U.S. patent application number 10/232799 was filed with the patent office on 2003-10-02 for self-actuating, foldable obstacle system on hydraulic lifting bodies.
This patent application is currently assigned to INSTITUTE OF HIGH PERFORMANCE COMPUTING. Invention is credited to Hung, Kin Chew, Yao, Qiang, Zhou, Li.
Application Number | 20030186599 10/232799 |
Document ID | / |
Family ID | 28450341 |
Filed Date | 2003-10-02 |
United States Patent
Application |
20030186599 |
Kind Code |
A1 |
Yao, Qiang ; et al. |
October 2, 2003 |
Self-actuating, foldable obstacle system on hydraulic lifting
bodies
Abstract
A foldable obstacle system is provided on a hydraulic lifting
body to alleviate cloud cavitation and so to reduce noise and
surface erosion that might otherwise result from unsteady sheet
cavitation and a re-entrant jet produced thereby. The system
includes one or more foldable obstacle devices or plates, each of
which is movable between a closed position at the surface of the
body and an open position away from the body. When a re-entrant jet
occurs, the jet acts to lift the plates and rotate them into the
open position in which they block the re-entrant jet so as to
reduce or prevent cloud cavitation. When cavitation does not occur,
the normal fluid flow over the body acts to close and keep closed
the obstacle devices or plates so that they do not interfere with
the normal fluid flow.
Inventors: |
Yao, Qiang; (Singapore,
SG) ; Zhou, Li; (Singapore, SG) ; Hung, Kin
Chew; (Singapore, SG) |
Correspondence
Address: |
HOGAN & HARTSON L.L.P.
500 S. GRAND AVENUE
SUITE 1900
LOS ANGELES
CA
90071-2611
US
|
Assignee: |
INSTITUTE OF HIGH PERFORMANCE
COMPUTING
|
Family ID: |
28450341 |
Appl. No.: |
10/232799 |
Filed: |
August 29, 2002 |
Current U.S.
Class: |
440/49 |
Current CPC
Class: |
B63B 1/244 20130101;
B63H 1/18 20130101 |
Class at
Publication: |
440/49 |
International
Class: |
B63H 001/14 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 2, 2002 |
SG |
200201837-2 |
Claims
What is claimed is:
1. A foldable obstacle device for blocking a re-entrant jet
produced by unsteady sheet cavitation on a hydraulic lifting body,
the foldable obstacle device being mounted at a surface of the body
and being movable between a closed position at the surface of the
body and an open position away from the body, the foldable obstacle
device being lifted from the closed position and moved into the
open position by the occurrence of a re-entrant jet to block the
re-entrant jet and reduce or prevent cloud cavitation.
2. A foldable obstacle device according to claim 1, wherein the
obstacle device is substantially perpendicular to the surface of
the body when in the open position and is mounted so that it cannot
be moved beyond the open position.
3. A foldable obstacle device according to claim 1, wherein the
obstacle device comprises a plate-like device which is pivotally
mounted adjacent the surface of the body.
4. A foldable obstacle device according to claim 3, wherein the
plate-like device is pivotally mounted at a first edge thereof and
has a plurality of feather-like flaps extending from a second edge
thereof opposite the first edge.
5. A hydraulic lifting body having a plurality of foldable obstacle
devices according to claim 1, wherein the plurality of foldable
obstacle devices being mounted at different locations on the
surface of the body and each being movable between closed and open
positions independently of other ones of the plurality of foldable
obstacles.
6. A hydraulic lifting body according to claim 5, wherein the
plurality of foldable obstacle devices are mounted for pivoting
movement along a common axis.
7. A hydraulic lifting body having a plate-like obstacle device
pivotally mounted adjacent a surface of the hydraulic lifting body
so as to be movable between a closed position in which the device
is generally continuous with the surface of the body and an open
position in which the device extends outwardly from the surface of
the body in response to a re-entrant jet at the surface of the
body.
8. A hydraulic lifting body according to claim 7, wherein the body
has a chord length and a leading edge, and the obstacle device is
located at a distance of 35-50% of the chord length from the
leading edge of the body.
9. A hydraulic lifting body according to claim 8, wherein the
obstacle device is located at a distance of approximately 37% of
the chord length from the leading edge of the body.
10. A hydraulic lifting body according to claim 7, wherein the body
has a chord length and the obstacle device extends upwardly from
the surface of the body by a height which is at least 1.2-2.0% of
the chord length.
