Natsuki Mizutani

Kinematics of overturning solitary waves and their relations to breaker types

Abstract Numerical simulations using a full-nonlinear BIM (Boundary Integral Method) potential-theory wave model are carried out to study the internal velocity and acceleration fields of an solitary wave overturning on a reef with vertical face (submerged breakwater) and their relation to breaker type. The simulations make it clear that the jet size normalized by the incident wave height is uniquely governed by the crown height of the reef, while the jet shape is similar and independent of the size. Further, they reveal that the overall internal kinematics of overturning waves is clearly related to the jet size. As the jet size increases and the breaker type changes from spilling to plunging, the kinematics thus become increasingly different from those of steady waves. Water particles with the greatest velocities or accelerations within the wave converge towards the jet. After the breaking, both of the velocities and accelerations almost simultaneously reach extreme values near locations beneath the jet. Some of the extreme values are closely related to the breaker type and can be uniquely determined by substituting the breaker type index into the regression equations suggested here.

RELATIONSHIPS OF PLUNGING JET SIZE TO KINEMATICS OF BREAKING WAVES WITH SPRAY AND ENTRAINED AIR BUBBLES

Wave breaking processes inducing violent transition of wave shapes and internal kinematics are investigated through laboratory experiments using flow visualization techniques, and through numerical...

INVESTIGATION OF DECOMPOSITION METHODS OF TURBULENT FLOW FIELD BENEATH WIND WAVES

The performance of two flow field decomposition techniques, the linear spatial filter and the proper orthogonal decomposition, in turbulence decomposition from a twodimensional velocity field was investigated by using experimental data of velocity field beneath laboratory-generated wind waves. The main interest was in the identification of turbulent eddy structures embedded in the unsteady velocity field by the decomposition techniques that are used. The linear spatial filter presented good results showing almost the same vorticity map from the decomposed velocity field as the one from the original velocity field. As long as the size of the window function implemented in the linear spatial filter is adequately determined, turbulent flow field is well extracted by the filter in terms of preservation of turbulent eddy structures. In contrast, somewhat distorted vorticity map was obtained from the turbulent velocity field decomposed by the proper orthogonal decomposition, which implies poor performance of the method in turbulence decomposition.

Wind Shear Stress Acting on Water Surface

This study investigated about the continuity of momentum flux through an air-water interface. We conducted to measure air-flow and water-flow very close to a water surface in a wind wave tank using PIV technique. To evaluate of the continuity of momentum flux calculating from the friction velocity was very difficult, because we could not detect exact a mean water level to fit a log-law profile for the experimental data. The viscous stress affected not only the viscous sub layer but also the turbulent boundary layer. The ratio of the viscous stress in water-flow is from several percent to about ten percent. We deduced that the continuity of momentum flux calculating from the total stress was strongly affected by three-dimensional turbulent water-flow.

PIV measurement of wind flow field over white-caps

Air flow over wind waves generated in a wind wave frame was visualized by water vapor, and instantaneous air flow fields over white-caps were obtained by a PIV system. Three air flow separation patterns were detected, which were decided by the situation of white-capping progress. The air flow separated from wind wave crest is accompanied by inverse flow at the forward face of wave crest. It was found that the frequency of air flow separation with inverse flow was about 6%. Also, the characteristics of the velocity distribution of the air flow field were compared with the mean wind velocity using conventional wind profiler.