Abstract:
A liner assembly for an aircraft engine housing includes a noise attenuation structure that is covered by a face sheet. The face sheet covering the noise attenuation structure includes a surface having a plurality of circumferentially spaced apart acoustic energy absorption areas that are interspersed between a corresponding plurality of acoustic energy reflective areas. The acoustic energy reflective areas scatter higher order acoustic modes into a plurality of lower order modes. The difficult to attenuate lower order acoustic modes produced by the various acoustic energy cancel each other out to provide significant improvement in liner noise reduction efficiency.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention generally relates to liners for aircraft engines. More particularly, this invention relates to an acoustic liner including acoustically inactive areas for attenuating noise generated by an aircraft engine. 
         [0002]    An aircraft engine typically includes a plurality of fan blades that rotate within an engine case. The engine case includes an acoustically active lining forward of the fan blades to reduce the level of noise emitted from the aircraft engine. Conventional acoustic linings include a face sheet covering a honeycomb structure. Acoustic energy generated by rotation of the fan blades is transmitted through a plurality of openings in the face sheet to the honeycomb structure where the acoustic energy dissipates to provide an overall noise reduction. 
         [0003]    Openings in the face sheet provide for the transfer of acoustic energy into the honeycomb structure. Therefore conventional acoustic liners are arranged to minimize splices, seams and other disruptions that prevent transmission of acoustic energy through the face sheet and scatter the noise energy into lower order acoustic modes that are difficult to attenuate by the liner. However, a splice free face sheet or honeycomb structure is not practical and minimizing the area of a splice or acoustically dead area of an acoustic liner requires efforts that in many instances are disproportionate to any realized benefits. 
         [0004]    Accordingly, it is desirable to develop and design an acoustic liner that provides improved sound attenuating characteristics without the need to minimize or eliminate acoustically inactive zones. 
       SUMMARY OF THE INVENTION 
       [0005]    A liner assembly for an aircraft engine housing includes a noise attenuation structure that is covered by a face sheet including a plurality of noise energy absorption areas that are interspersed between a plurality of noise reflective areas. The noise reflective areas scatter higher order acoustic modes into either a plurality of lower order acoustic modes that tend to cancel each other, or even higher order modes that are easier to attenuate. Scattering caused by splices in the honeycomb structure can be reduced in a similar manner by aligning the honeycomb splices directly under the splices in the face sheet. 
         [0006]    An example aircraft engine housing includes an acoustic liner assembly disposed forward of rotating fan blades. The number and configuration of rotating fan blades is utilized to determine the number of noise reflective areas within the face sheet. The noise energy absorption areas include a plurality of openings for transmitting acoustic energy through to a noise attenuation layer. Acoustic energy of higher modes is attenuated by the noise attenuation structure. 
         [0007]    Acoustic waves of higher modes are scattered into lower modes by splices or noise reflective areas. Acoustic waves of lower modes are not efficiently attenuated by the noise attenuation layer and therefore propagate through the liner assembly away from an inner surface. By introducing a large number of splices, the lower order mode noise can be substantially eliminated. An example liner assembly of this invention includes a plurality of noise reflective areas without openings. Each of the plurality of noise reflective areas scatters acoustic waves of higher modes into lower modes. The plurality of acoustic waves of lower modes cancels each other out, thereby reducing propagation of difficult to attenuate low order modes. 
         [0008]    Accordingly, an example housing assembly according to this invention includes a plurality of noise reflective areas that substantially reduce splice scattered noise without the difficulties accompanying fabrication of a completely acoustically active and absorptive liner assembly. 
         [0009]    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 
         [0010]      FIG. 1  is a schematic view of a housing including an example acoustic liner according to this invention. 
           [0011]      FIG. 2  is a schematic view of a portion of an example face sheet according to this invention. 
           [0012]      FIG. 3  is a schematic view of a portion of another example face sheet according to this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]    Referring to  FIG. 1 , an engine housing assembly  10  includes a leading edge  12  defined by a cowling  14 . The cowling  14  provides a smooth transition between an outer wall  16  and an inner surface  18  that defines an airflow path through the housing assembly  10 . The housing assembly  10  surrounds and protects a plurality of rotating fan blades  20 . The inner surface  18  includes a rub strip  22  adjacent the rotating fan blades  20  that protects the fan blades  20  from possible damage caused by contact. 
         [0014]    Forward of the rub strip  22  is an acoustically active liner assembly  25 . The acoustically active liner assembly  25  includes a noise attenuation layer  24 . The noise attenuation layer  24  is comprised of a plurality of honeycomb structures distributed annularly about the inner surface  18  of the housing assembly  10 . The noise attenuation layer  24  is covered by a protective face sheet  26 . The face sheet  26  is a metal sheet that includes a plurality of openings  32 . The openings  32  communicate acoustic energy through the face sheet  26  to the underlying noise attenuation layer  24  where acoustic energy is dissipated to reduce emitted noise. 
         [0015]    The plurality of openings  32  through the face sheet  26  are arranged in noise absorption areas  28  to communicate acoustic energy through the face sheet  26  to the underlying noise attenuation layer  24 . Interspersed between each of the noise absorption areas  28  are noise reflective areas  30 . The noise reflective areas  30  do not include openings and do not allow the transmission of acoustic energy through the face sheet  26 . Instead, the noise reflective areas  30  scatter acoustic waves of higher order modes into lower order modes. The large number of uniformly distributed noise reflective areas  30  causes the lower order modes to substantially cancel each other out. 
         [0016]    Rotating engine fan blades create a shock related noise that propagates forwardly in the housing  10 . The shock related noise is generated by a pressure field on the rotating fan blades  20 . Acoustic energy generated by this phenomenon is contained in spinning acoustic modes that have circumferential orders related to the number of fan blades  20  according to the relationship: 
         [0000]    
       
