CHAPTER I
                                  INTRODUCTION

    Realizing high speeds in underwater vehicles has been a challenge mainly because of propulsion limitations and skin friction drag. The problem of skin friction drag is compounded by the fact that it increases with the square of the velocity of the vehicle. However, the last half-century has seen significant advancement in technological concepts that makes achievement of greater speeds a definite possibility for underwater vehicles [Tulin 1961].   Such a possibility has been envisioned as a result of a phenomenon known as supercavitation. Under supercavitating conditions, an underwater vehicle is entirely encapsulated in a large gaseous bubble or cavity. Realizing this technology requires a thorough understanding of the physics of the problem. In addition, control and maneuverability of the vehicle pose new challenges because of the different dynamics involved.   In this situation, the measurement of the cavity shape with appropriate sensors is imperative. The characterization of these sensors demands an innovative test bed that is cost effective and, at the same time, able to simulate the supercavitation environment to a fair degree. This thesis aims at summarizing the development, design and characterization of such a test bed for sensor testing specific to supercavitating vehicles. The need for a unique test facility is built up from the definition of cavitation as given below.

    Cavitation is defined as the gas-liquid region formed due to pressure reduction as a result of the dynamic action of the fluid on the boundaries of a liquid system [Stutz and