Submarine stealth depends critically on the level and character of its radiated noise. In the past, as in the foreseeable future, acoustics will be the principal component of a submarine's signature and could lead to detection and classification by adversaries' sonar systems at relatively long ranges. Nonacoustic components of submarine signatures are more localized in space and are important at closer ranges.
In the absence of cavitation, submarine acoustic signatures generally include narrowband tonals at blade rate frequencies and broadband noise. These tonals are caused by interactions of the propeller with spatially and temporally unsteady flow fields and structural vibrations induced by the resulting time-dependent forces. Before the current proliferation of towed array sonars, only ocean surveillance systems could capture low-frequency blade rate signals from long ranges, but ships and submarines could not take immediate advantage of this information. The larger acoustic apertures of modern towed arrays and progress in flow noise control have overcome this restriction. Even though this source of noise has received much attention, there are still no cost-effective ways to control it.
Recent data acquired on very quiet ships reveal noise sources caused by turbulent boundary layer flow that were hitherto hidden by other, more intense radiation mechanisms. Although direct radiation from boundary layers is very weak, a turbulent fluid boundary layer along an elastic solid boundary can generate significant noise levels. This elastic solid boundary may be the hull or trailing edges of lifting surfaces. The structural vibrations excited may have distinct resonance peaks in the radiated noise spectrum.
Cavitation gives rise to bubbles of vapor or gas that collapse and oscillate. As a generator of acoustic monopoles, cavitation is a very efficient radiator. It is unacceptable on submarines and highly undesirable on surface ships. Separated flows caused by submarine maneuvers lead to premature cavitation inception and to significant increases in radiated noise levels. Flow-induced sonar self-noise is also adversely affected.
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An Assessment of Naval Hydromechanics Science and Technology 3 Technology Issues NAVAL NEEDS Submarine Stealth Submarine stealth depends critically on the level and character of its radiated noise. In the past, as in the foreseeable future, acoustics will be the principal component of a submarine's signature and could lead to detection and classification by adversaries' sonar systems at relatively long ranges. Nonacoustic components of submarine signatures are more localized in space and are important at closer ranges. In the absence of cavitation, submarine acoustic signatures generally include narrowband tonals at blade rate frequencies and broadband noise. These tonals are caused by interactions of the propeller with spatially and temporally unsteady flow fields and structural vibrations induced by the resulting time-dependent forces. Before the current proliferation of towed array sonars, only ocean surveillance systems could capture low-frequency blade rate signals from long ranges, but ships and submarines could not take immediate advantage of this information. The larger acoustic apertures of modern towed arrays and progress in flow noise control have overcome this restriction. Even though this source of noise has received much attention, there are still no cost-effective ways to control it. Recent data acquired on very quiet ships reveal noise sources caused by turbulent boundary layer flow that were hitherto hidden by other, more intense radiation mechanisms. Although direct radiation from boundary layers is very weak, a turbulent fluid boundary layer along an elastic solid boundary can generate significant noise levels. This elastic solid boundary may be the hull or trailing edges of lifting surfaces. The structural vibrations excited may have distinct resonance peaks in the radiated noise spectrum. Cavitation gives rise to bubbles of vapor or gas that collapse and oscillate. As a generator of acoustic monopoles, cavitation is a very efficient radiator. It is unacceptable on submarines and highly undesirable on surface ships. Separated flows caused by submarine maneuvers lead to premature cavitation inception and to significant increases in radiated noise levels. Flow-induced sonar self-noise is also adversely affected.
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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. Surface Platforms 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 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.
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An Assessment of Naval Hydromechanics Science and Technology Fast Ships Transporting troops and equipment at high speed is an attractive goal, but current technical barriers limit the likelihood of achieving it. It therefore exemplifies the critical need for an innovative and aggressive S&T program in hydromechanics and marine propulsion. An internal report by Colen G. Kennell of the Naval Surface Warfare Center, Carderock Division (NSWCCD) documents the results of an international meeting, the High-Speed Sealift Technology Workshop, hosted by the NSWCCD in October 1997. The report claims that “dramatic enhancements in sealift capabilities are possible if appropriate research and advanced development efforts are made.” It cites seven technology areas where such efforts are necessary. Those that involve research in hydromechanics include advanced high-speed hull forms, drag reduction, hull/propulsor integration problems, and sea-induced loads. The report also indicates that very substantial financial resources will be necessary in other areas, such as fuel-efficient power generation and propulsion machinery as well as lightweight ship structures. The high-speed ferry industry has demonstrated encouraging possibilities. Significant advances, however, will require the development and validation of analysis tools that can predict the performance of anticipated unconventional hull forms. Sea-induced loads, seakeeping, and propulsor/hull integration problems are likely to be significant and difficult to solve. They will require substantial research resources before analytical and numerical tools can be reliably used in design. More recently, the JASONs conducted a study entitled “Fast Ships” that was sponsored by the ONR and the Defense Advanced Research Projects Agency (DARPA).2 The study, conducted by a team of experts led by Paul Dimotakis of the California Institute of Technology, hypothesizes an extremely challenging future Navy mission and investigates ship concepts required to achieve the mission. The vehicle requirements are for a ship of about 10,000 tons with a payload of 1,000 to 2,000 tons and a range of 10,000 miles at a sustained ship speed of 75 to 100 knots. The ship should be of shallow draft and be able to transit the Suez Canal. One of the most stringent requirements is that the ship must be commercially viable. The JASONs study team determined that the performance goal of 100 knots cannot be achieved with the best current technical capabilities, but a speed of 75 knots may be attainable if advances in drag reduction and flow control that seem possible can indeed be made. For this concept to become viable, an aggressive S&T effort in turbulent drag reduction technology would need to be successful. Such an effort is not in place today. Additionally, major advances would be required in high-speed seakeeping, in cavitation technology (e.g., supercavitation), and in propulsion (probably in electric propulsion concepts). “Fast Ships,” in conjunction with the requirements for the nearer-term DD-21, points out the wide gap between the Navy's future hydromechanics needs, on the one hand, and the S&T programs in place to provide them, on the other. 2 Dimotakis, P., P. Diamond, F. Dyson, R. Garwin, J. Goodman, M. Gregg, D. Hammer, and R. Lelevier. To be published. Fast Ships: Hydrodynamics of Fast Ocean Transport. Arlington, Va.: Office of Naval Research and Defense Advanced Research Projects Agency.
