velocimetry image based on the signature sizes respectively.;The fluid velocity was approximately constant but sediment velocity varied with each trial for same wave-height-to-water-depth ratio and wave period respectively. For the experimental conditions, the loose particle motion experienced only on-shore motions. Sediment inertia, buoyancy, added mass, fluid acceleration and rolling friction force were of the same order of magnitude and all were one order of magnitude smaller than the dominant drag force. It was found that, the drag force governs the shape of resultant force on the right hand side of the equation of motions and is out of phase with the sediment inertia force. The bed load equation of motion was approximately balanced by replacing the rolling frictions with a viscous friction term in equation of motions. The viscous friction force was of the form F R = microUrdelta, where micro is the dynamic viscosity, Ur is the relative velocity between the fluid and sediment, and delta is a length scale. Two constant values of delta (one during sediment acceleration and the other during sediment deceleration) were in the viscous friction model to check the balance of equation of motion for rough bed. It was found that the friction model FR = microUrdelta did not balance all the trials of rough bed experiment. Secondly, it was observed that in those trials where the relative velocity was greater, the sediment displacements were found to be smaller suggesting the sediment moved close to bed. (Abstract shortened by UMI.)