Exhaust Jet Mixing:
Figure shows that the noise from the exhaust jet is the main contributor to the total noise generated by a low by-pass ratio turbo-fan. For a turbo-jet the noise from the exhaust is an even greater contributor to the whole. Fortunately it is comparatively easy to reduce the noise by increasing the mixture rate of the exhaust gases with the atmosphere. This can be achieved by increasing the contact area of the atmosphere with the gas stream by incorporating a corrugated or lobe-type suppresser in the propelling nozzle.
The addition of a corrugate nozzle gives the effect shown in figure 7.16. In the corrugated nozzle, atmospheric air flows down the outside corrugations and into the exhaust jet to promote rapid mixing. In the lobe-type nozzle, the exhaust gases are divided to flow through the lobes and a small central nozzle. This forms a number of separate exhaust jets which rapidly mix with the air entrained by the suppresser lobes. Deep corrugations or lobes give a greater noise reduction, but the penalties incurred limit the size of the suppressers, eg. to achieve the required nozzle area, the overall diameter of the suppresser may have to be so large that excessive drag results.
A nozzle may be designed to give a large reduction in noise level, but this could incur a considerable weight penalty due to the additional strengthening required. A compromise that gives a noticeable reduction in noise level with the minimum sacrifice of engine thrust or increase in weight is, therefore, the designer's aim.