Abstract:
An example turbofan engine sound control system includes a core nacelle ( 12 ) housing a compressor and a turbine. The core nacelle is disposed within a fan nacelle ( 34 ). The fan nacelle includes a turbofan. A bypass flow path downstream from the turbofan is arranged between the two nacelles. A controller ( 50 ) is programmed to manipulate the nozzle exit area to control sound propagating from the engine. In one example, the controller manipulates the nozzle exit area using hinged flaps ( 42 ) to control engine sound. The hinged flaps open and close to adjust the nozzle exit area and the associated bypass flow rate.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to controlling noise propagating from a gas turbine engine, and, more particularly, to controlling noise by effectively altering the nozzle exit area. 
     Gas turbine engines are widely known and used for power generation and vehicle (e.g., aircraft) propulsion. The engine produces engine noise due to the airflow moving through the engine and the various moving components within the engine. A person within the aircraft cabin may hear the engine noise. A person living near to an airport may often hear engine noise from the aircraft taking off and landing at the airport. Community noise is ordinarily defined as the aircraft noise perceivable by people located on the ground in the vicinity of the airport. Engine noise may limit an aircraft&#39;s ability to land at certain airports after certain hours, causing loss of revenue for an airline. 
     Noise from the engine primarily propagates fore and aft of the engine. The frequency content of the noise includes a tonal component and a broadband component. The fan section of the engine is a major contributor to overall engine noise, especially the tonal component. The size of the fan section relates, in part, to the desired bypass ratio for the engine, which is the ratio of fan bypass flow to core engine flow. The trend in commercial aircraft has been to increase the bypass ratio of the engine. However, increasing the bypass ratio generally requires increasing the size of the fan section within the turbofan engine, which may increase the noise contribution of the fan section. 
     What is needed is a method of optimizing engine noise for various flight conditions while maintaining engine thrust. 
     SUMMARY OF THE INVENTION 
     An example turbofan engine includes a core nacelle housing a compressor, combustor, and a turbine. A bypass flow path downstream from the fan section of the engine is a separate annular region radially outboard of and surrounding the core. A controller is programmed to manipulate the exit area of the fan nozzle to control noise propagating from the engine. In one example, the controller manipulates the fan nozzle exit area using hinged flaps to control engine noise. The hinged flaps open and close to adjust the nozzle exit area and the associated bypass flow rate. 
     Noise from the engine includes a tonal component and a broadband component. When combined with other engine parameters, such as fan speed, modifying the effective nozzle exit area enables an operator to achieve similar thrust through the bypass flow path with different overall noise levels. Further, changing the effective nozzle exit area also alters the combination of tonal and broadband components and the noise directivity. Depending on a flight condition, such as approach, cruise, or take-off, the overall level of engine noise can be optimized, as well as the combinations of the tonal and the broadband components. Directivity of the engine noise can also be controlled. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows. 
         FIG. 1  illustrates selected portions of an example gas turbine engine system. 
         FIG. 2  illustrates a variable area nozzle within the gas turbine engine system shown in  FIG. 1 . 
         FIG. 3  illustrates example lobes of fan noise extending from the gas turbine engine system shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A geared turbofan engine  10  is shown in  FIG. 1 . The engine  10  includes a core nacelle  12  that houses a low rotor  14  and high rotor  24 . The low rotor  14  supports a low pressure compressor  16  and low pressure turbine  18 . In this example, the low rotor  14  drives a fan section  20  through a gear train  22 . The fan section  20  rotates about an axis X and includes a plurality of fan blades  36 . The high rotor  24  rotationally supports a high pressure compressor  26  and a high pressure turbine  28 . A combustor  30  is arranged between the high pressure compressor  26  and high pressure turbine  28 . The low and high rotors  14 ,  24  rotate about the axis X, and at least a portion of the core nacelle  12  is disposed within a fan nacelle  34 . As is known, the engine  10  produces noise when running. 
