Use of bubble image velocimetry for measurement of plunging wave impinging on structure and associated greenwater

The measurement of velocity fields of a plunging wave impacting on a structure in a two-dimensional wave tank was investigated experimentally. As the wave impinged and overtopped the structure, a large highly aerated region was created in front of the structure and on top of the structure. The broken wave in front of the structure and associated greenwater on top of the structure are highly aerated containing not only a large number of bubbles but also very large sizes of bubbles. The highly aerated bubbly flow caused the traditional particle image velocimetry (PIV) technique to fail due to the uncontrollable scattering of laser light. A modified PIV method, called bubble image velocimetry (BIV), was introduced by directly using bubbles as the tracer and measuring the bubble velocity by correlating the 'texture' of the bubble images. No laser light sheet was needed while the depth of field was limited to minimize the error. Velocity measurements using BIV and fibre optic reflectometer were compared to validate the BIV technique. While the fluid velocity in the region where no or few bubbles exist can be successfully obtained using PIV, the velocity in the high void fraction region can be measured using BIV. Therefore, BIV can be seen as a complementary technique for PIV. The use of BIV is essential in the studied problem here due to the fact that in the vicinity of the structure the flow is almost entirely bubbly flow. From both the PIV and BIV measurements, it was found that the maximum fluid particle velocity as well as the bubble velocity in front of the structure during the impinging process is about 1.5 times the phase speed of the waves.