CMSE2015

Safe Gliding in Inflated Bubbles: Engineering solutions and numerical simulations of stable air-water-solid interfaces in supercavities providing a high-speed motion in water
Dr. Alexander Khotsianovsky
Pisarenko Institute of Problems of Strength of the National Academy of Sciences of Ukraine, Ukraine
In-water motion speed of modern surface vessels is limited by viscous resistance (drag) of water, which increases with the motion speed, while the available howercraft and airfoil solutions to this problem with reduced water-vessel contact area have their own limitations, including high energy expenses. This presentation outlines a well-known, but underdeveloped alternative of a drastic drag reduction by separating the vessel hull from water using a disk- or cone-shaped cavitator, which “opens an umbrella” over the major part of the hull to produce an ellipsoidal air bubble (supercavity) inflated by an artificial or natural air inflow, which provides a very high motion speed. This supercavitating principle was implemented in high-speed underwater vehicles (HSUV) and small water plane area twin hull (SWATH) vessels, as well as in famous military applications, such as Shkval (USSR, 1977) and Barracuda (Germany, 2005) high-speed torpedoes. This keynote report covers the latest experimental and numerical simulation results obtained for the air-water-solid interfaces of supercavities by a team of Ukrainian scientists within a framework of the US (DARPA/ONR) project called “Study of drag-reduction capabilities of cavitation applications for motion in water”. Those include experimental research techniques for investigation of cavitating flows, in particular, underwater photography and tensometric measurements, simulation of ventilated cavity bubbles near a free surface, experimental gas consumption and numerical predictions of cavity gas entrainment on the basis of assessment of characteristics of laminar-turbulent shear layer of gas at cavity surface, simulation of liquid weightiness and free border effects, organization of the developed cavitation flow by supply of aqueous solutions of high-molecular linear-chain polymers, and interaction of cavities formed by a conic cavitator and a cavitating strut for the SWATH application. The numerical simulation results are compared with the proposed engineering solutions. The most reliable cavitator and hull geometries and materials, providing the optimal supercavity closure around the hull, the minimal gas loss, as well as the maximal motion stability and maneuverability are identified. The deficiencies of the available solutions and follow-up project tasks are discussed.