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To better understand the extreme local amplification of tsunami, the experimental investigation on counter-propagating solitary wave collisions over a horizontal bottom was conducted using the optical measurement techniques: Particle Image Velocimetry (PIV), and Laser Induced Fluoresce (LIF). Head-on collisions and oblique collisions of equal-amplitude as well as unequal-amplitude waves were examined.
Precision measurements of water-surface variations were made with the LIF technique that revealed detailed features of the collision process of counter-propagating solitary waves. Comparisons of the laboratory results with the theory of Su and Mirie (1980) are made in terms of the maximum amplitude and found in good agreement. The phase shift resulting from collision is found to be dependent on wave amplitude, which is in a qualitative agreement with Su & Mirie’s (1980) immediate post-collision prediction. The solitary waves lose a small amount of energy after collision. Waves with larger amplitude before collision have greater amplitude reduction. The secondary waves resulting from collision is also compared with Su & Mires’ (1980) theory. The secondary wave form measured in the laboratory is in good agreement with the theory. Our laboratory results also include oblique collision cases, while Su & Mirie’s (1980) theory is developed for the head-on collision conditions only. The oblique collision cases that we investigated are those with small oblique angles (up to 20°) due to the limitation of our basin breadth. We find no difference essentially with or without oblique interaction angle.
PIV technique was used to explore variations of the boundary-layer formation under the collisions. With the resolved velocity field, which has high resolution both in space and time, flow acceleration, vorticity, and pathline for bottom boundary layer flow are computed and analyzed. Detailed description of the flow field transition during the collisions is presented. Layered formation of the boundary-layer structure immediately after the collision is observed for symmetric collisions. This layered formation is a consequence of the overshoot associated with the viscous diffusion in a flow reversal. We identify flow separation and attachment based on vorticity field along the bed. Flow separation happens prior to reaching collision peak when two solitary waves merge. Flow attachment occurs after the collision peak when two waves separate from each other. The separation/attachment point is fixed at the collision point for symmetric collision, while for asymmetric collision, the separation/attachment point advances in the direction of the larger wave.
The velocity field before and after collision peak causes a net displacement of water particles. This net displacement is always in the direction of the outgoing separating waves after the collision. The pathline displacement results from the overshoot of the rundown process, which must be the origin of the formation of trailing secondary waves.
This study used the visualization of the velocity-gradient-tensor field to analyze the flow behaviors and characteristics. The velocity-gradient tensors show that the fluid parcels are stretched vertically prior to reaching the maximum amplitude. After the collision peak, fluid particles are then stretched in the horizontal direction. The tensor magnitudes in the boundary layers are much larger than in the flow interior.
Available online from the National Sea Grant Library