Engine noise

In order to provide noise suppression, bumps or undulations are provided on a nozzle surface in order to vary the convergent-divergent ratio between that surface and an opposed nozzle surface. By such an approach, a circumferential variation in the shock cell pattern is created and the flow is deflected so as to enhance turbulent mixing thereby suppressing noise.

The present invention relates to gas turbine engine noise and more particularly to jet noise under cruise conditions.

The general stages of gas turbine operation are known. In particular, it will be understood that there is a downstream jet created as the various gas streams are forced out of the engine in order to create propulsion. Inherently such jet flows create noise as the jet shear layer breaks down. This shear layer breakdown along with other factors such as the presence of shock waves produces noise.

Clearly, noise is a detrimental factor with respect to gas turbine engine operation. Thus, there is a continuing objective to reduce engine noise in all phases of engine operation including whilst an engine is propelling an aircraft through the air at altitude and under cruise conditions.

At cruise conditions the nozzle of a jet engine is not perfectly expanded. As a result a shock structure occurs in the jet. This shock structure is strongest near the nozzle exit but extends several diameters downstream of the nozzle in a repeat but progressively fading shock cell pattern.

Shock cell noise is generated as the turbulence of the jet shear layer passes through and interacts with the shock structure of the jet (see Harper-Bourne, M. and Fisher, M. J., 1973, “The Noise from Shock Waves in Supersonic Jets”, Proceedings (No. 131) of the AGARD Conference on Noise Mechanisms, Brussels, Belgium). If one considers a cross section through half of a typical high bypass ratio civil engine nozzle system, with the bypass jet imperfectly expanded, there is a shock structure set up in the bypass stream. The shear layer between the flight stream and the bypass stream becomes turbulent as it develops and the turbulence that results convects through the shock structure generating noise. The region in which the shock cell noise is generated may be several nozzle diameters downstream of the nozzle exit plane.

Shock cell noise may be reduced by previous serrations at the nozzle exit that enhance the mixing of the shear layer so that the turbulence intensity is lower in regions where the turbulence interacts with the shock structure.

Noise suppression by serrations has been demonstrated but this has typically been for environmental noise at take off or landing conditions.

Generally, serrated nozzles consist of flaps or tabs added to or cut out of a nozzle so as to generate circumferential flow non-uniformities. The circumferential flow non-uniformities enhance mixing of the jet thereby breaking up coherent structures leading to lower noise.

In order for serrations to reduce the noise of a nozzle they need to disturb the nozzle flow. This typically requires the serrations to deflect the flow by having some incidence or insertion to the flow. This results in increased drag and an associated loss of performance. The performance loss and noise reduction mechanism are inherently linked for serrations.

The increased surface area of serrations also increases the drag. Increased surface area also increases overall weight.

In accordance with the present invention there is provided a nozzle for a gas turbine engine, the nozzle comprising a nozzle surface including a plurality of undulations to vary available cross sectional area across the nozzle between the nozzle surface and an opposed surface of the nozzle over a desired convergent-divergent ratio range for noise control of a jet passing through the nozzle in use.

Additionally, the undulations also provide variation in the angle of flow of the jet passing through the nozzle.

Generally, the variation in cross sectional area is adjusted to provide stimulation in mixing of a shear layer of the jet for relative noise reduction in comparison with that without mixing of the shear layer of the jet. Additionally, the undulations alter the repeat cycle and/or provide variation of intensity of shock cells generated by the jet.

Generally, the undulations comprise bumps formed in the nozzle surface.

Possibly, the undulations are sinusoidal in a circumferential direction or in a plane perpendicular to a jet flow direction in use.

Preferably, the undulations each comprise maximum amplitude, the maximum amplitude situated a distance between 2% and 15% of the nozzle diameter along the nozzle surface upstream from the nozzle exit plane.

Preferably, the maximum amplitude situated a distance equivalent to 6% of the nozzle diameter upstream of nozzle exit plane.

Alternatively, the undulations each comprise a maximum amplitude, the maximum amplitude situated within a distance equivalent to +/−2% of the nozzle diameter along the nozzle surface from the nozzle throat plane.

