SYSTEM AND METHOD FOR MITIGATING VORTEX-SHEDDING
VIBRATIONS OR STALL-INDUCED VIBRATIONS ON A ROTOR BLADE OF A
WIND TURBINE DURING STANDSTILL
FIELD
[0001] The present disclosure relates in general to wind turbines, and more
particularly to systems and methods for mitigating vortex-shedding vibrations or stallinduced vibrations on a rotor blade of a wind turbine during standstill.
BACKGROUND
[0002] Wind power is considered one of the cleanest, most environmentally
friendly energy sources presently available, and wind turbines have gained increased
attention in this regard. A modern wind turbine typically includes a tower, a
generator, a gearbox, a nacelle, and one or more rotor blades. The nacelle includes a
rotor assembly coupled to the gearbox and to the generator. The rotor assembly and
the gearbox are mounted on a bedplate support frame located within the nacelle. The
one or more rotor blades capture kinetic energy of wind using known airfoil
principles. The rotor blades transmit the kinetic energy in the form of rotational
energy so as to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is
not used, directly to the generator. The generator then converts the mechanical
energy to electrical energy that may be deployed to a utility grid.
[0003] During the lifespan of the wind turbine, there may, from time to time, be
periods where the rotor blades are parked or idled. For example, the wind turbine is
generally locked in a fixed position during installation or repair of the rotor blades. In
another instance, the rotor blades may be pitched so as to idle the wind turbine during
a maintenance procedure. Often, during such maintenance or installation procedures,
or in response to a system failure, the ability to pitch the rotor blades or yaw the
nacelle relative to the wind may be lost. Thus, when modern wind turbines are parked
or idled, they are put at risk for flow-induced oscillations caused by the wind.
[0004] In one instance, the flow-induced oscillations may be the result of the wind
primarily impacting a side, as opposed to an edge, of the rotor blade. When the wind
impacts the edge(s) in such a manner, the wind attempts to flow around the slender
structure of the rotor blade and form a vortex at the point where the flow separates
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from the rotor blade. Depending on the relative angle between the wind and the rotor
blade, a vortex-shedding phenomenon may occur where the vortices form on the
edges of the blade and are shed in a rhythmic pattern from alternating edges. The
alternating nature of the edgewise vortex shedding may result in a vibration of the
rotor blade. When the vortex shedding frequency coincides with, or is sufficiently
close to, a natural frequency of the structure, large and damaging vibrations may be
the result.
[0005] A known method for limiting vortex-induced vibrations is to feather the
rotor blade with respect to the wind so that the wind flows smoothly around the rotor
blade. However, without the ability to adjust the position of the rotor blade, the wind
direction generally shifts and increases the angle of attack of the rotor blade. At a
sufficiently high angle of attack, the flow over the rotor blade separates, thereby
resulting in a rotor blade in a stall condition. The separated, stalled flow is unstable
and can result in stall-induced vibrations.
[0006] In view of the aforementioned issues, the art is continuously seeking new
and improved systems and methods for mitigating vortex-shedding vibrations or stallinduced vibrations. Thus, a system and method for mitigating vortex-shedding
vibrations or stall-induced vibrations on one or more rotor blades of a wind turbine
during standstill would be advantageous.
BRIEF DESCRIPTION
[0007] Aspects and advantages of the invention will be set forth in part in the
following description, or may be obvious from the description, or may be learned
through practice of the invention.
[0008] In one aspect, the present disclosure is directed to an apparatus for
mitigating vortex-shedding vibrations or stall-induced vibrations on one or more rotor
blades of a wind turbine during standstill. The apparatus may have at least one
positioning element located between a blade tip section and a blade root section
thereof. The positioning element(s) is adapted for wrapping around at least a portion
of the rotor blade. The apparatus may also have at least one air flow modifying
element coupled to the positioning element(s). The airflow modifying element(s) may
define a height relative to a surface of the rotor blade. Additionally, the apparatus
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may include at least one securing element operably coupled to the positioning
element(s) for temporarily securing the airflow modifying element(s) to the rotor
blade.
[0009] In an embodiment, the positioning element(s) may include at least one
cord member helically wrapped around a portion of the rotor blade. In an additional
embodiment, the helically-wrapped cord element may have a pitch greater than or
equal to one half of a chord of the rotor blade up to less than or equal to 6 times the
chord of the rotor blade. At least a portion of the helically-wrapped cord element may
be separated from the surface of the rotor blade by a distance defined by the airflow
modifying element(s).
[0010] In an embodiment, the airflow modifying element(s) may include at least
one of a streamer, a fence, an inflatable cell, a serration, a vortex generator, and/or a
flow disruptor.
[0011] In an embodiment, the airflow modifying element(s) may include a
plurality of airflow modifying elements distributed along and secured to the
positioning element(s). In another embodiment, the plurality of airflow modifying
elements may include leading-edge airflow modifying elements and trailing-edge
airflow modifying elements. The leading-edge airflow modifying elements and the
trailing-edge airflow modifying elements may be distributed in an alternating pattern
along the positioning element(s).
[0012] In an embodiment, the airflow modifying element(s) may include at least
one leading-edge airflow modifying element and at least one trailing-edge airflow
modifying element disposed opposite thereof. The positioning element(s) may be
constructed of an elastic cord element coupled between the leading- and trailing-edge
airflow modifying elements. The elastic cord element may limit a separation between
the leading- and/or trailing-edge airflow modifying elements at a specified span
location of the rotor blade.
[0013] In an embodiment, the positioning element(s) may include a leading-edge
positioning element arranged parallel to a leading edge of the rotor blade and a
trailing-edge positioning element arranged parallel to a trailing edge of the rotor
blade. The airflow modifying element(s) may include at least one leading-edge
airflow modifying element coupled to the leading-edge positioning element and at
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least one trailing-edge airflow modifying element coupled to the trailing-edge
positioning element. The leading-edge airflow modifying element may define a
leading-edge notch corresponding to a leading-edge profile. The trailing-edge airflow
modifying element may define a trailing-edge mounting notch corresponding to a
trailing-edge profile.
[0014] In an embodiment, the airflow modifying element(s) may have a mounting
edge. The airflow modifying element may be coupled along the mounting edge to the
positioning element(s). The positioning element(s) may be oriented perpendicular to
a chordwise direction of the rotor blade. The positioning element(s) may be
temporarily coupled to at least one of a leading edge, a trailing edge, or along a line
upstream of a flow separation point on either a suction side or a pressure side of the
rotor blade. In an additional embodiment, the positioning element(s) may include a
cord oriented spanwise along the rotor blade. In such embodiments, the securing
element(s) may include a first securing element coupled to a first end of the
positioning element and a second securing element coupled to an opposing, second
end of the positioning element.
