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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.
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.
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.