Abstract:
A diffusing corner for fluid flow within a wind tunnel comprising an arrangement of diffusing vanes for diverting the flow of air through the corner in a plurality of air passages having increasing cross-sectional area, thereby increasing the overall cross-sectional area of the wind tunnel and slowing the air flow. The vanes are configured to minimize turbulence and the loss of energy through the corner, and to attenuate transmission of sound generated within the wind tunnel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to fluid flow systems. In one of its aspects, the invention relates to the control of fluid flow in a wind tunnel system. 
     2. Description of Related Art 
     Generally, the purpose of a wind tunnel is to measure the effect of the passage of a high velocity fluid, in this case air, over a body under controlled conditions. Such body may be an airplane, a building structure or an automobile. 
     In a wind tunnel, the prime consideration is to be able to control the velocity and the uniformity of the air flow. One very impractical method of doing this is to have a very long straight wind tunnel with the right combination of cross-sectional area and input wind generation power. This is impractical because each end of such a wind tunnel must be open to the atmosphere. Therefore, the common practice is to make the wind tunnel a loop so that no make up air is needed, debris can be prevented from entering the tunnel, energy is conserved, and other factors, such as air temperature, can be controlled. 
     In the past, it may have been a relatively simple matter of making a wind tunnel in a large loop. Perhaps the corners would be rounded, or perhaps some curved vanes would be placed in the corners to somewhat reduce the energy lost and the turbulence created in the corners. In a wind tunnel used only to determine forces of the wind on a body, and the aerodynamic effects, e.g. drag, on the body, the noise of turbulence occurring somewhere in the tunnel was less of an issue. 
     However, in a wind tunnel that is to be used for testing the acoustic effects of fluid flow over a body, the fluid flow must be “purified” to isolate the acoustic effects created by the tunnel from those created by the flow of fluid over the test body itself. The goal is therefore to minimize the “noise” generated by the wind tunnel itself. This “noise”, from the testing standpoint, involves both the actual audible noise generated by a wind generator (fan/turbine) and the noise created by the turbulence of the fluid flow as it traverses the passageways, transitions and corners of the tunnel. 
     For the purposes of aerodynamic and acoustic evaluation of solid bodies, it would be advantageous to provide a wind tunnel assembly that minimizes the noise generated and attributable to the wind tunnel itself. It would also be preferable to reduce the energy lost to inefficient corners and transitions so as to reduce the power consumption required to generate a given capacity for air movement. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a diffusing corner for fluid flow within a fluid conduit, comprising a plurality of vanes arranged to divide the fluid conduit into a plurality of fluid passages, each of the plurality of vanes having a first surface and a second surface, the first and second surfaces being defined by non-circular sections, wherein the second surface of a first vane and the first surface of a second vane define one of the plurality of fluid passages, and wherein the plurality of vanes is arranged to direct the fluid flow through an angular displacement and an increase in cross-sectional area of the fluid conduit. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a plan view of an aero-acoustic wind tunnel having a diffusing corner according to the invention. 
     FIG. 2 is a plan view of a pair of diffusing corner vanes from the aero-acoustic wind tunnel of FIG.  1 . 
     FIG. 3 is a graphic representation of the mathematic formulation of the surface of the diffusing corner vanes of FIGS.  1 - 2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An aero-acoustic wind tunnel  10  having a diffusing corner  100  according to the invention is shown in FIGS. 1-2. Referring with particularity to FIG. 1, the aero-acoustic wind tunnel  10  is formed in a closed loop for environmental control of the air flow. Air flows in this aero-acoustic wind tunnel  10  in a clockwise direction. 
     Beginning with the test section  20 , the air flows to the first corner  30  and second corner  40 , which contain substantially conventional turning vanes  35 ,  45 , for redirecting the air flow around each corner. The turning vanes  35 ,  45 , as well as the walls of each corner  30 ,  40  include acoustically insulating surfaces. From the second corner  40 , the flow of air then passes through the flow generator  50 . Flow generator  50  can comprise a fan blade/stator arrangement of conventional design. 
     After the flow generator  50 , the flow of air then enters the third, diffusing corner  100 . Diffusing corner  100  includes an inner wall  120  and an outer wall  130 . An innermost diffusing vane  105  is positioned next to inner wall  120 . An outermost diffusing vane  110  is positioned next to outer wall  130 . A plurality of interior diffusing vanes  115  are positioned equidistant between innermost vane  105  and outermost vane  110 . 
     After passage through the diffusing corner  100 , the air flows at a reduced velocity to pass through a heat exchanger  60  and fourth corner  70 . After passing around fourth corner  70 , the air flow passes through nozzle section  80  to increase its speed to the desired test speed, at which it re-enters the test section  20 . 
     Referring again to the third, diffusing corner  100 , and with particularity to FIG. 2, an air passage  200  is formed between each pair of adjacent vanes  105 ,  115 ,  110 . Inner wall  120  is configured to cooperate with innermost diffusing vane  105  to form a like air passage  200 . Outer wall  130  likewise is configured to cooperate with outermost diffusing vane  110  to form a like air passage  200 . 