[0015] In a further embodiment, the positioning element(s) may include a hollow
sleeve configured to encapsulate a portion of the rotor blade. The hollow sleeve may
define at least one inlet air hole.
[0016] In another aspect, the present disclosure is directed to a method for
mitigating vortex-shedding vibrations or stall-induced vibrations on one or more rotor
blades of a wind turbine during standstill. The method may include installing a
vibration-mitigation apparatus on the rotor blade. The installation of the vibrationmitigation apparatus may include, for example, positioning at least one airflow
modifying element at a desired location along an outer surface of a rotor blade with at
least one positioning element and securing the positioning element(s) to the outer
surface of the rotor blade via at least one securing element. The method may also
include performing a maintenance and/or a repair operation on the wind turbine with
the vibration-mitigation apparatus installed. Once the maintenance and/or repair
operation is complete, the method may include removing the vibration-mitigation
apparatus from the rotor blade.
[0017] In an embodiment, the airflow modifying element(s) may include at least
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one of a streamer, a fence, and inflatable cell, serration, a vortex generator, and/or a
flow disruptor.
[0018] In a further embodiment, the positioning element(s) may include a cord
element. In such embodiments, positioning the airflow modifying element(s) at the
desired location may include helically wrapping the cord element around a portion of
the rotor blade so as to disrupt airflow over a chordwise or spanwise section of the
rotor blade. At least a portion of the positioning element(s) may be separated from
the outer surface of the rotor blade by a distance defined by the airflow modifying
element(s).
[0019] In an additional embodiment, positioning the airflow modifying element(s)
at the desired location may include positioning an edge of the airflow modifying
element(s) adjacent to at least one of a leading edge, a trailing edge, or along a line
upstream of a flow separation point on either a suction side or a pressure side of the
rotor blade.
[0020] In an embodiment, the positioning element(s) may include a hollow sleeve
defining an inlet air hole and the airflow modifying element(s) may include the
inflatable cell. In such embodiments, positioning the airflow modifying element(s) at
the desired location may include encapsulating a portion of the rotor blade within the
hollow sleeve and coupling the inflatable cell to the hollow sleeve and operably
coupling the inflatable cell to the air hole. In addition, the inflatable cell may be
disposed adjacent to a pressure side or a suction side of the rotor blade.
[0021] In another aspect, the present disclosure is directed to a rotor assembly for
a wind turbine. The rotor assembly may include a rotor hub in standstill and at least
one rotor blade. The rotor blade(s) may include a leading edge and a trailing edge
disposed between a tip portion and a root portion opposite thereof. The root portion
may be coupled to the rotor hub. The rotor assembly may also include an apparatus
for mitigating vortex-shedding vibrations and/or stall-induced vibrations on the rotor
blade(s) during standstill. The apparatus may include at least one positioning element
located between a blade tip section and a blade root section thereof. The positioning
element(s) may be adapted for wrapping around at least a portion of the rotor blade(s).
The apparatus may also include at least one airflow modifying element coupled to the
positioning element(s). The airflow modifying element(s) may define a height
7
relative to a surface of the rotor blade. Additionally, the apparatus may include at
least one securing element operably coupled to the airflow modifying element(s) for
temporarily securing the airflow modifying element(s) to the rotor blade. It should be
understood that the rotor blade assembly may further include any of the additional
features described herein.
[0022] These and other features, aspects and advantages of the present invention
will become better understood with reference to the following description and
appended claims. The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate embodiments of the invention and,
together with the description, serve to explain the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] A full and enabling disclosure of the present invention, including the best
mode thereof, directed to one of ordinary skill in the art, is set forth in the
specification, which makes reference to the appended figures, in which:
[0024] FIG. 1 illustrates a perspective view of one embodiment of a wind turbine
with an installed apparatus for mitigating vortex-shedding vibrations or stall-induced
vibrations according to the present disclosure;
[0025] FIG. 2 illustrates a perspective view of one of the rotor blades of FIG. 1;
[0026] FIG. 3A illustrates a schematic view of a portion of the rotor blade of FIG.
2 according to the present disclosure, particularly illustrating one embodiment of an
apparatus for mitigating vortex-shedding vibrations or stall-induced vibrations
installed thereon;
[0027] FIG. 3B illustrates a cross-sectional view of the rotor blade and apparatus
of FIG. 3A according to the present disclosure;
[0028] FIG. 4 illustrates a schematic view of a portion of the rotor blade of FIG. 2
according to the present disclosure, particularly illustrating an alternative embodiment
of the apparatus for mitigating vortex-shedding vibrations or stall-induced vibrations
installed thereon;
[0029] FIG. 5A illustrates a schematic view of a portion of the rotor blade of FIG.
2 according to the present disclosure, particularly illustrating still another embodiment
of the apparatus for mitigating vortex-shedding vibrations or stall-induced vibrations
8
installed thereon;
[0030] FIG. 5B illustrates a cross-sectional view of the rotor blade and apparatus
of FIG. 5A according to the present disclosure;
[0031] FIG. 6A illustrates a schematic view of a portion of the rotor blade of FIG.
2 according to the present disclosure, particularly illustrating yet another embodiment
of the apparatus for mitigating vortex-shedding vibrations or stall-induced vibrations
installed thereon;
[0032] FIG. 6B illustrates a cross-sectional view of the rotor blade and apparatus
of FIG. 6A according to the present disclosure;
[0033] FIG. 7 illustrates a perspective view of a portion of the rotor blade of FIG.
2 according to the present disclosure, particularly illustrating a further embodiment of
an apparatus for mitigating vortex-shedding vibrations or stall-induced vibrations
installed thereon;
[0034] FIG. 8 illustrates a perspective view of a portion of the rotor blade of FIG.
2 according to the present disclosure, particularly illustrating another embodiment of
an apparatus for mitigating vortex-shedding vibrations or stall-induced vibrations
installed thereon; and
[0035] FIG. 9 illustrates a flow diagram of one embodiment of a method for
mitigating vortex-shedding vibrations or stall-induced vibrations on one or more rotor
blades of a wind turbine during standstill according to the present disclosure.