Preferably, the approximate cross-section shape of the undulations are from the group comprising triangular, trapezoidal, part-circular, sinusoidal and asymmetric-sinusoidal.

Alternatively, the undulations have an aerodynamically smooth gradual spline in an axial direction of jet flow in use. Typically, the aerodynamically smooth spline is between radii at three fixed axial locations along the nozzle surface from the nozzle exit plane, one radii within a distance upstream equivalent to 20% of the nozzle diameter, one radii at a point of maximum undulation amplitude within an upstream distance equivalent to 15% of the nozzle diameter and one radii within 10% of the nozzle exit plane diameter.

Generally, the convergent-divergent ratio is in the range of 1 to 1.01 preferably 1.008.

Typically, the undulations have amplitude39in the range 0.1-2.0% of the nozzle exit diameter.

Typically, the nozzle comprises a bypass nozzle of a gas turbine engine with the undulations on an inner surface of the outer wall and/or outer surface of an inner wall of the bypass nozzle.

Possibly, the nozzle is a core nozzle of a gas turbine engine with undulations on an inner surface of the outer wall and/or outer surface of the inner wall of the cone nozzle.

Possibly, the undulations are symmetrically regularly circumferentially distributed about the nozzle surface. Alternatively, the undulations are asymmetrically and/or irregularly circumferentially distributed about the nozzle surface. Further, the undulations may be at different axial positions relative to an exit plane of the nozzle. Possibly, the undulations have groups of differing amplitudes circumferentially and/or axially in the nozzle surface. Additionally, the undulations may have different groups of axial length and/or width relative to each other.

Possibly, the nozzle surface has an edge with serrations or tabs. Additionally, the undulations may be arranged reciprocally with the serrations for additional variation in convergent-divergent ratio range.

Alternatively, the tabs are deployable for noise reduction. Possibly, alternate tabs are deployable for noise reduction and the undulations are formed on any one or more of the tabs. Preferably, the undulations are transformable between a deployed position and a non-deployed position, the non-deployed position being less aerodynamically obtrusive than the deployed position. Further, the undulations may be transformable to a second deployed position, between the deployed and non-deployed positions.

Possibly, the undulations comprise a shape memory material element. Alternatively, the shape memory material element comprises two layers of SMM material, each layer having different switch temperatures and capable of deploying in a first shape and a second shape, the second shape having a greater amplitude that the first shape. Alternatively, the shape memory material element comprises two layers, one layer of SMM material and the other layer of resilient material to provide a spring force to the element.

Preferably, the undulations are integral with the nozzle. Alternatively, the undulations are created by attached elements individually or as part of an assembly secured to the nozzle surface.

Alternatively, the undulations are variable in terms of amplitude and/or position and/or distribution in the nozzle surface.

Alternatively, such variation is by use of inflatable features with the nozzle surface or deployable mechanical portions of the nozzle surface.

Preferably, the number of undulations is in the range one to forty-two and preferably twenty undulations distributed about the nozzle surface.

FIG. 1shows a half cross section through a typical high bypass ratio civil aero turbine engine exhaust nozzle1. The bypass stream, that is to say the outer stream that passes only through the fan has a nozzle exit area2. The area2is thus available for flow to pass through and exit a nozzle3. In the nozzle3there is a nozzle throat area4which is the minimum area for the flow to pass through at any point in the bypass nozzle1. This limits the possible mass flow rate so that there may be choking of the flow with expansion after the throat area4.

The throat area4and exit area2may be of different magnitudes and may occur at axially separated positions. A convergent nozzle is one in which a flow area5is continually decreasing in a direction of flow A (or axial direction) and therefore one in which the exit area2is the minimum area and thus also the throat area4. A convergent-divergent nozzle (FIG. 1) is one in which the throat area4is upstream of the exit area2so that the flow area2decreases in the direction A of flow until it reaches a minimum point at the throat area4and then increases to the exit area2.

