The VA-111 Shkval torpedo, along with its successors, represents a groundbreaking advancement in underwater weaponry, originally developed by the Soviet Union. These torpedoes utilize the phenomenon of supercavitation to create a bubble around the torpedo, allowing it to travel at astonishing speeds underwater. Supercavitating torpedoes can achieve velocities exceeding 200 knots (approximately 370 km/h or 230 mph), making them some of the fastest underwater projectiles ever created. Understanding the operational principles of the Shkval torpedo and its descendants is crucial for developing high-speed AUVs.
The Shkval torpedo has a length of 8.2 meters (26 feet 11 inches), a diameter of 532 millimeters (21 inches), a weight of 2,700 kilograms (6,000 pounds), and a warhead weight of 210 kilograms (460 pounds). The principle of supercavitation is fundamental to the operational technology of the torpedo. It occurs when the pressure around the torpedo drops sufficiently to form a gas bubble, which significantly reduces drag and allows for high-speed travel through water. The flow pattern associated with developed cavitation is influenced by several factors, including the size and resistance of the cavitator, the speed of movement, and the pressure in both the undisturbed medium and the cavity itself.
At lower speeds, artificial cavitation can be achieved by supplying forced air to the stall region behind a cavitator disc, leading to what is known as ventilated cavities. The characteristics of these cavities are defined by dimensionless criteria such as the cavitation number—a ratio of pressure differences to velocity pressure—and the Froude number, which represents the ratio of inertial forces to hydrostatic forces. The volumetric flow rate of booster gas is critical; at low flow rates, the cavitation number is high, resulting in smaller cavity dimensions. As gas flow increases, the cavitation number decreases, leading to larger cavities until a minimal threshold is reached where further increases in flow result in the destruction of cavity walls.
The behavior of gas within these cavities is also noteworthy. Gas can be expelled from the cavity either in portions along annular vortices or along longitudinal vortices, depending on whether the cavitation number is high or low. This behavior can be studied experimentally in hydraulic flumes using principles of motion reversal. Observations indicate that at low Froude numbers (around 3), cavities are large; as this number increases to 10 or 15, cavity size gradually decreases. At a Froude number of 25, cavities tend to take on an almost axisymmetrical shape.
The design of the cavitator itself influences performance significantly. When set at an angle of attack, the cavitator disrupts axial symmetry and causes curvature in the cavity’s axis. This asymmetry leads to changes in hydrodynamic forces acting on the torpedo; specifically, the normal force decreases proportionally to the cosine of the angle of attack while resistance force is proportional to its square. This results in a vertical component known as lift force.
Overall dimensions and shapes of cavities depend heavily on cavitator resistance, which varies with design features such as sharpening angles. High-speed filming has shown that smaller sharpening angles yield lower resistance and consequently smaller cavity sizes. As research continues into these advanced technologies, understanding these dynamics will be essential for future developments in both military applications and underwater exploration.