DETAILED DESCRIPTION
[0036] Reference now will be made in detail to embodiments of the invention,
one or more examples of which are illustrated in the drawings. Each example is
provided by way of explanation of the invention, not limitation of the invention. In
fact, it will be apparent to those skilled in the art that various modifications and
variations can be made in the present invention without departing from the scope or
spirit of the invention. For instance, features illustrated or described as part of one
embodiment can be used with another embodiment to yield a still further
embodiment. Thus, it is intended that the present invention covers such modifications
and variations as come within the scope of the appended claims and their equivalents.
[0037] As used herein, the terms “upstream” and “downstream” refer to directions
9
relative to the airflow. For example, a wind will contact an upstream feature prior to
contacting a downstream feature.
[0038] Generally, the present disclosure is directed to an apparatus and method
for mitigating vortex-shedding vibrations or stall-induced vibrations in a rotor blade
of a wind turbine while the rotor blade is in standstill. The apparatus may be
configured to temporarily attach to the rotor blade when the rotor blade is locked or
idled and removed when the maintenance, repair, or installation procedure is
completed. The apparatus may, in various embodiments, include at least one airflow
modifying element coupled to at least one positioning element. The apparatus may be
secured to the rotor blade and positioned so as to disrupt the flow of air over the
locked or idling rotor blade. The apparatus may disrupt the rhythmic shedding of the
vortices from the edges of the rotor blade by, for example, disrupting the formation of
the vortices, fixing the location of the vortex shedding, or causing many vortices to be
shed at fixed alternating locations, causing alternations of the mutual phase
relationship. Additionally, the apparatus may be placed to increase the drag of the
rotor blade, thereby destroying lift and preventing stall-induced vibrations. By
disrupting the flow of air over the locked or idling rotor blade, the apparatus may
mitigate the likelihood of large and damaging vibrations resulting the coincidence of a
vibration with the natural frequency of the structure.
[0039] Referring now to the drawings, FIG. 1 illustrates a perspective view of one
embodiment of a wind turbine 100 according to the present disclosure. As shown, the
wind turbine 100 generally includes a tower 102 extending from a support surface
104, a nacelle 106 mounted on the tower 102, and a rotor 108 coupled to the nacelle
106. The rotor 108 includes a rotatable hub 110 and at least one rotor blade 112
coupled to and extending outwardly from the hub 110. For example, in the illustrated
embodiment, the rotor 108 includes three rotor blades 112. However, in an alternative
embodiment, the rotor 108 may include more or less than three rotor blades 112.
Each rotor blade 112 may be spaced about the hub 110 to facilitate rotating the rotor
108 to enable kinetic energy to be transferred from the wind into usable mechanical
energy, and subsequently, electrical energy. For instance, the hub 110 may be
rotatably coupled to an electric generator positioned within the nacelle 106 to permit
electrical energy to be produced.
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[0040] Referring now to FIG. 2, a perspective view of the rotor blade(s) 112 of
the wind turbine 100 shown in FIG. 1 is illustrated. The rotor blade(s) 112 may
generally include a blade root 118 configured to be mounted or otherwise secured to
the hub 110 of the wind turbine 100 and a blade tip 120 disposed opposite the blade
root 118. A body shell 122 of the rotor blade(s) 112 may generally extend between
the blade root 118 and the blade tip 120 along a pitch axis 116. The body shell 122
may generally serve as the outer casing or surface or covering of the rotor blade(s)
112. The body shell 122 may define a substantially aerodynamic profile such as by
defining a symmetrical or cambered airfoil-shaped cross-section. The body shell 122
may also define a pressure side 124 and a suction side 126 extending between a
leading edge 128 and a trailing edge 130 of the rotor blade(s) 112. Further, the rotor
blade(s) 112 may also have a span (S), which defines the total length between the
blade root 118 and the blade tip 120. The rotor blade(s) 112 may also have a chord
(C) defining the total length between the leading edge 128 and the trailing edge 130.
As is generally understood, the chord (C) may vary in length with respect to the span
(S) as the rotor blade(s) 112 extends from the blade root 118 to the blade tip 120.
[0041] Referring now to FIGS. 3A-8, various embodiments of an apparatus 200
for mitigating vortex-shedding vibrations and/or stall-induced vibrations on the rotor
blade(s) 112 of the wind turbine 100 during standstill are illustrated according to the
present disclosure. For example, in certain instances, the rotor blade(s) 112 may be in
standstill when the hub 110 of the wind turbine 100 is braked, locked, or idling. As
shown, the apparatus 200 may include at least one positioning element 202 located
between the blade tip 120 and the blade root 118 sections of the rotor blade(s) 112.
More specifically, as shown, the positioning element(s) 202 may be adapted for
wrapping around at least a portion of the rotor blade(s) 112.
[0042] The apparatus 200 may also include at least one airflow modifying
element 204 coupled to the positioning element(s) 202. The airflow modifying
element(s) 204 may define a distance (H) relative to the body shell/surface 122 of the
rotor blade(s) 112. The apparatus 200 may also include at least one securing element
206 operably coupled to the positioning element(s) 202 for temporarily securing the
airflow modifying element(s) 204 to the rotor blade(s) 112. It should be appreciated
that, in an embodiment, the positioning element(s) 202 may be separated from the
11
surface, or body shell 122, of the rotor blade(s) 112 by a distance (H) defined by the
airflow modifying element(s) 204.
[0043] In at least one embodiment, the positioning element(s) 202 may be a cord
208 having a plurality of yarns, plies, fibers, or strands which are braided or twisted
together to form a generally circular cross section. For example, the cord 208 may be
a natural or synthetic rope having a single-braid, double-braid, plaited, twisted,
hollow, or diamond-braid construction. The cord 208 may, in an embodiment,
include a plurality of buoyant fibers so as to enable the apparatus 200 to float. In an
additional embodiment, maintaining a selected curvature of the positioning element(s)
202 may be facilitated by the inclusion of a malleable core material, such as a metal
wire. In further embodiments, the positioning element(s) 202 may, for example, be a
cable, a strap (e.g., a woven nylon strap), a ribbon, a pultrusion, or an elasticized cord
(e.g., a bungee cord).
[0044] In an additional embodiment, the positioning element(s) 202 may be a coil
spring. The positioning element(s) 202 may have a coiled length and an installed
length. The installed length may be greater than the coiled length. The positioning
element(s) 202 may have a resting coil diameter and an installed coil diameter which
is less than the resting coil diameter. It should be appreciated that, applying a tension
to stretch the positioning element(s) 202 to its installed length may result in the
positioning element(s) 202 tightening around the rotor blade(s) 112, and that releasing
the tension may allow the positioning element(s) 202 to return the resting coil
diameter and release the rotor blade(s) 112.
