GE researchers are claiming a world-record for the speed of an air jet, with a laboratory gadget measuring just 75mm square by 6mm thick putting out air at 220m/s, or about M 0.7.

The synthetic jet actuator (SJA), which looks like a pair of credit cards and works like bellows, opens and closes up as fast as 150 times per second and consumes "minimal" power, according to Sayed Saddoughi, principal engineer at GE's Aero-Thermal & Mechanical Systems laboratory in upstate New York.

The small size of these devices, combined with a robustness proven over billions of cycles in the laboratory, may open a path to active flow control. Injecting fast-moving air into the fluid flow over a surface is well known to dramatically reduce boundary layer drag, but the practical barriers to creating a system for cutting drag over, say, an aircraft wing have so far proved daunting.

An active flow control system might be able to cut skin drag by as much as 30%. Energy can be injected into the flow by air or plasma jets, or even "twitching" existing control surfaces. However, while some laboratory work has been encouraging, drag reduction tends to come at a prohibitive cost in terms of power consumption and mechanical wear. And, the physics of surface drag calls not for blanket flow enhancement, but rapid response to local, and fleeting, points of turbulence.

GE's devices may overcome the mechanical wear problem. Their motive force comes from a couple of slivers of piezoelectric ceramic, which change shape when an electric charge is applied, so the SJAs, or "dual piezoelectric cooling jets", effectively have no moving parts. "They are the highest TRL [technological readiness level] devices we have," says Saddoughi.

Lockheed Martin and the US Air Force used a version of GE's SJA's to demonstrate reduction of aeroacoustic vibration on F-16 fins in 2005, and Saddoughi has been able to show drag reduction over the hull of a ship model in his laboratory.

Lots of testing has also been done on wind turbine blades, he adds: "I can see an intelligent blade incorporating these devices."

Though Saddoughi stresses that his laboratory work is merely a precursor to any attempts to incorporate the devices into aerodynamic surfaces, he is optimistic that they could be used to practical effect. GE, he adds, is most likely to look first to its own area of competence; aircraft engines offer several opportunities for internal drag reduction.

Saddoughi's goal is to see his air jets fly. However, the first real-world application may be far more mundane. GE's piezoelectric cooling jets move as much air as a simple fan twice their size at half the power consumption, and are cheaper to make, so it is no surprise to learn that the technology has been licensed to a Japanese maker of computer components which is looking to replace laptop cooling fans.

GE estimates that the switch could extend laptop battery life for up to 30 minutes. That's not as dramatic as slashing airliner fuel burn, but for many people it would be a miracle all the same.

Source: Flight International