The respective variations in the available flow area is depicted graphically against axial distance in the flow direction A inFIG. 3. Thus, as can be seen, with a convergent nozzle the relationship is given by line10, whilst with a convergent-divergent nozzle the relationship is given by line11. The convergent-divergent ratio of a nozzle is therefore given by the ratio of the exit area2in comparison with the throat area4(FIG. 1). The convergent nozzle has a convergent-divergent ratio of unity (1) whilst, in general, a convergent-divergent nozzle has a convergent-divergent ratio greater than 1. With a typical engine used for aircraft, the nozzle configurations have a convergent-divergent ratio in the range 1.00 to 1.02 and for the Applicant's commercial production engines typically a range between 1.00 and 1.01.

FIG. 2is included to provide an illustration of a typical prior serrated nozzle in order to provide noise suppression. As can be seen, the nozzle20has serrations21which can take the form of flaps or tabs added to or cut from the nozzle in order to generate circumferential flow non-uniformities, which as indicated above break up coherent structures in the jet flow in order to give rise to noise suppression. In effect, the serrations21deflect the flow so as to enhance turbulent mixing thereby suppressing noise. However, as indicated above, serrations can add significantly to cost, weight and drag upon the engine reducing efficiencies.

The present nozzle provides a circumferentially varying convergent-divergent nozzle by incorporating a number of undulations or bumps into at least one nozzle surface. Typically, twenty sinusoidal and evenly spaced bumps are machined into an inner surface of the outer wall of a bypass nozzle such that the radius varies through the pitch of the sinusoidal bumps. As will be described later, a number of varying alternative embodiments will provide undulations and bumps in differing patterns and distributions in accordance with particular operational requirements. With regard to the first embodiment described, as indicated sinusoidal oscillations in the form of bumps in the circumferential direction about the nozzle will be provided.

Referring again toFIG. 1and also seen inFIG. 6, the bumps40comprise an upstream surface37, a point of maximum amplitude38and a downstream surface39. The undulations or bumps40will generally have a smooth spline in the axial direction with radii at three fixed axial locations along the nozzle surface. Defining the upstream surface37is a first fixed axial radii defined at a position up to 20%, but in a preferred example 10% of the nozzle exit plane diameter2, upstream of the nozzle throat position4. A second fixed radius is provided between the upstream and downstream surfaces37,39and defines the maximum bump amplitude38. The second fixed radius is positioned approximately 6% of the nozzle exit plane diameter upstream of the nozzle exit position, but in other embodiments may be positioned between 2% and 15% of the exit plane diameter upstream of the nozzle exit position. A third fixed radius preferably is positioned at the nozzle exit plane itself, but may be positioned up to 10% of the nozzle exit plane diameter upstream of the exit plane.

As indicated above in a first embodiment of a nozzle undulations or bumps will be provided circumferentially in a regular distribution pattern.FIGS. 4 and 5illustrate the first embodiment of the invention withFIG. 4providing a schematic perspective illustration of a nozzle whilstFIG. 5provides an enlarged view of a section of the nozzle bypass inner wall surface.

Undulations or bumps40are regularly circumferentially distributed about an inner surface41of an outer wall42of a bypass nozzle of an engine43. Thus, as can be seen in bothFIGS. 4 and 5, the effect of the undulations or bumps40is to provide a convergent-divergent nozzle form along the axial length of the undulation or bumps40with generally areas between the undulations or bumps40being flatter and therefore creating a convergent nozzle format. In such circumstances, as described previously, noise suppression is achieved through mixing of the shear layer so that the turbulence intensity is lower in the regions where the turbulence interacts with the shock structure from the nozzle. Furthermore, the shock cell repeat pattern will vary across the undulations again leading to noise suppression.

Referring toFIG. 5a, which shows the first embodiment in more detail, generally the undulations40are sinusoidal and have amplitude39in the range 0.1-3.0% of the nozzle exit diameter, but preferably in the range between 0.3% and 1.5%. Typically, the undulations have maximum amplitude point38positioned within ±2% nozzle exit diameter of the nozzle throat position4. The circumferential extent26of the undulations40is defined by a length equivalent to between 1° and 45° and the angular spacing27between maximum amplitude points38is between 2° and 90°, i.e. the total number of bumps is between 180 and 4, however, a preferred number of maximum amplitude points is between 12 and 45.