[0045] In at least one embodiment, the positioning element(s) 202 may be
helically wrapped around a portion of the blade 112. The helically-wrapped cord 208
may have a pitch greater than or equal to one half of the chord (C) of the rotor
blade(s) 112. The helically-wrapped cord 208 may also have a pitch less than or
equal to six times the chord (C).
[0046] It should be appreciated that, in additional embodiments, the positioning
element(s) 202 may be positioned in different orientations relative to the rotor
blade(s) 112. For example, the positioning element(s) 202 may be oriented generally
parallel to the pitch axis 116, the leading edge 128, and/or the trailing edge 130. In at
least one such embodiment, such as depicted in FIGS. 5A and 5B, the positioning
12
element(s) 202 may include at least one leading-edge positioning element 222
arranged parallel to the leading edge 128. The positioning element(s) 202 may also
include at least one trailing-edge positioning element 224 parallel to the trailing edge
130.
[0047] In an alternative embodiment, the cord 208 may be stretched between the
blade tip 120 and the hub 110 or between securing elements 206. The cord 208 may,
as depicted in FIG. 7, be positioned along a line upstream of an anticipated flow
separation point on either the suction side 126 or the pressure side 124 of the rotor
blade(s) 112.
[0048] As depicted in FIG. 6A, the positioning element(s) 202 may also be
oriented parallel to the chord (C) and generally perpendicular to the pitch axis 116.
For example, in at least one embodiment, the positioning element(s) 202 may include
an elastic cord element 226 stretched along the chord (C) between airflow modifying
elements 204 positioned along the leading and trailing edges 128, 130 respectively.
[0049] In an additional embodiment, such as shown in FIG. 8, the positioning
element(s) 202 may be a hollow sleeve 210. Thus, as shown, the hollow sleeve 210
may encapsulate a portion of the rotor blade(s) 112. Further, as shown, the hollow
sleeve 210 may have a first end 212 and a second end 214 disposed opposite thereof.
The first end 212 may define an opening corresponding to an airfoil shape at a first
specified spanwise location 216. The second end 214 may define an opening
corresponding to an airfoil shape at a second specified spanwise location 218. In an
alternative embodiment, such as depicted in FIG. 4, the second end 214 may be sealed
so as to encapsulate the blade tip 120. In certain embodiments, the hollow sleeve 210
may be a woven fabric, a mesh, a plastic, or a composite. In at least one embodiment,
the hollow sleeve 210 may be configured to be degradable over a specified interval of
time so that the hollow sleeve 210 may fall off the rotor blade(s) 112 with no
intervention.
[0050] Referring to FIGS. 3A, 4, 5A, 6, and 8, the securing element(s) 206 may,
in various embodiments, be configured to encircle a portion of the blade tip 120.
Thus, in certain embodiments, the securing element(s) 206 may, for example, be a
band, a cord, a belt, or a strap. In an embodiment, the securing element 206 may be a
loop having a diameter corresponding to the dimensions of an airfoil at a specified
13
spanwise location of the rotor blade(s) 112. Alternatively, the securing element(s)
206 may be an adhesive strip positioned between the positioning element(s) 202 and
the body shell 122. In at least one embodiment, the securing element(s) 206 may be a
high friction material, such as rubber, configured to resist a rotational movement
around the rotor blade(s) 112. It should be appreciated, that the securing element(s)
206 may, alternatively, be a sleeve, sack, or sock configured to encapsulate the blade
tip 120.
[0051] As particularly depicted in FIG. 6A, the securing element(s) 206 may, in
an embodiment, be a rigid securing element 220. The rigid securing element 220
may, for example, be formed from wood, metal, foam, plastic, composites, or any
other suitably rigid material. The rigid securing element 220 may define an opening
which corresponds to a cross-section of the rotor blade(s) 112 at a specified spanwise
location.
[0052] Still referring to FIGS. 3A-8, the airflow modifying element(s) 204 may be
any suitable structure configured to disrupt airflow over and/or around the rotor
blade(s) 112 in standstill. For example, in various embodiments, such as are depicted
in FIGS. 7 and 8, the airflow modifying element(s) 204 may include at least one of a
streamer 242, a fence, an inflatable cell 258, a serration, a vortex generator, a flow
disruptor, or a combination thereof. In several embodiments, the airflow modifying
element(s) 204 may be formed from wood, foam, plastics, composites, metal, fabric,
or any other suitable materials. In addition, the airflow modifying element(s) 204
may be buoyant. In further embodiments, the airflow modifying element(s) 204 may
be a solid structure or a hollow structure. The airflow modifying element(s) 204 may
have fixed external dimensions or may be inflatable. In at least one embodiment, the
airflow modifying element(s) 204 may have a diameter which is greater than or equal
to 5% of the chord (C). In an embodiment, the airflow modifying element(s) 204 may
have a diameter which is less than or equal to 15% of the chord (C) when installed.
[0053] In further embodiments, the airflow modifying element(s) 204 may, in an
embodiment, be any suitable three-dimensional shape. For example, in various
embodiments, the airflow modifying element(s) 204 may be a sphere, a prolate
spheroid, a hemisphere, a cube, a rectangular prism, a frustrum, or an irregular threedimensional shape. In additional embodiments, the airflow modifying element(s) 204
14
may be a flexible sheet oriented on a plane having one fixed edge or curving so as to
have at least two fixed edges. For example, in an embodiment, the airflow modifying
element(s) 204 may be a flexible sheet coupled to the trailing edge 130 at between 0.5
and two times the chord pitch and extending to a serrated edge. It should be
appreciated that the airflow modifying element(s) 204 may be formed so as to
establish a face conforming to a curvature of the rotor blade(s) 112.
[0054] Referring again to FIGS. 3A, 4, 5A, and 6, the apparatus 200 may include
at least one removal element 228. As shown, the removal element(s) 228 may be
configured to act on the securing element(s) 206 so as to initiate the removal of the
apparatus 200 from the rotor blade(s) 112 upon completion of the maintenance, repair,
or installation operation. In an embodiment, the removal element(s) 228 may be a
cord or line coupled to the securing element(s) 206 and extending to the support
surface 104. In another embodiment, the removal element(s) 228 may be at least one
of a separating feature (e.g., a pull-tab, a perforation, a zipper or other feature
configured to encourage a separation at a selected location), a quick-release coupling,
or a magnetic catch configured to release the securing element(s) 206 from the rotor
blade(s) 112.