11. A hydraulic lifting body according to claim 7, wherein a sheet
cavity of a certain thickness forms at the surface of the body when
cloud cavitation occurs at the surface providing a re-entrant jet
of a certain thickness at the surface of the body, and the obstacle
device extends outwardly from the surface of the body and has a
height which is greater than the thickness of the re-entrant jet
and less than the thickness of the sheet cavity.,
12. A hydraulic lifting body according to claim 7, wherein the
obstacle device is pivotally mounted at a first edge thereof
adjacent the surface of the lifting body and extends to an opposite
second edge thereof so as to be generally continuous with the
surface when in the closed position, and further including at least
one feather-like flap extending from the second edge of the device
so as to be generally continuous with the surface when in the
closed position.
13. A hydraulic lifting body according to claim 12, wherein each of
the feather-like flaps is has a shape selected from the group
consisting of rectangular, square, trapezoidal, semi-elliptic,
semi-circular.
14. A hydraulic lifting body according to claim 7, wherein the body
has a plurality of the plate-like obstacle devices mounted adjacent
a surface of the hydraulic lifting body and each being movable
independently of the other obstacle devices.
15. A hydraulic lifting body extending laterally between opposite
leading and trailing edges and having opposite surfaces, the body
comprising an elongated groove in a first one of the opposite
surfaces extending laterally along a portion of the body at a
location intermediate the opposite leading and trailing edges, a
plurality of generally cylindrical hubs rotatably disposed,
end-to-end, within the elongated groove and each having a shaft
extending from a first end thereof and a hole at an opposite second
end thereof for receiving the shaft of an adjacent hub, each of the
hubs having one of a plurality of plates mounted thereon, rotation
of each of the hubs within the elongated groove moving the plate
mounted thereon between a closed position in which an external
surface of the plate is generally continuous with the first one of
the opposite surfaces of the body and an open position in which the
plate is generally perpendicular to the first one of the opposite
surfaces of the body.
16. A hydraulic lifting body according to claim 15, wherein each
shaft is rotatable within the hole of an adjacent hub in which it
is received so that each plate may pivot between the closed and
open positions thereof independent of the other plates.
17. A hydraulic lifting body according to claim 15, wherein the
groove provides a clearance fit of each of the hubs therein which
prevents movement of the plates beyond the open positions
thereof.
18. A hydraulic lifting body according to claim 15, wherein each of
the plates has at least one feather-like flap extending from an
outer edge thereof opposite the hub on which the plate is mounted,
the flap being generally continuous with the first one of the
opposite surfaces of the body when the plate is in the closed
position.
19. A hydraulic lifting body according to claim 15, further
including a pair of covers secured on the first one of the opposite
surfaces of the body at opposite ends of the plurality of hubs to
rotatably secure the plurality of hubs within the groove.
20. A hydraulic lifting body which is subject to formation of a
cloud cavity on a surface thereof if a re-entrant jet is produced,
apparatus for minimizing or preventing formation of a cloud cavity
comprising means responsive to production of a re-entrant jet for
blocking the re-entrant jet from moving forward on the surface of
the body, and means responsive to the absence of a re-entrant jet
for allowing unblocked fluid flow over the surface of the body.
21. A hydraulic lifting body in accordance with claim 20, wherein
the body has a leading edge and an opposite trailing edge, fluid
flow normally occurs over the surface of the body from the leading
edge to the trailing edge, and a re-entrant jet, when produced,
flows over the surface of the body from an end of an unsteady sheet
cavity in a direction toward the leading edge.
22. A movable obstacle for installation on a hydraulic lifting
body, the foldable obstacle comprising: a blocking plate portion; a
hub at one end of the blocking plate portion; and mounting
structure for mounting the hub to the lifting body so that the
blocking plate portion is movable between a first position in which
it is closed against the hydraulic lifting body and a second
position in which it is opened away from the hydraulic lifting
body; wherein the hub is mounted to the mounting structure so that
a re-entrant jet flowing over the lifting body applies a force to
the movable obstacle and wherein the force of the re-entrant jet
urges the blocking plate portion to its opened position.
23. The movable obstacle of claim 22, wherein the blocking plate
portion is substantially perpendicular to a surface of the lifting
body when the blocking plate is in its opened position.
24. The movable obstacle of claim 22, and further comprising at
least one feather-like flap mounted on the blocking plate portion
at an end opposite the hub.
