Patent Publication Number: US-2021172373-A1

Title: Assembly for a compressor section of a gas turbine engine

Description:
TECHNICAL FIELD 
     The disclosure relates generally to gas turbine engines, and more particularly to assemblies including one or more struts and variable orientation guide vanes as may be present in a compressor section of a gas turbine engine. 
     BACKGROUND 
     In a gas turbine engine, air is pressurized by rotating blades, mixed with fuel and then ignited for generating hot combustion gases which flow downstream through a turbine for extracting energy therefrom. The air is channeled through circumferential rows of fan and/or compressor blades which pressurize the air in turn. Load bearing struts may be disposed in the gas path upstream of downstream rotors and interact with the flow of air. The presence of such struts in the flow of air may induce undesirable flow conditions and energy losses in the flow of air. 
     SUMMARY 
     In one aspect, the disclosure describes an assembly for a compressor section of a gas turbine engine. The assembly comprises: 
     an outer shroud and an inner shroud defining a substantially annular gas path therebetween, the gas path having a central axis and extending axially along the central axis; 
     a plurality of struts extending radially in the gas path, the struts angularly spaced-apart around the central axis, two adjacent struts defining a strut passage therebetween, the strut passage extending axially along the central axis between leading edges of the two adjacent struts and trailing edges of the two adjacent struts; and 
     a plurality of variable orientation guide vanes extending radially in the gas path, the variable orientation guide vanes uniformly angularly spaced-apart around the central axis, one or more of the variable orientation guide vanes having a leading edge axially overlapping the strut passage at one or more orientations of the one or more of the variable orientation guide vanes. 
     In another aspect, the disclosure describes a gas turbine engine comprising: 
     a compressor for pressurizing air; 
     a combustor in which the compressed air is mixed with fuel and ignited for generating a stream of hot combustion gases; and 
     a turbine section for extracting energy from the combustion gases; 
     the compressor including: 
     a substantially annular gas path having a central axis and extending axially along the central axis; 
     a strut extending radially in the gas path; 
     a strut passage defined in the gas path, the strut passage extending axially along the central axis between a leading edge of the strut and a trailing edge of the strut; 
     a plurality of variable orientation guide vanes extending radially in the gas path, the variable orientation guide vanes uniformly angularly spaced-apart around the central axis, one or more of the variable orientation guide vanes having a leading edge axially overlapping the strut passage and a trailing edge disposed downstream of the strut passage at one or more orientations of the one or more of the variable orientation guide vanes; and 
     a rotor disposed downstream of the plurality of variable orientation guide vanes and configured to compress the air. 
     In a further aspect, the disclosure describes a method of directing a flow of air through a compressor section of a gas turbine engine. The method comprises: 
     receiving the flow of air over a strut extending radially across a substantially annular gas path of the gas turbine engine, the gas path having a central axis; 
     at least partially confining a strut wake generated in the flow of air by the strut, the strut wake being at least partially confined between two adjacent variable orientation guide vanes angularly spaced-apart from the strut, the variable orientation guide vanes axially overlapping a portion of the strut relative to the central axis; and 
     modifying the strut wake using at least one of the two variable orientation guide vanes. 
