Abstract:
An engine assembly, an acoustical liner and an associated fabrication method are provided to address fan blade flutter and fan noise control simultaneously within the same liner area. Fan blade flutter is therefore controlled without necessarily increasing the weight of the engine, impairing the structural integrity of the engine, or increasing the noise generated by the engine. The acoustical liner may have additional acoustical degrees of freedom which permit these seemingly competing concerns to be addressed in a complementary manner. The acoustical liner may include inner and outer barrels with the inner barrel having a perforated face sheet, a perforated back skin and a core disposed between the perforated face sheet and the back skin. The fluid communication between the core and the space between the inner and outer barrels provides additional acoustical degrees of freedom which may be utilized to reduce fan blade flutter while concurrently limiting fan blade noise.

Description:
FIELD OF THE INVENTION 
     Embodiments of the present invention are directed generally to gas turbine engine assemblies and, more particularly, to an improved acoustical liner and an associated method of fabrication for providing passive control of both fan blade flutter and fan noise. 
     BACKGROUND OF THE INVENTION 
     Engines, such as aircraft engines, can generate significant noise. Such noise may be undesirable in populated areas and in other environments in which noise is desirably controlled. As such, acoustical liners for inlets, fan cases, fan nozzles, and other engine installation structures have been developed to reduce the amount of noise emanating from an engine. The acoustical liners generally are disposed within the nacelle of the aircraft engine. 
     In a turbofan engine, for example, the portion of the inlet portion of the nacelle forward of the fan includes inner and outer barrels separated by an air-filled space. In order to reduce the noise emanating from the engine, the inner barrel can incorporate an acoustical liner. An acoustical liner generally includes a cellular or honeycomb core positioned between a face sheet which faces the air flowing toward the fan and a back skin which faces the outer barrel. The face sheet may be perforated such that some of the acoustic air flow to or through the fan enters the honeycomb core through the perforations in the face sheet. As a result of the interaction of a portion of the air flowing to or through the fan with the honeycomb core, the noise emanating from the engine is reduced. In order to further reduce the resulting noise, a septum may be disposed within the honeycomb core. By controlling the size and number of the perforations as well as the volume of air within the honeycomb cells and the properties of the septum, the performance of the acoustical liner can be tuned to reduce noise in a particular frequency range. In this regard, the perforations through the face sheet provide acoustical inertia, while the volume of air contained in the honeycomb cells provide acoustical compliance, thereby providing a dynamic system with a limited number of acoustical degrees of freedom. 
     In addition to the noise generated by an engine, another issue associated with turbofan engines is fan blade flutter which may reduce the useful lifetime of the fan blades and, in some situations, may cause the fan blades to fail. In instances in which the fan blades are anticipated to flutter, the fan blades are generally scheduled to be inspected on a more frequent basis, and the lifetime of the fan blades is typically limited relative to fan blades that are not anticipated to flutter. In an effort to eliminate or reduce fan blade flutter, the fan nozzle geometry, that is, the converging/diverging characteristics of the nozzle, may be changed. However, such changes are constrained by the thrust requirements for the engine, and may disadvantageously add to the weight of the aircraft or undermine the structural integrity of the nacelle. Because fan blade flutter involves interaction between vibratory motion of the fan blades and vibratory motion of the surrounding fluid, it is in part an acoustical phenomenon, and the propensity of fan blades to flutter can be changed by manipulating the acoustical frequency response of the nacelle structures. Since the frequency at which fan blades flutter is different from the frequency of fan noise, efforts intended to modify the acoustical liner to improve fan blade flutter margin may be in conflict with the frequency response of the acoustical liner needed to reduce fan blade noise, thereby potentially leading to an increase in the fan blade noise. Discontinuities in acoustical frequency response between regions of the nacelle structure tuned to different frequencies for noise control and fan blade flutter control, respectively, can cause significant increases in noise. For example, acoustical resonators tuned to the fan blade flutter mode frequencies may be installed forward of the fan in an effort to reduce fan blade flutter. However, these acoustical resonators may conflict with the tuning of the inlet in regard to fan blade noise and result in a disadvantageous increase in the fan noise. 
     Accordingly, it would be desirable to provide for a mechanism for reducing or eliminating fan blade flutter without meaningfully increasing the weight of the engine, impairing the structural integrity of the engine and/or nacelle structures, or causing an increase in fan blade noise. 
     BRIEF SUMMARY OF THE INVENTION 
     An engine assembly, an acoustical liner and an associated method of fabricating an acoustical liner are therefore provided according to embodiments of the present invention in order to address at least some of the issues associated with conventional designs. In this regard, embodiments of the present invention provide an acoustical liner which permits fan blade flutter to be addressed without necessarily increasing the weight of the engine, impairing the structural integrity of the engine and/or nacelle structures, or increasing the noise generated by the engine. In this regard, embodiments of the present invention provide an acoustical liner having additional acoustical degrees of freedom which permit these otherwise sometimes competing concerns to be addressed in a complementary manner. As such, embodiments of the present invention permit simultaneous tuning of the acoustical frequency response of the nacelle structures to both fan noise control and fan blade flutter control frequencies over the same areas, thereby avoiding discontinuities in acoustical impedance. 