25. A hydraulic lifting body having at least one movable obstacle
according to claim 1 installed on the lifting body.
26. The hydraulic lifting body of claim 25, wherein a plurality of
the movable obstacles are installed on the lifting bodies.
27. A fluid lifting body comprising: a fluid lifting body surface;
and a movable obstacle comprising: a blocking portion; and mounting
structure for mounting the movable obstacle on the lifting body so
that the blocking plate portion is movable between a first position
in which blocking plate portion is closed against the fluid lifting
body surface and a second position in which the blocking plate
portion is opened away from the fluid lifting body surface; wherein
the movable obstacle is mounted on the lifting body so that a
re-entrant jet flowing over the lifting body applies a force to the
movable obstacle; wherein the force of the re-entrant jet urges the
blocking plate to its opened position; and wherein the blocking
structure in the opened position interferes with the flow of the
re-entrant jet over the lifting body surface.
28. The fluid lifting body of claim 27, wherein the lifting body
has a chord length and a leading edge, and wherein the movable
obstacle is mounted on the lifting body at a distance of 35-50% of
the chord length from the leading edge of the lifting body.
29. The fluid lifting body of claim 28, wherein the movable
obstacle is mounted on the lifting body at a distance of
approximately 37% of the chord length from the leading edge of the
lifting body.
30. The fluid lifting body of claim 27, wherein the lifting body
has a chord length, and wherein the movable obstacle in the opened
position extends upward from the lifting body surface to a height
of at least about 1.2% of the chord length.
31. The fluid lifting body of claim 30, wherein the movable
obstacle in the opened position extends upward from the lifting
body surface to a height of between 1.2% and 2.0% of the chord
length.
32. The fluid lifting body of claim 27, wherein a sheet cavity
having a sheet cavity thickness and a re-entrant jet having a
re-entrant jet thickness form over the fluid lifting body surface
under cloud cavitation conditions, and wherein the blocking portion
extends outward from the fluid lifting body surface to a height
that is greater than the re-entrant jet thickness and less than the
sheet cavity thickness.
33. The fluid lifting body of claim 27, wherein the movable
obstacle is mounted at a first end thereof adjacent the fluid
lifting body surface and extends to an opposite second thereof so
as to be generally continuous with the fluid lifting body surface
when the movable obstacle is in the closed position.
34. The fluid lifting body of claim 33, and further comprising at
least one feather-like flap extending from the second edge of the
movable obstacle so as to be generally continuous with the fluid
lifting body surface when the movable obstacle is in the closed
position.
35. The fluid lifting body of claim 27, wherein the fluid lifting
body includes a plurality of the movable obstacles, and wherein the
movable obstacles are movable independently of one another.
36. A method for reducing fluid cavitation over a fluid lifting
body, the method comprising: flowing fluid over a flow surface of
the lifting body, wherein flowing the fluid over the flow surface
forms a re-entrant jet over the flow surface; interposing a movable
blocking structure into the re-entrant jet to interfere with flow
of the re-entrant jet over the flow surface.
37. The method of claim 36, wherein the lifting body has a chord
length and a leading edge, and wherein the movable blocking
structure is located at a distance of 35-50% of the chord length
from the leading edge of the body.
38. The method of claim 37, wherein the movable blocking structure
is located at a distance of approximately 37% of the chord length
from the leading edge of the body.
39. The method of claim 36, wherein the lifting body has a chord
length, and wherein interposing the movable blocking structure into
the re-entrant jet includes extending the blocking structure away
from the flow surface to a distance that is at least 1.2% of the
chord length.
40. The method of claim 36, wherein a sheet cavity having a sheet
cavity thickness is formed over the flow surface; wherein the
re-entrant jet has a re-entrant jet thickness; and wherein
interposing the movable blocking structure into the re-entrant jet
includes extending the blocking structure away from the flow
surface to a distance between the re-entrant jet thickness and the
sheet cavity thickness.
41. The method of claim 36, wherein interposing the movable
blocking structure into the re-entrant jet includes pivoting the
movable blocking structure at a first edge that is toward a leading
edge of the lifting body to extend a second edge that is toward a
trailing edge of the lifting body away from the flow surface.