[0055] As depicted particularly in FIGS. 3A-4, the apparatus 200 may include a
plurality of airflow modifying elements 204 distributed along and secured to the
positioning element(s) 202. In an embodiment, as shown in FIG. 3A, the airflow
modifying elements 204 may be distributed at a specified interval (I) along the
helically-wrapped cord 208. The specified interval (I) may be selected to ensure that
the airflow modifying elements 204 may be distributed across the pressure side 124
and/or the suction side 126 of the rotor blade(s) 112. In an additional embodiment,
the plurality of airflow modifying elements 204 may include at least one leading-edge
airflow modifying element and at least one trailing-edge airflow modifying element
230, 232 distributed in an alternating pattern along the positioning element(s) 202 and
the specified interval (I) may be selected to position the airflow modifying elements
204 along the leading and trailing edges 128, 130. As such, the positioning of the
airflow modifying elements 204 relative to the rotor blade(s) 112 may be determined
by the specified interval (I) and the pitch of the helically-wrapped cord 208.
[0056] Still referring to FIGS. 3A-4, in an exemplary embodiment, the airflow
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modifying elements 204 may be spheres or prolate spheroids having the cord 208
passing through the center thereof and being fixed at the specified interval (I) along
the positioning element(s) 202. In such an embodiment, the distance (H) may be the
radius of the airflow modifying elements 204 minus the radius of the cord 208.
[0057] Referring particularly to FIGS. 5A and 5B, another embodiment of the
apparatus 200 for mitigating vortex-shedding vibrations or stall-induced vibrations in
a rotor blade(s) 112 in standstill is depicted. In such an embodiment, the positioning
element(s) 202 may include the leading-edge positioning element(s) 222 arranged
parallel to the leading edge 128 of the rotor blade(s) 112. The positioning element(s)
202 may also include the trailing-edge positioning element(s) 224 arranged parallel to
the trailing edge of the rotor blade(s) 112. The airflow modifying element(s) 204 may
include the leading-edge airflow modifying element(s) 230 coupled to the leadingedge positioning element(s) 222. The airflow modifying element(s) 204 may also
include the trailing-edge airflow modifying element(s) 232 coupled to the trailingedge positioning element(s) 224.
[0058] Thus, as shown, the leading-edge airflow modifying element(s) 230 may
define a leading-edge mounting notch 234 corresponding to a leading-edge profile
236, whereas the trailing-edge airflow modifying element(s) 232 may define a
trailing-edge mounting notch 238 corresponding to a trailing-edge profile 240. In at
least one embodiment, the leading- and trailing-edge airflow modifying elements 230,
232 may extend around the respective edges from the pressure side 124 to the suction
side 126. As such, the apparatus 200 may include additional leading-edge positioning
elements 222 and trailing-edge positioning elements 224.
[0059] It should be appreciated that, in at least one embodiment, the leading- and
trailing-edge airflow modifying elements 230, 232 may also be distributed along the
respective positioning elements 202 at specified locations calculated to fix the
location of the vortex shedding. The distribution of the leading- and trailing-edge
airflow modifying elements 230, 232 may place the respective modifying elements in
line along a specified chord (C), establish an alternating spanwise pattern, or a
combination thereof.
[0060] As depicted in FIGS. 6A-6B, in another embodiment, the apparatus 200
may include the leading-edge airflow modifying element(s) 230 and the trailing-edge
16
airflow modifying element(s) 232 disposed opposite thereof at a specified spanwise
location. In at least one embodiment, the leading- and trailing-edge airflow
modifying elements 230, 232 may extend around the respective edges from the
pressure side 124 to the suction side 126. The leading- and trailing-edge airflow
modifying elements 230, 232 may be coupled by the elastic cord element 226. The
elastic cord element 226 may limit a separation between the leading-edge airflow
modifying element 230 and the trailing-edge airflow modifying element 232 at the
specified spanwise location. For example, in an embodiment, the length (L) of the
elastic cord element 226 may be adjusted to conform to the length of the chord (C) at
a specified spanwise location. As such, in a tapered portion of the rotor blade(s) 112,
the length (L) of the elastic cord element 226 may determine the placement of the
leading- and trailing-edge airflow modifying elements 230, 232.
[0061] In an additional embodiment, such as depicted in FIG. 7, the airflow
modifying element(s) 204 may be a streamer 242 (e.g., a flag or other generally
planar, flexible sheet). The streamer 242 may have a surface density greater than 300
g/m². The streamer 242 may, in an additional embodiment, have a surface density
less than 750 g/m². In addition, as shown, the streamer 242 may have a mounting
edge 244 secured to the positioning element(s) 202, e.g. that is oriented perpendicular
to a chord-wise direction of the rotor blade(s) 112. In such embodiments, the
positioning element(s) 202 may be temporarily coupled to at least one of the leading
edge 128, the trailing edge 130, or along a line upstream of a flow separation point
246. The positioning element(s) 202 may be coupled to either the suction side 126 or
the pressure side 124 of the rotor blade(s) 112. In some embodiments, the streamer
242 may extend from the mounting edge 244 to a length three times the length of the
chord (C).
[0062] Still referring to FIG. 7, in at least one embodiment, the positioning
element(s) 202 may be a cord or strap oriented spanwise along the rotor blade(s). In
an embodiment, the securing element(s) 206 may include a first securing element 248
coupled to a first end 250 of the positioning element(s) 202. The securing element(s)
206 may also include a second securing element 252 coupled to an opposing, second
end 254 of the positioning element(s) 202. In an alternative embodiment, the securing
element(s) 206 may be an adhesive strip positioned between the positioning
17
element(s) 202 and the body shell 122.
[0063] Referring now to FIG. 8, still another embodiment of the apparatus 200 for
mitigating vortex-shedding or stall-induced vibrations in a rotor blade(s) 112 in
standstill is depicted. In such an embodiment, the securing element(s) 206 may be the
hollow sleeve 210. The hollow sleeve 210 may define at least one inlet air hole 256.
In an embodiment, such as depicted in FIG. 8, the airflow modifying element(s) 204
may be the inflatable cell 258. The inflatable cell 258 may be a flexible plenum
which may be fluidly coupled to the inlet air hole(s) 256 and inflated by air passing
therethrough. The inflatable cell 258 may be positioned adjacent to the pressure side
124 or the suction side 126. It should be appreciated that when not inflated, or
partially inflated, the inflatable cell 258 may act on the rotor blade(s) 112 in a manner
similar to the streamer 242 discussed above.
