Abstract:
A nacelle exhaust nozzle having a deployable noise-reducing component is described. The noise-reducing component includes an annular perforated sleeve coinciding with the inner nacelle loft line and circumscribing a mixed exhaust gas flow exiting the nacelle. The annular perforated sleeve is radially spaced apart from of a displaceable wall of an inflatable envelope that is displaceable between a deployed position, wherein noise-reduction is active, and a retracted position, wherein noise-reduction is inactive. When the inflatable envelope is pressurized, portions of the displaceable wall project through openings in the perforated sleeve and into the exhaust gas flow to form a rough surface at the loft line which causes a reduction in noise level. The portions of the displaceable wall that project through the openings in the perforated sleeve when the inflatable enveloped is pressurized include a plurality of dimples formed on the inner wall and forming the rough surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/407,055 filed on Feb. 28, 2012, the entire content of which is incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to gas turbine engines and more particularly to exhaust noise reduction. 
       BACKGROUND 
       [0003]    The exhaust jet of a gas turbine engine remains a significant noise source, particularly at high power conditions, despite the use of high by-pass ratios in modern aircraft engines which has brought several significant benefits particularly in fuel efficiency and noise. Jet noise in a turbofan engine is caused by the interaction of the air streams within the engine and also with the surrounding air. The jet noise comprises turbulent mixing noise, which is at least in part caused by: a) mixing of the core and bypass flow streams; and b) mixing of the mixed stream with the ambient air creating a shear layer. The turbulent shear flow mixing includes two types of noise, the first caused by fine scale turbulence and the second caused by large scale eddies. 
         [0004]    The prior art proposes reducing noise levels by modifying the exhaust nozzle area, such as by including bumps or wave-like surfaces or by adding chevron shaped tabs on the exhaust nozzle. 
       SUMMARY 
       [0005]    There is accordingly provided a gas turbine engine comprising an engine core and an annular by-pass duct defined between the engine core and a surrounding nacelle, the nacelle having an exhaust nozzle at an aft end thereof through which exhaust gas flow exits, the nacelle exhaust nozzle having a deployable noise-reducing component therewithin, the noise-reducing component including an annular perforated sleeve positioned to coincide with the inner nacelle loft line and circumscribing a mixed exhaust gas flow exiting the nacelle, the annular perforated sleeve being radially spaced apart from of a displaceable wall of an inflatable envelope, the displaceable wall being radially displaceable between a deployed position wherein noise-reduction is active and a retracted position wherein noise-reduction is inactive, the displaceable wall being displaced from the retracted position to the deployed position when the inflatable envelope is pressurized thereby forcing portions of the displaceable wall to project through openings in the perforated sleeve and into the exhaust gas flow to form a rough surface at the loft line of the nacelle exhaust nozzle which causes a reduction in the noise level of the gas turbine engine, wherein the portions of the displaceable wall that project through the openings in the perforated sleeve when the inflatable enveloped is pressurized include a plurality of dimples formed on the inner wall, the plurality of dimples being aligned with and protruding through the openings in the perforated sleeve to form said rough surface. 
         [0006]    There is also provided a gas turbine engine comprising an engine core and an annular by-pass duct defined within a surrounding nacelle, the nacelle having an exhaust nozzle which includes a selectively deployable noise-reduction section on an inner surface thereof, the noise-reduction section defining an inflatable envelope comprising a fixed outer wall, a displaceable inner wall radially inward of the fixed outer wall, and an annular perforated sleeve radially inward of the inner wall and facing an exhaust gas flow exiting the exhaust nozzle, the annular perforated sleeve defining a plurality of spaced-apart openings therein, at least a portion of the inner wall being radially inwardly displaced when the envelope is pressurized and retracted radially outwardly when the envelope is de-pressurized, wherein when the cavity is pressurized to inflate the inflatable envelope, a plurality of dimples on the inner wall protrude through the openings in the perforated sleeve and into the exhaust gas flow to form a rough surface on the inner surface of the nacelle exhaust nozzle and thereby reducing a level of jet noise produced, and wherein when said inner wall is retracted upon depressurization of the inflatable envelope, the dimples are to withdrawn from the exhaust gas flow via the openings in the perforated sleeve. 
