Abstract:
A subsonic diffuser is provided for decelerating airflow provided to an air-breathing engine. The subsonic diffuser includes a duct having an inlet and an outlet, and a splitter. The cross-sectional area of the duct increases from the inlet to the outlet, and the splitter delineates a plurality of passageways through the duct. During operation, the passageways divide and decelerate a subsonic airflow supplied to the subsonic diffuser, and the subsonic diffuser delivers a decelerated airflow to the air-breathing engine.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. provisional application Ser. No. 60/582,784 filed on Jun. 25, 2004, which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention generally relates to aircraft propulsion systems, and more particularly, to specially-configured subsonic diffusers for high-speed inlets.  
       BACKGROUND  
       [0003]     Recently, development of high-speed inlets has concentrated on supersonic diffuser designs with different types of compression, compression splits, bleed, etc. While some significant improvements in the performance of high-speed inlets have been documented using these supersonic diffuser designs, new approaches to the design of subsonic diffusers have received very little study. Some work with vortex generators, blowing, and bleed for subsonic diffuser boundary-layer control has been completed. However, this work has mostly concentrated on subsonic diffusers that have been designed using conventional design techniques which have been used for the last forty years. While currently used high-speed inlets have provided reasonable levels of performance, potential improvements in the performance that could be realized by concentrating development efforts on new subsonic diffuser designs have mostly been ignored. Consequently, there is a need for new subsonic diffusers that can be integrated with new high-speed inlets to provide significant improvement over conventional high-speed inlets.  
       SUMMARY  
       [0004]     In general, the present invention contemplates a subsonic diffuser including a duct having an inlet and an outlet, where the cross-sectional area of the duct increases from the inlet to the outlet, and a splitter in the duct for dividing the outlet into a plurality of outlet ports.  
         [0005]     The present invention also contemplates a method for decelerating airflow provided to an air-breathing engine using a subsonic diffuser, the method including dividing a subsonic airflow supplied to the subsonic diffuser between two passageways, decelerating the airflow in each of the two passageways, and delivering the decelerated airflow to the air-breathing engine.  
         [0006]     The present invention also contemplates a method of designing a subsonic diffuser, the method including providing the subsonic diffuser with a specified number of splitters, arranging each of the specified number of splitters to provide at least two passageways through the subsonic diffuser, and dimensioning the length of the subsonic diffuser according to the specified number of passageways  
         [0007]     The present invention also contemplates a subsonic diffuser including a duct having an inlet and an outlet, and a means for splitting an airflow between the inlet and the outlet.  
         [0008]     The present invention also contemplates a subsonic diffuser including a duct having an inlet and an outlet, where the cross-sectional area of the duct increases from the inlet to the outlet, and a splitter in the duct extending from the inlet partially through the length of the duct, the splitter dividing the inlet into a plurality of inlet port  
         [0009]     The present invention also contemplates a subsonic diffuser including a duct having an inlet and an outlet, wherein the cross-sectional area of the duct increases from the inlet to the outlet, and a splitter in the duct extending partially across the duct.  
         [0010]     Further embodiments, variations, and enhancements are also described herein. 
     
    
     DRAWINGS  
       [0011]      FIG. 1A  is a cross-sectional schematic view detailing the various sections of a bifurcated mixed compression high-speed (or supersonic) inlet including a conventional subsonic diffuser.  
         [0012]      FIG. 1B  is a cross-sectional view of single side mixed compression high-speed inlet including another type of conventional subsonic diffuser.  
         [0013]      FIG. 1C  is a cross-sectional schematic view of an external compression high-speed inlet including yet another type of conventional subsonic diffuser.  
         [0014]      FIG. 1D  is a cross-sectional schematic view of an all-internal compression high-speed inlet including yet another type of conventional subsonic diffuser.  
         [0015]      FIG. 2A  is an isometric view of the conventional subsonic diffuser depicted in  FIG. 1D .  
         [0016]      FIG. 2B  is a representational isometric view of the conventional subsonic diffuser of  FIG. 2A  as a conical frustum.  