[0064] It should be appreciated that the apparatus 200 may, in at least one
embodiment include various combinations of the embodiments discussed herein. For
example, in an embodiment, the apparatus 200 may include both the inflatable cell
258 and the airflow modifying element(s) 204 distributed along the helically wrapped
cord 208. Alternatively, the apparatus 200 may include leading- and trailing-edge
airflow modifying elements 230, 232 in combination with one or more streamers 242.
The apparatus 200 may, in additional embodiments, include additional combinations
of the positioning element(s) 202, the airflow modifying element(s) 204 and the
securing element(s) 206 as discussed herein.
[0065] Referring to FIG. 9, a flow diagram of one embodiment of a method 300
for mitigating vortex-shedding vibrationsand/or stall-induced vibrations on one or
more rotor blades of a wind turbine during standstill is illustrated. The method 300
may be implemented using, for instance, the apparatus 200 discussed above with
reference to FIGS. 3A-8. FIG. 9 depicts steps performed in a particular order for
purposes of illustration and discussion. Those of ordinary skill in the art, using the
disclosures provided herein, will understand that various steps of the method 300 or
any of the other methods disclosed herein may be adapted, modified, rearranged,
performed simultaneously or modified in various ways without deviating from the
scope of the present disclosure.
[0066] As shown at (302), the method 300 includes installing a vibration-
18
mitigation apparatus on the rotor blade. As shown at (302a), the installation of the
vibration-mitigation apparatus includes positioning at least one airflow modifying
element at a desired location along an outer surface of a rotor blade with at least one
positioning element. As shown at (302b), the installation of the vibration-mitigation
apparatus includes securing the at least one positioning element to the outer surface of
the rotor blade via at least one securing element. As shown at (304), the method 300
includes performing a maintenance and/or a repair operation on the wind turbine with
the vibration-mitigation apparatus installed. As shown at (306), the method 300
includes once the maintenance and/or the repair operation is complete, removing the
vibration-mitigation apparatus from the rotor blade.
[0067] In additional embodiments, positioning the at least one airflow modifying
element at the desired location may also, in accordance with the present disclosure,
include helically wrapping the cord element around a portion of the rotor blade so as
to disrupt airflow over a chordwise section of the rotor blade. In another embodiment,
positioning the at least one airflow modifying element at the desired location may also
include positioning an edge of the at least one airflow modifying element adjacent to
at least one of a leading edge, a trailing edge, or a long a line upstream of a flow
separation point on either a suction side or a pressure side of the rotor blade. In a
further embodiment, positioning the at least one airflow modifying element at the
desired location may also include encapsulating a portion of the rotor blade within the
hollow sleeve and coupling the inflatable cell to the hollow sleeve and operably
coupling the inflatable cell to the inlet air hole.
[0068] Furthermore, the skilled artisan will recognize the interchangeability of
various features from different embodiments. Similarly, the various method steps and
features described, as well as other known equivalents for each such methods and
feature, can be mixed and matched by one of ordinary skill in this art to construct
additional systems and techniques in accordance with principles of this disclosure. Of
course, it is to be understood that not necessarily all such objects or advantages
described above may be achieved in accordance with any particular embodiment.
Thus, for example, those skilled in the art will recognize that the systems and
techniques described herein may be embodied or carried out in a manner that achieves
or optimizes one advantage or group of advantages as taught herein without
19
necessarily achieving other objects or advantages as may be taught or suggested
herein.
[0069] This written description uses examples to disclose the invention, including
the best mode, and also to enable any person skilled in the art to practice the
invention, including making and using any devices or systems and performing any
incorporated methods. The patentable scope of the invention is defined by the claims,
and may include other examples that occur to those skilled in the art. Such other
examples are intended to be within the scope of the claims if they include structural
elements that do not differ from the literal language of the claims, or if they include
equivalent structural elements with insubstantial differences from the literal languages
of the claims.
[0070] For reasons of completeness, various aspects of the present disclosure are
set out in the following numbered clauses:
Clause 1. An apparatus for mitigating vortex-shedding vibrationsor stallinduced vibrations on one or more rotor blades of a wind turbine during standstill, the
apparatus comprising:
at least one positioning element located between a blade tip section and a
blade root section thereof, the at least one positioning element adapted for wrapping
around at least a portion of the rotor blade;
at least one airflow modifying element coupled to the at least one positioning
element, the at least one airflow modifying element defining a height relative to a
surface of the rotor blade; and,
at least one securing element operably coupled to the at least one positioning
element for temporarily securing the at least one airflow modifying element to the
rotor blade.
Clause 2. The apparatus of clause 1, wherein the at least one positioning
element comprises at least one cord member helically wrapped around a portion of the
rotor blade.
Clause 3. The apparatus of any preceding clause, wherein the helicallywrapped cord element has a pitch greater than or equal to one half of a chord of the
rotor blade up to less than or equal to six times the chord of the rotor blade, wherein at
least a portion of the helically-wrapped cord element is separated from the surface of
20
the rotor blade by a distance defined by the at least one airflow modifying element.
Clause 4. The apparatus of any preceding clause, wherein the at least one
airflow modifying element comprises at least one of a streamer, a fence, an inflatable
cell, a serration, a vortex generator, and/or a flow disruptor.
Clause 5. The apparatus of any preceding clause, wherein the at least one
airflow modifying element comprises a plurality of airflow modifying elements
distributed along and secured to the at least one positioning element.
Clause 6. The apparatus of any of clauses 1, 2, 3, or 5, wherein the
plurality of airflow modifying elements comprises leading-edge airflow modifying
elements and trailing-edge airflow modifying elements, wherein the leading-edge
airflow modifying elements and the trailing-edge airflow modifying elements are
distributed in an alternating pattern along the at least one positioning element.
Clause 7. The apparatus of any preceding clause, wherein the at least one
airflow modifying element comprises at least one leading-edge airflow modifying
element and at least one trailing-edge airflow modifying element disposed opposite
thereof, wherein the at least one positioning element is constructed of an elastic cord
element coupled between the leading- and trailing-edge airflow modifying elements,
wherein the elastic cord element limits a separation between the leading- and/or
trailing-edge airflow modifying elements at a specified spanwise location of the rotor
blade.