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An Assessment of Naval Hydromechanics Science and Technology MISSING OR INADEQUATELY ADDRESSED HYDROMECHANICS SCIENCE AND TECHNOLOGY Computational Simulation of Hydromechanics Phenomena Computational simulations are making significant contributions to many important areas of naval hydromechanics. Computational fluid dynamics (CFD) in particular is proving to be extremely useful in submarine and ship design. Positive impacts are also being made in computational hydroacoustics (CHA) and computational wave dynamics (CWD), but there has been less emphasis and less progress in these areas than in CFD. Because there are no numerical means to simulate exact three-dimensional wave propagation, one cannot make a numerical wave tank (to put ships or other bodies in) in three dimensions. Surface waves undergo very complicated nonlinear interactions over moderate and long time scales that are extremely important in many ocean problems. Similarly, one cannot deal realistically with wave breaking and splashing and air entrainment numerically. With ONR support, large eddy simulation (LES) is becoming an important new tool for CHA. However, LES requires modeling of small-scale phenomena, and there are important Navy applications (e.g., air entrainment at the waterline) where LES could be useful but is limited by the small-scale modeling. Since CHA and CWD are largely of interest only to the Navy, the primary responsibility for the research needed to develop these models rests with the Navy. Further progress will depend on improved modeling of the complex physical phenomena, including those that are unique to hydromechanics, such as air entrainment, wave breaking, cavitation, and turbulent interactions with the free surface. There is also a great need for better numerical prediction methods for complex, nonlinear, three-dimensional wave fields and their interactions with ships. The Navy centers all have ongoing efforts contributing to CFD, CHA, and CWD for Navy needs, and the ONR has sponsored substantial efforts at universities to develop CFD. These efforts have resulted in computational software and design tools that have contributed significantly to improved hull shapes and propulsor designs. Most of the software is focused on the solution of the unsteady Reynolds-averaged Navier-Stokes (URANS) equations, with modeling of the free-surface phenomena. However, URANS predictions are only as good as the turbulence models that they use. Current models do not do a very good job of predicting the location of separation induced by pressure gradients, do not handle the effects of frame rotation (as in propellers) properly, and do not handle the effects of microbubbles, polymers, and other small-scale elements that show great promise for flow control. LES is rapidly emerging as an alternative to URANS and is being actively explored by ONR. There is a clear need for new and better small-scale modeling methods for use in large-scale CFD. These methods are likely to be best if they are soundly based on small-scale physics and associated asymptotic analysis of the effects of this physics at large computational scales. Unfortunately, asymptotic analysis has taken a back seat to computation with the rise in CFD. New efforts are therefore needed to use small-scale physics and asymptotic theory to generate better models for use in CFD, CHA, and CWD. The committee recognized that a substantial community in applied mathematics and theoretical physics is intensely involved in studying small-scale turbulence, which can benefit modeling for naval hydromechanics applications. The direct numerical simulation of turbulent flow is one way to increase the knowledge base that is needed to develop improved models. The committee believes that one role of the university principal investigator is to develop innovative numerical solutions that address generic difficulties impeding progress. It is not to specifically design CFD modules that can be added on to operational codes.