     In the examples shown, the engine  10  is a high bypass turbofan arrangement. In one example, the bypass ratio is greater than 10:1, and the fan diameter is substantially larger than the diameter of the low pressure compressor  16 . The low pressure turbine  18  has a pressure ratio that is greater than 5:1, in one example. The gear train  22  can be any known suitable gear system, such as a planetary gear system with orbiting planet gears, planetary system with non-orbiting planet gears, or other type of gear system. It should be understood, however, that the above parameters are only exemplary of a contemplated geared turbofan engine. That is, the invention is applicable to other types of engines, including those with direct drive fans. 
     For the engine  10  shown  FIG. 1 , a significant amount of thrust may be provided by a bypass flow B between the core nacelle  12  and a fan nacelle  34  due to the high bypass ratio. Thrust is a function of density, velocity, and area. One or more of these parameters can be manipulated to vary the amount and direction of thrust provided by the bypass flow B. In one example, the engine  10  includes a nozzle structure  38  associated with the nozzle exit area A to change the physical area and geometry to manipulate the thrust provided by the bypass flow B. However, it should be understood that the nozzle exit area A may be effectively altered by other than structural changes, for example, by altering a boundary layer of the bypass flow B. Furthermore, it should be understood that effectively altering the nozzle exit area A is not limited to physical locations approximate to the exit of the fan nacelle  34 , but rather, includes altering the bypass flow B by any suitable means at any suitable location. 
     In the example shown in  FIG. 2 , an engine noise control system  54  includes multiple hinged flaps  42  arranged circumferentially about the rear of the fan nacelle  34 . The hinged flaps  42  form a portion of engine noise control system  54 , which further includes a controller  50  in communication with actuators  46  used to manipulate the hinged flaps  42 . The controller  50  also communicates with a driver  56 , which may be controlled by an aircraft operator or may operate automatically. In one example, the driver  56  monitors and communicates aircraft altitude and airspeed to the controller  50 . Based on the combination of altitude and airspeed, the controller  50  commands the actuators  46  to actuate the hinged flaps  42  and reduce engine noise for the particular combination of altitude and airspeed. The controller  50  thereby uses the altitude and airspeed of the aircraft to reduce the noise level, such as to reduce the noise level perceived in a community or within an aircraft cabin. Further, different combinations of the position of the hinged flaps  42  and the rotational speed of the fan section  20  produce similar amounts of thrust. The controller  50  may command the actuators  46  to actuate the hinged flaps  42  based on the thrust and/or rotational speed measurement of a component of the engine  10 . In so doing, the controller  50  controls engine noise while maintaining a desired thrust. 
     The hinged flaps  42  can be actuated independently and/or in groups using segments  44 . The segments  44  and individual hinged flaps  42  can be moved angularly using actuators  46 . The engine noise control system  54  thereby varies the nozzle exit area A ( FIG. 1 ) between the hinged flaps  42  and the engine  10  by altering positions of the hinged flaps  42 . In a closed position, the hinged flap  42  is closer to the core nacelle  12  for a relatively smaller nozzle exit area A. In an open position, the hinged flap  42  is farther away from the core nacelle  12  for a relatively larger nozzle exit area A. 
     When operating, the fan section  20  of the engine  10  produces sound waves that propagate as lobes of fan noise N fore and aft, as shown in  FIG. 3 . The lobes of fan noise N include both a broadband component and a tonal component. The broadband component is acoustic energy that is distributed across a range of frequencies, whereas the tonal component is acoustic energy focused within a narrow range of frequencies. The fan section  20  is the major contributor to the overall tonal component of noise propagating from the engine, although other portions of the engine  10 , such as the compressor  16  and turbine  18 , may contribute at certain conditions. In this example, the rotating portions of the fan section  20  generate the tonal component at approximately 1000 Hz. Although only fan noise N is shown in this example, many portions of the engine  10  (e.g., the combustor  30 , the rotors  14  and  24 ) contribute to the overall engine noise. Each portion has an associated intensity and directivity, and each portion is similarly modifiable with the invention. Although the example engine  10  includes hinged flaps  42  on the fan nacelle  34 , other portions of the engine  10  may include hinged flaps  42 , such as the core nacelle  12  ( FIG. 1 ). Positioning hinged flaps  42  on the core nacelle  12  may control noise from the compressors  16 ,  26 ; the combustor  30  and/or the turbines  18 ,  28 . 