Referring toFIG. 5b, alternate shapes of bumps40a-eare shown. These alternately shaped bumps40a-emay be either attached by suitable means to an existing smooth nozzle surface or may be machined into the nozzle wall. Where the bumps are machined into the nozzle wall they may either be proud of the wall surface23(as shown) or machined in as for the sinusoidal wave form having peaks38and troughs49defined by the respective inverse bump shape. Here the undulations40are preferably shaped in cross-section as shown by bump40a, which comprises maximum amplitude38defined by a radius24and blend radii25smoothing the shape into a circumferential profile23of the nozzle wall42. The maximum amplitude38is in this case the radial height above the existing or original wall surface23.

FIG. 5balso shows alternative shapes the undulations or bumps may take without departing from the scope of the invention. Bump40bis generally trapezoidal in cross-section; bump40cis triangular; bump40dis defined by a constant radius (part-circular) and bump40eis an asymmetric version of bump40aand similarly defined by three radii but radius25′ is greater than25″.

Each bump40a-e, in their respective array of bumps, are angularly spaced apart a corresponding distance27peak-to-peak (38) dependent on the number required around the nozzle's circumference.

For a rectangular nozzle or other non-circular nozzles the spacing of the bumps peak or maximum amplitude point38is the total length of side divided by the number of bumps.

In such circumstances, for a scale model tested by the Applicant, the nozzle exit diameter was 58 millimeters, the first radius is at approximately 11.2 millimeters, the second is at a position 3.6 millimeters upstream of the nozzle exit plane and the final radius at the exit plane itself. In such a situation, the amplitude of the undulations or bumps is in the order of 0.8 millimeters (1.38% of the nozzle exit diameter) with an axial position of maximum amplitude as indicated at 3.6 millimeters upstream of the nozzle exit plane (6% of the nozzle diameter). The scale model comprised 20 bumps40(FIG. 5a) evenly spaced at 18° intervals. A 2 db noise reduction was achieved over a similar nozzle without bumps.

In such circumstances, for a production gas turbine engine of the Applicant's, the nozzle diameter is 1450 millimeters, having a bump amplitude in the order of 4.5 millimeters (0.31% of the nozzle exit diameter) with the first radius at approximately 280 millimeters upstream of the nozzle exit plane, the second radius at approximately 90 millimeters upstream of the nozzle exit plane and the third radius at the nozzle exit plane. This nozzle comprised 20 sinusoidal bumps40a(FIG. 5a) evenly spaced at 18° intervals.

However, for other applications and depending on specific engine operating circumstances different distributions, amplitudes and axial lengths may be used within the ranges indicated throughout this specification and depending on particular noise reduction requirements.

FIGS. 6 and 7illustrate respectively a schematic cross-section through one of the undulations or bumps40in the inner nozzle surface41of an outer wall of a bypass nozzle42with respect to an opposed surface44of the nozzle (FIG. 6) and inFIG. 7a graphic representation illustrating differences in the available nozzle flow area relative to axial distance along the bump40. Thus, as can be seen, the available flow area45between the nozzle surface41and the opposed surface44is varied in the circumferential direction through the bump and the spaces in the area41between the bumps40. This is illustrated inFIG. 7through the representative relationship lines46showing the variation in available flow area45with axial distance in the direction46at different circumferential positions across one bump.

The above circumferential variation in available area45is further illustrated across, that is to say circumferentially around, the nozzle inFIG. 8where variations in the convergent-divergent ratio as well as the available flow area45are shown relative to the azimuthal angle across the undulation or bump40. As can be seen, with a sinusoidal undulation or bump40, a similar sinusoidal relationship is provided in the graphic representations depicted inFIG. 8. InFIG. 8a, the convergent-divergent ratio is depicted against circumferential angle across an undulation or bump without a flat space section between undulations such that there is a continuous sinusoidal variation from one bump to the next such that the convergent-divergent ratio oscillates sinusoidally around an average value47, but it will be understood where there is an undulation of bump formed with relatively flat spaces either side then a half sinusoidal relationship will be provided in terms of the variation in convergent-divergent in ratio as the bump or undulation amplitude moves into and out of the nozzle across the circumferential width of the bump or undulation. Similarly, the available flow area46will vary sinusoidally across the circumferential width of the bump or undulation and relative to a maximum exit area48defined at the exit plane of the nozzle.