     Further details of these and other aspects of the subject matter of this application will be apparent from the detailed description included below and the drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying drawings, in which: 
         FIG. 1  shows an axial cross-section view of an exemplary gas turbine engine; 
         FIG. 2  shows a perspective view, partly sectioned, of a portion of an exemplary compressor section of the gas turbine engine of  FIG. 1 ; 
         FIG. 3  shows a schematic side view of a variable orientation guide vane and a strut; 
         FIG. 4  shows a schematic view of an arrangement of struts and variable orientation guide vanes with the variable orientation guide vanes shown in a first (e.g., fully open) position; 
         FIG. 5  shows a schematic view of an arrangement of struts and variable orientation guide vanes with the variable orientation guide vanes shown in a second (e.g., partially closed) position; 
         FIG. 6A  is a top perspective view of Mach number contours (represented by shading) of an exemplary gas path of the gas turbine engine of  FIG. 1  with uniform angular spacing between variable orientation guide vanes and with uniform angular spacing between struts when the variable orientation guide vanes are at a design condition; 
         FIG. 6B  is a top perspective view of other Mach number contours (represented by shading) of another exemplary gas path of the gas turbine engine of  FIG. 1  with non-uniform angular spacing between variable orientation guide vanes when the variable orientation guide vanes are at the design condition; 
         FIG. 7A  is a cross-sectional view along a plane normal to a central axis of the gas path of  FIG. 6A  downstream of the variable orientation guide vanes; 
         FIG. 7B  is a cross-sectional view along a plane normal to the central axis of the gas path of  FIG. 6B  downstream of the variable orientation guide vanes; 
         FIG. 8A  is a combined enlarged cross-sectional view of the gas path downstream of the variable orientation guide vanes combining views along a plane normal to central axis (below dividing line) and a plane parallel to central axis (above dividing line) when the variable orientation guide vanes are at an off-design condition and are evenly spaced with respect to each other; 
         FIG. 8B  is a combined enlarged cross-sectional view of the gas path downstream of the variable orientation guide vanes combining views along a plane normal to central axis (below dividing line) and a plane parallel to central axis (above dividing line) when the variable orientation guide vanes are at the off-design condition and are unevenly spaced with respect to each other; 
         FIG. 9  is an illustrative graph of an angle (alpha) of the velocity of the air downstream of the variable orientation guide vanes with respect to the central axis, e.g. at a fixed radial location or an average across multiple fixed radial locations and as a function of an angular spacing (theta) between two adjacent struts for the gas paths of  FIGS. 8A and 8B ; and 
         FIG. 10  is a flowchart illustrating an exemplary method of directing a flow of air through a compressor section of a gas turbine engine. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure relates generally to gas turbine engines, and more particularly to assemblies including one or more struts and variable orientation guide vanes as may be present in a compressor section of a gas turbine engine. In some embodiments, the assemblies and methods disclosed herein can promote better performance of gas turbine engines, such as by improving flow conditions in the compressor section in some operating conditions, improving the operable range of the compressor, reducing energy losses and aerodynamic loading on rotors. 
     Aspects of various embodiments are described in relation to the figures. 
       FIG. 1  shows an axial cross-section view of exemplary gas turbine engine  10 . Gas turbine engine  10  generally comprises in serial flow communication: a fan  12  through which ambient air is propelled, compressor section  14  for pressurizing the air, combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and turbine section  18  for extracting energy from the combustion gases. Rotors of compressor section  14  and turbine section  18  rotate about central axis  20  of substantially annular gas path  22  (referred to hereinafter as annular gas path  22 ) extending between gas turbine engine inlet  21  and outlet  23 . In some embodiments, gas turbine engine  10  may not include fan  12 . 
     It should be noted that the terms “upstream” and “downstream” used herein refer to the direction of an air/gas flow passing through annular gas path  22  of gas turbine engine  10 . It should also be noted that the term “axial”, “radial”, “angular” and “circumferential” are used with respect to central axis  20  of gas path  22 , which may also be a central axis of gas turbine engine  10 . In particular, we can define axial direction  50  parallel to central axis  20  and radial direction  52 . 
       FIG. 2  shows a perspective view, partly sectioned, of a portion of an exemplary compressor section  14  of gas turbine engine  10  of  FIG. 1 . Compressor section  14  includes inner shroud  32  and outer shroud  34  with annular gas path  22  defined between inner shroud  32  and outer shroud  34 . Annular gas path  22  may be in serial flow communication with upstream gas turbine engine inlet  21 , rotors and stators, and downstream rotors, stators and downstream gas turbine engine  10  outlet  23 . Each of the rotors may include one or more rows of circumferentially spaced rotor blades. One or more rotors may have one or more associated stators located upstream thereof. Examples of stators may include struts and vanes. Vanes  24  can be fixed orientation or variable orientation guide vanes (referred hereinafter as VGVs). Examples of rotors include fans, compressor rotors (e.g. impellers), and turbine rotors (e.g. those downstream of the combustion chamber). VGVs  24  lie in the substantially annular gas path, extending radially between inner shroud  32  and outer shroud  34 . “Extending radially” as used herein does not necessarily imply extending perfectly radially along a ray perfectly perpendicular to the central axis  20 , but is intended to encompass a direction of extension that has a radial component relative to the central axis  20 . VGVs  24  are arranged to define a circumferential row (circular array) of VGVs  24  angularly spaced-apart around central axis  20 . VGVs  24  may be disposed downstream of another stator and upstream of a rotor. For example, VGVs  24  may be disposed upstream of bladed rotor  64 . Each of VGVs  24  may have leading edge  26  and trailing edge  28 . 
