Tackling Frictional forces and smoothing flow. Part 2

Controlling crossflow instabilities

Swept wings create an imbalance between the centripetal acceleration and pressure gradient experienced by air in the layer above the boundary layer, compared with the air within the boundary layer. This imbalance creates a secondary crossflow of air in the boundary layer, which runs in waves along the wingspan, perpendicularly to the air streaming over the chord. In attempts to achieve laminar flow, these crossflow waves are particularly difficult to control.

In two separate research initiatives— one a NASA Environmentally Responsible Aviation (ERA) project and the other an AFRL-funded effort with Lockheed Martin and Texas A&M—Saric’s team has experimented to suppress the most unstable crossflow wavelengths in different flight and wing conditions. Their approach has been to interfere with those unstable waves by inducing waves of other wavelengths along the span using two different techniques. The team’s experiments have used wind tunnels and the laboratory’s own Cessna O-2 testbed, fitted to carry a 30-deg swept airfoil section perpendicularly under its wing. The initial results have been promising.

Discrete roughness elements

One technique, funded under NASA’s ERA project, has been to use periodic discrete roughness elements (DREs) placed spanwise at regular intervals within the first 1% of the chord of the wing, to create interference waves. These DREs are tiny bumps, no more than 10-12 μm high and no more than 1 or 1.5 mm in diameter. They are spaced so the distances between their centers are from one-half to two-thirds of the wavelength of the most unstable crossflow wave, to create the maximum of interference with it.

In flight testing of the swept-wing airfoil—carried under the Cessna O-2 and painted to simulate a typical operational aircraft surface—the Texas A&M researchers found that the DREs suppressed the most unstable wave enough to move the transition point between laminar and turbulent airflow from 30% of chord to 60%.

Saric says the wavelength of the most unstable crossflow wave on a particular wing depends on the airfoil of the wing, the radius of its leading edge, the aircraft’s speed, and its condition of flight. (For instance, the most unstable wavelength might change with the aircraft’s angle of attack.)

In wind tunnel testing, Saric’s team found that the most unstable crossflow wavelength for their swept-wing airfoil model was 12 mm. However, in a wing-glove test on a Gulfstream 3 flying at Mach 0.75, the most unstable crossflow wavelength may be 7 mm; on the flight test model carried on the Cessna O-2 testbed the most unstable wavelength is 4.5 mm. Testing at Mach 1.85 with an F-15B had a 4-mm most unstable wave.

“My guess, if we had to make a transport wing, is that the most unstable wave would be in the 6-8-mm range,” says Saric. The ‘magic number’ of the wave needed to interfere with the most unstable wave would be from half to two-thirds of the most unstable wavelength, so the distance between the centers of the DREs would be from 3.5 mm to 4 mm.

More to follow.