Referring toFIG. 9, overall shock cell noise50is dependent on Mach number51of jet. A fully expanded nozzle has minimum noise (52) with over expanded and under expanded nozzles having greater noise (Tam, C. K. W. and Tanna, H. K., Journal of Sound and Vibration, 1982, 81(3), 337-358) shown inFIG. 9.

For a given pressure ratio there exists a convergent-divergent ratio to give a fully (perfectly) expanded jet and this will have the minimum noise as there will be no shock structure established. A fully expanded nozzle does not necessarily meet all operational requirements and so it is impractical for a fixed geometry nozzle to achieve a fully expanded jet at cruise conditions. For information solid line53shows the effect of a convergent-divergent nozzle whilst broken line54shows a simple convergent nozzle.

If some sectors of a nozzle operate at perfectly expanded conditions then no shock structure will be formed in those sectors and the mechanism for shock cell noise generation will disappear locally. The closer a sector of the nozzle is to being perfectly expanded the weaker the shock structure and the less shock cell noise will be generated. The undulations provide a range of available flow areas to increase the likelihood of a perfect or near perfect expansion for noise suppression.

For an imperfectly expanded supersonic jet from a nozzle of fixed geometry, the angle of the flow relative to the axis of the jet in the region just behind the nozzle exit is a function of the nozzle pressure ratio. This is a result of the flow emerging from the nozzle expanding to match the conditions outside of the nozzle. Moreover, the mass flow of the jet is fixed by the area of the nozzle throat. The final flow area of the jet (outside of the nozzle) is dependent on the mass flow and the freestream conditions. The freestream conditions are very nearly circumferentially uniform and so the flow area of the jet is proportional to the throat area these being linked by the mass flow. A circumferential variation in the throat area thus leads to a circumferential variation in the final flow area of the jet. This mimics the effects of serrations and produces a circumferentially non-uniform flow field.

The circumferential variation in convergent-divergent ratio as a result of a circumferential variation in throat area thus produces a circumferentially non-uniform flow field downstream of the nozzle exit. This enhances mixing of the shear layer reducing the extent of turbulent flow. The interaction of the turbulence and the shock cell structure responsible for the shock cell noise is thus further reduced as the turbulence is reduced.

In cases where the circumferential variation of convergent-divergent ratio is achieved with a circumferentially constant throat area (i.e. circular throat and sinusoidal variation of exit area) the circumferential non-uniformity in flow would be reduced but the circumferential variation in shock strength would persist and this would still reduce shock cell noise.

By contrast to serrations, circumferential variation of convergent-divergent ratio avoids the performance degradation due to tabs inserted into flow with incidence as this increases the drag on the serration. Serrations and tabs also have increased surface area exposed to the flow and this increases drag. The length (perimeter) of the trailing edge of the nozzle is a minimum for a circular nozzle in a plane perpendicular to the engine axis. The application of serrations or tabs increases the length of the nozzle trailing edge and thus increases the base drag.

The mixing achieved by varying the throat area circumferentially with undulations is as a result of manipulating the shock waves rather than deflecting the flow. Manipulating the shock waves to change flow directions is a near lossless process unlike deflecting the flow.

Serrations necessarily add weight to the design. The circular planar nozzle exit permitted by this invention is the minimum weight design. Mechanical challenge of tab and associated stress concentration are avoided. However, serration tabs suppress shock cell noise by enhanced mixing of shear layer.

A number of alternative embodiments to the regular sinusoidal or other shaped undulations presented circumferentially about the nozzle can be provided in accordance with the invention. Thus, the undulations or bumps may be provided on an inner surface of the outer wall of a bypass nozzle as described above, or alternatively the bumps or undulations can be provided on the outer surface of an inner wall of the bypass nozzle or bump undulations provided on the inner surface of the outer wall of the core nozzle or bumps and undulations provided on the outer surface of the inner wall of the core nozzle or combinations of these configurations. In the specific embodiment described above, it will be appreciated that there is a circular nozzle exit with sinusoidal variation in available throat flow area, but alternatively there could be a circular throat area with variation in the exit area by corrugating the nozzle exit area edge to create undulating correlations. Furthermore, there may be variation in the available throat flow area and variation in the exit area in such a way that leads to undulations that enhance shear layer turbulence and mixing as described above for noise reduction. Additionally, although described with regular spacing of the sinusoidal bumps or undulations in the embodiment described above, it will also be understood that there may be a range of different bump or undulation distributions as described below in a number of alternative embodiments.