     The orientation of VGVs  24  can be varied to change the direction towards which leading and trailing edges ( 26  and  28  respectively) are pointing. The orientation of VGVs  24  may be varied automatically and/or based on a pilot command via a controller (e.g., full authority digital engine controller (FADEC)) of gas turbine engine  10  for example. An orientation of one of VGVs  24  may be varied by changing its pitch angle about axis  30 . Axis  30  may be in a substantially radial direction with respect to central axis  20  and may define a pivot point P. The orientation of all VGVs  24  may be simultaneously varied by means of unison ring  36  connected to actuating system  40 . Connector links  38  may separately connect each of VGVs  24  to unison ring  36 . VGVs  24  may have an orientation where a flow of gas past VGVs  24  is substantially obstructed. In this orientation, the pitch angle of VGVs  24  may be such that the surface (or planform) area of VGVs  24  that is perpendicular to the oncoming flow is increased. For example, by varying the pitch of VGVs  24 , the angle between the oncoming flow and leading/trailing edges ( 26  and  28 , respectively) of VGVs  24  may be varied. At a larger angle, the flow may be obstructed more. VGVs  24  may have an orientation where a flow of gas past the VGVs  24  is substantially unobstructed. In this orientation, the pitch angle of VGVs  24  may be such that the surface (or planform) area of VGVs  24  that is perpendicular to the oncoming flow is decreased. For example, at a smaller angle between the oncoming flow and leading/trailing edges ( 26  and  28 , respectively) of VGVs  24 , the flow may be obstructed less. 
       FIG. 3  shows a schematic side view of exemplary VGV  24 A and strut  42 A. VGV  24 A is axially overlapping a portion  44  of strut  42 A, and may be positioned between two adjacent struts  42  (e.g. pair indicated by  66  in  FIGS. 4 and 5 ) in strut passage  54  defined therebetween. Strut  42 A is configured to interact with the flow of air  56 . 
       FIGS. 4 and 5  show schematic views of an arrangement of struts  42  and VGVs  24 . In  FIG. 4 , VGVs  24  arranged in a circumferential row around central axis  20  are in a first (e.g. fully open) position. In  FIG. 5 , VGVs  24  arranged in a circumferential row around central axis  20  are in a second (e.g. partially closed) position. The fully open position may be referred to a design condition. The second position may be referred to an off-design condition. In some embodiments, the design condition is defined as an operating condition for which a design of gas turbine engine  10  may be optimized and intended to spend most of its operational life. For example, the design condition may be a level-cruise condition of an aircraft to which gas turbine engine  10  is mounted. The design condition may include a throttle lever setting, a range of gas turbine engine speeds expressed as revolutions-per-minute (RPM) and/or other engine-related parameter. In some embodiments, the off-design conditions may include a takeoff, climb and/or landing phase of flight of the aircraft and its associate operating setting of gas turbine engine  10  for example. The off-design condition may be any condition different from (other than) the design condition. 
     In reference to both  FIGS. 4 and 5 , the exemplary assembly comprises a circumferential row of struts  42  extending radially in gas path  22 . Struts  42  may be configured to receive a flow of air  56  to impinge thereupon. Struts  42  may each have an airfoil profile with an associated leading edge  46  and trailing edge  48 . Struts  42  may be designed as relatively thick (in a circumferential dimension) and optionally hollow airfoils with a relatively large chord length which is measured between leading and trailing edges ( 46  and  48 , respectively) of the airfoil, in order to bear structural (e.g., thrust) loads between components (e.g., casings) of gas turbine engine  10 . The airfoil profile of struts  42  may be selected to modify the flow downstream of struts  42 . Struts  42  may extend between outer shroud  34  and inner shroud  32 . Struts  42  may be uniformly or non-uniformly angularly spaced-apart around central axis  20 . Struts  42  may be oriented based on the oncoming flow conditions or struts  42  may be oriented based on local flow conditions. Two adjacent struts (pair indicated by  66 ) have a space in between which defines strut passage  54 . Struts  42  may have respective strut wakes  68  in gas path  22  passing across the assembly. As referred to herein “wake” refers to a region of disturbed flow downstream of a solid body. 