         [0017]      FIG. 2C  is a lengthwise cross-sectional schematic view of the conical frustum of  FIG. 2B  including the dimensions used in evaluating performance characteristics.  
         [0018]      FIG. 3  is graph depicting the general performance characters associated with various subsonic diffusers having different dimensions.  
         [0019]      FIG. 4A  is a representational isometric view of a subsonic diffuser according to the present invention used in evaluating its performance characteristics.  
         [0020]      FIG. 4B  is an elevational view of one end of the subsonic diffuser of  FIG. 4A .  
         [0021]      FIG. 5A  is a representational isometric view of another subsonic diffuser according to the present invention used in evaluating its performance characteristics.  
         [0022]      FIG. 5B  is an elevational view of one end of the subsonic diffuser of  FIG. 5A .  
         [0023]      FIG. 6  is graph depicting the design characteristics of subsonic diffusers having various numbers of passageways.  
         [0024]      FIG. 7A  is a representational isometric view of another subsonic diffuser according to the present invention used in evaluating its performance characteristics.  
         [0025]      FIG. 7B  is a cut-away isometric view of the subsonic diffuser depicted in  FIG. 7A .  
         [0026]      FIG. 8  is a lengthwise cross-sectional schematic view of the subsonic diffuser of  FIG. 7A  including the dimensions used in evaluating its performance characteristics.  
         [0027]      FIG. 9A  is a cross-sectional schematic view of an all-internal compression high-speed inlet including a subsonic diffuser according to the present invention.  
         [0028]      FIG. 9B  is a cross-sectional view of the all-internal compression high-speed inlet taken along Line  9 B- 9 B of  FIG. 9A .  
         [0029]      FIG. 10  is an isometric view of another conventional subsonic diffuser having a square-shaped inlet throat.  
         [0030]      FIG. 11  is a lengthwise cross-sectional schematic view of the conventional subsonic diffuser of  FIG. 10  including the dimensions used in evaluating performance characteristics.  
         [0031]      FIG. 12A  is an isometric view of another subsonic diffuser according to the present invention having a square-shaped inlet throat.  
         [0032]      FIG. 12B  is a cut-away isometric view of the subsonic diffuser depicted in  FIG. 12A .  
         [0033]      FIG. 13  is a lengthwise cross-sectional view of the subsonic diffuser of  FIG. 12A  including the dimensions used in evaluating performance characteristics.  
         [0034]      FIG. 14  is an isometric view of an all-internal compression high-speed inlet having a conventional diffuser with a high aspect ratio rectangular-shaped throat.  
         [0035]      FIG. 15  is an isometric view of the conventional subsonic diffuser provided within the all-internal compression high-speed inlet of  FIG. 14 .  
         [0036]      FIG. 16A  is an isometric view of another subsonic diffuser according to the present invention having a rectangular-shaped throat.  
         [0037]      FIG. 16B  is a cut-away isometric view of the subsonic diffuser depicted in  FIG. 17A .  
         [0038]      FIG. 17A  is a lengthwise cross-sectional schematic view of another subsonic diffuser according to the present invention including one splitter providing two airflow passageways.  
         [0039]      FIG. 17B  is a cross-sectional view of one end of the subsonic diffuser taken along Line  17 B- 17 B of  FIG. 17A .  
         [0040]      FIG. 17C  is a cross-sectional view of the other end of the subsonic diffuser taken along Line  17 C- 17 C of  FIG. 17A .  
         [0041]      FIG. 18A  is a lengthwise cross-sectional view of another subsonic diffuser according to the present invention including two splitters providing four airflow passageways.  
         [0042]      FIG. 18B  is a cross-sectional view of one end of the subsonic diffuser taken along Line  18 B- 18 B of  FIG. 18A .  
         [0043]      FIG. 18C  is a cross-sectional view of the other end of the subsonic diffuser taken along Line  18 C- 18 C of  FIG. 18A .  