The particular combination of bump or undulation position in relation to distribution as well as exit plane area will depend upon particular engine design requirements.

FIG. 10illustrates a first alternative embodiment of a nozzle in which bumps140are arranged with a regular distribution about an inner nozzle surface141of an outer wall of a bypass nozzle. The bumps have undisturbed regions143between them in which the nozzle therefore acts as a simple convergent nozzle in these parts with the bumps140providing the convergent-divergent variation in available flow throat area as required for noise suppression. It would be appreciated that an opposed surface (not shown) of the nozzle may include itself undulations or bumps which may directly oppose the bumps140or interleave with those bumps140such that these bumps in the opposed surface directly oppose the undisturbed regions143.

FIG. 11illustrates a second alternative embodiment of a nozzle. Thus, bumps240are arranged with irregular spacing in an inner nozzle surface241of an outer wall242of a bypass nozzle. By such irregular spacing of the bumps240, it is possible that there is further disturbance with respect to circumferential modes for shear layer turbulence or there may be variation in the noise suppression level at certain directions of the nozzle in comparison with others dependent upon operational requirements.

FIG. 12illustrates a third alternative embodiment of the present nozzle. Thus, bumps340are located in groups or individually in an inner nozzle surface341. Thus in a similar fashion to that with regard to the second alternative embodiment depicted inFIG. 11, there is circumferential variation in the distribution of the bumps340acompared to bumps340bin order to again disturb the circumferential modes and vary the noise suppression level at different directions of the nozzle.

FIG. 13illustrates a fourth alternative embodiment of the present nozzle in which undulations or bumps are provided at different axial positions as compared to circumferential conditions in previous embodiments. Thus, bumps440are provided in an inner nozzle surface441of an outer wall442of a bypass nozzle. It will be noted that undulations or bumps440aare essentially based at an exit plane edge443of the nozzle, whilst bumps or undulations440bare slightly displaced from that exit edge443, whilst the undulations or bumps440care even further displaced from the edge443. Such an arrangement will provide a variation in the convergent-divergent ratio over a broader axial length of the nozzle and therefore provide different operational performance compared to previous embodiments.

For each of the bumps440a,b,c,dtheir first radius, which defines their upstream surface, are located at respectively 0%, 5%, 2.5% and 7.5% of the nozzle exit diameter, upstream of the nozzle throat. However, it should be appreciated that each of the bumps440a,b,c,dare located and sized within the ranges defined hereinbefore.

FIGS. 13aand13bshow alternative embodiments to that shown inFIG. 13, where a number of adjacent bumps (440a,b,c,dinFIG. 13) are merged circumferentially to form one or more larger and therefore more complex undulations443,444. For each of these embodiments the bumps443,444are generally sinusoidal or part sinusoidal in a circumferential sense and comprise their first radii axially located between 0% and 7.5% of the nozzle exit diameter, upstream of the nozzle throat. The second and third cross-section radii are accordingly located relative to the first radius at any given axial cross-section through each bump443,444. In these embodiments the maximum amplitude is constant (i.e. it forms a ridge of maximum amplitude) except at the circumferential extents, where each bump blends out to the nozzle wall. It should be appreciated that in other embodiments the maximum amplitude may be varied circumferentially along the bumps443,444.

FIG. 14illustrates a fifth alternative embodiment of the present nozzle in which bumps540are provided which have different amplitudes at different circumferential and axial locations in an inner nozzle surface541of an outer wall542. In such circumstances, the different amplitudes for the bumps or undulations540may provide differing levels of noise reduction in different directions of the nozzle, and through changing the available flow area throat could improve mixing to allow further noise control. As can be seen, bump or undulations540bhave a greater amplitude than bumps or undulations540a,540cand have a greater axial length whilst undulations540b,540crespectively have different circumferential widths and axial lengths compared to each other and undulation540a.

