Abstract:
The invention relates to an engine nacelle, including: a nacelle wall that has an inner side and an outer side; an inlet lip that is embodied at that end of the engine nacelle that is formed upstream; and an engine intake that takes in the air required for the respective engine and that is formed by the inner side of the nacelle wall. It is provided that the nacelle wall includes an air-permeable structure that extends from the outer side to the inner side of the nacelle wall, and that is configured for passing air that flows against the outer side from the outer side to the inner side. The invention further relates to a method for influencing the flows inside an engine nacelle.

Description:
REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to German Patent Application No. 10 2016 112 604.7 filed on Jul. 8, 2016, the entirety of which is incorporated by reference herein. 
       BACKGROUND 
       [0002]    The invention relates to an engine nacelle and a method for influencing flows inside an engine nacelle. 
         [0003]    The nacelle design for an engine is significantly impacted by the characteristics during the flight. This means that the definition of a nacelle&#39;s geometry is optimally adjusted to the flow conditions and the requirements of the fan, so that a minimal loss is created during the flight state and the specific fuel consumption is kept as low as possible. 
         [0004]    However, there are also operational states of an engine nacelle for which the nacelle and therefore also the fan are not ideally designed. These states include the case that there is a side wind flow during the start. In the event of a side wind flow during the start, the stationary or rolling aircraft is impinged by the side wind flow, which is for example oriented perpendicularly or obliquely with respect to the start direction. Such a side wind flow can cause a flow separation at the nacelle lip, which may have a negative impact on the operational behavior of the fan. Thus, flow separations can be generated at the inlet lip if the incident intake flow is not exactly axial, as may be the case in the event of a strong side wind. As a result, disturbances of the intake flow to the engine fan are created. 
         [0005]    EP 2 607 657 A2 describes an engine nacelle that counteracts a flow separation at the nacelle lip in the event of side wind, namely by pressurized gas extracted from the core engine being supplied to the engine intake. At that, the pressurized gas is supplied through a system of conduits of a chamber inside the nacelle that is formed in the area of the nacelle lip. From this chamber, the pressurized gas is supplied via defined openings to the engine intake. 
         [0006]    U.S. Pat. No. 8,967,964 B2 describes an arrangement of a plurality of air discharge openings at an engine surface. In an exemplary embodiment, in order to avoid a flow separation in the event of a side wind, air discharge openings are formed in the area of the lip of an engine intake or along the circumference of the fairing of the engine intake. 
         [0007]    The present invention is based on the objective to provide an engine nacelle and a method for influencing flows in an engine nacelle that avoid or reduce the flow separation behind the inlet lip in the event of a side wind flow. 
       SUMMARY 
       [0008]    According to an aspect of the invention an air-permeable structure is provided in the nacelle wall, with the air-permeable structure extending from the outer side to the inner side of the nacelle wall, and being configured for passing air, which is flowing against the outer side, from the outer side to the inner side. The air-permeable structure is in particular provided and configured for the purpose of making it possible for the air of a side wind flow to flow from the outer side into the nacelle directly through the nacelle wall. 
         [0009]    Through an air-permeable structure in the nacelle wall it is achieved that air can flow into the nacelle from the outside. At that, the air flowing from the outside into the nacelle supplies energy to the particles of the boundary layer of the main flow and accelerates them, so that the boundary layer material does not come to a standstill and a separation of the boundary layer does not occur or occurs only with a delay. A flow separation behind the nacelle lip is thus delayed or even avoided. Here, the air-permeable structure is designed in such a manner that an amount of air passed is considerably smaller than the main mass flow of the air that is suctioned in by the fan of the aircraft engine. 
         [0010]    The invention avoids a flow separation behind the nacelle lip in particular during the start. But it can also provide advantages in the case of a strong side wind flow during slow flight and cruise. 
         [0011]    According to one embodiment of the invention, it is provided that the air-permeable structure comprises a plurality of tubes that respectively extend all the way to the inner side. According to one embodiment, the tubes extend from the outer side to the inner side. However, this is not necessarily the case. Thus, it can for example be alternatively provided that the tubes extend from a collection volume, which is formed adjacent to the outer side and takes in inflowing air from the outer side, to the inner side. It can also be alternatively provided that the tubes extend from a further structure, which is formed at the outer side of the engine nacelle and defines a preferred passing direction, as will be explained in the following. 
         [0012]    In principle, the tubes can have any desired arrangement. For example, they can form a one-dimensional or a two-dimensional array in the nacelle wall. 