     In one embodiment, an acoustical liner is provided which includes an inner barrel having a perforated face sheet, a back skin comprising a plurality of perforations and a core, such as a cellular or honeycomb core, disposed between the perforated face sheet and the back skin. The acoustical liner of this embodiment also includes an outer barrel surrounding and spaced from the inner barrel to define a space therebetween. As a result of its design, the perforations through the inner barrel back skin place the core, such as the honeycomb cells in the core, in fluid communication with the space between the inner and outer barrels. As such, the fluid communication between the core and the space between the inner and outer barrels provides additional acoustical degrees of freedom which may be utilized to simultaneously tune the liner to frequencies needed to reduce fan blade flutter and to frequencies needed to control fan blade noise. 
     In another embodiment, an engine assembly is provided. The engine assembly includes a fan, a compressor downstream of the fan, a combustion section downstream of the compressor and a turbine downstream of the combustion section. The engine assembly of this embodiment also includes an acoustical liner forward of and surrounding the fan, including inner and outer barrels of the nacelle inlet and/or the engine fan case. The inner barrel includes a core and a back skin disposed on the core and including a plurality of perforations. The outer barrel surrounds and is spaced from the inner barrel to define an air-filled space therebetween. As before, the perforations through the inner barrel back skin place the core such as the honeycomb cells of the core, in fluid communication with the space between the inner and outer barrels thereby providing for additional acoustical degrees of freedom for the acoustical liner. 
     In either embodiment, the acoustical liner may also includes at least one extension tube extending from the back skin into the space between the inner and outer barrels. The extension tube is in fluid communication with a perforation through the back skin, thereby increasing the acoustical inertia and reducing one or more natural frequencies of the acoustical liner. Additionally or alternatively, the acoustical liner may include a baffle extending from the back skin into the space between the inner and outer barrels with the baffle positioned such that at least one perforation defined by the back skin is in fluid communication with the baffle. The baffle serves to increase the acoustical stiffness of the acoustical liner, thereby tuning the acoustical liner to have higher natural frequencies. The acoustical liner and, in particular, the inner and outer barrels may be configured to have at least one natural frequency within a frequency range associated with fan blade flutter and at least one natural frequency within a frequency range associated with fan noise and fan blade-pass frequencies. As such, the additional acoustical degrees of freedom permit these natural frequencies to be tuned to concurrently address both fan blade flutter and fan noise. 
     A core of one embodiment may include a plurality of honeycomb cells. As such, the perforations defined by the face sheet and the back skin may open into respective honeycomb cells. The core may also include a septum. 
     In accordance with another embodiment, a method of fabricating an acoustical liner is provided in which an inner barrel comprising a perforated face sheet, a back skin and a core disposed between the perforated face sheet and the back skin is provided. The inner barrel is positioned within an outer barrel which surrounds and is spaced from the inner barrel to define a space therebetween. A natural frequency of the acoustical liner is then tuned by perforating the back skin to place the core in fluid communication with the space between the inner and outer barrels. 
     As a result of the fabrication of the acoustical liner, one or more acoustical natural frequencies may be reduced in one embodiment by extending at least one extension tube from the back skin into the space between the inner and outer barrels with the extension tube remaining in fluid communication with the perforation defined by the back skin. Conversely, one or more natural frequencies may be increased by extending a baffle from the back skin into the space between the inner and outer barrels with the baffle being positioned such that at least one perforation defined by the back skin is in fluid communication with the baffle. In this regard, the tuning of the natural frequency may include tuning at least one natural frequency to be within a frequency range associated with fan blade flutter and tuning at least one natural frequency to be within a frequency range associated with fan noise at fan blade-pass frequencies. As such, the acoustical liner and associated engine assembly of embodiments of the present invention can be fabricated and tuned to concurrently address both fan blade flutter and fan noise, typically without meaningfully increasing the weight or sacrificing structural integrity. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  is a schematic representation of an engine including an acoustical liner in accordance with one embodiment of the present invention; 
         FIG. 2  is a cross-sectional representation of a portion of an acoustical liner in accordance with one embodiment of the present invention; 
         FIG. 3  is a cross-sectional representation of the portion of the acoustical liner depicted in  FIG. 2  with the addition of an extension tube; and 
         FIG. 4  is a cross-sectional representation of the portion of the acoustical liner depicted in  FIG. 2  with the addition of a baffle. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present inventions now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all, embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. 