FIG. 15illustrates a sixth alternative embodiment of a nozzle in which bumps or undulations640are presented in an inner nozzle surface641of an outer wall642of a bypass nozzle. Thus, the bumps or undulations640are grouped in regions with other undisturbed regions between them such that differing levels of noise suppression will be provided and therefore quieter areas achieved relative to normal nozzle operation. Such regionalisation of the bumps or undulations will provide a similar effect to a regular spacing and the bumps or undulations described with regard to the third alternative embodiment (FIG. 12) above.

FIG. 16illustrates a seventh alternative embodiment of a nozzle. Thus, undulations or bumps740are presented in a serrated outer wall742of a bypass nozzle. The bumps or undulations740are again presented on an inner nozzle surface741of the wall742. As described previously, the bumps or undulations could be provided in an inner or outer wall of a nozzle, butFIG. 16only illustrates provision of the bumps or undulations740in the inner surface741of the outer wall742. In such circumstances, the effects of serrations743are enhanced by this circumferential change in the convergent-divergent ratio created by the bumps and undulations740. Such an arrangement may provide enhanced noise suppression, although as described previously, provision of serrations743may add to drag and other factors with respect to engine operation.

FIG. 17provides a further eighth alternative embodiment of the present nozzle. Thus, as with the seventh alternative embodiment depicted inFIG. 16, bumps or undulations840are provided upon an inner nozzle surface841of an outer wall842of a bypass nozzle. In comparison with the embodiment depicted inFIG. 16, the bumps or undulations840circumferentially change the convergent-divergent, ratio, but do not extend into tab or serration portions843, but are upon a fixed portion of the nozzle prior to such serrations43. Again, such an approach will provide an alternative for particular operational requirements in terms of noise suppression and shear layer turbulence.

FIG. 18illustrates a further ninth alternative embodiment of a nozzle. Thus, in comparison with the embodiment depicted inFIG. 17, bumps or undulations940are again provided in an inner nozzle surface941of an outer wall942of a bypass nozzle. However, the bumps or undulations940still remain prior to serrations942in the exit plane of the nozzle and in comparison with the embodiment depicted inFIG. 17, these bumps940are out of phase with the serrations943in order to provide a further enhancement or variation in noise suppression performance dependent upon operational requirements.

It will be understood that noise and therefore noise suppression requirements will vary dependent upon an engine's operational state. In such circumstances, bumps or undulations in accordance with the present invention may be variable dependent upon operational conditions or desired requirements. In such circumstances, the bumps or undulations may have a shaped memory alloy type function and therefore vary according to temperature or other requirements in terms of amplitude and shape for variation in the turbulence created in the shear layer for noise suppression. Where possible, the bumps or undulations may be arranged to be electively deployable through use of inflation or deflectable mechanical panels or otherwise in order to change their amplitude, both in terms of inward deflection as well as axial length and circumferential spacing for operational requirements.

FIG. 19provides a schematic illustration of an engine nozzle arrangement in which the respective nozzle surfaces are illustrated. Thus, a bypass nozzle is provided by an outer wall1002and an inner wall1003such that surfaces1001and1004may incorporate bumps or undulations in accordance with the present invention in order to vary the convergent and divergent ratio across the nozzle surfaces1001,1004in accordance with the present invention. Similarly, an outer wall1005and an inner wall1006present opposing surfaces1007,1008of a core nozzle. These nozzle surfaces1007,1008may also incorporate undulations or bumps in accordance with the present invention in order to vary the convergent-divergent ratio across the core nozzle. In such circumstances, additional noise suppression may be provided by creating turbulence in the shear layer between jets for noise suppression as described above.

It will be understood that undulations particularly in a core nozzle will be subject to erosion at high temperatures, thus provision may be made for replacement of undulations as securable elements or assembly to a nozzle surface.

Alternative embodiments and modifications of the present invention will be understood by those skilled in the art. Thus, for example, rather than providing smooth splines for undulations or bumps as described above, more angular bumps or undulations may be provided. For example, a triangular cross-section bump, defining an apex at its point of maximum amplitude, may be used. Furthermore, there may be axial cycling in the bump or undulation amplitude axially or circumferentially if required in order to create mini turbulence in the jet flow for noise suppression.

