Abstract:
An example method of reducing a vibratory response of airfoils that support a shroud includes circumferentially misaligning some of the airfoils in a radially inner array of airfoils with all airfoils of a radially outer array of airfoils, and circumferentially aligning at least one of the airfoils in the radially inner array of airfoils with all airfoils of the radially outer array of airfoils.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to United States Provisional Application No. 61/939304 filed on Feb. 13, 2014. 
     
    
     STATEMENT REGARDING GOVERNMENT SUPPORT 
       [0002]    This invention was made with government support under Contract No. FA8650-09-D-2923-0021 awarded the United States Air Force. The government has certain rights in this invention. 
     
    
     BACKGROUND 
       [0003]    This disclosure relates generally to a concentric airfoil assembly and, more particularly, to mistuning a concentric assembly to reduce vibrations. 
         [0004]    Gas turbine engines are known and, typically, include a fan delivering air into a bypass duct as propulsion air and to be utilized to cool components. The fan also delivers air into a core engine where it is compressed in a compressor. The compressed air is then delivered into a combustion section where it is mixed with fuel and ignited. Products of the combustion pass downstream over turbine rotors, driving them to rotate. 
         [0005]    One type of gas turbine engine has multiple bypass streams. In such engine, there is typically a radially outer third stream bypass flow and a radially inner main bypass flow. Other types of gas turbine engines have other bypass flow arrangements. 
         [0006]    Gas turbine engines, and in particular gas turbine engines having multiple bypass streams, may include assemblies having a concentric airfoil arrays. These assemblies often have very little structural damping due to their symmetric nature. Lack of damping can lead to, among other things, high cycle fatigue issues resulting from high vibratory stresses, such as resonant or aero-elastic responses of the airfoils to dynamic pressure fluctuations. Mitigating these fatigue issues has involved increasing airfoil thicknesses, which can lead to localized hot spots and vibratory stresses and diminished aero-dynamic performance. 
       SUMMARY 
       [0007]    An airfoil assembly according to an exemplary aspect of the present disclosure includes, among other things, an annular shroud having a radially inner face and a radially outer face opposing the radially inner face. A radially inner array of airfoils extends from the radially inner face, and a radially outer array of airfoils extends from the radially outer face. At least one airfoil of the radially inner array is circumferentially aligned with a corresponding airfoil in the radially outer array. At least one airfoil of the radially inner array is circumferentially misaligned with the airfoils of the radially outer array. 
         [0008]    In another example of the foregoing assembly, one airfoil of the radially inner array is circumferentially aligned with a corresponding airfoil in the radially outer array at a twelve o&#39;clock position, and the remaining airfoils in the radially inner array are misaligned with the airfoils of the radially outer array. 
         [0009]    In another example of any of the foregoing assemblies, the radially inner array of airfoils are configured to guide flow within a radially inner bypass flow passage of a gas turbine engine, and the radially outer array of airfoils are configured to guide flow within a radially outer bypass flow passage of the gas turbine engine. 
         [0010]    In another example of any of the foregoing assemblies, the radially inner bypass flow passage and the radially outer bypass flow passage are both radially outside a core flow passage of the gas turbine engine. 
         [0011]    In another example of any of the foregoing assemblies, each of the radially inner array of airfoils at or above a horizontal midline is circumferentially aligned with a corresponding airfoil in the radially outer when is in an installed position. 
         [0012]    In another example of any of the foregoing assemblies, the shroud, the radially inner array of airfoils, and the radially outer array of airfoils are cast together as a single unitary structure. 
         [0013]    An airfoil assembly according to an exemplary aspect of the present disclosure includes, among other things, a shroud having a radially inner face and a radially outer face opposing the radially inner face. A radially inner array of airfoils extends from the inner face. A radially outer array of airfoils extends from the radially outer face. A distance between circumferentially adjacent airfoils of the radially inner array or circumferentially adjacent airfoils in the radially outer array varies. 
         [0014]    In another example of the foregoing assembly, the shroud is an annular shroud. 
         [0015]    In another example of any of the foregoing assemblies, the shroud, the radially inner array of airfoils, and the radially outer array of airfoils are cast together as a single unitary structure. 
         [0016]    In another example of any of the foregoing assemblies, at least one selected airfoil of the radially inner array is circumferentially aligned with a corresponding airfoil in the radially outer array, and at least one selected airfoil of the radially inner array is circumferentially misaligned with the airfoils of the radially outer array. 
         [0017]    In another example of any of the foregoing assemblies, one airfoil of the radially inner array is circumferentially aligned with a corresponding airfoil in the radially outer array at a twelve o&#39;clock position, and the remaining airfoils of the radially inner array are circumferentially misaligned with all airfoils in the radially outer array. 
         [0018]    In another example of any of the foregoing assemblies, the shroud is a middle shroud and the assembly further includes a radially inner shroud and a radially outer shroud. The radially inner array of airfoils extends from the middle shroud to the radially inner shroud. The radially outer array of airfoils extends from the middle shroud to the radially outer shroud. 