         [0044]      FIG. 19A  is a lengthwise cross-sectional view of another subsonic diffuser according to the present invention including one splitter providing two airflow passageways.  
         [0045]      FIG. 19B  is a cross-sectional view of one end of the subsonic diffuser taken along Line  19 B- 19 B of  FIG. 19A .  
         [0046]      FIG. 19C  is a cross-sectional view of the other end of the subsonic diffuser taken along Line  19 C- 19 C of  FIG. 19A .  
         [0047]      FIG. 20A  is a lengthwise cross-sectional view of another subsonic diffuser according to the present invention including two splitters providing three airflow passageways.  
         [0048]      FIG. 20B  is a cross-sectional view of one end of the subsonic diffuser taken along Line  20 B- 20 B of  FIG. 20A .  
         [0049]      FIG. 20C  is a cross-sectional view of the other end of the subsonic diffuser taken along Line  20 C- 20 C of  FIG. 20A .  
         [0050]      FIG. 21A  is a lengthwise cross-sectional view of another subsonic diffuser according to the present invention having one splitter moveable between two positions.  
         [0051]      FIG. 21B  is a cross-sectional view of one end of the subsonic diffuser taken along Line  21 B- 21 B of  FIG. 21A .  
         [0052]      FIG. 21C  is a cross-sectional view of the other end of the subsonic diffuser taken along Line  21 C- 21 C of  FIG. 21A .  
         [0053]      FIG. 22A  is a lengthwise cross-sectional view of another subsonic diffuser according to the present invention having a segmented splitter with a segment moveable between two positions.  
         [0054]      FIG. 22B  is a cross-sectional view of one end of the subsonic diffuser taken along Line  22 B- 22 B of  FIG. 22A .  
         [0055]      FIG. 22C  is a cross-sectional view of the other end of the subsonic diffuser taken along Line  22 C- 22 C of  FIG. 22A .  
         [0056]      FIG. 23A  is a lengthwise cross-sectional view of another subsonic diffuser according to the present invention having a splitter extending only partially along the length of the subsonic diffuser.  
         [0057]      FIG. 23B  is a cross-sectional view of one end of the subsonic diffuser taken along Line  23 B- 23 B of  FIG. 23A .  
         [0058]      FIG. 23C  is a cross-sectional view of the other end of the subsonic diffuser taken along Line  23 C- 23 C of  FIG. 23A .  
         [0059]      FIG. 24  is a cut-away isometric view of another subsonic diffuser according to the present invention.  
         [0060]      FIG. 25A  is a lengthwise cross-sectional view of the subsonic diffuser taken along Line  25 A- 25 A of  FIG. 24 .  
         [0061]      FIG. 25B  is a cross-sectional view of one end of the subsonic diffuser taken along Line  25 B- 25 B of  FIG. 25A .  
         [0062]      FIG. 25C  is a cross-sectional view of the other end of the subsonic diffuser taken along Line  25 C- 25 C of  FIG. 25A . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0063]     Typical high-speed (or supersonic) inlets used with various aircraft propulsion systems associated with air-breathing engines are depicted in  FIGS. 1A-1D . Each of the high-speed inlets depicted in  FIGS. 1A-1D  includes a subsonic diffuser which can be modified according to the present invention. As shown in  FIG. 1A , a bifurcated mixed compression high-speed inlet generally indicated by the numeral  10  has various sections including a supersonic diffuser  12 , a throat  14 , and a conventional subsonic diffuser  16 . Note that the cross-section of  FIG. 1A  could also represent an axisymmetric inlet in which the subsonic diffuser associated therewith transitions from an annular throat cross-section to the round air-breathing engine.  
         [0064]     The supersonic diffuser  12  decelerates the airflow entering the high-speed inlet  10  using a series of weak shock waves. In doing so, the supersonic diffuser decreases the speed of the airflow from a supersonic speed (i.e. high Mach number) to a low supersonic speed of about 1.2 to 1.3 times the speed of sound at the entrance to the throat  14 . Thereafter, the speed of the airflow is decreased from the low supersonic speed of about 1.2 to 1.3 times the speed of sound to a high subsonic speed by a terminal shock wave inside throat  14 . The high subsonic speed of the airflow is further reduced using the subsonic diffuser  16 .  
