Noise enfolded

Muffling the sound made by a supersonic jet during take-off could bring calm to areas where air traffic is high

Published: Monday 31 March 1997

Nosise pollution from superson THE ear-splitting roar of a supersonic passenger jet during take-off could be muffled by surrounding the jet's exhaust with a 'virtual shroud' of air, according to Dimitri Papamoschou, a professor of mechanical and aerospace engineering at the University of California at Irvine, in California, US. The shroud could be the key to an affordable successor to Concorde, says Papamoschou.

American aerospace companies and the US government are planning a new supersonic passenger plane called the high speed civil transport. The goal is to develop by2001, the technologies needed for a 300-seater passenger plane that could fly at Mach 2.4 (Mach 1 = 1193.3 km/hr) and travel as far as 9,000 km. But the noise limits imposed by major airports is one of the main problems.

A supersonic engine's exhaust is extensively noisy because the jet of gas moves at supersonic speed relative to the surrounding air even during takeoff. As the exhaust hits the air, it creates shock waves known as Mach waves, which make noise. One solution to the problem is to envelop engines in metal shrouds. when exhaust hits the shroud, it should slow down to subsonic speeds and thus should not produce shock waves when it is expelled from the engine.

Papamoschou's alternative is to surround the engine's exhaust with an envelope of air rather than metal. The air comes from the fan stream - air that flows around but not through the engine's compressor. Heating or accelerating the fan stream would require it to move the envelope of air around the exhaust jet fast enough for the difference in speeds to be less than supersonic, so there would be no shock waves.

Accelerating or hearing The fan stream would require the plane to burn some extra fuel. But that would only be necessary during take-off and the first few minutes of flight. After that the virtual shroud could be turned off. In a typical journey, the plane would need to burn only about 0.5 per cent extra fuel.

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