Breakthrough Could Lead to Faster Ships
By Rudi Williams
American Forces Press Service
ARLINGTON, Va., Nov. 8, 2002 The idea's not ready for prime time, but Defense Advanced Research Projects Agency researchers here have made a breakthrough that could mean faster, longer-range ships and millions of dollars in fuel savings.
DARPA program manager Lisa Porter said 14 teams are working on reducing "friction drag" as ships sail through water. Program participants include teams from the universities of Michigan, Delaware and Illinois; Stanford, Pennsylvania State, Washington (St. Louis), Texas A&M and Brown universities; and the Office of Naval Research.
There are different components of drag, but the program Porter is leading focuses on "friction drag." "It's the friction between the water and the ship's surface," she explained. "As you move through the water, your ship is dragging along a certain volume of water with it. That's because you have friction between the water and the ship's hull. And that's slowing you down."
As ships move through water, they create vortices that cause drag. "When you think of a vortex, you might think of a tornado and that's exactly analogous," Porter said. "We're talking about little, tiny swirling motions that mix up the flow and make it what we call turbulent."
Making ship's surfaces smoother was once considered as a way to reduce friction drag. In practice, Porter said, "It doesn't work too well because fouling and microorganisms keep the surface from remaining smooth enough to make a difference."
Decades of lab testing revealed that polymers and "microbubbles" were promising technologies that might reduce friction drag by as much as 80 percent. Up until now, however, no one had figured out how to apply the technologies to full-size ships, she said.
"What we're finding is, whether you add polymers or bubbles, they interact with those vortices and weaken them," Porter said.
Reducing friction drag by 60 percent would result in about a 30 percent reduction in overall drag, said Porter, who holds a bachelor's of science degree in nuclear engineering from Massachusetts Institute of Technology and a doctorate in applied physics from Stanford.
"What that means is it takes 30 percent less power to go from point A to point B. That also means it takes 30 percent less fuel to get there." she said.
Reducing the fuel load by 30 percent would significantly impact a ship traveling 7,000 to 8,000 nautical miles, she said. Up to half of a ship's total tonnage is fuel. Using less space for fuel makes room for more supplies and materiel. Likewise, keep all the fuel and "you can go 30 percent further before you have to refuel, which becomes very important operationally," Porter noted.
What the program isn't trying to do is create speedboats. Porter said a meaningful increase in speed would require at least a 50 percent reduction in friction drag. She said 30 percent overall drag reduction would only net about a 12 to 13 percent increase in speed.
"Some people have this vision of, oh, if you give us this drag reduction, we can go from 30 to 100 knots," Porter said. "DARPA is not claiming that, because it's not physically possible to do that by reducing friction drag."
What researchers are so enthusiastic about is that DARPA is undertaking a radically new approach in the friction drag reduction program, she continued. A multiscale modeling capability is being developed that will allow researchers to run full-scale experiments on computers.
Computers make it easier to develop techniques that will bring optimal results, according to Porter. For the first time, for instance, computers have helped teams realistically simulate polymers in turbulent flow, she noted.
They've also learned that polymers that organize into sheets or filaments can reduce friction drag dramatically, she noted. Also for the first time, researchers have learned that, like polymers, microbubbles must be located within a very thin layer near the ship's hull to be effective, she said.
In late fall, a near full-scale experiment will be run at the Navy's Large Cavitation Channel in Memphis, Tenn., to validate the large-scale models. The 1.4-million-gallon, stainless steel tunnel is regularly used to test the effects of water flow on ships, submarines, torpedoes, ship propellers and the like.
"The test is going to be the largest microbubble experiment ever done," she said.
Porter said the experiments are necessary because researchers don't understand enough about what's going on with polymers and microbubbles. She noted that they still have many unanswered questions such as: What are the polymers doing? What are the microbubbles doing? What are the optimal bubble sizes? What kinds of injectors should be used? How should we space them on the ship? Where should we put them? Should we use polymers and bubbles together?
"We can't answer those questions if we don't understand the physics," Porter noted. "DARPA's approach is, let's be scientific. Let's tackle the physics and try to understand what these things are doing. We need an engineering design tool that tells us exactly how to implement these additives."
She said before a lot of money is spent on a full-scale experiment, that engineering design tool is needed so experiments can be run on computers. And those results will tell researchers what to build without wasting money on something that doesn't work.
Porter is leaning toward pursuing an overall system like an advanced hull form, such as a hydrofoil that gives 35 to 40 knots, coupled with friction drag reduction.
She said, today, the average speed for transit from the United States to Saudi Arabia, for example, is about 20 to 25 knots. "The carriers, for example, can go faster, but they don't because you can only go as fast as your slowest ship. So if you can raise everybody else's speed to the carrier's, you've actually accomplished something."