         [0065]     As shown in  FIG. 1A , the high subsonic speed of the airflow is reduced in the subsonic diffuser  16  by an increase in cross-sectional area of the passageway or duct  18 . The conventional subsonic diffusers depicted in  FIGS. 1B, 1C , and  1 D are configured in a similar manner. For example,  FIG. 1B  depicts a single side mixed compression high-speed inlet  20  having a subsonic diffuser  21 ,  FIG. 1C  depicts an external compression high-speed inlet  22  having a subsonic diffuser  23 , and  FIG. 1D  depicts an all-internal compression high-speed inlet  24  including a subsonic diffuser  25 . Each of the conventional subsonic diffusers  16 ,  21 ,  23 , and  25  have different configurations, and each can be modified according to the present invention. In doing so, the resulting subsonic diffusers can have shortened lengths relative to the conventional subsonic diffusers  16 ,  21 ,  23 , and  25 , and have substantially similar performance characteristics. The shortened lengths of the resulting subsonic diffusers advantageously decrease the weight of the resulting high-speed inlets, thereby increasing the efficiency of the associated aircraft propulsion systems. The shortened lengths of the resulting subsonic diffusers also provide for shortened aircraft propulsion systems which offers more options for integration in aircraft.  
         [0066]     Basic subsonic diffuser nomenclature and diffusion characteristics are presented in  FIGS. 2A, 2B ,  2 C, and  3 . Performance characteristic curves according the dimensions of various subsonic diffusers are shown in  FIG. 3 . The performance characteristic curves of  FIG. 3  are a composite of many different research studies on a variety of subsonic diffusers having a variety of cross-sectional shapes as well as off-sets, etc. The performance characteristic curves of  FIG. 3  are used as a guide to determine acceptable dimensions for the length, entrance width, and diffusion angle to avoid airflow separation in subsonic diffusers. If the relationship between the length, entrance width, and diffusion angle on  FIG. 3  falls within the region of no appreciable stall, the subsonic diffuser having these dimensions should avoid airflow separation, and should yield acceptable performance and distortion.  
         [0067]     For example, a conventional subsonic diffuser  28  is shown in  FIG. 2A  having an inlet throat (or entrance)  30 , an outlet  32 , and a single duct  33  extending between the inlet throat  30  and outlet  32 . The typical approach to providing the subsonic diffuser  28  with acceptable dimensions according to the performance characteristic curves of  FIG. 3  is to define a circle representing the desired area of the inlet throat  30 , and, thereafter, represent the conventional subsonic diffuser  28  as the conical frustum  34  shown in  FIG. 2B  including a single duct  35 . Because one end  36  of the conical frustum  34  is defined by the aforementioned circle, and the other end  38  is defined by the air-breathing engine, the conical frustum  34  has known values for the minimum diameter and maximum diameter. As such, because diffusion angles of about 6° to 8° are typically provided, the appropriate length of the conical frustum  34  can be determined using  FIG. 3 . For simplicity, the conical frustum  34  can be reduced to a two-dimensional form depicted in  FIG. 2C  having a length L, an entrance width W 1  equal to its minimum diameter, an outlet width W 2  equal to its maximum diameter, and a half angle ⊖ (equal to one half the diffusion angle  2 ⊖).  
         [0068]     The performance characteristic curves of  FIG. 3  are the basis for the development of specially-configured subsonic diffusers according to the present invention. Generally, the specially-configured subsonic diffusers depicted in the accompanying drawings are formed as a duct including a splitter used to split the airflow passing through the diffuser between a plurality of airflow passageways. The ducts and/or the passageways defined by the splitters increase in cross-sectional area between the inlets and outlets of the specially-configured subsonic diffusers. For example,  FIGS. 4A and 4B  depict a subsonic diffuser  40  having a splitter  41  defining a first passageway  42  and a second passageway  43 . Furthermore,  FIGS. 5A and 5B  depict a subsonic diffuser  46  having two splitters  48  and  49  defining a first passageway  50 , a second passageway  51 , a third passageway  52 , and a fourth passageway  53 . As shown in  FIGS. 4A and 4B , the splitter  41  bisects the subsonic diffuser  40 , and, as shown in  FIGS. 4A and 4B , the two splitters  48  and  49  are arranged at ninety degrees with respect to one another.  
