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

Abstract

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.

Dynamics of large turbulent structures in a steady breaker

“…The synchro nization of the camera and the strobe light was performed with a Real Time Linux (RTAI) PC, which allows for accurate control of the timing. This technique, which has been referred to as Bubble Image Velocimetry (BIV) [27], is able to detect, although with some limitations, the rolling motion of the cloud of bubbles entrained by the large eddies, thus allowing the measurement of the velocity field wherever the bubble concentration is large enough. It must be pointed out that due to the nature of BIV, it is not suited to char acterize the fine scale velocity fluctuations of the flow.…”

Selection of Optical Tomography Parameters for Gas Bubble Shape Analysis

“…Assessment of the flow structure is often conducted by visual observation in a transparent channel (Leifer et al, 2003;Mayinger and Feldmann, 2001;Yonguk et al, 2005). It is not easy to include an observed flow to the given structure and such decisions are usually subjective, so averaging distribution of phase concentration at time and space is often accomplished by intuition.…”

Measurements of Surface Kinematics and Temperature in the Surf and Swash Zones Using Infrared Image Velocimetry

“…Chang and Liu, 1999;Qiao and Duncan, 2001;Melville et al, 2002;Huang et al, 2009). When performing measurements using a typical PIV, air bubbles also make the laser sheet uncontrollable in the aerated region (Ryu et al, 2005) and the high-power laser light may be reflected by bubb1les and damages the charge-couple device (CCD). A new measurement technique-bubble image velocimetry developed by Ryu et al (2005) assumes bubbles under breaking waves have a high traceability following to water fluid flow, may not be robust.…”

“…When performing measurements using a typical PIV, air bubbles also make the laser sheet uncontrollable in the aerated region (Ryu et al, 2005) and the high-power laser light may be reflected by bubb1les and damages the charge-couple device (CCD). A new measurement technique-bubble image velocimetry developed by Ryu et al (2005) assumes bubbles under breaking waves have a high traceability following to water fluid flow, may not be robust. In addition, recent progress of turbulence and current measurements in natural surf zones has been achieved using ADV (e.g., Ruessink, 2010;Feddersen, 2012).…”