     The assembly further includes a circumferential row of VGVs  24 . The number of VGVs  24  may exceed the number of struts  42 . Each of VGVs  24  may have an airfoil shape with associated leading edge  26  and trailing edges  28 . VGVs  24  may have pressure and suction surfaces defined between leading edge  26  and trailing edges  28 . In some embodiments, VGVs  24  may be symmetric so that at a zero angle of attack, the pressure and suction surfaces cannot be distinguished. Each of VGVs  24  may be configured to be smaller than one of struts  42  proximal thereto either in airfoil thickness (e.g., the maximum thickness between the pressure and suction surfaces) and/or in chord length. The circumferential row of VGVs  24  is upstream of bladed rotor  64 . 
     In some embodiments, the assembly includes a circular array of VGVs  24  about the central axis  20 . The VGVs  24  may, but not necessarily, all be positioned at substantially the same axial location relative to the central axis  20 . In various embodiments, the angular spacing of the VGVs  24  and struts  42  may be configured so that two, three or more VGVs  24  are angularly disposed between two neighbouring struts  42 . 
     The VGVs  24  may include a pair of VGVs  24 A,  24 B straddling a strut  42 A of the plurality of struts  42 . For example, VGVs  24 A,  24 B straddling the strut  42 A may refer to having a VGV on either side of the strut  42 A and both positioned so as to modify or interact with a wake of the strut  42 A by, for example, inducing channel-like flows in intervening regions between VGVs  24 A,  24 B and strut  42 A. The pair of VGVs  24 A,  24 B may be spaced apart from each other by a first circumferential distance  58 A. The VGVs  24  may include a first VGV  24 C disposed at a second circumferential distance  58 B (equal to the first circumferential distance  58 A) away from one VGV  24 B of the pair of VGVs in a first circumferential direction  53 A about the central axis  20 . The VGVs  24  may include a second VGV  24 D disposed at a third circumferential distance  58 C away from another VGV  24 A of the pair of VGVs in a second circumferential direction  53 B about the central axis  20 , the second circumferential direction  53 B being opposite the first circumferential direction  53 A. The first, second, and third circumferential distances ( 58 A,  58 B and  58 C) may be inter-vane spacings. The first, second, and third circumferential distances ( 58 A,  58 B and  58 C) may be measured along a radially outer surface of the inner shroud  32  and may be (substantially) equal to each other. 
     One or more VGVs  24  have respective leading edges  26  axially overlapping strut passage  54 . A portion of VGVs  24  may be contained within strut passage  54 . Respective trailing edges  28  of one or more VGVs  24  may be disposed downstream of strut passage  54 . In some embodiments, leading edges  26  of one or more VGVs  24  overlap strut passage  54  but are axially downstream of a location of maximum thickness (schematically marked a location  86 ) between opposing lateral (e.g., pressure and suction) surfaces of at least one of two adjacent struts  42  (pair indicated by  66 ) defining strut passage  54 . In some embodiments, VGVs  24  may be tandem VGVs  24  wherein a first row of vanes is directly upstream of a second row of vanes. 
     In the embodiment shown in  FIGS. 4 and 5 , VGVs  24  are uniformly angularly spaced-apart around central axis  20 . The angular distance between two adjacent (circumferentially closest) struts  42  (the pair indicated by  66 ) is defined as inter-strut spacing  62  extending between chords of the adjacent struts  42 . A single assembly may have a plurality of inter-strut spacings  62 , since struts  42  may not be uniformly angularly space-apart around central axis  20 . The angular distance between two adjacent (circumferentially closest) VGVs  24  is defined as inter-vane spacing  58 . Inter-vane spacing  58  associated with any two vanes  42  may be the same since VGVs  24  are uniformly angularly spaced-apart. The distance between one of VGVs  24  and one of struts  42  circumferentially closest thereto is defined as strut-vane spacing  60 A. A single assembly may include a variety of strut-vane spacings  60 A, since struts  42  may not be uniformly angularly space-apart around central axis  20 . An or any inter-vane spacing  58  may be different than at least one of the strut-vane spacings. 
