PIV Techniques for Velocity Fields of Internal Waves over a Slowly Varying Bottom Topography

Abstract

This paper reviews transformation processes and nonlinear properties of internal waves over a uniform slope in a two-fluid system, attempting to reveal water particle kinematics by using particle image velocimetry (PIV). Attenuation and setup induced by wave breaking are predicted using an energy-dissipation model including the radiation stress. In the present paper, the radiation stress is associated with the time-averaged momentum flux owing to the interaction between incident and reflected internal waves. These predictions agree with experimental data obtained by an image processing technique with which the boundary plane between the upper and lower layers could be detected as a density interface. A set of halogen lamps and three high-definition digital video cameras are used to illustrate velocity vector fields during one wave period. Instantaneous velocity fields measured by the PIV system are compared with the calculated velocity distribution by the method of characteristics in combination with the first-order Stokes theory. The rundown behavior is then investigated, and its influence upon the undertow in the lower layer is explained with a simple turbulent-jet model. Furthermore, the theoretical approach for examining mass transport and undertow is discussed in relation to the measured mean velocity.