         [0007]    There is further provided a method for reducing the level jet noise produced by a gas turbine engine having a engine nacelle from which an exhaust gas flows, comprising pressurizing an inflatable portion of an exit nozzle of the engine nacelle to displace an inner wall of the inflatable portion radially inwardly such that projecting portions thereof protrude into the exhaust gas flow at the loft line of the nacelle so as to form a rough surface at the loft line to thereby reduce the exhaust velocity as it mixes with ambient free air shear surrounding the nacelle and thus reduce the level of jet noise produced, wherein the projecting portions that protrude into the exhaust gas flow when the inflatable portion is pressurized include dimples formed on the inner wall; restricting the pressurized air to the inflatable portion at a predetermined altitude so that flight will proceed with the projecting portions retracted from the loft line and withdraw within the exit nozzle of the engine nacelle; and initiating the pressurized fluid to the inflatable portion in preparation for landing thereby re-forming the rough surface within the exit nozzle of the engine nacelle to thereby reduce the level of jet noise produced during landing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Reference is now made to the accompanying figures in which: 
           [0009]      FIG. 1  is a schematic cross-sectional view of a turbofan engine; 
           [0010]      FIG. 2  is a schematic cut-away view of a turbofan engine exhaust section; 
           [0011]      FIG. 3 a    is a fragmentary axial cross-section of a detailed shown in  FIG. 2 ; 
           [0012]      FIG. 3 b    is a fragmentary axial cross-section similar to  FIG. 3 a   , showing the detail in a different position; 
           [0013]      FIG. 4 a    is a schematic fragmentary axial cross-section of another embodiment of the detailed shown in  FIG. 2 ; 
           [0014]      FIG. 4 b    is a schematic fragmentary axial cross-section similar to  FIG. 4 a   , showing the detail in a different position; and 
           [0015]      FIG. 4 c    is an enlarged plan view of a detail shown in  FIG. 4   b;    
           [0016]      FIG. 5 a    is a schematic view of a further embodiment thereof; 
           [0017]      FIG. 5 b    is an exploded view of a detail shown in  FIG. 5   a;    
           [0018]      FIG. 5 c    is a schematic view of the detail shown in  FIGS. 5 a  and 5 b    in a different operative position; and 
           [0019]      FIG. 5 d    is an exploded view of the detail shown in  FIG. 5 c      
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  schematically depicts a gas turbine engine  10 , such as a turbofan for example, of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a multistage compressor  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  including at least one turbine for extracting energy from the combustion gases. 
         [0021]    Bypass duct  21 , defined within the surrounding engine nacelle  20 , may define an annular passage (e.g. defining bypass gas path  23 ) through which some of the airflow through engine  10  to bypass the core section  9  of engine  10 . Although the illustrated gas turbine engine  10  of  FIG. 1  is as a turbofan engine, it is understood that the devices, assemblies and methods described herein can also be used in conjunction with other types of gas turbine engines such as, for example, turboshaft and/or turboprop engines. 
         [0022]    The turbofan engine  10  includes a core exhaust nozzle  22  located downstream of the turbine  18  at the aft of the engine, which is at least partially surrounded by the annular by-pass duct  21  through which the by-pass air  23  flows. The core engine gas flow  19  from the engine exhaust nozzle  22  meets and mixes with the bypass air flow  23  from the bypass duct  21  within the exhaust nozzle  24  of the nacelle  20  to form a mixed exhaust gas flow  27 . This mixed exhaust gas flow then exits the nacelle  20  of the engine  10  and mixes with the surrounding ambient free air stream  25 . 
