Polymer Drag Reduction
Friction drag constitutes roughly 50% of the drag on surface ships and roughly 65% of the drag on submarines. Decades of research have identified two very promising techniques for reducing friction drag: polymers and microbubbles, which have shown 70-80% reduction in skin-friction drag coefficient in the laboratory. Polymers and microbubbles inhibit hairpin vortex formation - the source instability for boundary layer turbulence. But, success in the practical implementation of these techniques has eluded researchers for many decades. Too much polymer has to be carried, and the polymer degrades at high speeds. Power requirements for injecting microbubbles are below the break-even point.
Polymers have demonstrated 80% reduction in drag in small-scale lab experiments and 50% reduction for short periods in full-scale experiments. For the polymer solutions, it is possible to achieve up to 80 % drag reduction with only a few parts per million of polymer. Ionic and non-ionic surfactants can also offer similar drag reduction as polymer solutions but at a high concentration of few percent. Microbubbles are perhaps the cheapest and non-polluted drag reducer. However, the control of the bubble size and the angle of ejection can impose technical challenges.
Polymer ejection systems for reducing the drag on a moving vehicle by ejecting a fluid at or near the area of minimum pressure in the nose portion of the vehicle are known. Experiments show that the higher the molecular weight (MW), the more effective a given polymer as a drag reducer. Polymers with a MW below 100,000 seem to be ineffective. The longer polymer chain provides more chance for entanglement and interaction with the flow. It has been confirmed that the extension of the polymer chain is critical for drag reduction. The most effective drag reducing polymers are essentially in linear structure, with maximum extensivity for a given molecular weight. Poly(ethyleneoxide), polyisobutylene and polyacrylamide are typical examples of linear polymers. Polymers lacking linear structure, such as gum arabic and the dextrans, are ineffectivefor drag reduction.
The general concept is ejecting a polymer through ports which are spaced radially from the center line of the vehicle, and which are intended to introduce the fluid in the general area of minimum pressure where the surrounding fluid reaches its maximum velocity as the underwater vehicle is propelled through the fluid medium. The ejection of a polymer from a forward portion of an underwater body, as the body travels through a water environment, causing the polymer to stream along the exterior surfaces of the body, has a salutary effect upon quiet running of the underwater body through the water. It now is known that polymer flow rates sufficient to achieve the desired effect are quite small.
The potential of dilute aqueous solutions of long-chain polymer molecules to reduce drag, now known as the Toms' Effect, was introduced by B. A. Toms at the First International Congress on Rheology in Amsterdam in 1948 and was published in the proceedings of that conference. In 1949, Toms reported unusually low friction factors for dilute solutions of poly(methyl methacrylate) inmonochlorobenzene. He was the first to publish drag reduction data. P. S. Virk et al introduced the concept of drag reduction limits with polymer solutions in turbulent pipe flows in a paper entitled, "The Ultimate Asymptote and Mean Flow Structures in Toms' Phenomenon," published in the ASME Journal of Applied Mechanics, 37, pages 488 to 493, in 1970.
In the late ?fties and early sixties, the effect of dilute polymer solutions on drag reduction was actively investigated. Possible defence application was initiated by the work of Pruitt and Crawford and Savins. Hoyt and coworkers from U. S. Navy organizations have made significant contributionsto the drag properties of the dilute solutions of poly(ethylene oxide). Guar gum, which is a natural polymer ~ a polysaccharide derived from a plant, gave a similar reduction effects.
D.T. Walker, his professor W. G. Tiederman, and colleague T. S. Luchik, in a paper entitled, "Optimization of the ejection process for drag-reducing additives," which was published in Experiments in Fluids, 4, pages 114 to 120, in 1986, obtained drag reduction limits for slot ejection in a channel flow were 20 to 40 percent less than the maximum drag reduction observed in pipe flows. These observations were confirmed by others, such as Yu. F. Ivanyuta and A. A. Khomyakov in their article on the "Investigation of Drag Reduction Effectiveness with Ejection of Viscoelastic Polymer Solutions," which was published in the Proceedings of the International Shipbuilding Conference, KRSI, October, 1994, St. Petersburg, pages 163 to 170, in Russian.
While dilute solutions of polymer behave as Newtonian fluids in laminar flows, A. Gyr and H. W. Bewersdorff, in their text, Drag Reduction of Turbulent Flows by Additives, Kluwer Academic Publishers, 1995, point out that in certain laminar flows, such as laminar contraction flows, polymer solutions exhibit non-Newtonian behavior. The hypothesis cited is that in such a flow, as in turbulent flow, the long molecules of the additive become stretched (uncoiled and elongated) and aligned in the flow which are necessary conditions for the solution to exhibit non-Newtonian behavior. V. G. Pogrebnyak, Y. F. lvanyuta, and S. Y. Frenbel, in their paper, "The Structure of the Hydrodynamic Field and Directions of the Molecular Slope of Flexible Polymers Under Free-Converging Flow Conditions" published in Russian in Polymer Science USSR. Vol. 34, No. 3, 1992, define the conditions under which the polymer molecules can be uncoiled, aligned, and sufficiently stretched to become effective in drag reduction.