42. The method of claim 36, wherein interposing the movable
blocking structure into the re-entrant jet includes: locating the
movable blocking structure at a position in which the re-entrant
jet is directed against a feather-like flap to apply a force to the
feather-like flap that urges the movable blocking structure toward
an opened position; and interposing the blocking structure into the
re-entrant jet in a configuration in which the re-entrant jet urges
the blocking structure more fully toward the opened position.
43. The method of claim 36, and further comprising positioning the
movable blocking structure in a configuration in which the flowing
fluid is directed against the movable blocking structure in a
direction that urges the movable blocking structure to move from an
opened position to a closed position.
44. The method of claim 36, wherein interposing a movable blocking
structure into the re-entrant jet includes interposing a plurality
of movable blocking structures into the re-entrant jet, wherein the
plurality of movable blocking structures are urged into the
re-entrant jet by a force applied by the re-entrant jet; and
wherein the movable blocking structures are movable into the
re-entrant jet independently of one another.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates generally to hydraulic lifting bodies
such as the blades of marine propellers, hydro-turbines, pumps,
fins, or rudders of ships, and the like. More specifically the
invention provides apparatus and methods for preventing or reducing
cavitation noise and erosion on such lifting bodies.
[0002] Cavitation on the surface of a hydraulic lifting body causes
noise and erosion of that surface. These are caused by the
formation and collapse of bubbles adjacent to the surface due to
pressure changes as the fluid flows over the lifting body. The
situation is most severe when unsteady sheet cavitation and a
bubble cloud (cloud cavitation) occur on the surface of the
hydraulic lifting body under operating conditions. The bubble cloud
may contain thousands of tiny bubbles. These bubbles' collapse
often causes significant broad-band noise and surface erosion.
[0003] A re-entrant jet over the surface of the lifting body causes
periodic shedding of such a bubble cloud under a thick sheet
cavity. The re-entrant jet travels forward over the surface of the
body at a speed on the order of the inflow of fluid over the body.
A re-entrant jet rushing from the end of the unsteady sheet cavity
to the leading edge of the lifting body triggers the sheet cavity's
collapse. As a result, the sheet cavity is cut off in the vicinity
of its leading edge. The cut off portion of the sheet cavity then
moves downstream as a cluster of bubbles called a cloud cavity or
bubble cloud.
[0004] Cloud cavitation can be controlled by obstacles on the
surface of the lifting body. Such obstacles can reduce or eliminate
noise and surface erosion. When cavitation is not occurring,
though, these obstacles cause an undesirable increase in foil drag
and the efficiency of the lifting body is thereby reduced. Thus, if
one or more obstacles are provided on the lifting body, under
cavitation conditions the noise and erosion as well as drag can be
reduced. When the cavitation conditions are removed, though, the
continued presence of the obstacle or obstacles increases drag and
reduces the effectiveness of the lifting body. Such obstacles
should be removed, therefore, whenever cavitation is not
occurring.
[0005] It is difficult to determine, though, just when a lifting
body such as a propeller or a hydro-turbine blade might experience
cavitation. It is thus quite difficult to determine when the
obstacle is helpful to reduce cavitation noise and erosion, and
when it should be removed for greater efficiency. It is difficult,
too, to determine where on the lifting body's surface the
re-entrant jet will pass by. Obstacles provided in locations on the
lifting body where the re-entrant jet does not appear are of little
or no use. Even if one could predict cavitation conditions
correctly, obstacles on the lifting body will still disturb fluid
flow over the lifting body under conditions that do not cause
cavitation. Many lifting bodies such as propeller blades or
hydro-turbine blades are rotated at very high speeds, moreover, and
it is very difficult to install servo-control devices or the like
to control the presence and removal of obstacles for such lifting
bodies under operating conditions. Furthermore, sensors are
required on the lifting body to provide information about when and
where cavitation is occurring.
[0006] A need exists, therefore, for improved apparatus and methods
for providing removable obstacles selectively on hydraulic lifting
bodies under cavitation conditions. Such apparatus should be
simple, inexpensive, and effective in operation. The apparatus
should provide obstacles placed appropriately on the lifting body
surface under cavitation conditions. Those obstacles should then be
removed from the surface when cavitation is not in effect. The
invention provides these and other advantages, as will be
appreciated more fully in connection with the explanation
below.