         [0013]    In a further embodiment of the invention it is provided that the mentioned tubes are respectively embodied as a nozzle. For this purpose, they realize a cross-section reduction in the direction towards the inner side. Through the embodiment as nozzles, the velocity of the air flowing inside the tubes is increased, which leads to the boundary layer material of the main flow being more strongly accelerated, so that a flow separation is avoided in an even more effective manner. 
         [0014]    Instead of being formed by tubes, the air-permeable structure can also be formed in a different manner. In principle, the air-permeable structure can be provided by any kind of structure, air duct and perforation that make it possible for air to be guided from the outer side to the inner side, and to flow into the nacelle. Thus, it is provided in an alternative exemplary embodiment that the air-permeable structure is formed by a porous material or comprises a porous material. In contrast, what is not considered an air-permeable structure within the meaning of the present invention is a large-area recess inside the nacelle wall that can be covered by a movable flap, where necessary; such a recess does not have its own structure. 
         [0015]    According to one embodiment variant, the air-permeable structure is formed in such a manner that it has a defined blow-in direction from which the air can enter the structure. This can be achieved in such a manner that the air-conducting structures of the air-permeable structure transport the air that is entering at its outer side in the direction of the inner side along the preferred blow-in direction. Accordingly, the tubes or other structures that form the passage are oriented in the preferred blow-in direction adjacent to the outer side. 
         [0016]    According to one embodiment of the invention, the defined blow-in direction extends substantially transversely to the longitudinal direction of the engine nacelle, so that the air-permeable structure provides an air permeability for a side wind component that extends substantially transversely to the longitudinal direction of the engine nacelle. If the air-permeable structure is realized by means of tubes, these extend adjacent to the outer side substantially perpendicularly to the area of the outer side where they begin. As a result, it is achieved that the air-permeable structure is permeable for a side wind component that extends substantially transversely to the longitudinal direction of the engine nacelle. 
         [0017]    In another embodiment of the invention it is provided that the air-permeable structure is formed in such a manner that it has a defined blow-out direction in which the air flows from the inner side into the nacelle interior through corresponding openings inside the inner side. 
         [0018]    Here, it is provided according to one embodiment of the invention that the defined blow-out direction has a directional component in the longitudinal direction of the engine nacelle, so that air flowing through the air-permeable structure flows into the nacelle interior with a speed component in the direction of the main flow. Thus, the air is deflected in the direction of the main flow, i.e. towards the fan, already inside the passage, so that it has a speed component in the longitudinal direction of the engine nacelle when it leaves the air-permeable structure. This makes it possible that the boundary layer material of the main flow more strongly accelerated by means of the air that flows through the air-permeable structure into the nacelle interior, and that flow separation effectively avoided in this way. 
         [0019]    If the air-permeable structure is realized by means of tubes, it can be provided for this purpose that, at their ends that are facing the inner side, the tubes are curved in the longitudinal direction of the engine nacelle. In one embodiment, the tubes end at the nacelle inner wall in an approximately tangential orientation. 
         [0020]    In another embodiment of the invention it is provided that, at the outer side, the air-permeable structure comprises at least one material layer that if formed by a porous material with a defined passing direction. Here, the material layer forms the outer shell of the engine nacelle in the respective area. By providing a defined passing direction, it is achieved that only air of a side flow can flow inside and can pass the air-permeable structure. 
         [0021]    In one embodiment of the invention, the already mentioned tubes connect to the material layer of porous material formed with a defined passing direction at the outer side, in that case extending from this material layer to the inner side. 
         [0022]    The air-permeable structure is formed downstream of the inlet lip, wherein it can begin directly behind the inlet lip or can alternatively be realized in an axial distance to the inlet lip. Here, the air-permeable structure extends over a defined axial length that lies between 5% and 50% of the axial length of the engine intake between the inlet lip and the fan plane, for example. Its axial distance to the nacelle lip is one to ten times, in particular twice to five times, in particular twice to three times the nacelle lip diameter, for example, wherein the nacelle lip diameter is defined as twice the radius of the upstream curvature of the nacelle lip facing the flow. 
         [0023]    As for the extension of the air-permeable structure in the circumferential direction, it can be provided that the air-permeable structure is not formed along the entire circumference of the nacelle in the nacelle wall, but only in a certain circumferential area or in certain circumferential areas. In particular, it can be provided that the air-permeable structure is formed only in areas of the engine nacelle that are located at a side of the engine nacelle, when the engine nacelle is regarded in a state when it is mounted on a wing. What is thus regarded is a side of the engine nacelle that faces the wind if a side wind is present. However, in principle the air-permeable structure can also extend in other circumferential areas or in the entire circumferential direction. 
