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Aviation History
1953
1953 - 0761.PDF
FLIGHT, 12 June 1953 755 AERONAUTICAL ACOUSTICS The Problems of Noise Discussed at R.Ae.S. and Physical Society Joint Symposium A S we briefly recorded last week, noise problems of present- t\ day aircraft were the subject of an all-day joint symposium •i- -*• held recently in London by the Royal Aeronautical Society and the Acoustics Group of the Physical Society. The general title of the day's discussions was Aeronautical Acoustics—in particular Jet Noise. Jet-engine Noise—1: Under the chairmanship of Mr. N. Fleming, the first two of the morning's papers were concerned with engine noise. The opening speaker was Professor E. J. Richards, M.A., B.Sc, F.R.Ae.S. (Southampton University), who began his paper, Jet-Engine Noise, by reviewing work at present being carried out. On the jet side, he pointed out, this consisted of much theoretical and model work by Manchester and Southampton Universities and the College of Aero nautics, while other work had been done also by the N.P.L., Rolls-Royce, Faireys and Saunders-Roe. Aerodynamic noise from a jet, the speaker continued, arose from three general sources: (1) Subsonically, as a field of acoustic quadrupoles, giving rise to noise increasing roughly as V8, the high-frequency noise coming from the high velocity shear and turbulence at the edge of the jet close to the nozzle, and the low-frequency noise coming from the larger burbles downstream. (2) A further and more potent source of noise occurring when a jet was choked due to the standing shock-waves set up along the jet. Any eddy moving downstream with the jet would give a sound-wave as it passed through the shock-waves, the sound in turn travelling upstream. When the sound-wave reached the edge of the jet, the varying pressure in the sound-wave caused the over-expansion to fluctuate, and the ensuing fluctuating flow passed downstream. A resonance mechanism was thus established, and the growth of noise with increasing pressure ratio above choking took the form of a series of well-defined notes interspersed with regions where no single noise was apparent. (3) A further source of noise in a jet was that caused by noisy combustion in and aft of the flame tubes. Model experiments indicated that the maximum intensity of noise from an unchoked jet was in a downstream direction at an angle of between 30 and 40 deg to the jet axis, this angle being less for low frequencies than for high frequencies. These observations agreed with Lighthill's theory. Most modern jet engines were slightly choked at take-off, and heavily over-choked in cruising flight at altitude. Under these latter conditions, the predominant noise arose from shock-wave/ eddy interference and resonance, and was greatest at an angle somewhat more than 90 deg to the jet axis. The high c/der in velocity to which the noise corresponded (V to the eighth power subsonically, and far higher when the jet was well choked) gave the greatest clue to future noise suppression. As in flight it was the relative velocity to this power that was the essential parameter, a very good return should be obtained from any reductions in jet velocity, even when the mass-flow was increased to give a constant thrust. Turning to noise-suppression devices, the speaker mentioned the success obtained in model experiments. The College of Aeronautics had invented a jet-pipe with fingers inserted slightly into the jet stream around its periphery, which appeared to be most effective while causing little or no loss in thrust. Southampton had put forward a corrugated jet-pipe, possibly easier to maintain, which worked very well at over- choked jet speeds, giving noise reductions at some 15 to 20 decibels. In the over-choked case, it was thought that the primary object of noise reduction was the elimination of the resonance mechanism, which so greatly aggravated the noise, by preventing the formation of any discrete eddies. Subsonically, it was thought that noise reduction would be obtained by a reduction of the velocity shear at the jet exhaust. Since any device essentially gave rise to added turbulence, Professor Richards concluded, the noise reduction obtained depended on a fine balance between that given by shear velocity reduction and that lost by the added turbulence. Jet-engine Noise—2: The second paper on engine noise was given by Mr. F. B. Greatrex, B.A., A.M.I.E.E., A.F.R.Ae.S. (of Rolls-Royce), who began by describing the noise field around the jet from a jet engine. Velocity and temperature were seen from illustrations of typical traverses to remain uniform to within fin of the nozzle walls. Details of noise measurements made around a specially-mounted Derwent engine were then given, and typical results shown. The general pattern of noise did appear to be independent of the type of engine, which was to be expected if the noise pattern were entirely due to the interaction of the high- velocity jet with the surrounding air. The