Ashish Karn receives PhD for dissertation titled, " Bubbly Flow Physics for Applications in Aerated Hydroturbines and Underwater Transport"

PhD candidate Ashish Karn successfully defended his dissertation in Mechanical Engineering on Thursday, April 14th, 2016. Karn's advisors include Dr. Roger Arndt, Professor Emeritus of the St. Anthony Falls Laboratory and the Department of Civil, Environmental, and Geo-Engineering, and Dr. Jiarong Hong, Professor of the St. Anthony Falls Laboratory and the Department of Mechanical Engineering.  Congratulations, Dr. Karn!!

Dissertation Title: Bubbly Flow Physics for Applications in Aerated Hydroturbines and Underwater Transport

Ventilation technology is involved in a broad range of engineering problems. The current work deals with two such examples of ventilation technology: speed enhancement and controllability of next-generation high–speed underwater vehicles for naval defense applications and environment-friendly power generation through next-generation aerating hydroturbines. These research objectives are addressed through the investigations into fundamental fluid dynamics of bubbles at different size scales when air is entrained inside water. A large bubble is used to envelop an underwater vehicle causing tremendous reduction in flow resistance while the small bubbles are employed for aeration applications in a hydroturbine. Our experiments have provided critical insights into the design and development of operational strategies and models for these novel technologies. For the dispersed bubbly flow regime, bubble size characteristics in the wake of a ventilated turbine blade is measured using shadow imaging and a newly developed image processing approach. Simultaneous mass transfer measurements in the wake have shown an interrelationship with the bubble size and high speed imaging of the bubbles reveal the physical mechanisms of bubble breakup and coalescence and its effect upon the bubble sizes in the wake. In the supercavity bubble regime, systematic studies are carried out to investigate the supercavity closure mechanisms in detail, and a unified theory is proposed to predict different closure modes as a function of different operational variables. Further insights are provided into the interrelationship between supercavity closure, ventilation demand and gas entrainment behaviors of supercavity. The effect of ocean waves on the stability of supercavity bubbles and its closure are also investigated by replicating the ocean waves in the high speed water tunnel. Specifically, a novel aspect of the current research pertains to the visualization and quantification of the internal flows inside supercavity bubbles and drops.