         [0019]    In another example of any of the foregoing assemblies, the radially inner array of airfoils are configured to guide flow within a radially inner bypass flow passage of a gas turbine engine. 
         [0020]    In another example of any of the foregoing assemblies, the radially outer array of airfoils are configured to guide flow within a radially outer bypass flow passage of the gas turbine engine. 
         [0021]    In another example of any of the foregoing assemblies, the radially inner bypass flow passage and the radially outer bypass flow passage are both radially outside a core flow passage of the gas turbine engine. 
         [0022]    A method of reducing a vibratory response of airfoils that support a shroud according to an exemplary aspect of the present disclosure includes, among other things, circumferentially misaligning at least one of the airfoils in a radially inner array of airfoils with all airfoils of a radially outer array of airfoils. Circumferentially aligning at least one of the airfoils in the radially inner array of airfoils with all airfoils of the radially outer array of airfoils. 
         [0023]    In another example of the foregoing method, the method further includes aligning an airfoil of the radially inner array and an airfoil of the radially outer array at a twelve o&#39;clock position. 
         [0024]    In another example of any of the foregoing methods, the method further includes circumferentially misaligning the remaining airfoils of the radially inner array with all the airfoils of the radially outer array. 
         [0025]    In another example of any of the foregoing methods, the method further includes guiding flow through a radially inner bypass flow passage using the radially inner array of airfoils, and guiding flow through a radially outer bypass flow passage using the radially outer array of airfoils. 
         [0026]    In another example of any of the foregoing methods, the method further includes moving core flow through a core flow passage that is radially inside the radially inner bypass flow passage. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0027]    The various features and advantages of the disclosed examples will become apparent to those skilled in the art from the detailed description. The figures that accompany the detailed description can be briefly described as follows: 
           [0028]      FIG. 1  schematically shows a multiple bypass stream gas turbine engine. 
           [0029]      FIG. 2  is a highly schematic rear view of a concentric airfoil assembly from the engine of  FIG. 1 . 
           [0030]      FIG. 3  is a perspective view of a circumferential section of the assembly of  FIG. 2 . 
           [0031]      FIG. 4  is a perspective view of another circumferential section of the assembly of  FIG. 2 . 
           [0032]      FIG. 5  illustrates another example concentric airfoil assembly suitable for use with the engine of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0033]      FIG. 1  shows an exemplary engine  10  in a schematic manner. A fan section  12  delivers air into a core engine  16 , a radially inner bypass passage  20 , and a radially outer bypass passage  24 . 
         [0034]    A core engine flow C of air is delivered to the core engine  16  from the fan section  12  and moves along a core engine flow passage extending through a compressor section  28 , a combustor section  32 , a turbine section  36 , and then outwardly of a nozzle  40 . Compressed air from the compressor section  28  is mixed with fuel and ignited in the combustor section  32 . The products of combustion drive turbine rotors in the turbine section  36  to rotatably drive compressor rotors in the compressor section  28 , and fan rotors  44  and  48  about an axis A. 
         [0035]    The fan rotor  44  provides air to the main bypass flow B 1  and the core engine flow C. The main bypass flow B 1  flows through the radially inner bypass passage  20  inwardly of a main bypass flow outer housing  50 , and outwardly of a core engine outer housing  58 . 
         [0036]    The fan rotor  48  provides air to the main bypass flow B 1 , the core engine flow C, and a third stream bypass flow B 2 . The third stream bypass flow B 2  flows through a radially outer bypass passage  24  that is defined inwardly of an outer housing  54  and radially outwardly of the main bypass outer housing  50 . 
         [0037]    Referring now to  FIGS. 2-4  with continued reference to  FIG. 1 , the example engine  10  includes a concentric airfoil assembly  70  used to guide flow within the radially inner bypass passage  20  and the radially outer bypass passage  24 . The example airfoil assembly  70  includes a radially inner array  74  of airfoils  78  and a radially outer array  84  of airfoils  88 . The airfoils  78  extend from a radially inner face  90   i  of a mid-shroud  92  to a radially inner shroud  94 . The airfoils  88  extend from a radially outer face  90   o  of the mid-shroud  92  to a radially outer shroud  96 . The shrouds  92 ,  94 , and  96  are each annular in this example such that the assembly  70  is annular and extends circumferentially about the entire axis A. In other examples, the assembly  70  may be made of several individual assemblies extending circumferentially part of the way about the axis A. 
         [0038]    The radially inner shroud  94 , as can be appreciated, provides part of the core engine outer housing  58 . The mid-shroud  92  provides part of the main bypass flow outer housing  50 , and the radially outer shroud  96  provides part of the outer housing  54 . 
         [0039]    The example mid-shroud  92  can be supported radially within the engine by the arrays  74  and  84 . However, the primary function of the arrays  74  and  84  is to guide flow. One or both of the axial ends of the mid-shroud  92  may be rigidly attached to other structures within the engine  10 . 