     An engine assembly  10  according to one embodiment to the present invention is depicted in  FIG. 1 . Although described below in the context of an aircraft engine, the engine assembly can be employed in a variety of different applications, if so desired. As shown, the engine  10  may be a turbofan engine. In the illustrated embodiment, the engine includes two shafts, while other engines may include a single shaft or three or more shafts. As shown in  FIG. 1 , in instances in which the engine  10  is an aircraft engine, the engine is generally housed in a casing or cowl. The casing or cowl that is then disposed within a nacelle, and the engine and nacelle structures are mounted via a pylon to the wing or fuselage of an aircraft. 
     In accordance with embodiments of the present invention, the engine assembly  10  also includes an acoustical liner  24  which surrounds the fan  12 . In this regard, the acoustical liner may be disposed within the inlet  14  to the engine and/or may be disposed about the fan case  16 . However, the acoustical liner can be positioned elsewhere within the engine assembly including downstream of the fan, if desired. As described below, the acoustical liner has multiple acoustical degrees of freedom which permit the acoustical impedance of the liner to be tuned to have at least one natural frequency within a frequency range associated with fan blade flutter and at least one natural frequency within a frequency range associated with fan noise at fan blade-pass frequencies. As such, the acoustical liner can address the otherwise potentially competing issues associated with control fan blade flutter and fan noise without unnecessarily adding to the weight, introducing area discontinuities in acoustical liner frequency response or reducing the structural integrity of the engine or nacelle structures. 
     As shown in more detail in  FIG. 2 , the nacelle inlet  24  generally includes inner and outer barrels  26 ,  28 . Each barrel generally extends circumferentially about the longitudinal axis  30  of the engine  10  and, more particularly, about the fan  12  in one embodiment. The inner and outer barrels also extend in a lengthwise direction within the inlet or about the fan case. Further, the upstream and downstream ends of the inner and outer barrels are generally sealed or joined, such as by annular end members to thereby define an annular space  32  between the inner and outer barrels. 
     The inner barrel  26  generally includes a circumferentially extending face sheet  34 , a circumferentially extending back skin  36  and a core  38  positioned between the face sheet and the back skin. The face sheet and the back skin of the inner barrel, as well as the outer barrel  28 , may be formed of various materials including a metallic material, such as aluminum, or a laminated composite material, such as a carbon or glass reinforced plastic material. The core generally includes a plurality of honeycomb cells  40 . Additionally, the core may include a septum  42  extending through the plurality of honeycomb cells. The core, including the honeycomb cells and the septum, may also be formed of various materials including a metallic material, such as aluminum, or a composite material, such as a NOMAX® material available from E. I. du Pont de Nemours and Company. 
     The face sheet  34  may define a plurality of perforations  44  that open into and are in fluid communication with respective honeycomb cells  40 . As such, air flowing to or through the fan  12  may enter the core through the perforations, thereby reducing engine noise. The interaction of the perforated face sheet and the core provide one or more acoustical degree of freedom with acoustical inertia provided by the perforations through the face sheet and acoustical compliance provided by the volume of air contained within the honeycomb cells. The natural frequency of this system may be tuned to give noise reduction in the desired frequency band. 
     In accordance with embodiments of the present invention, the back skin  36  of the inner barrel  26  may also be perforated. In the illustrated embodiment, the back skin is perforated such that a perforation  46  is defined which opens into each honeycomb cell  40  of the core  38 . However, the back skin of other embodiments may define more or fewer perforations. As shown, the perforations defined by the back skin place the honeycomb cells into which the perforations open in fluid communication with the space  32  between the inner and outer barrels  26 ,  28 . The perforations defined by the back skin therefore provide one or more additional acoustical degrees of freedom having acoustical inertia due to the perforation through the back skin of a finite thickness and acoustical compliance due to the additional fluid communication with the air within the space between the inner and outer barrels. 
     By appropriately sizing and spacing the perforations  46  defined by the back skin  36 , a natural frequency of the acoustical liner  24  may be deigned to be within a frequency range associated with fan blade flutter, thereby effectively reducing fan blade flutter. In this regard, increasing the size of the perforations generally decreases the natural frequency of the acoustical liner, while decreasing the size of the perforations generally increases the natural frequency of the acoustical liner. Additionally, decreasing the spacing between the perforations generally decreases the natural frequency of the acoustical liner, while increasing the spacing between the perforations generally increases the natural frequency of the acoustical liner. Moreover, by providing a perforated back skin which permits the space  32  between inner and outer barrels  26 ,  28  to be in fluid communication with the core  38 , the acoustical liner of embodiments of the present invention has additional acoustical degrees of freedom relative to a conventional acoustical liner, thereby permitting the acoustical liner to be tuned to have multiple natural frequencies which appropriately align with the different frequency ranges associated with fan blade flutter and with fan noise. In this regard, the size and spacing of the perforations of the face sheet  34  and the back skin of the inner barrel may be configured such that the resulting acoustical liner has at least one natural frequency within a frequency range associated with fan blade flutter, such as 120 to 150 Hertz, and at least one natural frequency within a frequency range associated with fan noise in a fan blade-pass frequency, such as 800 to 1000 Hertz, thereby concurrently addressing issues associated with both fan blade flutter and fan noise. 
