圖19-3 改變排氣流形狀以降低噪聲
Fig. 19-3 Change of exhaust jet pattern to reduce noise level.
7.對純噴氣發動機來說，排氣流噪聲是如此之高，以致于除了在著陸進場工程中推力較小時，渦輪和壓氣機的噪聲相比之下微不足道了。對于渦輪風扇發動機的而言，由于排氣速度減少，排氣流噪聲下降，但是由于發動機內部處理的功率較大，使低壓壓氣機和渦輪噪聲加大。
Noise suppression
8.采用單級低壓壓氣機（風扇）大大降低了壓氣機噪聲，這是因為整個紊流和相互作用的程度都減少了。當涵道比超過約5：1時，排氣流噪聲降低到這樣一個水平，使內部噪聲源增加，而且占主導地位。低和高的涵道比發動機噪聲源之間的比較見圖19-4。
produced by the reaction of each blade to the passage of air over its surface, even with a smooth airstream. Turbulence in the airstream passing over the blades increases the intensity of the broadband noise and can also induce tones.
7.
With the pure jet engine the exhaust jet noise is of such a high level that the turbine and compressor noise is insignificant at all operating conditions, except low landing-approach thrusts. With the by-pass principle, the exhaust jet noise drops as the velocity of the exhaust is reduced but the low pressure compressor and turbine noise increases due to the greater internal power handling.
8.
The introduction of a single stage low pressure compressor (fan) significantly reduces the compressor noise because the overall turbulence and interaction levels are diminished. When the by-pass ratio is in excess of approximately 5 to 1, the jet exhaust noise has reduced to such a level that the increased internal noise source is predominant. A comparison between low and high by-pass engine noise sources is shown in fig. 19-4.
9. Listed amongst the several other sources of noise within the engine is the combustion chamber. It is a significant but not a predominant source, due in part to the fact that it is 'buried' in the core of the engine. Nevertheless it contributes to the broadband noise, as a result of the violent activities which occur within the combustion chamber.
METHODS OF SUPPRESSING NOISE
10. Noise suppression of internal sources is approached in two ways; by basic design to minimize noise originating within or propagating from the engine, and by the use of acoustically absorbent linings. Noise can be minimized by reducing airflow disruption which causes turbulence. This is achieved by using minimal rotational and airflow velocities and reducing the wake intensity by appropriate spacing between the blades and vanes. The ratio between the number of rotating blades and stationary vanes can also be advantageously employed to contain noise within the engine. 高涵道比
（單級風扇）
9.在發動機內部，其它幾個噪聲源之一是燃燒室。這是一個重要的但不是占支配地位的噪聲源，其部分原因是因為，它被“埋”在發動機的核心。然而，由于在燃燒室內方式的激烈的活動，它產生寬頻噪聲。
圖19-4 低和高涵道比發動機的噪聲源比較
Fig. 19-4 Comparative noise sources of low and high by-pass engines.
11.如前所述，在純噴氣發動機和低涵道比發動機中，噪聲的主要來源是尾噴氣流，這可采用一迅速或較短的混合區予以降低。這樣減小了低頻噪聲，但可能會增大高頻噪聲。幸而高頻噪聲會很快在大氣中被吸收，一些傳到聽者的噪聲已超出聽覺范同，因而使人覺得這是一種噪聲較小的發動機。這是在推進噴管上，采用一波紋形或瓣形的消聲器(圖19-5)以增大大氣與排氣流的接觸面積來達到的。
Noise suppression
12.在波紋形噴口中，自由流的大氣沿著外部的波紋流動，并進入排氣流，以促進迅速混合。在瓣形噴口中，排氣被分開，流過各瓣和一個小的中央噴管。這形成許多單獨的噴氣流并迅速與消聲器瓣帶來的空氣混合。這種原理可以擴展到利用一系列管子，使其總面積與基術的圓形噴管相等。
13．深的波紋、瓣或多個管子使噪聲降低得最多，但招致的性能損失限制了波紋或瓣的深度和管子的數目。例如，為了達到所需的噴口面積，消聲器的總直徑可能不得不增大，這導致阻力和重量增加過多。因此，設計師的目標是采用折衷辦法，既使噪聲顯著降低，同時又盡可能減小發動機推力和油耗方面的損失或重量的增加。
11.
As previously described, the major source of noise on the pure jet engine and low by-pass engine is the exhaust jet, and this can be reduced by inducing a rapid or shorter mixing region. This reduces the low frequency noise but may increase the high frequency level. Fortunately, high frequencies are quickly absorbed in the atmosphere and some of the noise which does propagate to the listener is beyond the audible range, thus giving the perception of a quieter engine. This is achieved by increasing the contact area of the atmosphere with the exhaust gas stream by using a propelling nozzle incorporating a corrugated or lobe-type noise suppressor (fig. 19-5).
12.
In the corrugated nozzle, freestream 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 that rapidly mix with the air entrained by the suppressor lobes. This principle can be extended by the use of a series of tubes to give the same overall area as the basic circular nozzle.
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