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Researchers Stabilize Active Drag Reduction for Swept Airfoils; 10-15% Gains in Fuel Economy Possible

by Rafael Seidl

German news magazine Der Spiegel reports that researchers at the University of Stuttgart (Germany) have succeeded—at least in computer simulations—in stabilizing active drag reduction for the airfoils of commercial airliners. They claim their approach could lead to fuel economy gains of 10-15%. For a major carrier such as Lufthansa, that would translate to savings of hundreds of millions of Euros per year and a corresponding reduction in CO2 emissions.

The concept of active drag reduction has been around for a while: the surface of an aircraft wing moving through the air at high speed generates a boundary layer that increases in thickness toward the trailing edge. Along the way, the layer becomes increasingly turbulent and eventually sheds eddies. Both the layer’s thickness and the eddies it sheds generate significant drag.

If, however, the airfoil is perforated with a very large number of tightly spaced microscopic perforations (~50 microns diameter), a vacuum can be applied from the inside to manipulate the shape and internal structure of the boundary layer. The manufacture of the perforation patterns adds significant cost (and affects mechanical stresses)—one reason why it is only considered worth pursuing if jet fuel are expected to remain high.

The other reason is technical: traditional hole patterns work fine for straight wings, such as those found on ridge soarers. Applied to the swept wings typical of commercial jets, Boeing discovered they can actually produce chaotic vortices that interfere with stable airflow over the wing. The crux of the present breakthrough (patent pending) is the discovery of subtly different hole patterns that keep the boundary layer both thin and the vortices inside it from shedding. Stable vortices can also be achieved with passive surface dimples [cp. golf balls] and artificially increased surface roughness, but only the vacuum technique also keeps the thickness of the boundary layer small.

The team leaders, Markus Kloker and Ralf Messing, are careful to stress that they have hitherto limited their analyses to wing sections, due to the massive compute power required for whole-wing CFD simulations of the internal structure of the boundary layer. Also, the scope for savings still has to be confirmed in real-world experiments. The researchers have entered into talks with Airbus (whose A380 and A350 models must compete against Boeing’s lightweight 7E7 with regard to fuel economy).

If successful, the technique could be valuable for other airfoil applications, such wind turbine blades (and helicopter rotors, race car spoilers, compressor vanes etc.)

Resources:

Comments

Lucas

About 40-50 years ago a lot of experimental work was done in this area in the US. For the reasons enumerated here the research finally ended. I always thought that to be a mistake and am glad to see someone else take it up.

I wonder what would be the effect of putting a 50 micron hole in the middle of a dimple? Certainly someone needs to take a look at that in a windtunnel.

pizmo

Great news. Hope it works out.

Neil

I can see why they gave up on this research some years ago. The amount of computing power required would be enormous. Even with the machines today which are several orders of magnitude faster than the ones I first used (I won't say how long ago that was but lets just say that I know about flipping bytes to do math on a PDP 8), you'd have to have something much larger than the desktop I'm on to do the job.

Patrick

With multi-core processor chips becoming more pedestrian obtaining the type of computing power necessary for CFD simulations will become more accessible without having to lease time on a supercomputer. In fact, multi-core processors are on course to allow Moore's law to continue on even though they initially thought it would be de-railed sometime in the beginning of the coming decade.

Cell-processor based IBM blade servers are rolling out soon and just four of these processors working together can equal the processing power of Cray supercomputers from the early 90s.

Rafael Seidl

Patrick -

instationary CFD problems involving millions of nodes and sophisticated, highly nonlinear turbulence models (Reynolds stresses etc.) are still best solved on big iron supercomputers with very fast data interconnects, e.g. something with a ccNUMA architecture and hundreds of processors. CPU clock speed as such isn't everything - beyond a certain size, data transport architecture becomes the limiting performance factor for monolithic problems. Disclosure: I used to work for SGI, a supercomputer maker.

When Mssrs. Kloker and Messing say they don't have enough compute horsepower, I suspect they don't mean they need a fancier PC or commodity cluster.

For something like this, even more money has to go into the experimental validation. Scale models will only get you so far. Also, fluid dynamics calculations in particular are still as much of an art as a science; any given turbulence model only covers only certain classes of problems and certain parameter ranges. Pick the wrong one and you end up with garbage out.

Roger Pham

Boundary layer manipulation by suction slots in the upper airfoil surface has been studied before, and determined that the energy required to create the partial vacuum for suctioning is quite considerable, thereby negating much of the gain in aerodynamic efficiency. Furthermore, for a jetliner cruising at above Mach 0.8, pressure wave drag at near the limit of compressibility become more predominant and reduces the significance of vortex turbulence drag.
There are natural laminar flow airfoil designs that allows for laminar flow to over 1/2 of the wing surface that does not require any energy input in creating partial vacuum suction.
Studies of unducted fan propulsion in the 1980's has shown that improvement in fuel efficiency of 20-30% could be realized over turbofan jets, however, perhaps the drop in fuel prices and noise has eliminate interest in these unducted fan.

However, fast-forwarding into the future, it seems that high-speed train is hard to beat in term of energy efficiency. However, for rapid long distance or over-water travel, there is no choice but air travel.