         [0040]    The arrays  74  and  84  are concentric about the axis A. The airfoils  78  of the radially inner array  74  are disposed within the radially inner bypass passage  20 . The airfoils  88  of the radially outer array  84  are disposed within the radially outer bypass passage  24 . Thus, the airfoils  78  and  88  are disposed within concentric bypass flow passages. 
         [0041]    In other example assemblies, the radially inner array  74  can be disposed in the core engine  16  and the radially outer array  84  within the radially inner bypass passage  20 . A person having skill in this art and the benefits of this disclosure could develop other areas of the engine suitable fur utilizing the assembly  70 . 
         [0042]    The example assembly  70  is cast as a single unitary structure. In other examples, the airfoils  78  and  88  are produced separately from the shrouds  92 ,  94 , and  96 . The airfoils  78  and  88  are then fastened to the shrouds  94 ,  92 , and  96 . Fastening may include welding. 
         [0043]    In this example, the airfoils  78  extend radially further than the airfoils  88 . More specifically, the airfoils  88  have a radial length that is about half a radial length of the airfoils  78 . An overall diameter of the example assembly may be from 24 to 36 inches (610 to 914 millimeters). 
         [0044]    Flow moving through the assembly  70  may cause vibration. The example assembly  70  provides a circumferential distribution of the airfoils  78  and  88  that mitigates such vibrations. 
         [0045]    In this example, the inner array  74  includes airfoils  78   m  and an airfoil  78   a.  The airfoils  78   m  are circumferentially misaligned from each of the airfoils  88 . By contrast, the airfoil  78   a  is circumferentially aligned with an airfoil  88   a  of the array  84 . The aligned airfoils  78   a  and  88   a  are at a twelve o&#39;clock position in this example. 
         [0046]    As used herein, twelve o&#39;clock position refers to circumferential position relative to the axis A when the assembly  70  is in the orientation seen in  FIG. 2 , i.e. an orientation representative of the engine  10  or associated aircraft being on level ground or in straight and level flight. 
         [0047]    Regarding the airfoils  78   m,  a circumferential distance between the airfoils  78   m  and the circumferentially closest airfoil  88  of the array  84  varies. That is, D 1  is less than D 2 . By varying the circumferential spacing of the airfoils  78  relative to the airfoils  88 , the fundamental bending frequencies of the airfoils  78  and  88  are varied. This is because the stiffness of an area  90  where the airfoils  78  and  88  interface with the mid-shroud  92  is influenced by the circumferential alignment of the airfoils  78  relative the airfoils  88 . 
         [0048]    If the airfoils are aligned, such as the airfoil  78   a  and the airfoil  88   a,  the interface area  90  is stiffened, which increases a fundamental mode frequency in the area  90 . If the airfoils are not aligned, stiffness of the areas  90 ′ where the airfoils  78  and  88  interface with the mid-shroud  92  is reduced and the fundamental mode frequency in the area  90 ′ is lowered relative the area  90 . Varying the circumferential spacing of the airfoils  78  and  88  at the mid-shroud  92  also varies the fundamental natural frequencies of the airfoils  78  and  88 , which results in the assembly  70  being mistuned. 
         [0049]    The assembly  70  includes nine airfoils  78  in the inner array  74  and eight airfoils  88  in the outer array  84 . The airfoils  78  of the inner array  74  are distributed circumferentially equally about the axis A. The airfoils  88  in the outer array  84  are also distributed circumferentially evenly about the axis A. The difference in the number of total airfoils in the inner array  74  and the array  84  enables the circumferential misalignment of the airfoils  78  and  88 . Since different numbers of airfoils are used in the inner array  74  and the outer array  84 , and the airfoils are distributed evenly, the circumferential spacing between the airfoils of the arrays differs. 
         [0050]    Referring now to  FIG. 5 , another example assembly  170  is suitable for use within the engine  10 . The example assembly  170  is mis-tuned by providing asymmetric patterns of airfoils  178  in the inner array  174  and airfoils  188  in the outer array  184 . 
         [0051]    In the example assembly  170  airfoils  178  that are at or above a horizontal midline H of the assembly  170  are aligned with a corresponding airfoil  188  of the array  184 . Airfoils  178  that are below the horizontal midline H are misaligned with all airfoils  188  of the airfoil array  184 . The example array  174  includes five airfoils  178  at or above the horizontal midline H, and four airfoils  178  below the horizontal midline H. 
         [0052]    For the airfoils  178  below the horizontal midline H that are misaligned with airfoils in the array  184 , the circumferential distance D between the airfoil  178  and the circumferentially closest airfoil  188  also varies in this example. That is, D 1 ′ is less than D 2 ′, for example. 
         [0053]    Features of the disclosed examples include mistuning an assembly using variations in circumferential airfoils spacing between concentric airfoil arrays. The mistuning reduces resonant or aero-elastic response with minimal to no impact to the aerodynamic performance of the assembly. 
         [0054]    The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. Thus, the scope of legal protection given to this disclosure can only be determined by studying the following claims.