         [0069]     Each of the plurality of passageways shown in  FIGS. 4A, 4B ,  5 A and  5 B should exhibit performance characteristics (e.g. providing a specified rate of diffusion) according to the performance characteristic curves of  FIG. 3 . For example, basic geometric calculations show that, when a subsonic diffuser formed as a conical frustum having a single duct is modified to incorporate a splitter defining two passageways (and the length of the conical frustum remains unchanged), conical diffusion calculations for the resulting two passageways will indicate that the diffusion angle  2 ⊖ has been reduced by one half. Due to the reduction of the diffusion angle  2 ⊖ by one half, a subsonic diffuser resulting from the use of the single splitter, such as subsonic diffuser  40  shown in  FIGS. 4A and 4B , offers some improvement in performance according to  FIG. 3 , but also has increased weight. The same holds true for subsonic diffuser  46  shown in  FIGS. 5A and 5B . That is, the subsonic diffuser shown in  FIGS. 5A and 5B  offers some improvement in performance according to  FIG. 3 , but also has increased weight.  
         [0070]     According to  FIG. 3 , however, the subsonic diffusers  40  and  46  can be shortened to compensate for the increased weight of their splitters, and still benefit from the performance provided by the use of splitters. The shortened lengths of the subsonic diffusers  40  and  46  should provide for weight reductions, even after accounting for the weight of the splitters. For example, the length of the subsonic diffusers  40  and  46  can be reduced so that the diffusion angles  2 ⊖ associated with their passageways are the same as that of the subsonic diffuser formed as a conical frustum having a single duct. The resulting subsonic diffusers will have shortened lengths, and because of the performance improvements provided by the splitters, have similar performance characteristics as the longer subsonic diffuser formed as a conical frustum having a single duct.  
         [0071]     A graph depicting the design characteristics of subsonic diffusers having various numbers of passageways provided by the splitters is shown in  FIG. 6 . The graph of  FIG. 6  shows the dimensional relationship between subsonic diffusers with various numbers of passageways N, and an equivalent subsonic diffuser formed as a conical frustum with a single duct. The subsonic diffuser formed as an equivalent conical frustum with a single duct would have a length L 1 , a weight X 1 , and wetted area A 1 , and the subsonic diffusers with various numbers of passageways N would have lengths L N , weights X N , and wetted areas A N . For example, the graph of  FIG. 6  indicates that a subsonic diffuser having four passageways, such as the subsonic diffuser  54  shown in  FIGS. 7A and 7B , when compared to subsonic diffuser formed as an equivalent conical frustum with a single duct, will result in a length ratio of about 50%, a weight reduction ratio of about 16%, and a wetted area ratio of about 89%.  
         [0072]     As shown in  FIGS. 7A and 7B , the subsonic diffuser  54  includes an inlet throat (or entrance)  55 , an outlet  56 , and two splitters  58  and  59 . Each of the two splitters  58  and  59  are provided in different planes that intersect with one another. The two splitters  58  and  59  are oriented at ninety degrees with respect to one another, and define four passageways  60 ,  61 ,  62  and  63 . The subsonic diffuser  54  can be reduced to a two-dimensional form depicted in  FIG. 8 . Using  FIG. 8 , the dimensions of the subsonic diffuser  54  can be compared to the dimensions of a subsonic diffuser formed as an equivalent conical frustum having a single duct. For example, if the subsonic diffuser  54  had a diffusion angle  2 ⊖ equivalent to that of the conical frustum  34  (the dimensions of which are shown in  FIG. 2C ), the subsonic diffuser  54  and the conical frustum  34  would have the same entrance width W 1  and outlet width W 2 , but the subsonic diffuser  54  would have one half the length (L/2) of the length L of the conical frustum  34 .  