speaker continued by comparing model test results with theory; the variation with frequency of the peak noise angle generally agreed with the variation obtained on model tests, the actual figures being 14 deg from the jet centre line in the 37.5 to 75 c.p.s. band, varying round to 70 deg in the 2,400 to 4,800 c.p.s. band. The apparent sources of the noise also seemed to be farther downstream at the lower frequencies, as in the model tests. The loudest noise appeared to occur at approximately 30 deg to the jet axis. A point on this 30-deg line at 20 yd from the jet nozzle had therefore been chosen for a more detailed investigation, and the results were extremely interesting when compared with the main points of the Lighthill theory: (i) the theory implied that the noise produced by a jet was independent of the jet temperature, and model tests had supported this. In the full-scale tests on the Derwent and Avon engines, jet temperature variations had produced no deviation from a straight forward power law relationship between noise and velocity, (ii) The results concerning this power law relationship were particularly striking; the index was exactly eight for the Derwent results, agreeing precisely with the theory. Very good agreement was found in the case of the Avon also. Any shock-wave phenomena that might have occurred did not seem to have had any appreciable effect on the noise, (iii) The theory predicted that the noise should vary as the square of the jet nozzle diameter. Such a correction did not account for the difference between the engines however—there remained a difference of some seven decibels —but this could possibly be explained by the different test conditions and the different noise analyzers used. Clearly, the speaker continued, it was high jet velocity that was the real culprit in producing noise. It was therefore unfortunate that one of the most useful methods of getting a large thrust out of a small engine, by the use of re-heat or afterburning in the jet pipe, depended entirely on increasing this velocity. Mr. Greatrex went on to illustrate the principles involved in the re-heat process. The amount of fuel needed to produce the given thrust (with pressure ratio and hence Mach number in the jet constant) increased rapidly, but the hotter engines were smaller, requiring a reduced airflow, and this was the great advantage of this type of engine. The maximum noise increase to be expected, due to the increase in velocity alone, was about i8£db. In practice, re-heat was applied to an existing engine, increasing the nozzle area so that the airflow was maintained constant. With the further noise contributed by the increased nozzle area and thrust, the maximum possible noise increase due to applying re-heat to an existing engine would be about 21 decibels—giving an overall intensity at 20 yd and 30 deg from an Avon of 160 decibels. Fortunately (as far as noise and vibration were concerned), the very large thrust increases theoretically possible with re-heat were still not realized in practice. Practical evidence concerning re-heat, taken from an R.A.E. report on noise tests on a Derwent engine, supplied further confirmation of the Lighthill theory. It was clear that the use of re-heat presented a formidable problem in its associated noise, a problem which must be mastered in order that its performance advantages could be effectively used. Concerning noise reduction methods, there was firstly the possibility of designing an engine from the start to use a lower jet velocity. Such a low-temperature engine, however, would inevitably be larger, but the fuel consumption would be reduced. The large civil jet airliner was an extremely suitable application for an engine of this type, such as the Conway, and it was hoped that measurements shortly to be made would confirm the expected noise reduction. Secondly, there were a number of devices which had been found to give substantial noise reductions when tested on model jets at Cranfield, Manchester and Southampton. The most promising of these, at any Effect on Noise at High r.p.m. Type of device 1 2 3 4 5 Low Frequencies 15-45 deg from jet axis (db) -8 -6 -5 -3 unchanged a5-120 deg from jet axis (db) + 2-3 slight increase unchanged -2 unchanged High Frequencies 15-45 deg from jet axis (db) -3 -2 unchanged -4 + 2 45-120 deg from jet axis (db) + 6 + 4 + 4 -2 unchanged Fig. 1. The results of full-scale tests on a Derwent engine, using various noise-reduction devices: (1) Equi-spaced square teeth, three parallel to axis, three inclined inwards at 30 deg; (2) three square teeth parallel to axis, three triangular teeth inclined inwards; (3) six triangular teeth arranged as in (1); (4) square teeth, six parallel to axis, six inclined inwards; (5) eight vee-notches cut in standard convergent nozzle. Dimensions of all teeth are 1112 circumference (sides of square, base and height of triangle) except those of (4) which are 1\24 circumference.
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