US2936846A - Ground exhaust noise suppressors - Google Patents

Description

y 1960 J. M. TYLER ETAL 2,936,846

GROUND EXHAUST NOISE SUPPRESSORS Filid April 30, 1956 3 Sheets-Sheet 2 F I 3. 6 3, i6

iz a s 7d iN' /ENTORS JCDHN M- TYLER "C3EOF3GE B- TOWLEI AT TORPJEY y 1960 J. M. TYLER ETAL 2,936,846

GROUND EXHAUST NOISE SUPPRESSORS Filed April 30, 1956 3 Sheets-Sheet 3 2124441575? Jff f/VG/A/E IVUZZZE FIGJO FiCaJl HQVENTORS QHN EV! TYLER United States Patent GROUND EXHAUST NOISE SUPPRESSORS John M. Tyler and George B. Towle, Glastonbury, Conn.,

assignors to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Application April 30, 1956, Serial No. 581,418

9 Claims. (Cl. 181-60) pression of low frequency audible noise from the exhaust of any type of a jet and in particular an aircraft jet engine wake.

Since a modern jet aircraft engine produces noise energy of approximately 200 horsepower and since such noise would interfere with conversations at a distance of as much as 3000 feet from the engine, it is obvious that steps must be taken to control the formation of noise from the jet exhaust. The novel method of accomplishing this noise control taught herein is to shift the noise created to a super audio spectrum so that it will not interfere with airport personnel and the populace in the vicinity of airports.

The jet acoustic creation is called noise as opposed to sound since the latter is normally composed of a spectrum of but a few frequencies, while the former is normally composed of a frequency spectrum reaching from sub-audible to ultrasonic and including the objectionable audible noises.

The noise produced by a reciprocating engine such as the normal piston engine in an automobile, or by a rifle is brought about by explosive discharges coupled with the participation of solid objects such as the engine pistons, the engine exhaust valves, or, in the latter case, by the cartridge or shell, the movement of such solid objects and explosive discharges creating noise waves which emanate into the atmosphere or pass through the exhaust manifold and exhaust pipe in engines.

In the past, with regard to the noises produced by the solid object and explosions, such equipment as mufflers and silencers have been used to reduce the noise levels, once the noise has been created. In our invention, we are not reducing the noise level once the noise has been created but are preventing the creation of the low frequency audible noise by regulating the jet exhaust in the manner to be described.

A theory of the aerodynamic origin of noise has indicated that much of the noise created by jet engines can be attributed to turbulent mixing in the jet wake. The unsteady, turbulent airflow occurs in the jet wage and results from a steady state discharge of high velocity gases into quiescent air.

This theory has been verified by both model tests and full scale jet engine tests. As a result it is now known that, the noise created within the jet engine is a very small proportion of the total noise generated. Therefore, conventional silencers incorporating resonant chambers and other damping devices can have little effect on reducing the noise unless they are of such size as to enclose a large portion of the jet wake of the engine. Since the noise creating portion of the wake may extend some 60 odd feet beyond the discharge nozzle, a conventional silencer becomes excessively cumbersome.

. Experimental tests show that the total noise power is distributed throughout the sub-audible, audible and super 2,936,846 Piitented May 17, 1960 audible frequency ranges. However, the distribution is unique and shows a peak noise power at a preferred frequency. This preferred frequency is determined by the size of the jet discharge nozzle. Large nozzles pro duce primarily audible noise, whereas very small nozzles produced noise which is above the audible limit.

In our invention, we are preventing the creation of low frequency audible noise associated with a large nozzle, by discharging the jet engine exhaust through a multiplicity of small holes or nozzles.

It is an object of this invention to prevent the creation of audible noises, or control the creation of audible noises during ground operation of aircraft jet engines and other jets.

It is an object of this invention to provide one piece portable or mobile detachable noise control means for use with aircraft jet engines during ground operation to control jet noises during ground operation yet not change engine operating characteristics and which may be removed to permit the aircraft jet engine to operate normally in flight.

It is the further object of this invention to control the noises created by aircraft jet engines by placing a perforated receptacle downstream of the exhaust outlet of said engine in which the perforation diameter and spacing is selected for optimum noise suppression.

It is a further object of this invention to control the creation of noise from an aircraft jet engine by the use of a perforated receptacle used in combination with the necessary equipment to prevent fuselage damage.