     As shown in FIG. 2, each vane  115  has an inner surface  140  and an outer surface  150 . The air passage  200  is defined by the outer surface  150  of a first vane  115  and the inner surface  140  of the next outwardly positioned vane  115 . Inner wall  120  of the diffusing corner  100  substantially matches the profile of the outer surface  150  of a vane  115  so as to cooperate with innermost vane  105  to form the air passage  200 . Likewise, outer wall  130  matches the profile of an inner surface  140  to cooperate with outermost vane  110  to form an air passage  200 . 
     Each air passage  200  has an upstream entrance  160  and a downstream exit  170 . Each entrance  160  of the plurality of air passages  200  is substantially equal in width. Each exit  170  of the plurality of air passages  200  is also substantially equal in width, and is wider than the entrance  160  to provide an overall increase in air passage cross-sectional area, as will be further discussed below. 
     Normally in ducts of this type, circular arcs are used, which, when intersecting with walls that are straight, cause an adverse pressure gradient resulting in flow separations on the wall, poor flow uniformity, flow pulsation and a degradation in flow efficiency. By utilizing higher order curves such as ellipses, conics or cubics, the degree of adverse pressure gradient is greatly reduced, minimizing the possibility of flow separation on the corners or on the vane surfaces themselves. 
     The diffuser function of the corner and vanes design is not normally done in conjunction with the turning function. It is conventional wisdom that you cannot turn and diffuse the flow at the same time. This however has been proven not to be true when the disclosed design is utilized. Each of the passages between the vanes, between the inner vane and the inner wall surface, and between the outer vane and the outer wall surface, is a high aspect ratio (small width and long length) diffuser which can be configured with a constant angle of diffusion or a variable angle of diffusion. By breaking down the diffusion into these several passages, the desired degree of duct expansion and flow deceleration can be accomplished in a much shorter length than would be the case with a single flow passage. 
     The diffusing corner for fluid flow according to the invention uses higher order mathematically formulated curves to define the profile of the transition regions between the upstream and downstream wall angles of the inner and outer walls of the duct. Instead of using a circular arc, an ellipse, cubic or conic is utilized. The contour of the curved transition of the inner corner of the duct forms the shape of the outer surface of multiple turning vanes which are inserted into the flow. The contour of the curved transition of the outer corner of the duct is also a higher order curve such as an ellipse, a cubic or conic, and forms the inner surface of the vanes inserted into the flow. This contour is defined by mathematically formulating the surface of an expanding channel whose inner surface is defined by the curved transition on the inner wall of the duct. This expansion can be constant or variable in degree of expansion along the length of the turning vanes. By defining the amount of expansion or area change desired, the maximum degree of expansion allowed and the distance available, the corner will naturally result in a certain number of these curved vanes being inserted between the inner and outer walls of the corner. 
     The new method employed in generating the contours of the corner and vane curves is composed of the following steps: 
     A. Determining the passage width of the duct upstream of the corner. 
     B. Determining the minimum width of the upstream end of the vanes to be used from structural or acoustic considerations. 
     C. Determining the desired width of the duct downstream of the corner. 
     D. Determining the number of vanes/passages needed to accomplish the desired width/area change between the upstream portion of the duct and the downstream portion of the duct. Keeping in mind the high aspect ratio desired, there is a balance between the minimum number of vanes necessary (or a maximum air passage width) to keep the required length of the vanes down, while still having the air passages long enough keep the diffusion angle between vanes low enough to avoid flow separation. 
     E. Designing/calculating the surfaces of the vanes: 
     1. Selecting the major and minor half axis lengths of the cubic curve to be used for the inside corner transition. 
     2. Mathematically generating the points along the contour of the curve according to the equation x{circumflex over ( )}3/a+y{circumflex over ( )}3/b=1. 
     3. Finding the slope of the curve at each of the points describing the generated cubic curve. 
     4. Generating a normal to the curve at each point. 
     5. Selecting a nominal offset of the outside passage curve, which is the upstream distance between the vane surfaces. 
     6. Mathematically generating the point locations for the simply offset curve. 
     7. Selecting the amount of diffusion angle desired to take place in the passage between vanes and determining if this diffusion angle will be constant or variable. A nominal diffusion angle of around 2.5 to 3 degrees is generally desirable. 
     8. If the diffusion angle is to be variable, defining the mathematical formulation which yields the diffusion angle at each point. 
     9. Mathematically determining the coordinates of each of the points defining the offset plus diffusion curve, which is the inner curve of the turning vane and the curve utilized between the outside walls at the intersection. 
     10. Constructing the inner and outer corners of the duct and the vane contours with the mathematically defined curves. 
     FIG. 3 depicts graphically the coordinates of the resultant cubic curve  300 , the constant offset curve  310 , and the expanding offset curve  320 . The new result is that the flow is turned the desired angle, in this application 90 degrees and the area of the duct is increased by approximately 40% within the area of the corner between the inner cubic curve  300  and the expanding offset curve  320 . These two effects are accomplished while maintaining good flow uniformity and with much less pressure loss than would be the case for a corner with conventional vanes and a separate area of diffusion. 
     It is also advantageous to introduce sound dampening in the air passages, particularly in the surface of the vanes, to attenuate the acoustic noise generated by the fluid flow generator. 
     While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the scope of the appended claims.