         [0023]    As seen in both  FIG. 1  and  FIG. 2 , the aft portion of the nacelle  20  which defines the exhaust nozzle  24  includes an annular section  26  therewithin which circumscribes and faces the mixed exhaust gas flow (i.e. the mixed core engine exhaust flow  19  and the bypass air flow  23 ) and which, as will be described in more detail below, is said to be “expandable” in that it permits a flow-modifying element to be selectively deployed such as to produced radially into the mixed exhaust gas flow, when desired, in order to reduce the level of jet noise produced by the exhaust gas flow from the engine  10 . This annular expandable section  26  of the exhaust nozzle  24  of the engine  10  provides, as will be seen, a surface texture which can deployed and retracted or otherwise modified, when required, in order to actively control the flow within the exhaust nozzle and thus control a desired amount of noise level reduction. For example, because the amount of jet noise is particularly undesirable at takeoff and landing, the expandable section  26  of the exhaust nozzle  24  may be deployed into an active position thereof for takeoff and landing, but retracted during flight at cruising altitude when jet noise is less problematic. It is also of note that the expandable section  26  of the nacelle exhaust nozzle  24  could constitute part of a thrust reverser assembly of the gas turbine engine  10 . 
         [0024]    Referring now to  FIGS. 3 a  and 3 b    which depict a first embodiment of the selectively deployable noise-reduction section  26  of the gas turbine engine exhaust nozzle  24 , an annular hollow metal envelope  28  forms part of the nacelle  20  and includes an inlet  29  for pressurizing the envelope  28  at an upstream end of the envelope  28 . The envelope  28  includes a fixed outer circumferential wall  32  and a displaceable inner circumferential wall  34  which are substantially concentric and define therebetween and annular cavity  33  which is in fluid communication with the inlet  29  and which is adapted to be pressurized. A perforated sleeve  30  having a plurality of perforations or openings  35  is located radially inwardly of the inner wall  34  in the loft line of the nacelle  20 . The inner wall  34  is provided with a plurality of flow obstructing elements, such as pins  36 , thereon which are aligned with the openings in the perforated sleeve  30  and sized such as to be received within and insertable through the openings  35 . Accordingly, when the envelope  28  is in its non-pressurize state (i.e. the annular cavity  33  is not pressurized), the pins  36  do not project beyond the sleeve  30 , as shown in  FIG. 3   a.    
         [0025]    When noise reduction is desired, such as on takeoff or landing, the envelope  28  of the noise-reduction section  26  of the nacelle exhaust nozzle  24  is pressurized by allowing high pressure air to enter into the annular cavity  33  via the inlet  29 . The high pressure air may come from the engine or from an independent source. The pressurization may also be provided by using oil pressure or other hydraulic systems. This pressurization of the cavity  33  of the envelope  28  will cause the inner wall  34  to move radially inwardly so that the inner wall  34  abuts the perforated sleeve  30  and the pins  36  extend through the openings  35  in the perforated sleeve  30  and protrude into the mixed exhaust airflow  27  which exists the exhaust nozzle  24  of the nacelle  20 , as shown in  FIG. 3 b   . This constitutes the “active” state of the selectively-deployable noise-reduction section  26 . 
         [0026]    When the noise-reducing section  26  is in the active state as seen in  FIG. 3 b   , the protruding pins  36  thereby form a rough surface on the inner surface of the exhaust nozzle  24 , which increases the boundary layer thickness at the inner loft line, thereby slowing down the exhaust stream close to this inner aft surface of the exhaust nozzle  24 . Since the mixing phenomenon between the mixed engine exhaust flow  27  and the ambient free stream air  25  is initiated by the velocity differential between the two streams, a reduction in the velocity of the mixed exhaust gas stream  27  will reduce the velocity differential therebetween, thereby reducing the mixing, and thus reducing the noise generated by this mixing. It is noted however that this operation will result in an increase in drag and fuel consumption. Accordingly, the noise-reducing section  26  is only selectively deployable, rather than being permanently deployed in its active (i.e. noise-reducing) mode, such that the flow disrupting pins  36  only protrude into the exhaust gas flow  27  when noise reduction is desired, but can be withdrawn when reduced drag and fuel consumption are desired. This is achieved, as noted above, by pressurizing the cavity  33  within the envelope  28 , which allows the crew to deploy the pins  36  of the noise-reducing section  26  of the exhaust nozzle  24  to create the rough boundary surface only when the noise level is considered undesirably high, for example, when the aircraft is near airport buildings or near populated areas, such as at takeoff and landing. When the aircraft is at higher elevations, the engine noise level is less conspicuous and not noticeable to the passengers within the aircraft, such that the pins  36  may be retracted, thereby creating a smooth surface at the nacelle loft line, thus reducing the drag during cruising altitudes. This will also restore reduced fuel consumption. 