     In some embodiments, an angular position of strut  42 A may be substantially equidistant from two adjacent vanes  24  when vanes  24  are disposed in the fully open position as shown in  FIG. 4 . In such embodiments, the angular spacings  60 A and  60 B may be substantially equal. Alternatively, strut  42 A may not be angularly equally spaced between adjacent vanes  24  so that the angular spacings  60 A and  60 B may be different. The angular spacings  60 A and  60 B may be taken from a pivot point P of a vane  24  to a chord of the strut  42 . 
     VGVs  24  may also have respective VGV wakes  76  in gas path  22  flow passing across the assembly. Strut wakes  68  and VGV wakes  76  may interact and influence a downstream velocity. In some situations, channel-like flows between struts  42  and VGVs  24  may also interact. VGVs  24  thus distributed with respect to struts  42  and protruding within strut passage(s)  54  may cause a downstream overall component of velocity non-parallel to central axis  20  to be modified by the interaction with the flow of air in gas path  22 . In some embodiments, strut wakes  68  are modified by interaction due to channel-like flows between struts  42  and VGVs  24 . In some embodiments, the overall circumferential component of velocity is reduced. The downstream velocity may be a velocity in region  78  proximal to and upstream of rotor  64 . Without being bound by a particular theory, in some embodiments, the interaction of VGVs  24  and strut wakes  68  may cause more desirable flow conditions upstream of rotor  64 . Strut wakes  68  shown in  FIGS. 4 and 5  are illustrative. In various embodiments, strut wakes  68  may be narrower or wider. In some embodiments, strut wakes  68  may only weakly interact (e.g. impinge) on the VGVs  24  or only weakly influence a flow proximal to the VGVs  24 . 
     Overall velocity (or overall component of velocity) refers to a characteristic velocity. Velocity is a field quantity comprising a 3-D vector function of space and time, and particularly in a turbulent flow as may be present in gas turbine engine  10 , acquires a chaotic pattern. For example, although a flow may be moving in a direction from inlet  21  of gas turbine engine  10  to outlet  23  at a certain speed on average, it may, locally in space-time, be moving in a completely different direction and at a different speed. For this reason, a characteristic velocity is used to characterize the flow. Thus, in some embodiments, an overall velocity (or overall component of velocity) may be a maximum or average velocity (or component of velocity) over some time period in an associated portion of gas path  22 . In other embodiments, an overall velocity (or overall component of velocity) may be a conditional average, p-norm (for some integer p&gt;0), filtered and averaged, mass-averaged velocity, or some other processed velocity (or component of velocity) over some time period in an associated portion of gas path  22 . In an exemplary embodiment, the overall component of velocity non-parallel to central axis  20 , downstream of VGVs  24  and in region  78  close to rotor  64 , is the integral of the circumferential velocity over slice  80  of gas path  22  close to rotor  64  divided by the volume of the slice  80  and over a time period greater than a characteristic time associated with gas turbine engine  10  (e.g. time-scale associated with an engine RPM). 
     In embodiments having more numerous VGVs  24  than struts  42 , for VGVs  24  overlapping strut passage(s)  54 , the pattern of circumferential spacings  60 A between struts  42  and VGVs  24  leads to a strut-vane spacing  60 A smaller than any inter-vane  58  or inter-strut spacing  62 . The respective small strut-vane spacing  60 A may confine strut wake  68 , which may eventually impinge on one of VGVs  24  or otherwise interact with a flow around a VGV  24  and disrupt strut wake  68  flow structures, e.g. by attenuating large-scale (and more energetic) turbulent eddies. The strut-vane spacing  60 A may also confine channel-like flows between struts  42  and VGVs  24  and thereby may, in some embodiments, cause strut wakes  68  to be modified by interaction due to the channel-like flows between struts  42  and VGVs  24 . In some embodiments, a wake vortex (e.g. a coherent vortex flow structure) shedding from a strut  42  may be disrupted by the interaction (e.g., impingement). As a result of disrupted strut wake  68  downstream of the assembly, an overall circumferential component of velocity non-parallel to central axis  20  of gas turbine engine  10  may be reduced, thereby potentially increasing efficiency of downstream rotor  64 . In some embodiments, reduced distortion or disruption in the flow may also help in improve the operable range of the compressor. 