It is a further object of this invention to control the noises created by an aircraft jet engine during ground operation by controlling the size of the exhaust jets and to substitute a plurality of small exhaust jets for the one normal exhaust jet by means of a detachable exhaust accumulator or receptacle, which receptacle causes the jet noises to be formed in a predominately high frequency, super-audio spectrum as opposed to a predominately low frequency, audible spectrum.

It is a further object of this invention to control the perimeter to area ratio of jet exhaust nozzles so as to control the creation of exhaust noises and keep them in the super-audio as opposed to the audio ranges.

It is a further object of the present invention to control the noise created by the exhaust of a jet aircraft engine by increasing the exhaust nozzle perimeter to area ratio and by using a plurality of exhaust nozzles to accomplish this.

It is a further object of our invention to control the creation of noise from the exhaust of a jet aircraft engine by causing the engine to exhaust into an accumulator or receptacle having radial perforations in its walls and being separated a slight distance from the engine exhaust duct to permit thrust measurements during ground operation of the engine.

It is a further object of the present invention to discharge the exhaust gases of a jet through a plurality of small orifices after first passing the exhaust gases through a diffuser for static pressure recovery purposes.

It is a further object of the present invention to discharge the exhaust gases of a jet through a plurality of small orifices after first passing the exhaust gases through a diffuser for static pressure recovery purposes and also passing the exhaust gases through a cylindrical duct for flow stabilizing purposes. i 7

While we choose to show our invention with respect to the control of the creation of noises by an aircraft turbojet engine, such is done for purposes of illustration only, and it should be borne in mind that the principles disclosed herein are applicable to noise creation control i any type of anaerodynamic exhaust.

noises.

In the drawings:

Fig. 1 shows a cross sectional view of an aircraft jet engine with our detachable one piece ground noise suppressor consisting of a cylindrical conduit, a diffuser section and the perforated discharge receptacle or chamber.

Fig. 2 shows a side elevation of our noise suppressor without a diffuser section but with a rotary perforated matching valve.

Fig. 3 shows a side elevation of our noise suppressor without a diffuser and in which the perforated suppressor chamber is of shortened length and increased diameter.

Fig. 4 shows a mobile version of our exhaust noise suppressor in which the cylindrical section connecting the jet engine with the perforated suppressor discharge chamber or receptacle is split and joined by a bellows type structure so that the suppressor will not impose an appreciable weight overhang moment on the engine.

Fig. 5 is an enlarged section of the perforated wall of our exhaust noise suppressor discharge chamber to demonstrate more clearly the perforations and their spacing.

Fig. 6 shows an embodiment of our ground exhaust noise suppressor which is a mobile, detachable unit providing vertical discharge to eliminate re-circulation and other heat problems and which includes a high frequency reflector for personnel protection.

Fig. 7 shows another embodiment of our ground exhaust noise suppressor which is mobile and provides vertical discharge through a perforated sheet.

Fig. 8 is a graph demonstrating the attenuation of noises of various requencies in the atmosphere.

Fig. 9 is a graph (not to scale) showing the noise in decibels and frequencies created by jet nozzles of various diameters.

Figs. 10 and 10A show two versions of a protective shroud surrounding our ground noise suppressor.

Fig. 11 shows the characteristics of a jet wake which causes or creates the aerodynamic jet noises.

Referring to Fig. 11 we see a diagram showing the turbulent region downstream of the tail pipe of the normal jet engine in which the entire exhaust passes through the exhaust outlet 50 of the tail pipe 28. Experience has shown that in the region within one diameter of the tail pipe 21 high velocity shear, because of the difference in velocity between the jet exhaust and the atmosphere, occurs causing fine grain turbulence and high frequency It will be noted that a constant velocity profile exists at the tail pipe exit and dissipates by turbulent mixing from the periphery inward, forming a cone 2 of constant velocity. The apex of the cone 2 extends approximately five diameters downstream of the outlet of the tail pipe. The high velocity shear and the other turbulence set up within a few diameters of the tail pipe cause small scale eddies which change to large scale eddies and nearly isotropic turbulence to generate low frequency noises in the region from 5 to 30 diameters downstream of the tail pipe. These low frequency noises are audible and therefore objectionable.

We are not concerned with the high frequency noises since the atmosphere is generally accepted as attenuating sound energy by means of absorption. The atmosphere converts the orderly mass motion of sound energy into the molecular motion of heat energy. Referring to Fig. 8 we see a chart showing the attenuating of noises of various frequencies in air. It will be noted that high frequency noise dissipates into heat almost immediately. This is an additional reason why high frequency noise, in addition to its superaudio frequency, is not undesirable.