     Although the engine  10  in the example embodiment produces engine noise, those skilled in the art and having the benefit of this disclosure will understand that engine noise is not limited to uncomfortable levels of sound produced by the engine  10 . That is, the disclosed example may be used to control various levels of sound from the engine  10 . 
     Fan noise N extends from the engine  10  in all directions, but the highest concentrations extend in the area of these lobes. When seated in an aircraft cabin, the directivity angle of an aircraft passenger relative to the engine  10  is fixed. If the seated position of the passenger is not within the fan noise N lobes, the passenger may not perceive uncomfortable levels of fan noise N from the engine  10 . As an example, a passenger seated toward the front of an aircraft cabin may be positioned within the fan noise N lobe extending forward from the engine and, more specifically, seated at an angle of about 50 degrees relative to the axis X. Such a passenger would experience a relatively large amount of fan noise N within the cabin. Altering the effective nozzle area A alters the intensity and the position of the lobes of fan noise N. As such, the effective nozzle area A may be adjusted to direct the peak of fan noise N away from the passenger seated toward the front of the cabin, as well as lessen the intensity. 
     Regarding the lobes of fan noise N extending rearward from the engine  10 , airflow communicating through the engine  10  experiences a wake deficit, or non-uniform flow, after moving over the plurality of fan blades  36 . Each fan blade  36  creates a wake deficit, or pocket of lower velocity airflow. Stators  40 , placed in the bypass flow path B, streamline the airflow and remove the swirl from the airflow through the bypass flow path B. Airflow over the stators  40  may have a vortex flow pattern, but the stators  40  straighten the airflow such that the airflow has a substantially axial flow pattern when communicating through the nozzle exit area A. 
     The wake deficits from the rotating fan blades  36  cause a time-dependent variation of pressure on the stators  40 , which in turn generates the tonal component of the fan noise N propagating aft of the engine  10 . Modifying the effective nozzle exit area A affects the structure of the wake deficits from the fan blades  36  and the associated fan noise N. As a result, an operator can modify the effective nozzle exit area A to change the associated fan noise N. 
     Modifying the effective nozzle exit area A increases the potential operating points for an engine  10  that are capable of achieving similar levels of thrust through the bypass flow path B. As a result, the operating point of the engine  10  can be tuned to facilitate overall noise reduction. As an example, a typical cruising altitude for an aircraft is about 35,000 feet. Different combinations of effective nozzle exit area A and fan section  20  speed and other engine  10  parameters may produce the same desired airspeed at this altitude. As a result, the operator is free to choose the combination of nozzle exit area A and fan section  20  speed to control overall perceived engine noise while maintaining required thrust. Because of the altitude, community noise is not an issue, thus the specific conditions may be further refined to control cabin noise. 
     In another example, during the climbing flight stage, many sizes of the effective nozzle exit area A produce desired thrust. Thus, the effective nozzle exit area A can be sized to minimize noise from the engine  10 . During climb, community noise remains a factor especially at lower altitudes, thus the effective nozzle exit area may be sized to minimize the tonal component propagating from the engine  10 , as the tonal component is an undesirable contributor to community noise. Thus, modifying the effective nozzle exit area A affects perceived noise from the engine  10  and provides a degree of freedom for designers or operators to control noise N, and the noise level may be reduced for the particular flight stage, e.g., take-off, climb, cruise, descent. 
     Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art may recognize certain modifications falling within the scope of this invention. For that reason, the following claims should be studied to determine the true scope of coverage for this invention.