       FIG. 6A  is a top perspective view of Mach number contours (shading) of an exemplary gas path of gas turbine engine  10  of  FIG. 1  with uniform angular spacing between VGVs  24  where inter-vane spacings  158 A and  158 B are the same, and with uniform angular spacing between struts  42  (i.e. uniform inter-strut spacings), when VGVs  24  are at an exemplary design condition. 
       FIG. 6B  is a top perspective view of Mach number contours (shading) of another gas path of gas turbine engine  10  of  FIG. 1  with non-uniform (e.g., irregular) angular spacing between VGVs  24  where inter-vane spacings  258 A and  258 B are not the same, when VGVs  24  are at the exemplary design condition. 
     In the exemplary embodiment of  FIG. 6A , the assembly has strut-vane spacing  160  smaller than both inter-vane spacings  158 A and  158 B. In contrast, in the assembly of  FIG. 6B , strut-vane spacing  260  is equal to inter-vane spacing  258 A. As a result of this difference, the characteristics of the flow field in a region  88  downstream of struts  42 , and particularly in region  78  close to rotor  64 , are different in the exemplary embodiment of  FIG. 6A  as compared to the assembly shown in  FIG. 6B . At the design condition of VGVs  24 , the swirl of the flow field in region  78  or  88  of the exemplary embodiment of  FIG. 6A  may be similar or higher than the swirl in the flow in region  78  or  88  in the embodiment of  FIG. 6B . At an off-design condition of VGVs  24 , the swirl of the flow field in region  78  or  88  of the exemplary embodiment of  FIG. 6A  may be lower than the swirl in the flow in region  78  or  88  in the embodiment of  FIG. 6B . The swirl may be the angle the velocity makes with streamwise direction, the maximum vorticity, or any other measure of the strength of non-parallel flow. 
       FIG. 7A  is a cross-sectional view, along a plane normal to central axis  20  of gas path  22  of  FIG. 6A  downstream of VGVs  24 . The cutting plane for the cross-sectional view is indicated by broken line P-P′ in  FIG. 6A . As in  FIG. 6A , contour shading indicates Mach number. 
       FIG. 7B  is a cross-sectional view, along a plane normal to central axis  20 , of gas path  22  of  FIG. 6B  downstream of VGVs  24 . The cutting plane for the cross-sectional view is indicated by the broken line P-P′ in  FIG. 6B . As in  FIG. 6B , contour shading indicates Mach number. 
       FIG. 8A  is a combined enlarged cross-sectional view of the gas path  22  downstream of VGVs  24  and close to rotor  64 , combining views along a plane normal to central axis  20  (below dividing line  500 ) and a plane parallel to central axis  20  (above dividing line  500 ), when VGVs  24  are at an off-design condition and are evenly spaced with respect to each other (compare to the configuration of  FIG. 6A ). 
       FIG. 8B  is a combined enlarged cross-sectional view of the gas path  22  downstream of VGVs  24  and close to rotor  64 , combining views along a plane normal to central axis  20  (below dividing line  500 ) and a plane parallel to central axis  20  (above dividing line  500 ), when VGVs  24  are at an off-design condition and are evenly spaced with respect to struts  42  and each other, i.e. unevenly spaced with respect to each other (compare to the configuration of  FIG. 6B ). 
     In reference to both  FIGS. 8A and 8B , the combined views show an angular portion of the annular gas path (the portion encompassing one strut  42 ). Contour shading indicates the angle (alpha) of the velocity with respect to axial direction  50 . An angle (alpha) of zero may be desirable in some embodiments. Regions of relatively low alpha are shown at  91 A and  91 B. Regions of relatively high alpha are shown at  93 A and  93 B.  FIG. 8A  shows the exemplary embodiment of  FIG. 6A  at an off-design condition.  FIG. 8B  shows the assembly of  FIG. 6B  at an off-design condition. As mentioned previously, the swirl (here quantified using alpha) may be lower in the embodiment of  FIG. 6A  at an off-design condition compared to the assembly of  FIG. 6B . 