InFIG. 20, the nozzle42comprises an arrangement of deployable noise reducing tabs80,82which are described in U.S. Pat. No. 6,813,877, the teachings of which are incorporated herein. Briefly, circumferentially alternate tabs80are rigidly fixed in a ‘deployed’ position as shown in the figure, where they interact with the gas streams to enhance mixing out the noise creating shear layer between gas streams. The deployable tabs82comprise shape memory material and are moveable between a deployed position as shown inFIG. 20and a non-deployed position, where they are aligned and abutting tabs80. During take-off and climb the tabs82are deployed, angled radially outwardly, for noise reduction purposes and the exit area of the nozzle12is enlarged. This enlargement reduces the velocity of the gas stream issuing from the bypass duct12and which intrinsically reduces exhaust noise. At aircraft cruise the tabs82are in the non-deployed position, where adjacent tabs' edges88,90are in sealing engagement with one another, and the exit area is therefore reduced. This reduction increases the velocity of the exhaust gas stream and improves engine efficiency.

As the bumps40are designed primarily for reducing aircraft cabin noise at cruise, the exhaust exit plane36in this case is defined by a downstream edge86of the tabs80,82when in their non-deployed position. The bumps40are still positioned within the range of positions specified hereinbefore and may therefore be situated on one or more of the tabs' radially inner surface, depending on the axial length of the tab and the convergent-divergent ratio.

FIG. 21show a bump40situated on each of the tabs80,82around the nozzle42circumference. The shape and configuration of the bumps40are as hereinbefore described.

In further embodiments of the present invention shown inFIGS. 22,22A and23, the bumps40are deployable and preferably comprise shape memory material as a means for actuating the bumps between a deployed position40′ and a non-deployed position40″. Shape memory material (SMM) is well known in the industry and is not discussed further except that its operation is similar to that for the deployable nozzle tabs as disclosed by the present applicant in U.S. Pat. No. 6,813,877, the teachings of the use of shape memory material are incorporated herein. The main advantage of having deployable bumps is to reduce aerodynamic drag when they are not required.

In the non-deployed position40″ the gas stream through the nozzle42is not disturbed by any bump40as would otherwise be the case and described hereinbefore. In the deployed position40′, particularly used at aircraft cruise, the bumps40interact with the gas stream and reduce exhaust noise as herein described.

In each figure the bumps40are formed from a shape memory material element60which is prestressed to a particular shape and changes shape, at a predetermined temperature, between the deployed and non-deployed positions. InFIG. 22securing means61attaches a continuous ring of SMM defining bump elements60. However, individual SMM elements may equally be used and attached to the nozzle wall by the securing means61. The securing means61may be a nut and bolt, weld, screw or other capturing member. The dashed lines define the non-deployed position40″ of the SMM element60. In the left hand part of the figure, the nozzle wall42defines a bump62having amplitude between the maximum amplitude38and the otherwise ‘original’ nozzle wall profile indicated by the dashed line63. This arrangement is advantageous in that there are two bump amplitudes which are help to attenuate cabin noise at two different engine operating points.

InFIG. 22Ashows a further embodiment of the SMM element60, where there are two layers of SMM material64,65which have different switch temperatures. At a first temperature element64switches and the bump obtains a first shape40′″ and at a second temperature element65obtains a second shape, the second shape having a greater amplitude that the first shape.

In an alternative embodiment ofFIG. 22A, the layer64is a spring element, which comprises titanium or other suitable resilient material, such that the spring element provides a force to retain or return the bump in the non-deployed position or perhaps in the deployed position. The element60is arranged such that the change in modulus of the SMM element65is capable of bending the element60into the desired shape.

In the embodiments shown all the downstream surfaces of the bumps blend out at or just upstream of the final exit plane36. Thus the exit plane itself is a smooth and in these cases circular shape. However, it is possible that the downstream surface is intersected by the nozzle exit plane particularly where the convergent-divergent is 1.00 or very close thereto.