         [0024]    Another aspect of the present invention relates to a method for influencing flows in an engine nacelle. It is provided in the method that, in the event of a side wind, air is guided from the outer side to the inner side through an air-permeable structure formed in the nacelle wall, and energy is thus supplied to the boundary layer of the main flow that is present at the inner side. In particular, the air flowing through the air-permeable structure accelerates the boundary layer of the main flow, so that a flow separation behind the nacelle lip, as it occurs in the event of a side wind, is reduced or avoided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The invention will be explained in more detail on the basis of exemplary embodiments with reference to the accompanying drawings in which: 
           [0026]      FIG. 1  shows a simplified schematic sectional view of a turbofan engine in which the present invention can be realized; 
           [0027]      FIG. 2  shows, in a schematic manner, a front view into an engine nacelle according to the state of the art, also rendering a flow separation in the event of a side wind flow; 
           [0028]      FIG. 3  shows, in a schematic manner, a partially sectioned front view of a first exemplary embodiment of the invention, wherein a side passage is formed in the nacelle wall; 
           [0029]      FIG. 4  shows an enlarged rendering of the sectioned area A-A of  FIG. 3 , wherein the sectioned area is shown in front view; 
           [0030]      FIG. 5  shows a sectional view along the line B-B of  FIG. 4 , wherein the sectioned area is shown in top view; and 
           [0031]      FIG. 6  shows an alternative exemplary embodiment of the invention in a sectional view corresponding to  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 1  shows, in a schematic manner, a turbofan engine  100  that has a fan stage with a fan  10  as the low-pressure compressor, a medium-pressure compressor  20 , a high-pressure compressor  30 , a combustion chamber  40 , a high-pressure turbine  50 , a medium-pressure turbine  60 , and a low-pressure turbine  70 . 
         [0033]    The medium-pressure compressor  20  and the high-pressure compressor  30  respectively have a plurality of compressor stages that respectively comprise a rotor stage and a stator stage. The turbofan engine  100  of  FIG. 1  further has three separate shafts, namely a low-pressure shaft  81  which connects the low-pressure turbine  70  to the fan  10 , a medium-pressure shaft  82  which connects the medium-pressure turbine  60  to the medium-pressure compressor  20 , and a high-pressure shaft  83  which connects the high-pressure turbine  50  to the high-pressure compressor  30 . However, this is to be understood to be merely an example. If, for example, the turbofan engine has no medium-pressure compressor and no medium-pressure turbine, only a low-pressure shaft and a high-pressure shaft would be present. 
         [0034]    The turbofan engine  100  has an engine nacelle  1  that has an inlet lip  14  and forms an engine inlet  11  at the entry side, supplying inflowing air to the fan  10 . The fan  10  has a plurality of fan blades  101  that are connected to a fan disc  102 . Here, the annulus of the fan disc  102  forms the radially inner delimitation of the flow path through the fan  10 . Radially outside, the flow path is delimited by the fan housing  2 . Upstream of the fan-disc  102 , a nose cone  103  is arranged. 
         [0035]    Behind the fan  10 , the turbofan engine  100  forms a secondary flow channel  4  and a primary flow channel  5 . The primary flow channel  5  leads through the core engine (gas turbine) which comprises the medium-pressure compressor  20 , the high-pressure compressor  30 , the combustion chamber  40 , the high-pressure turbine  50 , the medium-pressure turbine  60 , and the low-pressure turbine  70 . At that, the medium-pressure compressor  20  and the high-pressure compressor  30  are surrounded by a circumferential housing  29  which forms an annulus surface at the internal side, delimitating the primary flow channel  5  radially outside. Radially inside, the primary flow channel  5  is delimitated by corresponding rim surfaces of the rotors and stators of the respective compressor stages, or by the hub or by elements of the corresponding drive shaft connected to the hub. 
         [0036]    During operation of the turbofan engine  100 , a primary flow flows through the primary flow channel  5 . The secondary flow channel  4 , which is also referred to as the partial-flow channel, sheath flow channel, or bypass channel, guides air sucked in by the fan  10  during operation of the turbofan engine  100  past the core engine. 
         [0037]    The described components have a common symmetry axis  90 . The symmetry axis  90  defines an axial direction of the turbofan engine. A radial direction of the turbofan engine extends perpendicularly to the axial direction. 