Referring to Fig. 9 we see a chart showing the noise spectrum generated by jets discharged from engines having exhaust nozzles of varying diameter sizes. It will be noted that the spectrum of noise generated by a jet passing through a 22 inch diameter exhaust nozzle (see curve A) produces its loudest noise at approximately 87 cycles per second (c.p.s.) and the bulk of the total noise which has been created by this jet (see the area under curve A), is below the audible limit of 19,200 c.p.s. We further note that the noise spectrum (see curve B), generated by a jet passing through an exhaust nozzle of 2 inch diameter produces a lesser total noise (see area below curve B), below the audibility limit. Now referring to the noise spectrum created by a jet discharged through a .085 inch diameter nozzle (see curve C) we see that it peaks at a frequency of 23,600 c.p.s. and that almost all of its total noise (see area below curve C), lies above the audibility limit. As progressively increasing numbers of .085 inch nozzles are employed through which to discharge the jet exhaust, the noise level increases progressively, see curves D, E, F, G, H, but continues to peak at 23,600 c.p.s. and the bulk of the noise created continues to lie above the audibility limit. This graph demonstrates the desirability of employing a plurality of small diameter jets through which to exhaust the discharge gases of a jet engine as opposed to exhausting the entire gas volume through a single large diameter nozzle. Our invention utilizes the principle demonstrated by this graph and discharges the exhaust gases of a jet through a plurality of small nozzles as opposed to one large nozzle, thereby preventing the creation of low frequency, audible noise.

A preferred embodiment of our invention is shown in Fig. 1 in conjunction with an aircraft jet engine. The engine is designated as 10 and consists of air intake section 12, compressor section 14, combustion chamber section 16, turbine section and exhaust outlet section 20. Air enters the engine 10 through air intake section 12 and is compressed as it passes through compressor section 14. The air is then heated in combustion chamber section 16 due to the combustion which is occurring in combustion chambers 22 which are fed fuel by fuel nozzles 24- and fuel manifold 26. The fuel is ignited by spark plug 27. The air then passes into turbine section 18 and a power extraction function occurs to drive cornpressor 14 while the remainder of power is manifested as thrust as the exhaust gas is discharged through exhaust section 20 which consists of tail pipe 28 and exhaust outlet or nozzle 50. This is a typical aircraft jet engine, with which our ground exhaust noise sup pressor will be used. Our one piece ground exhaust suppressor 30 consists of cylindrical duct 32, conically diverging diffuser 34, perforated cylindrical receptacle or chamber 36 and conically converging and perforated rear section 38. Suppressor unit 30 is attached to engine 10 by means of any conventional type of system, for example, clamp unit 40 which is a 2 piece split clamp which pieces are bolted together by bolt unit 42 to form a hoop of concave cross section so as to engage flanges 44 of tail pipe 28 and 46 of cylindrical section 32.

After the jet engine exhaust passes through discharge section 20 of engine 10, it then passes through cylindrical chamber 32. The function of cylindrical chamber or straight section 32 is to promote or induce a steady state flow of the exhaust gases since unsteady flow, in itself, is a noise creator. After passing through straight section 32 the exhaust gas then passes through conically outwardly diverging diffuser 34. The function of diffuser 34 is to cause the exhaust gas to recover a high static pressure and to reduce its velocity so that when the exhaust gas enters perforated chamber 36, because of the high static pressure the gas will be induced to pass through perforations 48 in the walls of cylindrical section 36 and rear section 38.

By causing the engine gas to discharge through the plurality of small diameter perforations 48, in preference to passing through the normal tail pipe exhaust outlet 50, we accomplish the result of creating the noise spectrum generated by the jet engine exhaust at high frequencies as shown in Fig. 9 as opposed to the low frequencies noise spectrum creation which would have been brought about by discharging the exhaust gas through a single large diameter nozzle, such as 50.

a To prevent the premature re-joining of the exhaust wakes after they pass through perforations 48, the perforations are spaced a minimum of 3 perforation diameters apart. This is best shown in Fig. 5 in which perforations 48 are indicated to be of diameter d while the space separating adjacent perforations centers is indicated to be a minimum of 3d. Should the exhaust wakes be permitted to re-join too early externally of the engine and suppressor, a low frequency and audible noise spectrum would be created. If less noise reduction is desired, the spacing may be decreased and/or the perforation diameters increased.