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An Assessment of Naval Hydromechanics Science and Technology In addition, there is a need for carefully coordinated experiments and CFD simulations designed to improve understanding of the basic physical phenomena. Such an element is largely absent from the present program, where most of the CFD is directed to the development of design tools. On the experimental side, there is a need to perform full-scale trials to resolve some of the scaling issues. These trials can take advantage of the existing LSV program. Scaling to High Reynolds Numbers When a new submarine or surface ship is being designed, the required performance parameters that are predicted by analytical or numerical methods must be validated by scale model tests. The data acquired experimentally are intended to demonstrate that the ship's specifications will be met. The parameters that are affected by hydromechanics include powering, maneuvering and control, seakeeping, cavitation, and acoustic and nonacoustic stealth. The capabilities of the available test facilities and the cost of manufacture restrict the size of models to a small fraction of the full-scale ship. Experimental data are therefore obtained at values of Reynolds, Froude, and cavitation numbers at least one of which is very different from that of the real vessel. Although the basic scaling laws are well known, their application, especially by extrapolation, is still largely empirical. For conventional designs, the predictions generally agree well with full-scale measurements. However, even in these cases there have been important exceptions where extrapolations from the model scale have failed, with potentially severe consequences. The Department of the Navy Large Cavitation Channel in Memphis, Tennessee, has some very exciting possibilities for fundamental research. However, so far it has not been used very much for such research. There is, accordingly, a need for new methods based on first principles for scaling experimental data from model systems to full-scale systems and for full-scale measurement programs to validate these results. The new methods will probably incorporate new abilities in flow prediction for full-scale systems of the type described above, but these predictive tools will themselves probably need to incorporate field data on full-scale systems. To solve these problems, the Department of the Navy could mount a concerted effort to develop the new scaling methods, determine the sort of field data needed, and develop the instrumentation to acquire these data in the field. This aspect of naval hydromechanics research is crucial for evaluating new concepts and will not be initiated or supported by any agencies other than the Department of the Navy. Interface Physics, Chemistry, and Biology In his white paper,3 Marshall P. Tulin provides a cogent overview of research issues that distinguish naval hydromechanics from other branches of fluid mechanics. In his summary, the major subdivisions of naval hydromechanics included free-surface hydrodynamics, cavitation, effects of stratification, resistance of ships, ship wakes, aeration, and remote sensing. Ship waves, wind waves, and aeration are topics that are of continuing interest and importance. The understanding of the interaction of complex turbulent flows is far from complete. For example, a wake 3 Tulin, Marshall P. 1999. “Naval Hydrodynamics: Perspectives and Prospects.” Santa Barbara, Calif: Ocean Engineering Laboratory, University of California. September 14.
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An Assessment of Naval Hydromechanics Science and Technology with a free surface requires a detailed understanding of the vortex interactions at the free surface. Free-surface turbulence has features that are quite different from the turbulence in fully submerged flow because of the complex vortical interactions at the free surface. Aeration due to ship waves, wave breaking, and boundary layer entrainment are also not well understood. A complete knowledge of the source of bubbles in the wake of a ship is far from within our grasp. All these topics are of crucial interest to the stealth problem of surface vessels and submerged vessels running at shallow depths. Surfactants or contaminants on the free surface require special consideration, because they alter surface tension. Surface tension gradients have insidious effects such as the well-known Reynolds ridge phenomenon. Aeration physics and cavitation are also affected by the presence of surfactants. The chemical makeup of the ocean, in conjunction with the thermal gradients, affects the stratification of the ocean, which in turn has a major impact on the formation and decay of ship and submarine wakes. Internal waves, driven by gravitational restoring forces on density gradients, have an impact on acoustic propagation and the operation of submarines in the ocean environment. It is well known that viscous resistance is modified substantially by the presence of long-chain polymer additives (the Thoms effect). Naturally occurring algae, plankton, and other biomass can also affect ship resistance substantially. Outgassing from small animals in the sea and bubbles entrained by breaking at the surface account for the presence of cavitation nuclei at depth.4 Bubble formation and cavitation in seawater (rather than freshwater) have not been explored in depth. These physiochemical and biological effects are clearly of importance to the Department of the Navy and are not typically supported by the research programs of other agencies. Driving home this point, Tulin says that we have failed to learn enough about fundamental hydrodynamic phenomena related to surface effects and about how these phenomena relate to remote detection. Two excellent sources of information on fluid dynamics research are Research Trends in Fluid Dynamics, published by the U.S. National Committee on Theoretical and Applied Mechanics, and Annual Review of Fluid Mechanics, published by Annual Reviews. These sources, however, mention very little about the physicobiochemical impact on naval hydromechanics. What is mentioned may be characterized as still unknown. An example is the chapter by A. Prosperetti,5 who says that “detailed mechanics [of cavitation damage] and possibly physicochemical aspects are not completely understood,” and “the role of surface forces and contamination appears to be essential [to the processes of bubble splitting and coalescence].” Thirty-one volumes of the Annual Review of Fluid Mechanics have been published, yet it is difficult to find a specific reference to this topic. In short, physicobiochemical effects on the hydromechanics of the ocean environment are highly relevant to the Department of the Navy. It is a topic that has received relatively little attention in the context of naval hydromechanics and is, moreover, clearly a topic that if not supported by the ONR will not be supported elsewhere. 4 O'Hern, T.J., J. Katz, and A.J. Acosta. 1985. “Holographic Measurement of Cavitation Nuclei in the Sea.” ASME Cavitation and Multiphase Flow Forum. Albuquerque, N.Mex. 5 Prosperetti, A. 1996. “Multi-phase Flow, Cavitation, and Bubbles.” Research Trends in Fluid Dynamics. J.L. Lumley, A. Acrivos, G.L. Leal, and S. Leibovich, eds. Woodbury, N.Y.: AIP Press.