For that, liquid hydrogen (LH2) can offer reduction in energy use as much as 35%. LH2 has 1/3 the weight of kerosene per BTU energy, and for a long-range flight, as much as 1/3 to 1/2 of the aircraft gross takeoff weight is fuel. Now, if you reduce the fuel weight to 1/3 and further reduce the airframe weight and power plant weight to accomodate a much lighter fuel load, then it calculates to reducing the gross takeoff weight by 1/2. Now, assuming that the efficiency of reforming H2 from crude oil is close to that of kerosene refining, then we must substract 30% efficiency loss in liquifying H2, and come up with a 35% energy reduction for long-range flights. The foam insulation for the LH2 fuel tank adds negligible weight gain, and the 1.75x larger fuel tank volume would still add negligible increase in aerodynamic drag. The fuselage dimension may have to be increased slightly to accommodate the increase in fuel volume.

The advantage of LH2 is that it can be produced from the most diverse of sources, such as coal, natural gas, nuclear energy, wind, solar, geothermal, biomass, etc...thereby greatly increase energy security into the long future, as well as the potential for GHG reduction.

Patrick

Rafeal,

I wasn't referencing clock speed per se but actual computational capacity.

4 Cell processors properly addressed (each of the 8 cores on each processor similar to hyperthreading in a multi pipeline processor) are capable of turning out 1TFLOP of calculations per second.

A.Syme

The X-21 was a research aircraft that tried laminar flow using suction on the upper surface of the wing. In the real world, this was much harder to do than in theory. The plane showed that this type of boundary layer control was not practical. If it had been, I’m sure the airliner of today would have such a system.

John

Lucas,
Funny you mentioned the whole "hole in a dimple" thing. I have connections with Callaway golf. The recently started using small dimples within dimples. The result has been impressive. I cannot provide any imperical data, though I will try and obtain some.

Roger,
Interesting stuff, but....
How do you store the LH2? High pressure no doubt.
What about localized storage of LH2? Hydrogen has a nasty tendancy to leak.

Andrey

Manipulation of boundary layer was successfully employed on some torpedoes. External thin metal torpedo skin was laser-bored with billions of small holes, and during underwater travel compressed gases (in some models from pyrotechnic device) were bled through the holes. Torpedo virtually traveled in cloud of small bubbles, and hydrodynamic drag was drastically reduced. It translated in much higher speed and longer range.

Rafael Seidl

John -

LH2 is liquid hydrogen at 20 Kelvin, at atmospheric pressure. In an aviation application, you would want to minimize the weight of the fuel tank (not as negligible as Roger suggests) by using a cylindrical tank capped by hemispheres. This sharply reduces the surface-to-volume ratio relative to in-wing tanks, which were found to be impractical for cryoapplications.

http://www.flug-revue.rotor.com/FRheft/FRH9809/FR9809k.htm

The optimum would be a single, large spherical tank with internal baffles to keep the liquid phase from sloshing around. This might be feasible in a blended wing-body design (cp. B2 bomber), if placed immediately behind the cockpit. BWB designs are also aerodynamically efficient.

As for boil-off, an airport needs a lot of electrical power so it could operate its own genset/SOFC. The fuel would be delivered frequently to limit the volume that must be stored on site. Similarly, all commercial aircraft feature APUs (currently small gas turbines) to generate power for on-board systems. An LH2-based aircraft with a fuel cell and electric motor would not need a separate APU, given high efficiency in part load. Any excess electricity produced while on the ground could be fed into the airport's grid.

The one snag is that you need very high power levels to take off in the first place. You don't want to overengineer your fuel cell stack if at all possible. In-wheel electric motors and perhaps, inductive couplers embedded in the runway for grid connectivity might do the trick.

Andy

Something else to note regarding Liquid H2 storage on aircraft as well.

Most airliners already have a perfect location on board for storage of the large cylindrical tanks. The luggage hold.

If passengers could accept a lower limit on baggage size (and it became carry on baggage, even on long haul) then this would free up the sizable baggage hold to contain fuel.

Aviation may be one of the only sane applications of hydrogen as a fuel once oil is scarce/expensive.

allen_Z

Andrey,
Supercavitation. They are fitting them on autocannon shells as well (to kill inbound torpedos, naval mines and other underwater targets).

bert

Regarding the practical aspects, I'm assuming they have the maintenance aspects worked out for keeping a large array of 50-micron holes uniformly clean for a surface that sees 24/7 environmental exposure?

Jens

Hydrogen for planes are new to me. The idea of using a fuel cell and an electric engine is talented. What you need is to be able to boost energy at take of and landing. A ceramic condensator can contain a lot of energy and discharge it rapidly - and it is not heavy.

In the calculation you also have to take into account that both a fuel cell and an electric engine can deliver continous torque regardsless of how thin the air becomes whereas a jet looses torque rapidly with altitude. This allows a plane with fuel cell and electric engine to fly much higher where there are much lower air resistance which translates into higher speed and lower power consumption.

This combination may do the trick for the SAX plane concept as both noise and power could be reduced.

Regarding the hydrogen this can be produced very cheaply at location by low cost plants that tap into the electric grid outside peak hours or when windmills produce excess energy. This way the capacity factor of electricity plants can be kept high which will benefit their cost effectivenes and actually also their power efficiency because stop and go running of a power plants is very inefficient.

Also the air pollution in the form of unburnt fuel can be reduced.

I think Roger Pham should take this further.

Regarding dynamic fluid dynamics then please note that Moores law is still in function and that Intel has stated that we will be able to handle 100 times more cycles at the same cost for every ten years. It will be a while before dynamic fluid dynamics will be employed in standard planes but when it is time for it the computing power will be at hand.

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