Because this is a ground exhaust noise suppressor which is intended to be detached before the airplane employing the aircraft jet engine takes off, we are not concerned with thrust generation and therefore radially extending perforations 48 may consist merely of a plurality of holes, substantially circular, which are punched or in some way placed in the walls of cylindrical section 36 and rear section 38 so as to discharge substantially radially.

So that engine 10 maybe fully tested operationwise on the ground, it is necessary that the number and size of perforations.48 be so selected that the total flow characteristics through silencer unit 30, including perforations 48, is substantially the same as the total flow characteristics through outlet 50 of tail pipe 28. To accomplish this, it is sometimes necessary to use a perforation, port or nozzle regulating valve such as is shown at 52 in Fig. 2 which may be actuated by pilot controlled lever 53 so as to govern the total area presented by the plurality of perforations 48 to insure that the total flow characteristics through the plurality of perforations 48 is substantially the same as the total flow characteristics through exhaust outlet 50 of tail pipe 28. If valve 52 is solid, it may be actuatecl axially to block oif the necessary number of perforations 48 to vary total area of and the total flow characteristics through perforations 48 to match the total flow characteristics through exhaust outlet 50. If valve 52 is perforated, it may be moved axially or circumferentially so that its perforations 55, which may correspond in size and shape with perforations 48, may be aligned with or block off the necessary number of perforations 48 to bring about this total flow characteristics matching.

It has been determined through experience that in certain installations, the upstream portion of perforated cylinder 36 performs a sufficient diffuser function as regards the reduction of velocity and the increasing of specific pressure that a separate diffuser unit for specific pressure recovery purposes solely is not always necessary. The embodiment of our invention shown in Fig. 2 demonstrates an installation of this type in which the exhaust gas, having passed through engine 10', is discharged through exhaust outlet 50' of tail pipe 28' and then through flow stabilizing straight passage 32 and into the chamber formed by perforated sections 36' and 38 to be discharged through the plurality of perforations 48'. Under certain conditions, to permit the upstream sections of perforated cylinder 36 to perform a diffuser function, it is necessary that the overall length of the perforated section be increased. The unit shown in Fig. 2 operates without the need of a special diffuser section, but is normally of increased length over the embodiment shown in Fig. 1.

Fig. 3 employs no diffuser section either but does employ straight cylindrical section 32" and a shortened but increased diameter perforated cylindrical section 36 together with an increased diameter, perforated rear section 38". This is normally used in installations where there is a large diameter available in exhaust outlet 50". Because there is no pressure recovery afforded either through a diffuser section or the upstream stages of the perforated cylinder 36", this configuration requires that a large discharge area be provided by the plurality of perforations 48".

The embodiment shown in Fig. 4 is substantially the same as that shown in Fig. 2 and consists of tail pipe 28", straight section 32, perforated cylinder 36 and perforated rear section 38" both of which carry a plurality of perforations 48". The unique feature about this installation is that it is shown to be mobile or transportable on carriage unit 54 which consists of wheels 56, platform 58 and adjustable mounting bars 60 which carry the noise abatement unit 30". Further, this unit has the unique feature of having straight section 32" divided into two straight cylindrical sections 62 and 64 which are joined through expansion unit 66, which is possibly a bellows. This permits suppression or noise abatement unit 30 to be attached to the engine and permits small amplitudes of motion in the engine and the bellows also permits the attachment of the suppressor to the engine without causing an appreciable weight overhang on the engine.

Referring to Fig. 10, We see our noise suppressor or noise abatement unit 30"" connected to tail pipe 28"" by clamp 40"" and surrounded by protective shroud 68 to prevent the jet wakes or discharges, caused by the exhaust gas from the engine, passing through perforations 48", from causing damage by impinging against the fuselage or some other fragile material or structure.

Since more than one type of protective shroud has been developed, the lower portion, Fig. 10A, demonstrates -a second configuration. This configuration consists of at least one frustoconical sheet unit 69 which envelops noise suppressor 30" and projects divergently downstream; Each unit 69 may be of two piece construction and joined by bolts, as shown. Unit 69 is provided with band or ring 71 at its upstream or forward end. Band 71 engages suppressor 30" circumferentially in a friction fit to position unit 69. So that unit 69 is not forced forward on suppressor 30 due to gas loading, lug 73 projects from and attaches to suppressor 30" to abut against the forward end of ring 71. When silencer unit 30 is not attached to engine 10, it must be spaced in axial alignment with and in close proximity to engine 10 such that the inlet of suppressor unit 30 is substantially the same diameter as that of engine exhaust outlet 50 thereby causing the cylindrical exhaust gas stream being discharged through engine outlet 50 to enter the inlet of suppressor unit 30 with no radial clearance between the exhaust gas stream and the suppressor inlet.

