Aeronautics and astronautics – Aircraft sustentation – Sustaining airfoils
Reexamination Certificate
2000-04-13
2002-02-12
Barefoot, Galen L. (Department: 3644)
Aeronautics and astronautics
Aircraft sustentation
Sustaining airfoils
C244S130000, C244S199100
Reexamination Certificate
active
06345791
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates in general to an improved apparatus for reducing the skin friction drag of an aerodynamic or hydrodynamic surface, and in particular to an improved riblet design for reducing the skin friction drag coefficient of aerodynamic or hydrodynamic surfaces.
2. Description of the Prior Art
It is relatively well known that the aerodynamic drag of a surface may be reduced by applying a microscopic “texture” to the otherwise smooth surface. Although the exact fluid dynamic mechanism at work in this drag reduction is not well understood, it is speculated that the reduction relates to controlling the turbulent vortices in the boundary layer adjacent to the surface. The microscopic texture reduces the skin friction drag of solids moving through fluids (e.g., aircraft, ships, cars, etc.), and of fluids moving along solids (e.g., pipe flow, etc.). Although the practical use of such texturing has been very limited, one well known exception was the application of a texture to the racing yacht, Stars and Stripes.
The geometric form for the microscopic, friction-reducing texture is known as “riblets.” As shown in
FIG. 1
, riblets
11
are typically an integrated series of groove-like peaks
13
and valleys
15
with V-shaped cross-sections. Riblets
11
always extend along the aerodynamic surface
17
in the direction of fluid flow
19
. The height of the riblets and the spacing between the riblets are usually uniform and on the order of 0.001 to 0.01 inches for most applications. Dimensionless units, sometimes referred to as wall units, are conventionally utilized in describing fluid flows of this type. The wall unit h+ is the non-dimensional distance away from the wetted surface or more precisely in the direction normal to the surface, extending into the fluid. Thus h+ is a non-dimensional measurement of the height of the riblets. The wall unit s+ is the non-dimensional distance tangent to the local surface and perpendicular to the flow direction, thus the non-dimensional distance between the riblets. In the prior art riblets, h+ and s+ are in the range between 10 and 20. Previous riblet designs consisted of an adhesive film applied to a smooth solid surface. However, with advanced manufacturing techniques, the same shapes may be directly formed and integrated into the structure of the aerodynamic surface.
The interaction of riblets with the structure of the turbulent boundary layer of the fluid reduces the skin friction drag coefficient (Cdf) of the surface by approximately 6% compared to an identical smooth surface without riblets. This reduction occurs despite the significant increase in “wetted area” (the surface area exposed to the fluid stream) of a riblet-covered surface over a smooth surface. In attempts to further reduce the Cdf, modifications to conventional V-shaped riblets have been proposed. Examples include rounding of the peaks
21
and/or valleys
23
(FIG.
2
), as well as even smaller V-shaped notches
31
in the sides of the larger V-shaped riblets
33
(FIG.
3
). In summary, all of the work has been with riblets having a constant geometry or cross-section in the streamwise direction. An improved riblet design that decreases skin friction drag with less concomitant increase in wetted area than conventional riblets would be desired.
SUMMARY OF THE INVENTION
A series of parallel riblets extend from a smooth, aerodynamic surface for reducing the skin friction drag of the surface an airstream flows around it. The riblets extend longitudinally along the surface and have a triangular cross-section in the transverse direction. The apex of the cross-section defines a continuous, undulated ridge with peaks and valleys. Measured from the surface, the peaks have a greater height than the valleys. The interaction of the riblets with the structure of the turbulent boundary layer of the airstream reduces the skin friction drag coefficient of the surface by approximately 8% to 20% over an identical smooth surface without the riblets. The reduction is better than the 6% reported for conventional riblets because the wetted area is increased less with this invention than with conventional riblets.
REFERENCES:
patent: 4650138 (1987-03-01), Grose
patent: 4706910 (1987-11-01), Walsh et al.
patent: 4736912 (1988-04-01), Loebert
patent: 4750693 (1988-06-01), Lobert et al.
patent: 4753401 (1988-06-01), Bechert
patent: 4759516 (1988-07-01), Grose
patent: 4863121 (1989-09-01), Savill
patent: 4865271 (1989-09-01), Savill
patent: 3609541 (1987-09-01), None
patent: 730121 (1955-05-01), None
WO 93/19981, Savill, “Control of Fluid Flow” Oct. 1993.
Barefoot Galen L.
Bracewell & Patterson L.L.P.
Lockheed Martin Corporation
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