      
       m 
       f 
       =nBPF*B  
      
     
         [0017]    Where nBPF is an integer representing the fan blade passing frequency harmonic order; and 
         [0018]    B is the number of fan blades. 
         [0019]    The spinning acoustic modes that carry the noise do not propagate through the housing assembly  10  at lower rotational speeds of the fan blades  20  (Cut-off Modes). However, as the fan blades  20  increases in rotational speed, the acoustic modes that cause propagation of acoustic energy cuts on and propagates within the housing  10 . 
         [0020]    Although, the noise begins to propagate within the housing  10 , these propagating modes are relatively high order modes that are still close to cut-off and most of their acoustic energy is concentrated near the inner surface  18  in a direction substantially normal to the inner surface  18 . The spinning acoustic modes of higher orders that are close to cut-off are easily attenuated by the noise attenuation layer  24  of the acoustic liner assembly  25 . However, splices  34  in the face sheet  26  that do not include openings scatter incident acoustic modes into other circumferential orders (m) according to the relationship: 
         [0000]    
       
      
       m=m 
       f 
       −k*s  
      
     
         [0021]    Where s is the number of splices or non-acoustically active areas 
         [0022]    m f  is the circumferential order; 
         [0023]    m is the number of circumferential modes caused by the non-acoustically active areas; and 
         [0024]    k is an integer that can be positive or negative. 
         [0025]    Because, the number of splices is always much smaller than the number of blades in prior art liner assemblies, the scattered modes are lower acoustic modes that propagate axially within the housing  10  and reduce the attenuation effects of the acoustic liner assembly  25 . 
         [0026]    The example acoustic liner assembly  25  of this invention includes multiple noise reflective areas  30  within the face sheet  26  that scatters high order acoustic modes caused by pressure concentrated near the rotating fan blades  20  into either even higher order modes that are easier to attenuate or into a plurality of lower acoustic modes that cancel each other out. Rather than eliminate scatter of acoustic energy of higher modes into lower modes by minimizing the number of splices  34  as is commonly attempted in prior art designs, the acoustic liner  25  according to this invention utilizes multiple noise reflective areas  30  to increase the scatter of higher order acoustic modes into a plurality of lower order acoustic modes that substantially cancel each other out and prevent low order noise propagation axially through the housing. 
         [0027]    The multiple noise reflective areas  30  generate multiple scatterings of acoustic energy within the housing assembly  10  that cancel out the pressure field associated with lower order modes and keep the acoustic energy in the easily attenuated high order modes. The acoustic energy of higher modes is efficiently attenuated by the noise attenuation structure  24 . 
         [0028]    Referring to  FIG. 2 , the noise reflective areas  30  are disposed in an axial orientation within the housing and parallel with the direction of air flow and substantially evenly spaced about an inner circumference of the housing assembly  10 . The direction of airflow is perpendicular to the plane of rotation of the fan blades  20 . The noise reflective areas  30  are of a width  38  that is greater than the spacing between adjacent openings  32  in the noise absorption areas  28 . The width  38  of the reflective areas  30  can be equal to a width of the noise absorption areas as is illustrated in the example shown in  FIG. 2 , but may also be arranged in varying widths to provide a uniformly orientated repeating pattern within the housing. The width  38  of the reflective areas  30  may also be greater than the width of the nose absorption areas  28  to tailor noise attenuation to application specific requirements. 
         [0029]    The number of noise reflective areas  30  is determined with respect to the number of fan blades  20  and operation of the engine assembly to produce the desired scattering effect that cancels the scattered low order acoustic modes. 
         [0030]    The number of noise reflective areas  30  is related to the number of fan blades  20  and the frequency of noise desired to be cancelled. For fan blade frequency harmonic orders of BPF or lower frequencies, the number of noise reflective areas is determined according to the relationship: 
         [0000]      S≧2B 
         [0031]    where B=the number of fan blades; and 
         [0032]    S=the number of noise reflective areas. 
         [0033]    For fan blade frequency harmonics of 2BPF or lower frequencies, the number of noise reflective areas is determined according to the relationship: 
         [0000]      S≧4B 
         [0034]    where B=the number of fan blades; and 
         [0035]    S=the number of noise reflective areas. 
         [0036]    These relationships provide only modes that are of an order of m f  or higher. Significant noise reduction is further provided with smaller numbers of noise reflective areas  30 . For example the number of strips could be varied between S≧1.5 B for frequency harmonics of BPF, and S≧3 B for frequency harmonics of 2BPF and lower. As appreciated, the specific number of splices is determined according to a relationship accounting for the number of fan blades and the specific noise attenuation application such that the number of noise reflecting areas  30  can be varied to tailor the noise attenuation properties of the liner assembly  25 . 
         [0037]    Referring to  FIG. 3 , in another example liner assembly  40 , a face sheet  42  includes the plurality of acoustically absorption areas  28  and the noise reflective areas  30  disposed at an angle  44  relative to the air flow through the housing assembly  10 . The noise absorption areas  28  and noise reflective areas  30  are disposed at an angle  44  relative to the air flow to provide an angular orientation that aids in tailoring the direction in which acoustic energy is reflected back into the housing assembly  10  to provide the desired noise attenuation performance. 
         [0038]    Accordingly, the noise attenuation liner assembly for the aircraft housing according to this invention provides for the reduction of noise emitted from the fan case housing without the accompanying complication required for creating a zero splice or completely acoustically transparent connections of sections of the face sheet utilized to cover the noise attenuation structure within the aircraft housing. 
         [0039]    Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.