         [0027]    In another embodiment as shown in  FIGS. 4 a  and 4 c   , the noise-reducing section  126  operates similarly to that described above and shown in  FIGS. 3 a   - 3   b,  however in this alternate embodiment the noise-reducing section  126  comprises dimples  136 , in lieu of the previously described pins  36 , which can be deployed such that they protrude into the exhaust gas flow  27  (as seen in  FIG. 4 b   ) and therefore act to reduce the noise produced or can be retracted (as seen in  FIG. 4 a   ) such as to reduce drag and fuel consumption but not jet noise levels. As seen in  FIGS. 4 a  and 4 b   , the dimples  136  may be formed by the deformation of the inner wall  134  when the envelope  128  is pressurized. In this embodiment, the inner wall  134  may be formed of a flexible material which allows elastic deformation, within the openings  135 , when the cavity within the envelope  128  is pressurized thereby forcing the inner wall  135  inward to form the radially protruding dimples  136  when in the device is in the active mode. The dimples  136  thus protrude through the openings  135  in the perforated sleeve  130  to provide the rough boundary layer surface in the mixed exhaust gas flow  27  exiting from the exhaust nozzle  24 . As seen in  FIG. 4 c   , the dimples  136  in their deployed or active mode may be provided in a staggered configuration across the surface of the noise-reducing section  126  within the exhaust nozzle  24  of the nacelle  20 , such as to produce the rough surface of the inner loft line which reduces jet noise. 
         [0028]    When the engine is started, and the engine control instruments detect a “weight-on-wheels”, air is supplied to the inlet port  29  of the envelope  28 ,  128  of the noise-reducing section  26 ,  126  within the exhaust nozzle  24  or thrust reverser, thus pressurizing and/or inflating the envelope  28 ,  128  and thereby causing the pins  36  or the dimples  136  to project through the perforated sleeve  30 ,  130  and into the exhaust gas flow  27 . These projecting pins or dimples  136  accordingly act as flow disturbing elements which provide a rough boundary layer surface which results in relatively lower noise levels being produced. The envelope  28 ,  128  will remain pressurized and/or inflated, with the pins  36  or dimples  136  deployed in their “active” mode, throughout the taxiing and at least through the initial part of the takeoff, thereby providing a reduction in the mixing noise at the critical sideline and flyover conditions. Upon reaching a certain altitude, whereby noise is less of a concern, the air supply to the inlet  29  is restricted or otherwise stopped and the residual air within the envelope  28 ,  128  is discharged through an exit port  40  (see  FIGS. 3 a -3 b   ). The exit port may be located anywhere on the nacelle. This allows for the retraction of the protruding pins  36  or dimples  136  back into the envelope  28 ,  128  (as seen in  FIGS. 3 a  and 4 a   ) thereby leaving the inner surface of the perforated sleeve  30 ,  130  relatively smooth, which reduces the drag and fuel consumption. 
         [0029]    In a further embodiment, as shown in  FIGS. 5 a  to 5 d   . The wall  234  of the exhaust nozzle  24  is structurally supportive of the nozzle eliminating the need for sleeve  30 . A plurality of inflatable tubes  230  are integrated into the inner wall  234 . Flaps  236  are hinged at  237 , just upstream of the inflatable tubes  230 . The flaps  236 , when retracted, lie in the loft line. In order to deploy the flaps  236 . A pressurized fluid is injected into the tubes  230 , thus expanding the tubes  230  and causing the flaps  236  to deploy thus forming a roughened surface at the loft line. At the opportune time, the tubes  236  are depressurized, thus allowing the flaps  236  to be retracted, by the force of the exhaust gases, against the inner wall  234 . 
         [0030]    When the landing gear is deployed upon descent of the aircraft, the air supply to the inlet port  29  is once again resumed and the envelope  28 ,  128  is again pressurized and/or inflated such as to deploy the projecting pins  36  or dimples  136  through the perforated sleeve  30 ,  130  and thereby providing noise reduction under approach conditions. As noise reduction is more desirable at lift-off and landing than the resultant penalties in increased drag and fuel consumption, the increased fuel consumption is acceptable given the improvements in noise levels. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.