Clause 8. The apparatus of any preceding clause, wherein the at least one
positioning element comprises a leading-edge positioning element arranged parallel to
a leading edge of the rotor blade and a trailing-edge positioning element arranged
parallel to a trailing edge of the rotor blade, the at least one airflow modifying element
comprises at least one leading-edge airflow modifying element coupled to the
leading-edge positioning element and at least one trailing-edge airflow modifying
element coupled to the trailing-edge positioning element, the leading-edge airflow
modifying element defining a leading-edge mounting notch corresponding to a
leading-edge profile, and the trailing-edge airflow modifying element defining a
trailing-edge mounting notch corresponding to a trailing-edge profile.
Clause 9. The apparatus of any preceding clause, wherein the at least one
airflow modifying element has a mounting edge, the airflow modifying element is
21
coupled along the mounting edge to the at least one positioning element, the at least
one positioning element being oriented perpendicular to a chordwise direction of the
rotor blade, the at least one positioning element being temporarily coupled to at least
one of a leading edge, a trailing edge, or along a line upstream of a flow separation
point on either a suction side or a pressure side of the rotor blade.
Clause 10. The apparatus of clause 9, wherein the at least one positioning
element comprises a cord oriented spanwise along the rotor blade, and wherein the at
least one securing element comprises a first securing element coupled to a first end of
the positioning element and a second securing element coupled to an opposing,
second end of the positioning element.
Clause 11. The apparatus of any of clauses 1 or 4, wherein the at least one
positioning element comprises a hollow sleeve configured to encapsulate a portion of
the rotor blade, the hollow sleeve defining at least one inlet air hole.
Clause 12. A method for mitigating vortex-shedding vibrationsor stallinduced vibrations on one or more rotor blades of a wind turbine during standstill, the
method comprising:
installing a vibration-mitigation apparatus on the rotor blade, wherein
installation of the vibration-mitigation apparatus further comprises:
positioning at least one airflow modifying element at a desired location
along an outer surface of a rotor blade with at least one positioning element,
and
securing the at least one positioning element to the outer surface of the
rotor blade via at least one securing element,
performing an installation, a commissioning, a maintenance and/or a repair
operation on the wind turbine with the vibration-mitigation apparatus installed; and,
once the installation, commission, maintenance and/or the repair operation is
complete, removing the vibration-mitigation apparatus from the rotor blade.
Clause 13. The method of clause 12, wherein the at least one airflow
modifying element comprises at least one of a streamer, a fence, an inflatable cell, a
serration, a vortex generator, and/or a flow disruptor.
Clause 14. The method of any of clauses 12 or 13, wherein the at least one
positioning element comprises a cord element, wherein positioning the at least one
22
airflow modifying element at the desired location further comprises helically
wrapping the cord element around a portion of the rotor blade so as to disrupt airflow
over a chordwise section of the rotor blade, wherein at least a portion of the at least
one positioning element is separated from the outer surface of the rotor blade by a
distance defined by the at least one airflow modifying element.
Clause 15. The method of any of clauses 12, 13, or 14, wherein positioning
the at least one airflow modifying element at the desired location further comprises
positioning an edge of the at least one airflow modifying element adjacent to at least
one of a leading edge, a trailing edge, or along a line upstream of a flow separation
point on either a suction side or a pressure side of the rotor blade.
Clause 16. The method of any of clauses 12 or 13, wherein the at least one
positioning element comprises a hollow sleeve defining an inlet air hole and the at
least one airflow modifying element comprises the inflatable cell, wherein positioning
the at least one airflow modifying element at the desired location further comprises
encapsulating a portion of the rotor blade within the hollow sleeve and coupling the
inflatable cell to the hollow sleeve and operably coupling the inflatable cell to the
inlet air hole, wherein the inflatable cell is disposed adjacent to a pressure side or a
suction side of the rotor blade.
Clause 17. A rotor assembly for a wind turbine, comprising:
a rotor hub in standstill;
at least one rotor blade comprising a leading edge and a trailing edge disposed
between a tip portion and a root portion opposite thereof, the root portion being
coupled to the rotor hub; and,
an apparatus for mitigating vortex-shedding vibrationsand/or stall-induced
vibrations on the at least one rotor blade during standstill, the apparatus comprising:
at least one positioning element located between a blade tip section and a
blade root section thereof, the at least one positioning element adapted for wrapping
around at least a portion of the rotor blade,
at least one airflow modifying element coupled to the at least one positioning
element, the at least one airflow modifying element defining a height relative to a
surface of the rotor blade, and,
at least one securing element operably coupled to the at least one positioning
23
element for temporarily securing the at least one airflow modifying element to the
rotor blade.
Clause 18. The rotor assembly of clause 17, wherein the at least one
airflow modifying element comprises at least one of a streamer, a fence, an inflatable
cell, a serration, a vortex generator, and/or a flow disruptor.
Clause 19. The rotor assembly of any of clauses 17 or 18, wherein the at
least one positioning element comprises a cord helically wrapped around a portion of
the rotor blade, wherein the at least one airflow modifying element comprises a
plurality of airflow modifying elements distributed along the at least one positioning
element.
Clause 20. The rotor of assembly any of clauses 17, 18, or 19, wherein the
at least one airflow modifying element has a mounting edge, the airflow modifying
element is coupled along the mounting edge to the at least one positioning element,
the at least one positioning element being oriented perpendicular to a chordwise
direction of the rotor blade, the at least one positioning element being temporarily
coupled to at least one of a leading edge, a trailing edge, or along a line upstream of a
flow separation point on either a suction side or a pressure side of the rotor blade

WHAT IS CLAIMED IS:
1. An apparatus for mitigating vortex-shedding vibrations or stall-induced
vibrations on one or more rotor blades of a wind turbine during standstill, the
apparatus comprising:
at least one positioning element located between a blade tip section and a
blade root section thereof, the at least one positioning element adapted for wrapping
around at least a portion of the rotor blade;
at least one airflow modifying element coupled to the at least one positioning
element, the at least one airflow modifying element defining a height relative to a
surface of the rotor blade; and,
at least one securing element operably coupled to the at least one positioning
element for temporarily securing the at least one airflow modifying element to the
rotor blade.
2. The apparatus of claim 1, wherein the at least one positioning element
comprises at least one cord member helically wrapped around a portion of the rotor
blade.
3. The apparatus of claim 2, wherein the helically-wrapped cord element
has a pitch greater than or equal to one half of a chord of the rotor blade up to less
than or equal to six times the chord of the rotor blade, wherein at least a portion of the
helically-wrapped cord element is separated from the surface of the rotor blade by a
distance defined by the at least one airflow modifying element.