         [0073]     A similar comparison can be illustrated using  FIGS. 10, 11 ,  12 A,  12 B, and  13 . For example,  FIG. 10  depicts a subsonic diffuser  66  with a square-shaped inlet throat, a circular-shaped outlet, and a single duct extending therebetween, and  FIGS. 12A and 12B  depict an equivalent subsonic diffuser  68  with four passageways (formed by two splitters  70  and  71 ), a square-shaped inlet throat, and a circular-shaped outlet. Because the subsonic diffusers  66  and  68  have equivalent diffusion angles  2 ⊖, the subsonic diffusers  66  and  68  (represented in two-dimensional form in  FIGS. 11 and 13 , respectively) will have the same entrance width W 1  and outlet width W 2 , but the subsonic diffuser  68  would have one half (L/2) of the length L of the subsonic diffuser  66 .  
         [0074]     Because subsonic diffusers according to the present invention have shortened lengths, the lengths of the resulting high-speed inlets incorporating such subsonic diffusers will also be shortened. For example, an all-internal compression high-speed inlet is generally indicated by the numeral  72  in  FIGS. 9A and 9B . Rather than incorporating the conventional subsonic diffuser  25  associated with the all-internal compression high-speed inlet  24  depicted in  FIG. 1D , the high-speed inlet  72  includes a subsonic diffuser  73  according to the present invention. The subsonic diffuser  73  includes two splitters  74  and  75 , each provided in different planes that intersect with one another. The two splitters  74  and  75  are oriented at ninety degrees with respect to one another, and define four passageways  76 ,  77 ,  78  and  79  ( FIG. 9B ). Because of the performance improvements provided by the four passageways  76 ,  77 ,  78 , and  79 , and because the subsonic diffuser  73  has a diffusion angle  2 ⊖ equivalent to that of the subsonic diffuser  25 , the subsonic diffuser  73  would have one half (L/2) the length L of the subsonic diffuser  25 . As such, the high-speed inlet  70  would have a shorter length than the subsonic diffuser  25 , and, therefore benefit from the resulting weight reduction.  
         [0075]     Additionally, a subsonic diffuser according to the present invention could be incorporated into a high-performance, low-sonic-boom, high-speed  80  inlet depicted in  FIG. 14 . As shown in  FIG. 14 , the high-speed inlet  80  includes a supersonic diffuser  82 , a throat  84 , and a conventional subsonic diffuser  86  ( FIG. 15 ). The supersonic diffuser  82  of the high-speed inlet  80  should have very high performance and operability. If the supersonic diffuser  82  was integrated with a subsonic diffuser according to the present invention, rather than the subsonic diffuser  86 , a significant increase in the efficiency for a high-speed inlet could nevertheless result. As shown in  FIG. 15 , the subsonic diffuser  86  has a rectangular-shaped inlet throat  88 , a circular-shaped outlet  89 , and a single duct  90  extending therebetween. In the manner described above, the subsonic diffuser  86  could be shortened using splitters. The resulting subsonic diffuser  92  incorporating two splitters  94  and  95  is shown in  FIGS. 16A and 16B . The subsonic diffuser  92  has a rectangular-shaped inlet  96 , a circular-shaped outlet  97 , and four passageways  98 ,  99 ,  100 , and  101  extending therebetween. Provided the subsonic diffuser  92  has a diffusion angle  2 ⊖ equivalent to that of the subsonic diffuser  86 , the subsonic diffusers will have the same entrance width and outlet width, but the subsonic diffuser  92  would have one half the length of the subsonic diffuser  86 .  
         [0076]     For illustrative purposes, other embodiments of the subsonic diffusers according to the present invention are depicted in the remaining drawings. The subsonic diffusers depicted in the remaining drawings include splitters in various arrangements.  FIGS. 17A-17C  depict a subsonic diffuser  104  having one splitter  105 . The subsonic diffuser  104  includes a rectangular-shaped entrance throat  108 , a circular-shaped outlet  109 , and two passageways  110  and  111  defined by the splitter  105 .  
