טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentArad Ludar Lotan
SubjectCavitational Flows and Supercavitation Bubbles
DepartmentDepartment of Applied Mathematics
Supervisor Professor Emeritus Alon Gany


Abstract

Cavitation is a unique phenomenon in hydrodynamics where vapor appears within a homogenous liquid medium. It occurs in different situations commonly characterized by the pressure decrease under the vapor pressure of the liquid. As a consequence, the liquid starts to evaporate locally, generating volumes of vapor, referred to as “cavitation bubbles”. The vapor structures are often unstable, and when reaching a zone of increased pressure, they may violently collapse. Under certain conditions of velocity and geometry a cavitation bubble formed over a moving underwater vehicle may grow and extend, enveloping the entire body. Such situation is called supercavitation and may be used purposefully to reduce friction and drag, allowing moving underwater at extremely high speeds. To prevent a sudden collapse of the supercavitation bubble, and ensure its completeness, mechanisms for gas or liquid injection from the body may be applied. By means of these injection mechanisms, supercavitation bubbles can be created in different conditions than those of bubbles occurring naturally. This phenomenon is called "artificial supercavitation" and it is of interest to researchers as the basis for developing underwater vehicles, which can implement the phenomenon, as well as a research tool allowing for future investigation of supercavitation in conditions conducive for research.

Understanding the geometry of a supercavitation bubble is essential to the design of supercavitational underwater vehicles and applications, and it is the focus of this work. To enhance understanding of the supercavitation bubble shape and structure, a series of experiments have been conducted on cylindrical slender bodies in a uniform flow of water, in different flow speeds. A comparison of supercavitation bubbles created on bodies with different nose geometries (i.e. cavitators), has been made. The comparison was referred to the conditions of the bubbles’ creation and collapse, and also their shape and development. It was found that the different cavitators produce the same bubble geometries, though at somewhat different flow velocities. As opposed to the common assumption that the flow and bubble shape are determined by the cavitator, it was found that the cavitator only determines the local pressure that leads to the bubble's creation, but does not dictate the entire pressure field of the supercavitation bubble and flow.

A hysteresis phenomenon was observed, showing different bubble development path when increasing versus decreasing the flow velocity. We conclude that hysteresis may occur not only for naturally developed or artificial supercavitation bubbles closing on a solid body, as was suggested before, but also in different situations that have to be further explored. 

Theoretical analyses of the bubble's geometry have also been made. The work involved a static stability examination of a fully developed supercavitation bubble in steady flow. A few general solutions of supercavitation bubbles in equilibrium have been obtained. A number of geometries of bubbles exposed to different possible disturbances, have been examined.

For studying the pressure field of a supercavitation bubble, its creation and development, another experiment has been made, in which we measured the pressure inside and outside the supercavitation bubble.