SUMMARY OF THE INVENTION
[0007] In accordance with the invention, a foldable obstacle device
is provided for blocking a re-entrant jet produced by unsteady
sheet cavitation on a hydraulic lifting body. The foldable obstacle
device is mounted at a surface of the body and is movable between a
closed position at the surface of the body and an open position
extending outwardly from the body. The foldable obstacle device is
lifted from the closed position and moved into the open position by
the occurrence of the re-entrant jet, to block the re-entrant jet
and reduce or prevent cloud cavitation. The foldable obstacle
device may comprise a plate which is pivotally mounted adjacent the
surface of the body at one edge thereof. The obstacle device may be
substantially perpendicular to the surface of the body and mounted
so that it cannot be moved beyond such orientation, when in the
open position. The foldable obstacle device may have one or more
feather-like flaps mounted thereon so as to extend from an edge
thereof opposite the pivoting edge. The re-entrant jet blows up the
feather-like flaps with an initial driving force. The torque caused
by the initial driving force forces open the obstacle device. Once
partially open, the re-entrant jet also exerts force on the
obstacle device. The torques caused by force on the obstacle device
and the initial driving force further open the device to the fully
opened position.
[0008] The hydraulic lifting body may have a plurality of the
foldable obstacle devices, which devices may be mounted at
different locations on the surface of the body or may be mounted
along a common axis. In either event, each obstacle device is
movable between the closed and opened positions independently of
the other obstacle devices.
[0009] The hydraulic lifting body has leading and trailing edges
and a chord length. In accordance with the invention, the obstacle
device is located at a distance of 35-50% of the chord length from
the leading edge of the body, and preferably at a distance of
approximately 37% of the chord length from the leading edge of the
body, for optimum results. Additionally, it is preferred that the
obstacle device have a height when in the open position which is at
least 1.2-2.0% of the chord length. Preferably, the obstacle device
has a height which is greater than the thickness of the re-entrant
jet and less than the thickness of the sheet cavity, for optimum
results.
[0010] A preferred arrangement for mounting a plurality of the
obstacle devices along a common axis on the body utilizes an
elongated groove in a first one of the opposite surfaces of the
body. The groove extends laterally along a portion of the body at a
location intermediate the opposite leading and trailing edges. A
plurality of generally cylindrical hubs are rotatably disposed,
end-to-end, within the elongated groove, and each has a shaft
extending from a first end thereof and a hole in an opposite second
end thereof for receiving the shaft of an adjacent hub. Each of the
hubs has a different one of a plurality of plates mounted thereon
to form the obstacle devices. Rotation of each of the hubs within
the elongated groove moves the plate which is mounted thereon
between a closed position in which an external surface of the plate
is generally continuous with the surface of the body and an open
position in which the plate is generally perpendicular to the
surface of the body. The groove provides a clearance fit for each
of the hubs therein so as to prevent movement of the plates beyond
the open positions thereof.
[0011] In such arrangements according to the invention, the
obstacle devices or plates are closed by fluid flow over the
surface of the body from the leading edge, for low drag in the case
of non-cavitation. However, in the case of cavitation, the
re-entrant jet which flows from the end of the unsteady sheet
cavity toward the leading edge of the body creates lifting forces
which move the obstacle plates into the open positions, thereby
blocking the re-entrant jet and preventing or substantially
reducing the noise and surface erosion that might otherwise
result.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a perspective view of a hydraulic lifting body
having a plurality of foldable obstacle devices in accordance with
the invention, with the obstacle devices being shown in enlarged
fashion relative to the body for clarity of illustration.
[0013] FIG. 2 is a broken-apart, perspective view, partly in
cross-section, of portions of the body of FIG. 1, illustrating one
arrangement for movably installing the obstacle devices.
[0014] FIG. 3 is a cross-sectional view of the body of FIG. 1
illustrating the manner in which a re-entrant jet produces an
initial driving force to rotate the obstacle devices toward an open
position and thereby block the re-entrant jet.
[0015] FIG. 4 is a perspective view, partly in cross-section, of
the body of FIG. 1, showing three of the foldable obstacle devices
in the open position so as to block the re-entrant jet.