       FIGS. 8A and 8B  show that the exemplary embodiment of  FIG. 6A  has a less overall non-parallel flow in relation to the assembly of  FIG. 6B , at an off-design condition. Such a flow condition may arise because of the confinement of strut wake  68  between VGVs  24 , which may lead to attenuation of strut wake  68 , in the exemplary embodiment, at an off-design condition. Additionally, confinement may encourage mixing of VGV wakes  76  and strut wakes  68  (see  FIG. 5 ), which may cause attenuation of strut wake  68  in region  78 . For example, such an attenuation may be caused by vortex interaction between strut wakes  68  and VGV wakes  76  which in turn may accelerate a turbulent cascade breaking up large-scale (and more energetic) vortical structures into small-scale (and less energetic) vortical structures that can be more efficiently viscously dissipated. Without such confinement-specific effects, the vortical structures may be longer-lived and may decay more slowly. When strut wake  68  is (e.g., tightly) confined between two VGVs  24 , as in the exemplary embodiment, the flow in region  78  may be more diffuse and sharp gradients in Mach number may not be as prevalent as when there is less confinement, as in the assembly of  FIGS. 8B and 6B . Sharp gradients may be associated with increased vorticity (shear tilting of the flow), which may lead to undesirable non-parallel flow further downstream in region  78  close to rotor  64 . 
       FIG. 9  is an illustrative graph of the angle (alpha) of the velocity downstream of VGVs  24  with respect to central axis  20 , e.g. at a fixed radial location or an average across multiple fixed radial locations, and as a function of the angular spacing (theta) between two adjacent struts  42 , for the gas paths of  FIGS. 6A and 6B . Plots  82  and  84  represent or are indicative of the values of the contours of  FIGS. 8A and 8B  (respectively) at a fixed radial distance. An example fixed radial distance  90  is indicated in  FIG. 8B  for illustrative purposes. Plot  84  is associated with the exemplary embodiment of  FIG. 8A , and plot  82  is associated with the assembly of  FIG. 8B . As shown in the graph, when the strut-vane spacing  60 A is smaller than inter-vane spacing  58  (see  FIGS. 4 and 5 ), the maximum circumferential component of velocity may be reduced as compared to when the strut-vane spacing  60 A is equal to inter-vane spacing  58 . 
       FIG. 10  is a flowchart illustrating an exemplary method  300  of directing a flow of air through compressor section  14  of gas turbine engine  10 . Method  300  may be carried out using assemblies as disclosed herein. Method  300  includes:
         receiving the flow of air over strut  42  extending radially across a substantially annular gas path  22  of gas turbine engine  10 , gas path  22  having central axis  20  (see block  310 );   at least partially confining strut wake  68  generated in the flow of air by strut  42 , strut wake  68  being at least partially confined between two adjacent variable orientation guide vanes  24  angularly spaced-apart from strut  42 , variable orientation guide vanes  24  axially overlapping a portion of strut  42  relative to central axis  20  (see block  320 ); and   modifying strut wake  68  using at least one of the two variable orientation guide vanes  24  (see block  330 ).       

     In some embodiments, modifying strut wake  68  includes permitting strut wake  68  to interact (e.g. impinge) on at least one of the two variable orientation guide vanes  24 . 
     Some embodiments include permitting at least one vortex shed from strut  42  to interact (e.g. impinge) on the at least one of the two variable orientation guide vanes  24  to reduce an overall component of velocity of the flow of air non-parallel to central axis  20 . 
     In some embodiments, an angular spacing between the two variable orientation guide vanes  24  relative to central axis  20  is configured to reduce an overall component of velocity of the flow of air downstream of strut  42 . 
     In some embodiments, the overall component of velocity is normal to central axis  20  in a region downstream of the two variable orientation guide vanes  24 . 
     In some embodiments, modifying the strut wake  68  includes attenuating strut wake  68  using a variable orientation guide vane wake  76  generated in the flow of air by at least one of the two variable orientation guide vanes  24 . 
     The above description is meant to be exemplary only, and one skilled in the relevant arts will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. The present disclosure may be embodied in other specific forms without departing from the subject matter of the claims. The present disclosure is intended to cover and embrace all suitable changes in technology. Modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims. Also, the scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.