A shrouded propulsor is a directional collar around the submarine's propeller. It lowers the passive sonar signature of the sub and provides improvement over exposed propeller designs in both efficiency. The Improved Los Angeles (SSN-751 onwards) have a shrouded propulsor, as do all subsequent classes of American attack submarines. At least one Russian KILO has had its propeller replaced by a shrouded propulsor, which is now used by new US and British nuclear submarines. A shrouded propulsor is identical to a standard shafted propeller, with a cylindrical ring of metal attached at the the tips of the propeller blades around the full circumference.
Cavitation occurs behind the leading edge of an exposed propeller blade as it travels through water, especially around the propeller tips. There is a decrease in pressure behind the blades rotating at high speed, and in some cases bubbles can form when the pressure drops below the saturated vapor pressure-locally. This is called cavitation. Vortex lines are generated at the edge of boat propellers in motion. Toward the core of the vortices, the liquid velocity increases and the pressure decreases. The lower pressure triggers cavitation, and the resulting bubbles produce noise, vibrations, and erosion of the propellers.
The cavitation noise can compromise the submarine's position. In general, the production of cavitation has been a phenomena many have tried to avoid. Cavitation in a liquid is the formation, growth, and collapse of gaseous and vapor bubbles due to the reduction of pressure below the vapor pressure of the liquid at the working temperature. Pump impellers, boat props, and similar applications experience cavitation which can produce rapid damage and erosion of surfaces. Studies by a number of authors have revealed that one significant element in producing the damage caused by cavitation occurs when a cavitation bubble collapses in the vicinity of a surface, launching what is called a re-entrant micro-jet toward the surface. This liquid jet can produce velocities as high as 1500 m/s, and is capable of damaging the hardest materials known.
The collapse of these bubbles gives a continuous spectrum of noise, which dominates the higher frequency range of ship's noise and is speed related. That is, cavitation is greater at higher speeds, because the propellers produce more bubbles Cavitation is a much more significant factor in surface ships than in submarines. This is because submarine cavitation, which is relatively slight in shallow water, can be almost completely eliminated in deeper water. The deeper the submarine is, the greater the hydrostatic pressure, thus the less cavitation.
A propeller is essentially a set of fan blades that turn in the water. These blades direct water away from the blades in a flame shaped plume, which elongates and increases in efficiency as the speed of the vessel increases. Despite this action, which does propel a ship forward, the propeller blades also throw water from the blades in a tangential pattern extending from the blade edge. Thus, water is thrown outward from the propeller at angles ranging from 0 degrees to 90 degrees. Moreover, the blades even let water fall over the face of the blades, which creates an inefficient back eddy. All water that is not propelled directly perpendicular to the blades reduces the efficiency of the propeller.
In addition, large swirls of water called vortices are shed by the propeller tips, causing preventable propulsion inefficiencies. A propeller actually works just like a sail, creating lift on the leading edge, which "pulls" the propeller forward. Vortices are spiralling bodies formed by the 'leaking' of pressures at the tip of the fin. A vortice is formed when the high pressure underneath the blade curls around the blade tip to the top side of low pressure. The blade tip has a tip vortice just like any airfoil or hydrofoil, and this excess energy is swept away by the passing water.
The performance characteristics of a hydrodynamic propulsor are determined, to a large extent, by complex three-dimensional viscous flow in the blade rows. The ability to compute the complex flow field for use in the design and analysis of performance of a propulsor would be a valuable tool for improving performance. Research has addressed computation of the steady flow field in shrouded propulsor blade rows. A prime consideration in the choice of a computation procedure is the multiplicity of length scales in the shrouded propulsor flow field that need to be adequately resolved. The differing dominant flow mechanisms at the hub and tip of the blade, blade boundary layers, and a core inviscid flow characterize the multiple length scales.
Improved design in submarine screws (propellers) significantly reduces cavitation. Cavitation is reduced by the shroud, as the duct maintains higher pressure around the blade tips and prevents cavitation bubbles from forming. One method of improving the efficiency of propellers uses a pipe shroud to encase the propeller. This shroud or nozzle forces more of the water directly out from the propeller. And by ducting water flow through the shroud, tip vortices can be harnessed to provide thrust.
Marine propulsors have much more complex geometry than usual turbomachinery blades such as gas turbine engine compressor cascade or helicopter propellers due to the constraints of cavitation and vibratory excitation. Therefore, the rotating blade shape is highly skewed and curved, and much wider in relation to its diameter than would be found in aircraft propellers. The operating conditions of these propulsors are in the very high Reynolds number regime and the flow is fully turbulent.
The performance characteristics of a hydrodynamic propulsor are determined, to a large extent, by complex three-dimensional viscous flow in the blade rows. The ability to compute the complex flow field for use in the design and analysis of performance of a propulsor would be a valuable tool for improving performance. A prime consideration in the choice of a computation procedure for the steady flow field in shrouded propulsor blade rows is the multiplicity of length scales in the shrouded propulsor flow field that need to be adequately resolved. The differing dominant flow mechanisms at the hub and tip of the blade, blade boundary layers, and a core inviscid flow characterize the multiple length scales. One approach is to study the flow field in the tip and hub regions of the blade in separate tractable computations (zonal approach).
On merchant ships this is called a Kort Nozzle. The Kort Nozzle, first introduced in the 1930's, yields greater performance for a variation of the propeller at low boat speeds. It applies a simple ringed nozzle around the periphery of an underwater propeller. By use of carefully designed angled airfoil shapes to the nozzle ring it is possible for the Kort nozzle to actually gain thrust from external forces acting on the nozzle. A well designed Kort nozzle shows noticeable performance gains over a standard underwater propeller at speeds up to, say, 16-20 knots. Beyond those speeds, the drag of the nozzle itself rules out use of the Kort nozzles. As such, Kort nozzles are widely applied to tug boats and other low speed mostly work boats. For purposes of this application, low speed is defined as boat speeds up to and including 20 knots and high speed as boat speeds of over 20 knots.
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