[0016] FIG. 5 is a perspective view, partly in cross-section, of
the body of FIG. 1, illustrating the manner in which some but not
all of the foldable obstacle devices may be opened depending on the
location of the re-entrant jet on the lifting body.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0017] When fluid flow over the surface of a hydraulic lifting body
is not cavitated there is no re-entrant jet. When the hydraulic
lifting body operates under certain operating conditions, however,
the unsteady sheet cavity and the cloud cavity can appear on the
surface of the body. The cloud cavitation is caused by a re-entrant
jet, which moves upstream (toward the leading edge of the body)
from the end of the unsteady sheet cavity. When the re-entrant jet
reaches a location near the leading edge, it cuts the sheet cavity
off. The cut off part of the sheet cavity then moves downstream as
a cluster of bubbles called a cloud cavity or a bubble cloud. The
bubble cloud may contain thousands of tiny bubbles, whose collapse
can cause broadband noise and erosion on the surface of the
hydraulic lifting body. After the cut-off, the small sheet cavity
on the leading edge will grow again and gradually reach its maximum
length. The re-entrant jet will re-form and move again from the end
of the unsteady sheet cavity upstream toward the leading edge of
the body. This process can repeat indefinitely, or until conditions
conducive to cavitation no longer exist.
[0018] As described in detail below, the present invention provides
a hydraulic lifting body with a self-actuating, foldable obstacle
system. The system includes a plurality of foldable obstacles or
plates that rotate outwardly from the surface of the body to block
the re-entrant jet if and when it occurs, thereby preventing or
substantially reducing noise and surface erosion. The obstacle
devices are designed so that when the re-entrant jet flow moves
forward over the body (which occurs at a flow velocity of roughly
the same order as that of the inflow velocity) over the body,
feather-like flaps on the obstacle devices are lifted up from the
surface of the body. The lifting force acting on the feather-like
flaps from the re-entrant jet acts as an initial driving force.
Torque produced by this initial driving force opens the obstacle
devices or plates. As soon as an obstacle device is partially open
and thus projecting out from the surface of the body, the
re-entrant jet can then exert a force on the obstacle device
itself. Torque applied both to the obstacle device and the
feather-like flap act to further open the obstacle device until the
device is fully opened and the plane of the obstacle device becomes
approximately normal to the surface of the lifting body. The
re-entrant jet is blocked by the obstacle device and cannot move
further forward. The re-entrant jet cannot reach a location near
the leading edge of the body, and thus will not cut off the sheet
cavity at that position. As a result, cloud cavitation is
suppressed or extensively reduced. Noise caused by the cloud cavity
is similarly reduced and blade surface erosion largely
eliminated.
[0019] If the operating conditions for the hydraulic lifting body
then change so that they are no longer favorable for cavitation,
the force of the fluid flowing in the usual way from the leading
edge of the body toward the trailing edge closes the foldable
obstacle device so that the obstacle device does not disturb the
fluid flow unnecessarily.
[0020] FIG. 1 shows a hydraulic lifting body 10 which extends
laterally between an outer end 12, and which has a leading edge 14
and a trailing edge 16. The body 10 has an upper surface 18 and an
opposite lower surface 20. The upper surface 18 has curvature equal
to or greater than that of the lower surface 20, and is thus
generally the low-pressure or suction side of the body 10, if the
angle of attack .alpha. is positive for a symmetric configuration,
or if .alpha.>.alpha..sub.0 for a non-symmetric (cambered)
configuration, where .alpha..sub.0 is the angle of attack for zero
lifting force and is negative for a non-symmetric configuration and
zero for a symmetric configuration. The body 10 may be, e.g., a
blade of a marine propeller, a hydro-turbine, or a pump, a fin or
rudder of a ship, or another hydraulic lifting body. The body 10
moves through a fluid to create the desired lifting action. Under
normal conditions, the fluid passes over the body 10 from the
leading edge 14 to the trailing edge 16.
[0021] This embodiment includes a self-actuating, foldable obstacle
system 22 on the upper surface 18 of the body 10. The system 22
includes a plurality of foldable obstacle devices or plates 24
mounted in the upper surface 18 of the body 10. Each of the plates
24 has a plurality of feather-like flaps 26 extending in a rearward
direction from a trailing edge of the plate 24. In the embodiment
of FIG. 1, the feather-like flaps 26 are generally semicircular in
shape. In other embodiments, the feather-like flaps may be
rectangular, square, trapezoidal, semi-elliptical, semi-circular,
or of any other shape appropriate to the flaps' function as
described herein.
[0022] As can be seen in FIG. 2, the plates 24 are mounted to fold
or pivot along a common axis extending in a lateral direction
across the body 10. Opposite end covers 28 are mounted on the upper
surface 18 of the body 10 to secure the obstacle system 22 with its
plates 24 in place. Fasteners 30 secure the end covers 28 in place
on the upper surface 18 of the body 10.
