Nozzle Area and Shape Control
In a high by-pass ratio engine with a single-stage fan without inlet guide vanes, the predominant sources governing the overall noise level are the fan and turbine. If the fan speed can be reduced without loss of thrust, then the engine noise level would be reduced. At conditions below maximum thrust, the multi-spool engine enables this to be accomplished by using a variable area nozzle to mechanically reduce the area of the hot stream final nozzle. This causes the speed of the LP turbine and its associated compressor spool to be reduced, producing a corresponding reduction in fan and turbine noise levels. However, the velocity of the hot stream will increase, producing a corresponding rise in exhaust jet noise. If the final nozzle area is reduced until the noise level of the fan, turbine and exhaust are of the same order, the optimum mean noise level for the engine will have been achieved. This normally occurs when the area of the hot stream final nozzle is reduced by approx. 50%. At the optimum nozzle area, the noise radiated towards the ground can be further reduced by a change in the geometrical shape of the nozzle.