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An Assessment of Naval Hydromechanics Science and Technology
Traditionally, full-scale cavitation inception was based on visual observations in water tunnels. This method, however, is not suitable for modern submarine propulsors as indicated by measurements made on the Large Scale Vehicle (LSV), a ¼-scale powered model of the SSN 21 submarine, at Lake Pend Oreille. Even with the relatively large size of the LSV, substantial scaling corrections for the cavitation inception number are necessary, because of a mismatch in Reynolds number. Differences in both scale and kinematic viscosity (due to temperature differences) contribute to the differences in Reynolds number. This is an important issue since laboratory research and field studies indicate that the inception index is strongly dependent on this parameter. Unfortunately, a precise, scientifically based scaling relationship is not available, making it problematic to predict the cavitation performance of some major weapon systems. A physics-based method for predicting cavitation inception would enable better and quicker design and reduced model and full-scale testing costs. Research in the Large Cavitation Channel in Memphis, Tennessee, should include fundamental work aimed at developing the needed physical models.
Although the discussion has so far concentrated on submarines, it applies generally to weapons silencing as well. In addition to hydrodynamics, the critical technologies are hydroacoustics and structural acoustics. Progress in all three technological areas is essential if future stealth requirements are to be met.
To prevent the detection and classification of surface platforms at long ranges, electromagnetic, hydrodynamic, and acoustic sources must be controlled. The hydrodynamic and thermal wakes of surface ships can be detected by a wide variety of electromagnetic sensors with frequencies ranging from visual to radar. Submarines generally detect and classify surface ships from the modulated cavitation noise generated by the propellers. In spite of very significant progress, propeller cavitation still begins at relatively moderate ship speeds. The level of radiated noise also adversely interferes with towed array beams directed toward the towing vessel. As in the case of submarines, maneuvers significantly degrade the acoustic signature of surface ships. The magnetic field of surface platforms extends to shorter ranges but is clearly critical for mine warfare.
To achieve the required stealth performance for surface ships, water tunnel and lake testing needs to be supplemented by model or full-scale measurements at sea to address specific stealth and signature problems. Air entrainment and bubbly flows cannot be adequately modeled in freshwater. It should be stressed that the tools are available to conduct almost laboratory-quality experiments in the field, and these could be conducted on a noninterference basis using naval vessels. The hydromechanics program also has to recognize that stealth and signature problems must be addressed in the context of the operational environment, and this is generally not well represented by towing tank wave fields. Surface ship (and submarine) signatures depend on the marine and atmospheric environments, and these must be measured or modeled for results to be useful.
Technology areas affecting surface ship stealth include hydrodynamics, hydroacoustics, and electromagnetics. Seakeeping and speed are other important considerations in ship design, and here hydromechanics is important: “The ability to develop hull forms capable of sustained operations at high speed in heavy seas would yield tremendous tactical benefits, and the peak performance of any crew is enhanced if the adverse effects of roll and pitch can be minimized.”1
Naval Studies Board, National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force, Vol. 6, Platforms. Washington, D.C.: National Academy Press, p. 26.