     In order to further tune the natural frequencies of the acoustical liner  24 , the acoustical liner may include one or more extension tubes  48  and/or baffles  50 , as shown in  FIGS. 3 and 4 , respectively. The extension tubes and baffles can be formed of various materials, such as a metallic material, e.g., aluminum, or a composite or plastic material. With reference to  FIG. 3 , for example, the acoustical liner may include one or more extension tubes which extend from the back skin  36  of the inner barrel  26  into the space  32  defined between the outer and inner barrels  26 ,  28 . Each extension tube may define a passageway that is associated with and in fluid communication with one or more perforations  46  defined by the back skin. For example, each extension tube of the embodiment of  FIG. 3  is in fluid communication with a single respective perforation and, in turn, with a single respective honeycomb cell  40 . The extension tubes effectively increase the acoustical inertia without any meaningful increase in the acoustical stiffness of the liner, thereby reducing the natural frequencies of the liner. As such, there is generally an inverse relationship between the changes in the natural frequencies of the liner to the length of the extension tubes with longer extension tubes generally reducing the natural frequencies of the liner more so than shorter extension tubes. The tubes in the embodiment shown in  FIG. 3  are straight. In general, the tubes could be formed into any shape to permit tubes of the desired length to be made to fit in the space available between the inner and outer barrels. 
     As shown in  FIG. 4 , an acoustical liner  24  of one embodiment may include one or more baffles  50  which extend from the back skin  36  into the space  32  between the inner and outer barrels  26 ,  28 . While the acoustical liner of the illustrated embodiment includes only a few baffles, the acoustical liner could include any plurality of baffles, if so desired. Each baffle at least partially encloses one or more perforations  46 . In this regard, each baffle generally includes an opening  52  into the remainder of the space defined between inner and outer barrels, but otherwise defines a relatively closed region  54  that is substantially smaller than the space between inner and outer barrels. The baffles tend to increase the acoustical stiffness of the liner, thereby increasing the natural frequencies of the liner. As such, there is generally an inverse relationship between the change in the natural frequencies of the liner to the volume defined or contained within a baffle with smaller baffles generally increasing the natural frequencies of the liner more so than larger baffles. The baffles can take on any shape desired to provide the desired volume. 
     While the acoustical liner  24  of the illustrated embodiment includes a baffle  50  which is in fluid communication with two perforations  46  defined by the back skin  36 , each baffle may be in fluid communication with any number of perforations, such as a single perforation or three or more perforations. Moreover, in embodiments which include a plurality of baffles, the baffles may be in fluid communication with different numbers of perforations, if so desired. As a result of the various configurations of the baffles, the flexibility with which the natural frequencies of the liner may be tuned may be even further increased. 
     Although  FIGS. 3 and 4  illustrate acoustical liners  24  which include extension tubes  48  and baffles  50 , respectively, the acoustical liner of one embodiment may include both extension tubes and baffles if further acoustical degrees of freedom are desired. By appropriate designing the acoustical liner, such as by appropriately sizing and spacing the perforations  46  defined by the back skin  36  and, in some embodiments, by including one or more extension tubes and/or one or more baffles, the natural frequencies of the acoustical liner can be tuned to the desired frequency range. In this regard, the natural frequencies of the acoustical liner can be tuned to have at least one natural frequency within a frequency range associated with fan blade flutter and at least one natural frequency to be within a frequency range associated with fan noise at fan-blade pass frequencies. As such, the acoustical liner of embodiments to the present invention can concurrently reduce fan blade flutter and fan noise with acoustical liner material over a given area. This avoids area discontinuities in the acoustical response of the liner, which are detrimental to both noise control and fan blade flutter control performance. Moreover, the acoustical liner can address both fan blade flutter and fan noise without meaningfully, if at all, adding to the weight of the engine  10  and without reducing the structural integrity of the engine or nacelle structures. 
     In the embodiments described above, the tuning is primarily accomplished by selection of the size and number of perforations  44 ,  46  in the inner and outer face sheets  34 ,  36  of the inlet inner barrel  26 , by the thickness of the inner and outer face sheets (usually constrained by strength and weight considerations), and by the volume of the core  38 , such as the cells  40  in a honeycomb core. However, the tubes  48  and baffles  50  are secondary devices that permit tuning beyond practical limitations of hole size/spacing, and core size. 
     Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.