         [0038]    In the context of the present invention, the embodiment of the engine nacelle  1  in the axial area located upstream of the fan  10  is of particular importance. 
         [0039]      FIG. 2  shows an engine nacelle  1  in a rendering from the front, i.e. with view onto the fan. In the schematic rendering of  FIG. 2 , the nose cone  103  is the only part of the fan that is shown. The plane  104  represents the fan plane. The nacelle  1  has a nacelle wall  12  with an inner side  11  and an outer side  13 . Here, the inner side  11  forms an engine intake in the area in front of the fan, with the engine intake taking in the air required by the engine and supplying it to the fan. The nacelle interior, i.e. the area in front of the fan that is delimited by the nacelle wall  12 , is indicated by reference sign  19 . 
         [0040]    The nacelle  1  comprises an inlet lip  14  (also referred to as the nacelle lip) that is formed in a rounded manner. The inlet lip  14  forms the front end of the engine nacelle  1 . At the inner side  11 , it transitions into the engine intake. In the axial direction, it ends at the narrowest inner cross-section (also referred to as the “throat”) of the engine nacelle  1 . In a subsonic engine intake, as it is regarded here, the engine intake  11  beginning behind the narrowest inner cross-section is embodied as a diffusor. 
         [0041]    What is further shown in  FIG. 2  is a side wind component A of a side wind flow. Due to the side wind component A, the air intake flow towards the fan does not occur in the engine intake  11  in an exactly axial manner, wherein the side wind flow additionally flows around the inlet lips  14  in the area that is facing the side wind component A. As a result, flow separations may be generated at the inlet lips  14 . Such a flow separation  15  shown in a schematic manner. 
         [0042]      FIG. 3  shows an engine nacelle  1 , in which a schematically shown air-permeable structure  16  (also referred to as the passage) is formed in the nacelle wall  12 , extending from the outer side  13  to the inner side  11  of the nacelle wall  12 . The air-permeable structure  16  makes it possible for the air of a side wind flow A to flow from the outer side  13  directly (that is, not through the nacelle lips  14 ) into the nacelle interior  19 . The air-permeable structure  16  extends over the defined axial length and defined angular range in the circumferential direction. 
         [0043]    In the axial direction, the air-permeable structure  16  begins directly behind the inlet lip  14 , or alternatively at a certain distance to the inlet lip  14 . For example, the air-permeable structure is formed at an axial distance to the nacelle lip  14  that is twice to three times the nacelle lip diameter, wherein the nacelle lip diameter is defined as twice the radius of the upstream curvature of the nacelle lip  14  facing the flow. 
         [0044]    As for the extension of the air-permeable structure  16  in the circumferential direction, it is provided in the shown exemplary embodiment that the air-permeable structure  16  is formed only in that area of the engine nacelle  1  that is facing towards the side wind component A. That is one of the two lateral areas when referring to the engine nacelle \ mounted on a wing. Alternatively, an air-permeable structure is formed at both side areas. However, in principle the air-permeable structure  16  can extend around the entire circumference of the nacelle  1 . 
         [0045]      FIG. 4  shows an exemplary embodiment of an air-permeable structure  16 . The air-permeable structure  16  comprises a plurality of tubes  161 , which respectively extend from the outer side  13  to the inner side  11  in the shown exemplary embodiment. Here, the tubes  161  are formed in a defined area inside the nacelle wall  12  extending in the axial direction and the circumferential direction. For example, they may form a two-dimensional array in the nacelle wall  12 . The tubes  161  are formed in a material  121  that forms a component of the nacelle wall  12 . For example, the air-permeable structure  16  comprising the material  121  with the tubes  161  is prefabricated and inserted into a corresponding recess inside the nacelle wall  12 . Alternatively, the tubes  161  are formed in a material  121  that also forms the nacelle wall  12  in other areas. 
         [0046]    The tubes  161  have a circular cross-section, for example. However, this is not necessarily the case. For example, they may have a maximum diameter in the range between 5 mm and 10 cm. The tubes  161  end in circular holes inside the inner wall  11 , for example. 
         [0047]    The size and number of the individual tubes  161  is designed in such a manner that the total mass flow, which maximally (i.e., in the event of a strong side wind in the transverse direction) flows into the nacelle interior  19  through the air-permeable structure  16 , is considerably smaller than the main mass flow that moves in the intake area  11  in the direction of the fan and flows through the fan plane  104  (cf.  FIGS. 2 and 3 ). For example, the maximum mass flow that flows through the air-permeable structure  16  is no more than 10%, in particular no more than 5%, in particular no more than 1% of the main mass flow. 
