Abstract:
A turboengine that reduces rear noise emissions has a thickness of a rear part of a sound-attenuating coating, which is borne internally by an external fan cowl, that is increased toward a front part of the sound-attenuating coating. An increased-thickness zone is connected to a pan of the coating, located in a critical area of a jet nozzle, by a surface having a curved profile.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of reducing the sound emissions at the rear of a bypass turboengine for aircraft, as well as to a turboengine improved by the implementation of this method. 
     BACKGROUND OF THE RELATED ART 
     It is known that bypass turboengines comprise a nacelle delimiting at the front an air inlet and containing a cold stream fan, a hot stream central generator and a fan channel with annular section provided with a nozzle for the cold stream, said fan channel being formed between an internal cowl surrounding said hot stream central generator and the internal tubular face of an acoustic attenuation coating with annular section carried internally by an external fan cowl forming the rear of said nacelle, said coating comprising a front part, disposed upstream of said nozzle and exhibiting an optimal thickness for the acoustic attenuation of the noise produced by said fan and conveyed by said cold stream, as well as a rear part, contiguous with said front part and disposed on either side of the throat of said nozzle, said rear coating part exhibiting a thickness which decreases towards the rear edge of said external fan cowl delimiting the annular ejection orifice for said cold stream, and said front coating part having, in the vicinity of its junction with said rear coating part, a convergent zone in which its internal tubular face begins to converge towards said nozzle. 
     Since the rear part of said acoustic attenuation coating exhibits a decreasing thickness which is less than said optimal thickness of the front part—except possibly at the junction with the latter—this rear part may not exhibit optimal attenuation characteristics. 
     Moreover, the shape of the internal tubular face of the acoustic attenuation coating, in particular opposite said nozzle—that is to say at the level of said rear part—is determined in order that, in combination with the shape of said internal cowl of the hot stream central generator, the performance of said nozzle—and therefore that of said turboengine—is optimal. It is not therefore possible to modify the shape of said internal tubular face of the acoustic attenuation coating without degrading the performance of the turboengine. 
     SUMMARY OF THE INVENTION 
     The Applicant has however found that, under certain conditions, it was possible greatly to increase the acoustic attenuation of the rear part of said coating by modifying the shape thereof, while only slightly degrading, in an acceptable manner, the performance of the turboengine. 
     To this end, according to the invention, the method of reducing the sound emissions at the rear of a bypass turboengine of the type recalled below is noteworthy in that:
         a critical zone of the fan channel, beginning at said nozzle throat and extending frontwards is determined, in which any possible geometric modification of said fan channel, and therefore of the internal tubular face of the rear coating part, is impossible without demanding a modification of the parameters of said nozzle;   in said convergent zone of the front coating part, the internal tubular face is modified in the sense of a progressive increase in the thickness of said coating towards said rear coating part and this progressive modification in the internal tubular face of the contiguous rear coating part is continued until a zone of the latter with increased thickness is approximately equal to said optimal thickness; and   the rear end of said zone with increased thickness is linked to the front end of said critical zone by an internal tubular face with inflection profile.       