         [0077]      FIGS. 18A-18C  depict a subsonic diffuser  114  having two splitters  115  and  116  provided in different planes that intersect with one another at ninety degrees. The subsonic diffuser  114  includes a rectangular-shaped entrance throat  118 , a circular-shaped outlet  119 , and four passageways  120 ,  121 ,  122  and  123  defined by the two splitters  115  and  116 .  
         [0078]      FIGS. 19A-19C  depict a subsonic diffuser  124  having one splitter  125  which can be formed as an airfoil. The subsonic diffuser  124  includes a rectangular-shaped entrance throat  128 , a circular-shaped outlet  129 , and two passageways  130  and  131  defined by the splitter  125 .  
         [0079]      FIGS. 20A-20C  depict a subsonic diffuser  134  having two splitters  135  and  136  which can be contoured. The two splitters  135  can be oriented parallel to one another ( FIGS. 20B and 20C ), and  136  can be shaped as airfoils or have some other configurations. The subsonic diffuser  134  includes a rectangular-shaped entrance throat  138 , a circular shaped outlet  139 , and three passageways  140 ,  141  and  142  defined by the two splitters  135  and  136 .  
         [0080]      FIGS. 21A-21C  depict a subsonic diffuser  144  having one splitter  145  which can be formed as an airfoil. The subsonic diffuser  144  is adapted for inlet variable geometries. For example, the splitter  145  is configured to move with a moveable wall  146  between a design position P 1  and an off-design position P 2 . The subsonic diffuser  144  includes rectangular-shaped entrance throat  148  (of variable dimensions), a circular-shaped outlet  149 , and two variably-sized passageways  150  and  151 .  
         [0081]      FIGS. 22A-22C  depict a subsonic diffuser  154  having a segmented splitter  155  with a stationary segment  157  and a moveable segment  156 . The subsonic diffuser  154  is also adapted for inlet variable geometries. For example, the moveable segment  156  is configured to move with a moveable wall  158  between a design position P 3  and an off-design position P 4 . The subsonic diffuser  154  includes a rectangular-shaped entrance throat  160  (of variable dimensions), a circular-shaped outlet  161 , and two passageways  162  and  163 . Movement of the moveable segment  156  to the off-design position P 4  joins the two passageways  162  and  163  to further alter the airflow.  
         [0082]      FIGS. 23A-23C  depict a subsonic diffuser  164  having a rectangular-shaped inlet  166  and a circular-shaped outlet  167 . One splitter  168  extends from the rectangular-shaped inlet  166  only partially through the length of the subsonic diffuser  164 . As such, the splitter  168  defines two passageways  170  and  171  adjacent the rectangular-shaped inlet  166 . Additional splitters extending partially through the subsonic diffuser  164  can also be provided. For example, two splitters can be oriented parallel to one another to provide three passageways adjacent the rectangular-shaped inlet  166 . Furthermore, two splitters can be provided in different planes that intersect with one another to provide four passageways adjacent the rectangular-shaped inlet  166 .  
         [0083]      FIGS. 24 , and  25 A- 25 C depict a subsonic diffuser  180  having at least one splitter extending partially across the span thereof. The subsonic diffuser  180  includes a rectangular-shaped inlet  182  and a circular-shaped outlet  183 . As shown in  FIGS. 24, 25B  and  25 C, two splitters  184  and  185 , although only one can be provided, extend partially across the subsonic diffuser  180 . A first passageway  186  is effectively defined above the two splitters  184  and  185 , and a second passageway  187  is effectively defined below the two splitters  184  and  185 . The first and second passageways  186  and  187  communicate with one another via a gap  188  between the two splitters  184  and  185 . Like the splitter  168  of the subsonic diffuser  164 , the two splitters  184  and  185  can be configured to extend only partially through the length of the subsonic diffuser  180 .