Simulation of wave spray cloud

Bodaghkhani, Armin (2018) Simulation of wave spray cloud. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Wave impacts on vessels and offshore structures can induce significant spray. This process leads to topside icing in sufficiently cold and windy conditions. This study establishes the current state of the art understanding of the physical behavior of wave impact and the process of spray cloud formation upstream of a ship or marine structure. The process of spray formation is related to several complicated phenomena including wave slamming, jet formation after impact, sheet and droplet breakup, and production of the spray cloud on the top surface of the ship bow. The process of spray-cloud production and flow kinematics arising from breaking wave impact on a lab-scaled model is investigated using the Bubble Image Velocimetry (BIV) method to measure the wave run-up velocity. In addition to the BIV method, spray characteristics were examined using the Digital Particle Image Velocimetry (DPIV) method. Measurements of droplet size, and velocity, as well as wave run-up velocity, were major elements of this study. Progress has been made in modeling wave spray phenomena, including numerical methods for modeling the free surface, and consideration of slamming, air entrainment, and water breakup. Further, the interaction of single nonlinear wave with a solid vertical surface was numerically simulated in three dimensions. Complex behavior of the wave impact as well as the resulting water sheet and high-speed jet were captured in the numerical model. The maximum wave run-up velocity, instant wave run-up velocity in front of the vertical surface, the break-up length of the water sheet, and the maximum impact pressure were all computed for several input wave characteristics. In addition to the experimental and numerical work, conservation constraints that govern the flow behavior, which is important in the process of spray cloud formation resulting from wave impact, were developed. The size and velocity distribution of spray droplets arising from the maximization of the entropy is subject to these constraints. The prediction is based on a statistical tool called the Maximum Entropy Principle (MEP), and the resulting droplet size distribution is in agreement with the general empirical distributions. The prediction distribution applied to both one- and two-dimensional cases. Finally, four stages of wave spray production are added together to produce a more comprehensive analytical model for predicting the final average droplet diameter from the information related to the inlet wave conditions. These mathematical stages are; 1) the formulation of wave impact velocities based on the input wave characteristics, 2) the formulation of air entrapment at the moment of impact based on the Bagnold-Mitsuyasu scaling law, which calculates the maximum impact pressure, 3) a mathematical relationship between the maximum wave impact pressure and the maximum wave run-up velocities, and finally, 4) the breakup phenomena.

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