Referring to Fig. 6 we see another embodiment of our invention in which jet engine 10, which is carried in aircraft 70, discharges gases through outlet 50 which gases then pass through a flexible connection 72 which consists of a series of small, sudden expansion diffusers or chambers formed by telescoping cylindrical units 74, 76 and 78 and then pass through discharge tube 80, is then turned by turning vanes 82 to pass vertically through discharge tube 84 and then into perforated suppressor 30 from whence it is discharged through the plurality of perforations 48. A suppressor unit for this type, since intended solely for ground operation, is mobile because of the use of wheel units 56 and discharges the exhaust gases at a substantial height above the ground. A high frequency reflector 86 causes the sound waves generated by the exhaust gas passing through perforations or nozzles 48 to be reflected in a vertical direction or in direction away from the ground.

Fig. 7 shows another embodiment of our invention in which engine 10 is carried by airplane 70. The exhaust gases from engine 10 pass through flexible connection and expansion diffuser 72, thence pass through diffuser tube 80, is turned by turning vanes 82 and then passes through diffuser 88 and then is discharged through perforated noise suppressor sheet 90 which contains a plurality of perforations or nozzles 48.

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described, but may be used in other ways without departure from its spirit as defined by the following claims.

We claim:

1. Means to control the frequency of the noise created in a turbulent exhaust gas wake caused by discharging to atmosphere a high velocity stream of nonpulsating, heated and pressurized exhaust gas which is substantially free of low frequency sound waves comprising receptacle means having an inlet adapted to receive all of the exhaust gas stream before exhaust gas mixing with atmosphere occurs, said receptacle means including a hollow discharge portion, said discharge portion having side and downstream end Walls with a plurality of spaced perforations in said walls connecting the interior of said discharge portion directly to atmosphere, said inlet and said perforations constituting the sole apertures in said receptacle means so that all exhaust gas passes into said receptacle means thru said inlet and from said receptacle means directly to atmosphere thru said perforations as high frequency jet Wakes, said perforations being positioned to direct the high frequency jet wakes to project in directions which generate substantially no forward thrust.

2. Apparatus according to claim 1 including exhaust gas static pressure recovery means in said receptacle means side walls adjacent said inlet. I

3. Apparatus according to claim 1 wherein the total flow characteristics of said inlet and said perforations are substantially equal. 7

4. Apparatus according to claim 1 wherein said perforations are spaced sufficiently to prevent premature rejoining of the high frequency jet wakes.

5. Apparatus according to claim 1 including gas deflectors of circular cross section positioned externally of said receptacle means and spaced substantially therefrom so as not to affect discharge ofsaid jet wakes thru said perforations but to prevent the high frequency jet wakes from impinging against fragile parts.

6. Apparatus according to claim 1 including expansion means located between said inlet and said perforations.

7. Apparatus according to claim 1 including means to vary the total area of said perforations.

8. Apparatus according to claim 2 including exhaust gas flow stabilization means in said receptacle means side walls between said inlet and said static pressure recovery means.

9. Apparatus according to claim 1 wherein said receptacle means consists of a single, hollow body defined by said Walls and wherein said perforations extend over the entire surface of said discharge portion.

References Cited in the file of this patent UNITED STATES PATENTS 1,110,040 Chatain Sept. 8, 1914 1,401,368 Schneebeli Dec. 27, 1921 1,638,087 Clark Aug. 9, 1927 2,370,259 Rippingille Feb. 27, 1945 2,388,924 Mercier Nov. 13, 1945 2,418,488 Thompson Apr. 8, 1947 2,685,936 Brenneman et al. Aug. 10, 1954 FOREIGN PATENTS 546,538 Germany Mar. 15, 1932 851,012 France Sept. 25, 1939 901,803 France Nov. 13, 1944 473,327 Italy July 23, 1952 OTHER REFERENCES Van Nostrands Scientific Encyclopedia (3rd edition), 1958, by D. Van Nostrand Company, Inc., Princeton, New Jersey (page 517).

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