Experiments by C. S. Wells and J. G. Spangler, described in their paper, "Injection of a Drag-reducing Fluid into Turbulent Pipe Flow of a Newtonian Fluid" published in The Physics of Fluids, Vol. 10, No. 9, pages 1890 to 1894, September, 1967, by M. M. Reischman and W. G. Tiederman described in an article, "Laser-Doppler Anemometer Measurements in Drag-reducing Channel Flows," published in the Journal of Fluid Mechanics, Vol. 70, Part 2, pages 360 to 392, in 1975, and by W. D. McCombs and L. H. Rabie in "Local Drag Reduction Due to Injection of Polymer Solutions into Turbulent Flow in a Pipe," Parts I and II, published in the AlChE Journal, Vol. 28, No. 4, pages 547 to 565, in July 1982, have clearly demonstrated that polymer additives can reduce drag when they are in the near-wall region of the turbulent boundary layer, known as the buffer zone.
Research in the Soviet Union [described by B. F. Dronov and B. A. Barbanel in their paper "Early Experience of BLC Techniques Usage in Underwater Shipbuilding," published in the Proceedings of Warship 99. Naval Submarine 6, by the Royal Institute of Naval Architects, London in June, 1999] used a wide array of angled slots or circular apertures to eject sufficient material to flood the entire boundary layer. Because of the acceptance of rapid diffusion, not only through but even outside the boundary layer, the amount of material ejected was often several times that calculated to flood the entire boundary layer at its greatest extent.
W. B. Amfilokhiev, B. A. Barbarnel, and N. P. Mazaeva in their paper on "The Boundary Layer with Slot Injection of Polymer Solutions," prepared for the Tenth European Drag Reduction Working Meeting, Mar. 16 to 17, 1997, point out that experience had demonstrated that a single slot with very high concentration was superior to the same amount or more additive being ejected from multiple slots along the length of the vessel.
The effectiveness and efficiency of drag reduction obtained by ejecting non-Newtonian additives in "external" turbulent boundary layer flows has been limited relative to the effectiveness and efficiency observed in "internal" or pipe flows. In high Reynolds number turbulent pipe flows, reductions in friction drag of 70 to 80 percent are observed, while for ejection into high Reynolds number turbulent flows over a flat-plate, the maximum observed reduction in friction drag has been only about 40 to 60 percent. Further, the high additive expenditure rates experienced for external boundary layers have limited the economic benefit of implementing additive systems on maritime transport craft. Ejection techniques to introduce additives into external flows also have introduced unsteadiness and, in some cases, unfavorable viscosity gradients into the boundary layer, such that the penalties associated with the ejection process resulted in a greatly reduced net benefit. A more efficient method for introducing additives into the near-wall region of the boundary layer for drag reduction is needed.
In one implementation longitudinal riblets were combined with polymer ejection to predictably control the rate of diffusion of the polymer. However, the maximum downstream distance at which the material has completely diffused away from the riblets was identified as about 400 riblet widths, which scales to the order of centimeters for a marine vehicle, while the diffusion distance for another device was shown to be on the order of tens of meters.
Systems for storing and ejecting polymers into the boundary layer of an underwater vehicle are known. Generally speaking, such systems provide for a rigid storage tank in the vehicle, and suitable internal piping for ejecting liquid polymer into the fluid moving over the vehicle nose portion to provide a more laminar flow of the fluid through which the underwater vehicle is traveling. In one such system, a pump is provided to pressurize sea water drawn through an inlet at the aft end of the vehicle and mixed with the polymer so the mixture is available for ejection at the nose of the vehicle to thereby reduce the tendency of the fluid flow around the vehicle to separate and/or become turbulent, the polymer and sea water mixture providing for a more laminar flow of the fluid mixture along the external surface of the underwater vehicle.
These prior art systems teach the necessity for pressurizing a mixing chamber which is of rigid geometry and therefore occupies a substantial portion of the interior of the vehicle. Further, these prior art systems are not acoustically quiet enough to satisfy current requirements for underwater vehicles of the type used by the United States Navy for example. Air pockets trapped in such prior art systems, where the pressure is rapidly decreasing, create popping sounds that result in undesirable noise that can lead to premature detection of the vehicle in a combat situation.
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