Aeronautics and astronautics – Aircraft structure – Details
Reexamination Certificate
2000-12-27
2002-11-26
Jordan, Charles T. (Department: 3641)
Aeronautics and astronautics
Aircraft structure
Details
C244S199100, C244S200000, C244S204000
Reexamination Certificate
active
06484971
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to a method, and structure, which suppresses separation of fluid (e.g., gas or liquid, such as air) flow adjacent a surface of a body when passing the fluid along an exterior surface of the body, such as in the case of a fluid flowing across the wing of an airplane, or when passing the fluid along an interior surface of the body, such as in the case of a fluid flowing in a diffuser along a surface of the diffuser. In particular, the present invention is directed to a method, and structure, wherein instabilities or oscillations associated with separation of fluid flow adjacent the surface of the body is suppressed, and drag associated with this fluid flow separation is suppressed.
It is well known in fluid dynamics that when a fluid passes along a solid body, the fluid will form a viscous layer, known as a “boundary layer”, adjacent the body surface. This boundary layer possesses a much lower energy level than the flow outside it. Inside the boundary layer, the flow is distorted under viscous effects and there exists a large velocity gradient in the direction normal to the body surface.
When a large adverse pressure gradient exists, that is, when pressure increases in the direction of the fluid flow, the boundary layer may not be able to tolerate the pressure gradient, and will start to separate from the body surface, as schematically illustrated in FIG.
1
. Thus, shown in
FIG. 1
is surface
1
of the body, having fluid flow adjacent surface
1
. The flow is shown schematically by reference character
3
, and includes, initially, flow adjacent surface
1
of the body (attached flow, e.g., in an attached boundary layer flow region), shown by reference character
5
, and separated flow, e.g., in separated boundary layer flow region shown by reference character
7
, where the flow is separated from surface
1
of the body. The separation point, where the flow initially separates from the body, is shown by reference character
9
in FIG.
1
. This point is defined as the point where the velocity gradient in the direction normal to the body surface is observed for the first time to be continuously less than or equal to zero.
FIGS.
18
(
a
) and
18
(
b
) respectively show, in more detail (with respect to fluid flow) than shown in
FIG. 1
, such separation from an external surface
1
of a body
2
(e.g., an airplane wing) and from an internal surface
1
a
of a body
2
a
(e.g., a diffuser). This phenomenon is called “flow separation”, or “separation”. Shown in FIG.
18
(
a
) is separation point
9
, where the fluid first separates from surface
1
of body (airfoil)
2
. Downstream from the separation point
9
, in the direction of fluid flow, the fluid exhibits separated flow in a separated flow region
13
(separated boundary layer flow region), where the fluid flow forms eddies
17
. Even downstream of separation point
9
, in regions spaced from surface
1
or
1
a
of the body of the fluid exhibits smooth flow, e.g., smooth outer flow, in smooth outer flow regions
16
shown in each of FIGS.
18
(
a
) and
18
(
b
). The separated boundary layer flow region is defined as the region in which the local flow velocity is directed essentially in the direction opposite to the direction of the main flow prior to the separation point. This is compared to the smooth flow region (smooth outer flow region), which is the flow region outside (relative to the body) the separated boundary layer flow region.
Moreover, as the fluid flow continues over time this flow separation propagates upstream from an initial separation point, as seen in FIGS.
2
(
a
) and
2
(
b
), and as shown in more detail with respect to fluid flow patterns in FIGS.
19
(
a
) and
19
(
b
). That is, FIG.
2
(
a
) shows an initial stage of fluid flow, and FIG.
2
(
b
) shows a later stage (later in time). At the initial stage, separation point
9
is toward the rear end of surface
1
of the body, with respect to fluid flow direction
3
. At a later time, separation point
15
has moved upstream, as shown in FIG.
2
(
b
). And, as shown in FIG.
19
(
a
) disturbance waves
11
propagate upstream from inside the separated flow region
13
, exerting influences on the upstream flow region. Separated flow region
13
is extended upstream, so that separation point
9
changes to an “adjusted” separation point
15
, as shown in FIG.
19
(
b
).
In almost all cases, flow separation is associated with disadvantages, and is therefore to be avoided. If the body is the wing of an airplane, flow separation may cause the airplane to lose its lifting force, a situation known as “stall”. Flow separation also increases the drag force acting on the wing, which is particularly disadvantageous when the airplane is in a cruise condition. If separation occurs inside a diffuser, the diffuser loses its diffusing ability.
In many cases, flow separation also leads to the occurrence of unsteady phenomena, which may cause control problems. Aerodynamic unsteadiness could alternately lead to structural failure of the body in question. This is particularly true for an airplane flying in the flow regime known as the “transonic” regime, or maneuvering a landing approach, where a phenomenon known as “buffeting” can arise due to flow separations. Flow separation also often leads to other interfering phenomena, such as the disturbing wind noises around transport vehicles, or the noise interference to electrical/power transmissions through cables/power lines.
In the transonic regime, where the flow speed is close to the speed of sound, separation induced flow oscillations (usually in conjunction with the formation of shock-waves) can be very severe. One method that has been proposed to stop these oscillations is disclosed in U.S. Pat. No. 5,692,709 to Mihora and Cannon. In this particular patent a method is described in which flow oscillations are stopped by fixing simple devices at prescribed positions on the surface of the aerodynamic body. These devices force shock-waves to form prematurely at fixed locations, which are the locations of these devices.
In addition to the foregoing, many other methods have been proposed to suppress flow separation under specific flow conditions. These include, but are not limited to, vortex generators (such as disclosed in U.S. Pat. No. 5,253,828 to Cox), riblets (such as disclosed in U.S. Pat. No. 4,863,121 to Savill), large-eddy break-up devices, porous or slotted walls, fluid blowing and/or suction, moving surfaces, actuators (such as disclosed in U.S. Pat. No. 5,209,438 to Wygnanski), vibrating flexible structure (such as U.S. Pat. No. 5,961,080 to Sinha), and stepped body surfaces.
As mentioned previously, flow separation is associated with the existence of vortical structures (eddies)
17
, of various sizes, inside separated flow region
13
, as shown in FIGS.
18
(
a
) and
18
(
b
). These eddies
17
give rise to disturbance waves
11
(see FIG.
19
(
a
)), which can travel in the upstream direction. The disturbance information in the disturbance waves
11
is received by the flow upstream of the original separation point. The flow will adjust itself to this information, and the separation point is shifted upstream, until some kind of balance (e.g., steady-state) is achieved. This adjustment of original separation point
9
to adjusted separation point
15
(see FIG.
19
(
b
)), at an upstream location from original separation point
9
due to this disturbance information, has been previously mentioned. In the case that the disturbances are large, the whole flow field is said to be “unsteady”, and the point of separation fluctuates about a mean position. The flow field then constitutes a feed-back system.
As mentioned previously, on the surface of the body the fluid particles form a very thin viscous layer, known as the “boundary layer”. Because flow inside the boundary layer is mostly much slower than the flow outside it, it is easier for disturbances to travel inside this layer. In the case that the flow outside the boundary layer is supersonic, ups
Antonelli Terry Stout & Kraus LLP
Semunegus Lulit
LandOfFree
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