Air Entrainment by Bow Waves

Creators: Waniewski, Tricia Ann

Abstract

Experimental studies of air entrainment by breaking waves are essential for advancing the understanding of these flows and creating valid models. The present study used three-dimensional simulations of a bow wave to examine its air entrainment process. The simulated waves were created by a deflecting plate mounted at an angle in a super-critical free surface flow. Since the air entrainment process is closely coupled with breaking wave dynamics, the present study included both air entrainment and free surface measurements. Measurements of the free surface were obtained from the simulated bow waves at two scales, and also from the bow wave created by a towed wedge model. Contact line and bow wave profile measurements for the different experiments were compared, demonstrating the similarity of the experimental simulations to the towed model experiments. The plunging wave jet shape was measured in the larger scale stationary model and towed model experiments and used to calculate jet thickness, velocity, and impingement angle. The bow wave profile data from the towed model experiments were used to investigate the scaling of the wave with the flow and geometric parameters. Surface disturbances were observed on the plunging wave face, and their wavelength, frequency, and velocity were measured. The primary mechanisms for air entrainment were the impact of the plunging wave jet and individual droplets in the splash region on the free surface. The air entrainment process was observed in the larger scale stationary model experiments, and the air bubbles were entrained in spatially periodic bubble clouds. Due to the shallow depth in these experiments, measurements of only the larger bubbles in the initial stages of air entrainment were obtained. An impedance based void fraction meter, developed specifically for the purpose, was used to measure void fractions and bubble size distributions beneath the wave. The bubble cloud size and void fraction increased with downstream distance. There were indications that the surface disturbances control the periodicity of the bubble clouds. Namely, the surface disturbances divide the plunging liquid jet sheet into a series of plunging wave jets, each entraining air into a separate bubble cloud beneath the free surface.

Additional Information

© 1999 Tricia Ann Waniewski. Report number E200.36 on contract N-00014-94-1-1210. My thesis would not possibly be complete without acknowledging many sources of assistance. First, I am grateful for the financial support of the Office of Naval Research under grant number N00014-94-1-1210, and the American Society of Mechanical Engineers through a graduate teaching fellowship. Second, I am grateful to the many people whose names appear in the following paragraphs. I would like to recognize the contributions of the Thomas Building staff, especially Jackie Beard, Cecilia Lin, and Dana Young and also Fran Matzen of the Keck Building staff. They all helped me in a friendly way through mazes of paperwork so that things were finished on time. My experiments required a great deal of technical assistance. Special thanks go to Rich Eastvedt for his cheerful help with seemingly every aspect of the Keck SB lab, especially for patiently cutting a 4'x3' hole in 1" thick stainless steel and for figuring out how to ship hundreds of pounds of fragile lab equipment across the country. Thanks also go to Hai Vu for designing and building numerous ingenious and neatly packaged electronic circuits required for my experimental measurements. In addition, I am thankful for the machining and design expertise of Joe Fontana, Russ Green, Rodney Rojas, and John VanDeusen. Finally, I would like to thank Rod Barr, Bob Kowalyshyn, and Jim McGurrin at Hydronautics Research for their assist a nce in a series of experiments in March 1998. My fellow graduate students were both my friends and my colleagues; they all made the basement of Thomas a cool place to spend five years. I gratefully acknowledge their help and friendship - especially Robert Behnken, Michael Kaneshige, Anna Karion, Amy Warncke Lang, Bruce Nairn, N.V.V. Rajan, Clancy Rowley, Roberto Zenit, and the members of the "Thomas Ensemble." I would also like to thank Christopher Hunter, Hilla Shaviv, and all of the undergraduate students who provided much needed assistance with my experiments and good company in the lab. My advisor, Christopher Brennen, extended his professional guidance and support to me since my very first day at Caltech. His vast experience in experimental fluid mechanics was indispensable. I was also very fortunate to have a second advisor, Fredric Raichlen. Many productive research ideas originated in conversations with him, often as I was doing experiments in the lab. I express my deepest gratitude to both of my advisors. In addition to offering many thoughtful research ideas, Allan Acosta. was a fellow flutist who encouraged my interests in music and engaged in many delightful hours of chamber music with myself and other graduate students. Morteza Gharib and Theodore Wu served on my thesis committee and made many interesting comments and suggestions. Finally, I am grateful for the continued encouragement of John Gardner at the Pennsylvania State University. Most importantly, I would like to acknowledge the loving support of my family. My brother, Brian Waniewski, who always had a humorous story to tell. My parents, David and Nancy Waniewski, who never failed to call and write to see how I was doing every week. And to my husband, Sudipto Sur, whom I met while I was a graduate student at Caltech. I thank you all with much love.

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