         [0048]    In the following, it is referred to  FIG. 5 , which shows a section along the line B-B of  FIG. 4 . The axial or longitudinal direction is indicated by X in  FIG. 5 . As can be seen, the individual tubes  161  are curved in the axial direction towards the inner side  11 , so that the air flowing therein has a speed component in the direction of the main flow inside the engine intake  11 . The air that is flowing from the tubes  161  into the engine interior  19  supplies additional energy to the air particles located in the boundary layer  18  which is present at the inner side, thus accelerating the same. This leads to a separation of the boundary layer  18 , and thus a flow separation behind the inlet lip  14 , being delayed or even avoided. The flow C in the boundary layer  18  is present at the inner wall  11  despite the side wind component A. 
         [0049]      FIG. 5  also shows, in a schematic manner, the situation that would arise without the air-permeable structure  16 . Here, a flow separation B would occur due to the side wind component. 
         [0050]    As can be further seen in  FIG. 5 , the tubes  161  are respectively formed as a nozzle  17 , and taper off in the direction of the inner wall  11  or have a tapering cross-sectional surface for that purpose. This leads to an acceleration of the air that is transported in the tubes  161 . As a result, the acceleration of the air in the boundary layer  18  is even increased, so that a flow separation is avoided even more effectively. 
         [0051]    In contrast, where they adjoin the outer side  13 , the tubes  161  extend substantially transversely to the longitudinal direction X of the engine nacelle. As a result, the blow-in direction into the air-permeable structure  16  is defined transversely to the longitudinal direction X. In this way, it is ensured that a side wind component A of a side wind that is oriented transversely to the longitudinal direction X is coupled in and can be transported through the air-permeable structure  16 . Due to the shape of the tubes  161  being curved towards the inner wall  11 , the blow-out direction into the nacelle interior  19  that is thus defined has an axial speed component, so that the air flows into the boundary layer  18  with an axial speed component. 
         [0052]    It is to be understood that the embodiment and arrangement of the tubes  161  in the  FIGS. 4 and 5  is to be understood merely as an example. The air-permeable structure  16  can in principle also be realized by means of other structures which are suitable and provided for the purpose of transporting air from the outer side into the nacelle interior. For example, for this purpose the air-permeable structure can alternatively be formed by any material with open pores in which the individual pores are connected to each other and the environment, and in which an air-permeable structure with a defined blow-in direction and a defined blow-out direction is provided. 
         [0053]    In a further embodiment variant, it is provided that an embodiment of the air-permeable structure  16  is a combination of a porous material and a plurality of tubes. Such an exemplary embodiment is shown in  FIG. 6 . It differs from the exemplary embodiment of  FIG. 5  insofar as the air-permeable structure  16  comprises a layer  162  as a further element, which forms the passage  16  at the outer side  13  and consist of a porous material with a defined passing direction. Here, the passing direction is perpendicular to the longitudinal direction X. The layer  162  forms the outer shell of the engine nacelle  1  in the respective area. 
         [0054]    Radially inside, a tube arrangement comprising tubes  161  connects to the layer  162  according to  FIGS. 3 to 5 . By using a layer  162  with a defined passing direction, it is ensured that air can flow through the air-permeable structure  16  into the nacelle interior  19  only when a side wind component A is present, while air with a different directional component cannot flow into the air-permeable structure  16 . 
         [0055]    What can be used as the porous material forming the layer  162  with a defined passing direction are air-permeable composite materials with perforations, for example. For instance, one may use air-permeable laminates that are manufactured by using blowing agents for controlled expansion of the fiber architecture. The perforation may for example be provided by pins that are contained in the composite material and that are removed after the composite material has be cured. It can also be provided that the perforation is formed by subsequent removal of sewing threads. Here, the porous material only forms the layer  162  or the outer shell, and does not extend along the tubes  161 . 
         [0056]    The present invention is not limited in its design to the above-described exemplary embodiments, which are to be understood merely as examples. For instance, it can alternatively be provided that the air inside the passage  16  is first guided into a collection volume, and is then conducted from the same into the nacelle interior via a plurality of tubes. 
         [0057]    Further, it is to understood that the features of the individual described exemplary embodiments of the invention can be combined with each other in different combinations. As far as ranges are defined, they comprise all values within these ranges as well as all partial areas falling within a range.