[0023] Each of the plates 24 rotates independently of the other
plates to open or close against the lifting body 10. In operation,
fluid flows over the body 10 from the leading edge 14 to the
trailing edge 16. When there is no cavitation, or when a sheet
cavity is not severe enough to produce a re-entrant jet, the plates
24 remain in their closed or folded positions as shown in FIG. 1.
The flow field on the surface 18 of the body 10 is not disturbed by
the plates 24 and the feather-like flaps 26, because the external
surfaces of the plates 24 have a curvature like that of the upper
surface 18 of the body 10, and the feather-like flaps 26 are
flexible and very thin.
[0024] The foldable obstacle devices or plates 24 are mounted on
the body 10 so as to be disposed at the central portion of an area
of possible cavitation along the lateral direction of the body 10
so as to achieve the best effect. Location of the plates 24 in this
manner acts to extensively reduce cavitation noise, although an
unsteady cavitation may extend over the entire low pressure surface
of the body 10 in its lateral or span-wise direction. It is also
desirable to mount the plates 24 at a central location on the body
10 because the thickness of the body 10 is large enough to
accommodate the rotating apparatus for the plates 24.
[0025] The body 10 has a chord length between its leading and
trailing edges 14 and 16. The positions of the plates 24 in the
chord-wise direction are preferably at a distance of approximately
35%-50% of the chord length, measured from the leading edge 14. In
particular, locating the plates 24 at 37% of the chord length from
the leading edge 14 has been found to reduce the noise levels most
effectively, although location of the plates 24 at other chord
length distances can also reduce the noise level.
[0026] FIG. 2 shows an arrangement of the self-actuating, foldable
obstacle system 22 in which the obstacle devices or plates 24 are
mounted for independent rotation along a common axis extending
along the body 10. An elongated groove 32 is formed within the body
10, and is curved in cross-section so as to rotatably seat a
plurality of generally cylindrical hubs 34. Each hub 34 has one of
the plates 24 mounted thereon at a first or leading edge of the
plate. An opposite trailing edge of each plate 24 has a plurality
of the feather-like flaps 26 mounted thereon (not shown in FIG. 2).
Each hub 34 has a shaft 36 extending from one end thereof and
rotatably received within a hole 38 in an adjacent hub 34. A shaft
40 at the left end of the obstacle system 22 as viewed in FIG. 2
extends into a hole 38 in a left-most one of the hubs 34 to
facilitate rotating motion of such hub. A right-most one of the
hubs 34 as viewed in FIG. 2 has the shaft 36 thereof rotatably
disposed within a hole 42 at a right-hand end of the elongated
groove 32. The opposite covers 28 help to secure the arrangement of
foldable obstacle devices or plates 24 within the body 10. Also,
the groove 32 is configured so as to rotatably retain the hubs 34
therein and to define a rotational limit for each of the plates 24.
As a result, each of the plates 24 is rotatable through an angle of
approximately 90.degree. between a closed position at the upper
surface 18 of the body 10 and an open position in which the plate
24 extends outwardly so as to be substantially perpendicular to the
upper surface 18 of the body 10.
[0027] Referring to FIG. 3, when an unsteady sheet cavity 44 is
fully developed on the body 10, a re-entrant jet 46 appears. The
pressure within the sheet cavity 44 is the saturated vapor pressure
P.sub.v, which is always lower than the pressure of nearby fluid.
The re-entrant jet 46 is comprised of running fluid. The jet 46
flows in a direction from the trailing edge 16 to the leading edge
14 of the body 10, and does so at a velocity on roughly the same
order as that of the inflow. When the re-entrant jet 46 reaches the
feather-like flaps 26, the jet 46 blows the feather-like flaps 26
upward and produces a force F.sub.0 on the feather-like flaps 26.
The torque generated by the force F.sub.0 causes some rotation of
the plates 24. Once each plate 24 is partially opened so as to
extend outwardly from the upper surface 18 of the body 10, the
re-entrant jet 46 exerts a force on the plate itself. The torques
caused by the forces on the plate 24 and the initial driving force
F.sub.0 act to further open the plate 24 until it is fully opened
and approximately normal or perpendicular to the upper surface 18
of the body 10. With the plate 24 opened in this fashion, the plate
blocks the re-entrant jet 46 so that the jet 46 cannot move further
forward to a location near the leading edge 14 and cannot cut off
the sheet cavity 44 near the leading edge. Accordingly, cloud
cavitation is prevented or extensively reduced. The noise caused by
a cloud cavity is reduced and surface erosion of the body 10 is
largely eliminated.