4. The apparatus of claim 1, wherein the at least one airflow modifying
element comprises at least one of a streamer, a fence, an inflatable cell, a serration, a
vortex generator, and/or a flow disruptor.
5. The apparatus of claim 1, wherein the at least one airflow modifying
element comprises a plurality of airflow modifying elements distributed along and
secured to the at least one positioning element.
6. The apparatus of claim 5, wherein the plurality of airflow modifying
elements comprises leading-edge airflow modifying elements and trailing-edge
airflow modifying elements, wherein the leading-edge airflow modifying elements
and the trailing-edge airflow modifying elements are distributed in an alternating
pattern along the at least one positioning element.
25
7. The apparatus of claim 1, wherein the at least one airflow modifying
element comprises at least one leading-edge airflow modifying element and at least
one trailing-edge airflow modifying element disposed opposite thereof, wherein the at
least one positioning element is constructed of an elastic cord element coupled
between the leading- and trailing-edge airflow modifying elements, wherein the
elastic cord element limits a separation between the leading- and/or trailing-edge
airflow modifying elements at a specified spanwise location of the rotor blade.
8. The apparatus of claim 1, wherein the at least one positioning element
comprises a leading-edge positioning element arranged parallel to a leading edge of
the rotor blade and a trailing-edge positioning element arranged parallel to a trailing
edge of the rotor blade, the at least one airflow modifying element comprises at least
one leading-edge airflow modifying element coupled to the leading-edge positioning
element and at least one trailing-edge airflow modifying element coupled to the
trailing-edge positioning element, the leading-edge airflow modifying element
defining a leading-edge mounting notch corresponding to a leading-edge profile, and
the trailing-edge airflow modifying element defining a trailing-edge mounting notch
corresponding to a trailing-edge profile.
9. The apparatus of claim 1, wherein the at least one airflow modifying
element has a mounting edge, the airflow modifying element is coupled along the
mounting edge to the at least one positioning element, the at least one positioning
element being oriented perpendicular to a chordwise direction of the rotor blade, the
at least one positioning element being temporarily coupled to at least one of a leading
edge, a trailing edge, or along a line upstream of a flow separation point on either a
suction side or a pressure side of the rotor blade.
10. The apparatus of claim 9, wherein the at least one positioning element
comprises a cord oriented spanwise along the rotor blade, and wherein the at least one
securing element comprises a first securing element coupled to a first end of the
positioning element and a second securing element coupled to an opposing, second
end of the positioning element.
11. The apparatus of claim 1, wherein the at least one positioning element
comprises a hollow sleeve configured to encapsulate a portion of the rotor blade, the
hollow sleeve defining at least one inlet air hole.
26
12. A method for mitigating vortex-shedding vibrations or stall-induced
vibrations on one or more rotor blades of a wind turbine during standstill, the method
comprising:
installing a vibration-mitigation apparatus on the rotor blade, wherein
installation of the vibration-mitigation apparatus further comprises:
positioning at least one airflow modifying element at a desired location
along an outer surface of a rotor blade with at least one positioning element;
and,
securing the at least one positioning element to the outer surface of the
rotor blade via at least one securing element;
performing an installation, a commissioning, a maintenance and/or a repair
operation on the wind turbine with the vibration-mitigation apparatus installed; and,
once the installation, commissioning, maintenance and/or the repair operation
is complete, removing the vibration-mitigation apparatus from the rotor blade.
13. The method of claim 12, wherein the at least one airflow modifying
element comprises at least one of a streamer, a fence, an inflatable cell, a serration, a
vortex generator, and/or a flow disruptor.
14. The method of claim 12, wherein the at least one positioning element
comprises a cord element, wherein positioning the at least one airflow modifying
element at the desired location further comprises helically wrapping the cord element
around a portion of the rotor blade so as to disrupt airflow over a chordwise or
spanwise section of the rotor blade, wherein at least a portion of the at least one
positioning element is separated from the outer surface of the rotor blade by a distance
defined by the at least one airflow modifying element.
15. The method of claim 12, wherein positioning the at least one airflow
modifying element at the desired location further comprises positioning an edge of the
at least one airflow modifying element adjacent to at least one of a leading edge, a
trailing edge, or along a line upstream of a flow separation point on either a suction
side or a pressure side of the rotor blade.
16. The method of claim 13, wherein the at least one positioning element
comprises a hollow sleeve defining an inlet air hole and the at least one airflow
modifying element comprises the inflatable cell, wherein positioning the at least one
27
airflow modifying element at the desired location further comprises encapsulating a
portion of the rotor blade within the hollow sleeve and coupling the inflatable cell to
the hollow sleeve and operably coupling the inflatable cell to the inlet air hole,
wherein the inflatable cell is disposed adjacent to a pressure side or a suction side of
the rotor blade.
17. A rotor assembly for a wind turbine, comprising:
a rotor comprising a rotatable hub in standstill;
at least one rotor blade comprising a leading edge and a trailing edge disposed
between a blade tip and a blade root opposite thereof, the blade root being coupled to
the rotatable hub; and,
an apparatus for mitigating vortex-shedding vibrations and/or stall-induced
vibrations on the at least one rotor blade during standstill, the apparatus comprising:
at least one positioning element located between a blade tip section and
a blade root section thereof, the at least one positioning element adapted for
wrapping around at least a portion of the rotor blade;
at least one airflow modifying element coupled to the at least one
positioning element, the at least one airflow modifying element defining a
height relative to a surface of the rotor blade; and,
at least one securing element operably coupled to the at least one
positioning element for temporarily securing the at least one airflow
modifying element to the rotor blade.
18. The rotor assembly of claim 17, wherein the at least one airflow
modifying element comprises at least one of a streamer, a fence, an inflatable cell, a
serration, a vortex generator, and/or a flow disruptor.
19. The rotor assembly of claim 17, wherein the at least one positioning
element comprises a cord helically wrapped around a portion of the rotor blade,
wherein the at least one airflow modifying element comprises a plurality of airflow
modifying elements distributed along the at least one positioning element.
20. The rotor of assembly claim 17, wherein the at least one airflow
modifying element has a mounting edge, the airflow modifying element is coupled
along the mounting edge to the at least one positioning element, the at least one
positioning element being oriented perpendicular to a chordwise direction of the rotor
28
blade, the at least one positioning element being temporarily coupled to at least one of
a leading edge, a trailing edge, or along a line upstream of a flow separation point on
either a suction side or a pressure side of the rotor blade.