     Thus, by virtue of the invention, the acoustic attenuation properties of said rear coating part are augmented by endowing the front zone of the latter—front zone which in certain cases can exhibit an axial length of the order of a quarter of the total axial length of said rear coating part—with a thickness equal to said optimal thickness of the front coating part. 
     The extent of said critical zone is preferably determined by the fact that the Mach number of the cold stream thereat goes from about 0.8 (at the front) to about 1 (at the throat). Any geometric modification of the internal tubular face of said acoustic attenuation coating in this critical zone must be avoided, since it would modify the parameters of the nozzle in a non-negligible manner. 
     Moreover, as regards the progressive shape modification of the acoustic attenuation coating, it is advantageous that it begin in said convergent zone, in which the cold stream accelerates, since said modification begins at a relatively low Mach number, lying for example between 0.4 and 0.55. It follows from this that, from said convergent zone of the front coating part to the front end of the critical zone, the shape modification (including said internal tubular face with inflection profile) takes place in a span of Mach numbers lying between about 0.45 and 0.8. 
     Of course, said internal tubular face with inflection profile must in no case produce an inversion of the pressure gradient, which would have the immediate effect of causing the boundary layer to detach. For this purpose, the shape parameter Hi of said inflection profile must remain less than 1.6. 
     From the foregoing, it is noted that the bypass turboengine improved according to the method of the invention is noteworthy in that the acoustic attenuation coating with annular section carried internally by said external fan cowl comprises an inflection profile between an upstream zone, in which the thickness of said coating is at least approximately equal to an optimal thickness E, and said critical zone of the nozzle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures of the appended drawing will clearly elucidate how the invention can be carried out. In these figures, identical references designate similar elements. 
         FIG. 1  is a diagrammatic axial cross-section through a bypass turboengine. 
         FIG. 2  is a magnified diagrammatic cross-sectional view illustrating the known acoustic attenuation tubular coating envisaged in the fan channel of the turboengine of  FIG. 1 . 
         FIG. 3  shows, in a view similar to  FIG. 2 , the acoustic attenuation tubular coating improved in accordance with the present invention. 
         FIG. 4  is a magnified view, dilated orthogonally to the axis of said turboengine, of a part of  FIG. 3  at the level of the improvement of the acoustic attenuation tubular coating in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The known bypass engine for aircraft, diagrammatically shown in  FIG. 1  in cross-section passing through its longitudinal axis L-L, comprises a nacelle  1  delimiting, at the front, an air inlet  2 . The nacelle  1  contains a fan  3 , a hot stream central generator  4  and an annular fan channel  5  traversed by the cold stream. 
     The fan channel  5  is provided with an annular ejection orifice  6  corresponding to the trailing edge of the nacelle  1 . This fan channel  5  is formed between a cowl  7 , surrounding said hot stream central generator  4 , and the internal face  8  (see  FIG. 2 ) of an internally by an external fan cowl  10 , forming the rear part of said nacelle  1 . 
     In the fan channel  5 , the cowl  7  and the internal face  8  form a nozzle  11 , which emerges through the annular ejection orifice  6  and whose throat  12  is situated in a plane  13  transverse with respect to the longitudinal axis L-L. 
     The acoustic attenuation tubular coating  9 , for example of known type with absorbent cells, consists of two contiguous parts  9 A and  9 R, having respective internal faces  8 A and  8 R forming said internal face  8 , and adjacent along a line  14 , whose plane is orthogonal to said axis L-L. The front part  9 A, disposed well upstream of the nozzle  11 , exhibits a thickness E, at least approximately constant, corresponding to an optimal attenuation of the noise produced by the fan  3  and conveyed by the cold stream circulating in the fan channel  5 . On the other hand, the rear part  9 R, which is disposed on either side of the throat  12  of the nozzle  11  and which extends over an axial length D, exhibits a thickness which decreases in a uniform manner from said line  14 —where it is equal to the optimal thickness E—to the annular ejection orifice  6 . Of course, on account of its decreasing thickness, which is less than the optimal value E (except on the line  14 ), the rear part  9 R could not offer an optimal acoustic attenuation. 
     In the arrangement described above, the cold stream in the fan channel is subsonic and such that:
         opposite the rear part  9 R of the acoustic attenuation coating  9 , the Mach number goes from about 0.55, at the level of the line  14 , to about 1.0, at the level of the annular ejection orifice  6 ; and   opposite the front part  9 A of said coating  9 , there exists:   a divergent upstream zone  17 U, in which said cold stream slows, the Mach number thereat going from about 0.5 to about 0.4; and   a convergent downstream zone  17 D, in which said cold stream accelerates, the Mach number thereat going from about 0.4 to about 0.55.       

     As indicated above, the object of the present invention is to increase, towards the rear, the front part  9 A of optimal thickness E of a zone  9 A′ of length d so as to reduce the rear part  9 R with decreasing thickness to a zone  9 R′ of reduced length D-d (see  FIG. 3 ), while only negligibly degrading the performance of the turboengine. 
     Therefore, as illustrated on a larger scale in  FIG. 4 :
         we begin by determining a critical zone  15  of the fan channel  5 , beginning with the plane  13  of the nozzle throat  12  and extending towards the front as far as a front boundary  16 , critical zone in which any geometric modification of the internal tubular face  8 R of the rear coating part  9 R is impossible without demanding a modification of the parameters of said nozzle  12 . The critical zone  15  is for example determined by the fact that, at said front boundary  16 , the Mach number of the cold stream already attains a value at least approximately equal to 0.8, to attain a value of about 1 at the nozzle throat  12 ;   in the downstream zone  17 D opposite the front coating part  9 A in which the latter begins to converge towards the nozzle throat  12 , and in which the Mach number of the cold stream lies between about 0.4 and about 0.55, the internal tubular face  8 A is modified in the sense of a progressive increase in the thickness (see the line  8 A′) of said coating, towards the rear coating part  9  and this progressive increase in the internal tubular face of the contiguous rear coating part is continued until a zone  18  of the latter, of length d and with increased thickness, is obtained in which the thickness is at least equal to said optimal thickness E; and   the rear end  19  of said zone  18  is linked to the front end  16  of the critical zone  15  by an internal tubular face with inflection profile  20 , whose shape parameter Hi is at most equal to 1.6.       

     Thus, the length d of the zone  18  is defined by the position of the nozzle throat  12 , the axial extent of the critical zone  15  and the axial extent of the internal tubular face with inflection profile  20 . This length d can, in certain cases, be in the vicinity of a quarter of the length D of the rear coating part  9 R, so that a significant increase in acoustic attenuation is achieved without however overly degrading the operation of the turboengine.