We have tons of stuff on this at the website, but here is an interesting third party article on the topic pasted below. As an aside, the 3M riblet film mentioned in the article (which requires much higher Reynolds number flows that we see in bicycles unfortunately, but we’ve tried it anyway) has been outlawed in pretty much every type of boat and aircraft racing worldwide, if we rode underwater we would all be paying a fortune for riblet films. The key to making the surface work, is based on dimple pattern, dimple shape and depth of dimple. In the old days, many golfballs had positive protrusions on them, and that worked too, just not as well. Now companies spend millions trying to make the dimples on the balls more efficient, generally packing them tighter and trying odd shapes. There are actually balls that are illegal for tournament play because they fly too far in the PGA ‘Iron Byron’ test. For the record, the Zipp dimples are icosohedral (20 sided) and a few thousandths of an inch deep, the dimples at the perimeter and towards the top of the wheel are the most important, and letters, actually form a very random and efficient pattern, but their location near the hub minimizes their importance and effect. We tried about 10 different patterns in the wind tunnel before arriving at the pattern currently in production, and they were definitely all not created equal, and yes, some of them were worse.
In the wind tunnel, the result is between 2 and 6 watts of efficiency improvement over the smooth disc between 5 and 30 degrees of yaw, and between 0 and 2 watts improvement from 0-5 degrees. As an average that’s like 6-10 seconds per 40km, more if it’s windy. We see even better results in the rims due to the sidewall curvature and rim shaping but that’s another story. Lastly, the new Lexus LS400 uses a dimpled underbelly not so much for drag, but to minimize noise. Since it takes energy to create sound (we all don’t want higher wattage speakers for nothing…) this means it also operates at a higher efficiency though this clearly was not the primary goal, I think this is a really clever use of the technology.
Providence, RI–More than a century ago, golfers noticed a curious phenomenon: Used golf balls flew better than new, smooth ones. Random patterns of dents on the used balls actually stabilized their flight and, in some cases, made them go farther. Eventually, the golf world incorporated that knowledge into the design of new balls, providing them with dimples.
Now, engineers may benefit from a similar phenomenon that offers potential improvements in disciplines ranging from aircraft design to pump operation. A recently published study in the journal Nature describes a technique for reducing skin friction drag by up to 13% on aerodynamic surfaces. Aircraft engineers say that if such reductions could be achieved on production aircraft, it would be a significant step forward. “When we try to reduce drag, we normally work in second decimal places,” notes William W. Greer, vice president of engineering and quality assurance for Learjet (Wichita, KS). “So 13% represents a revolution to us.”
Drag reduction has long been an area of concentration among aircraft designers, for good reason. Some estimate that a typical commercial aircraft can save between $100,000 and $120,000 a year in fuel costs by cutting overall drag just 1%. Worldwide, a 1% drag reduction could translate to fuel savings of more than $1 billion a year. What’s more, the new technique could apply to a wide variety of other applications, including designs of ships, submarines, torpedoes, pumps, HVAC ducting, mixing machinery, and pipe flow systems.
Random pattern is key. The new reductions in skin friction drag involve the use of a simple aerodynamic surface geometry. Researchers Sture Karlsson, an emeritus professor of engineering at Brown University, and Lawrence Sirovich, director of the applied mathematics laboratory at CUNY/Mt. Sinai in New York, found that a random pattern of small bumps could lower skin friction drag. The size of the bumps, or so-called “chevrons,” is determined by the thickness of the viscous wall layer on the aerodynamic surface, they say. Though the precise size of the chevrons is proprietary, they do describe them as being on the order of a “fraction of a millimeter.”
The real key to the extraordinarily high reductions, however, is not the size of the bumps, but their overall pattern. Sirovich and Karlsson found that the chevron pattern must be random. When using a random pattern, they achieved skin friction drag reductions between 12% and 13%. In contrast, aligned bumps not only failed to lower the drag, they raised it by as much as 20%.
The theory behind their findings is complex, but it basically says that the random chevrons reduce “bursting” near an aerodynamic surface. Bursting, which is caused by low speed air streaks near the wall, is generally believed to be responsible for intense turbulence and, therefore, greater drag.
Sirovich and Karlsson aren’t the first to employ surface geometry changes as a means of lowering drag. Researchers have been trying to find ways to “energize the boundary layer” for almost two decades. They’ve tried putting microscopic holes on aerodynamic surfaces to suck the boundary layer off, and they’ve applied grooves to wing surfaces to decrease turbulence. 3M Aerospace is currently testing Drag Reduction Film, which employs small riblets in the air flow direction. Used on an Airbus A-340, the film has been successful in reducing drag. Cathay Pacific Airlines applied it on one commercial aircraft that flies long routes, and it has achieved drag reductions of approximately 0.8%. “It has allowed Cathay to fly from Hong Kong to Toronto without a refueling stop,” notes Tom Ihbe, 3M Aerospace market development supervisor. “Sometimes in the past, they had to refuel in Anchorage.” Ihbe adds that the riblet film offered drag reductions ranging from 2-4% in wind tunnel tests.
Up to now, however, no one has achieved skin friction drag reductions as great as 12-13%. Because the figures are so high, some experts believe that airframe manufacturers might be inclined to invest substantial capital in it if the concept proves out.
Still, they warn, much remains to be learned about it. “Its success will depend on how it affects an aircraft’s handling and sound characteristics,” Greer says. “It will also depend on how maintainable it is and what it costs.” One way to implement the concept, Greer says, is to program CNC systems to ma-chine the random chevrons into the wing skins during regular machining operations.
Business jet manufacturers might be willing to make such accommodations, not because of fuel savings, but because of the additional range the concept could provide. Greer says that the newest Learjet could extend its 2,000-mile range by as much as 120 miles by employing the concept. “For business jet operators, that’s where the real advantage is,” Greer says.
**More than aerospace. **Even if the concept ultimately loses its appeal to the aircraft industry, Sirovich and Karlsson believe that it offers advantages elsewhere. Like 3M’s riblet film, which was used on Stars and Stripes in the America’s Cup competition in 1988, the random chevrons could find application in the design of large ships and racing boats. Sirovich also believes that the concept could play a role on submarines. By reducing drag on a submarine’s hull, the concept could enable submarine powerplant designers to employ less powerful pumps. As a result, submarines employing the concept would be more difficult to pick up on enemy sonar.
The concept could also play a key role in water pumping equipment. If used on the inside of a pipe, for example, it could reduce the horse-power required to pumpwater. As a result, it could be valuable for fire fighting equipment and water hydraulics systems.
Ironically, Sirovich believes that the aligned chevron concept, which* raises* drag by 20%, could eventually be as important as the random chevron idea. Because the aligned concept causes greater turbulence, it could serve in applications that require fluid mixing and heat transfer. In HVAC, for example, designers might employ it on the insides of air ducts. There, greater turbulence could cause more effective heat transfer. As a result, furnaces could use less powerful blower motors. The same technique could also serve in electronic package design, where more efficient heat transfer enables chips to run cooler, as well as in chemical mixing equipment.
Ultimately, the concept’s success will depend on how well it can be replicated in further wind tunnel tests, and how effectively it translates to real-world engineering. If Sirovich and Karlsson’s results are correct, a 13% skin friction drag reduction could lead to overall aircraft drag reductions of as much as 6.5%. (Skin friction drag typically comprises about half of an aircraft’s overall drag.) But even if the concept only achieves one-third of that in actual application, aircraft manufacturers would be elated. “It’s not yet clear how much real-world drag reduction we can achieve with this concept,” Sirovich says. “But in our experiments it’s been a factor of two better than any technique out there now.”