[0028] Because the obstacle devices or plates 24 are foldable and
rotate to the closed position in the case of non-cavitation, the
heights of the plates 24 can be greater than in a case where fixed
obstacles are provided on the surface of a body. As soon as the
plates 24 disturb the flow of fluid from the leading edge 14 to the
trailing edge 16 of the body 10, in the case of non-cavitation, the
force produced by the flow closes the plates 24. For this reason,
the height of the plates 24 can be between the thickness of the
re-entrant jet 46 and the thickness of the sheet cavity 44.
Experimentation can be used to determine the ideal plate height.
Alternatively, the thickness of the re-entrant jet 46 and the
thickness of the sheet cavity 44 can be observed experimentally. A
still further method of determining the optimum height of the
plates 24 is to predict the thickness of the re-entrant jet 46 and
the thickness of the sheet cavity 44 using numerical methods. It is
preferred that the obstacle device have a height when in the open
position which is at least 1.2-2.0% of the lifting body's chord
length, though this will depend somewhat upon the characteristics
of a particular lifting body and the fluid through which it
moves.
[0029] FIG. 4 depicts a condition in which all of the obstacle
devices or plates 24 have been fully rotated to the open position.
The re-entrant jet 46 lifts the feather-like flaps 26 to produce
the initial driving force F.sub.0. The initial driving force
combines with forces acting on the half-opened plates 24 to rotate
the plates 24 to the fully open position, as shown in FIG. 4. The
plates 24 cannot be rotated further, because the configuration of
the elongated groove 32 prevents this. In the example of FIG. 4,
all of the plates 24 are open because the re-entrant jet 46 extends
across all of the positions where the plates 24 are located.
[0030] FIG. 5 illustrates a condition in which some but not all of
the obstacle devices or plates 24 are open. More specifically, two
of the plates 24 are opened, but a third of the plates 24 remains
closed. This is because the re-entrant jet 24 in this figure
extends only across the positions of the first two plates 24 and
not to that of the third such plate. The plates 24 open and close
independently of each other, and take their effect only where
necessary.
[0031] Self-actuating, foldable obstacle systems according to the
invention can be designed to suit the particular surface of the
hydraulic lifting body with which they are mounted. Where a
plurality of obstacle devices or plates are provided, they need not
necessarily be of the same size, although a common size or shape
can be chosen for simplicity. In the embodiment described herein, a
plurality of the obstacle devices or plates 24 are mounted along a
common axis. However, the obstacle devices or plates can be mounted
in different locations on the body 10 for rotation about different
axes if desired. Also, the shafts 36 and the holes 38 can be
hemispherical or otherwise curved in shape in order to mount the
plates on a hydraulic lifting body that may be skewed, twisted or
raked.
[0032] The close clearance fit between the hubs 34 and the walls of
the elongated groove 32 constrains the motion of the hubs 34 in the
radial direction relative to the body 10 and allows the hubs 34 to
rotate relative to the body 10, even without the shafts 36 and 40
and the holes 38 and 42. The clearance fits between the shafts 36
and 40 and the holes 38 and 42 make the system more reliable,
however. Also, for certain applications it may be desirable to
utilize bearings where the various shafts and holes are connected
to facilitate the desired rotational movement.
[0033] In FIGS. 1-5, the self-actuating, foldable obstacle system
22, with its obstacle devices or plates 24, is shown in enlarged
fashion relative to the body 10 for clarity of illustration. In
actual practice, the system 22 with its obstacle devices or plates
24 is usually substantially smaller relative to the body 10 than is
indicated in the figures.
[0034] The obstacle devices or plates 24 are preferably made of
material that is light, hard, tough, and resistant to erosion and
fatigue. ABS plastic is a suitable material for many applications.
The feather-like flaps 26 can be made of a flexible material such
as Teflon.RTM..
[0035] An exemplary embodiment of the invention has been described
herein for purposes of illustration. The invention is not limited
to this embodiment, though, and numerous and diverse changes,
modifications, or additions may be made by those of skill in the
art without departing substantially from the inventions basic
principles. The true scope of the invention should be determined,
therefore, primarily by reference to the appended claims, along
with the full scope of equivalents to which those claims are
legally entitled.
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