Mach Wave Elimination
A new method for quieting high-speed jets
(U.S. Patents 5590520 and 5916127)
In high-speed jets, the dominant source of noise is Mach wave radiation. Mach waves are shock waves generated by the supersonic motion of turbulent eddies relative to the surrounding air. As a very rough rule of thumb, the velocity of the jet eddies is 80% of the jet exhaust velocity. For instance, if the jet speed is 700 m/s, the eddy speed is around 560 m/s. This is 1.6 times the speed of sound of the ambient air (350 m/s). The eddies essentially become a myriad of bullets traveling at Mach 1.6, creating an intense system of shocks that propagates noise to very far distances.
Surrounding the jet with a secondary flow (coflow) can reduce the relative velocity of the eddies (the eddy velocity relative to the secondary flow) to subsonic values. For example if the secondary flow is supplied at room temperature and velocity of 300 m/s, the eddies feel a relative motion of only 560-300=260 m/s. This is less than the speed of sound of the secondary flow (350 m/s), so the eddies become subsonic relative to the secondary flow. This prevents the formation of shocks. One must make sure that the secondary flow does not create Mach waves of its own, so its speed has an upper limit.
This principle is illustrated by the sketches below and by the schlieren photographs taken in the UCI facilities.
DEFINITION OF METHOD
When the jet velocity approaches or exceeds the speed of sound (relative to the ambient medium), Mach wave emission becomes the dominant source of jet mixing noise. Mach wave radiation is controlled by the convective Mach number Mc1 of the jet eddies. When Mc1 is close to 1.0 or higher than 1.0, Mach wave emission occurs.
Addition of a secondary flow adjacent to the jet reduces the convective Mach number Mc1 of the primary eddies. When Mc1 drops to subsonic values, Mach wave emission is substantially suppressed. The convective Mach number of the secondary flow should also be subsonic. In other words, we require Mc1 < 1 and Mc2 < 1.
SCHLIEREN IMAGES FROM UCI TESTS
We have an extensive database of acoustic results covering a large range of jet Mach numbrs and velocities. Some are shown on this page. Others are in journal papers and downloadable conference papers.
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