U.S. patent application number 12/125380 was filed with the patent office on 2009-05-07 for enhancement of vortex induced forces and motion through surface roughness control.
Invention is credited to MICHAEL M. BERNITSAS, KAMALDEV RAGHAVAN.
Application Number | 20090114001 12/125380 |
Document ID | / |
Family ID | 40075439 |
Filed Date | 2009-05-07 |
United States Patent
Application |
20090114001 |
Kind Code |
A1 |
BERNITSAS; MICHAEL M. ; et
al. |
May 7, 2009 |
ENHANCEMENT OF VORTEX INDUCED FORCES AND MOTION THROUGH SURFACE
ROUGHNESS CONTROL
Abstract
Roughness is added to the surface of a bluff body in a relative
motion with respect to a fluid. The amount, size, and distribution
of roughness on the body surface is controlled passively or
actively to modify the flow around the body and subsequently the
Vortex Induced Forces and Motion (VIFM). The added roughness, when
designed and implemented appropriately, affects in a predetermined
way the boundary layer, the separation of the boundary layer, the
level of turbulence, the wake, the drag and lift forces, and
consequently the Vortex Induced Motion (VIM), and the
fluid-structure interaction. The goal of surface roughness control
is to increase Vortex Induced Forces and Motion. Enhancement is
needed in such applications as harnessing of clean and renewable
energy from ocean/river currents using the ocean energy converter
VIVACE (Vortex Induced Vibration for Aquatic Clean Energy).
Inventors: |
BERNITSAS; MICHAEL M.;
(Saline, MI) ; RAGHAVAN; KAMALDEV; (Houston,
TX) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Family ID: |
40075439 |
Appl. No.: |
12/125380 |
Filed: |
May 22, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60931957 |
May 25, 2007 |
|
|
|
Current U.S.
Class: |
73/105 ;
73/861.22 |
Current CPC
Class: |
Y02E 10/30 20130101;
B63B 2021/504 20130101; Y10T 137/2087 20150401; F15D 1/12 20130101;
F05B 2240/32 20130101; Y02E 10/20 20130101; Y02E 10/72 20130101;
F03B 17/06 20130101; F05B 2240/201 20130101 |
Class at
Publication: |
73/105 ;
73/861.22 |
International
Class: |
G01B 5/28 20060101
G01B005/28; G01F 1/32 20060101 G01F001/32 |
Goverment Interests
GOVERNMENT INTEREST
[0002] This invention was made with government support under
N00014-03-1-0983 awarded by the Office of Naval Research and
DE-FG36-05GO15162 awarded by the Department of Energy. The
government has certain rights in the invention.
Claims
1. A system for enhancing vortex induced forces on a bluff body
disposed in a fluid, the fluid moving relative to the bluff body,
said system comprising: a bluff body having a surface, said bluff
body being shaped to define a linear body dimension being the
largest linear dimension of a cross section of said bluff body in
the plane of the flow of the fluid; and a roughness zone disposed
on said surface, said roughness zone defining a roughness height
extending above said surface that is less than or equal to 5% of
said linear body dimension.
2. The system according to claim 1 wherein said roughness zone
comprises a base and a grit, said grit being disposed on said
base.
3. The system according to claim 1 wherein said roughness zone is
disposed on only a portion of said surface.
4. The system according to claim 1 wherein said roughness zone
comprises a member coupled to said bluff body.
5. The system according to claim 4 wherein said member comprises
sandpaper.
6. The system according to claim 1 wherein said roughness zone is
integrally formed on said surface of said bluff body.
7. The system according to claim 1 wherein said roughness zone
comprises an actively controllable roughness zone operable between
a first roughness state and a second roughness state, said first
roughness state being different than said second roughness
state.
8. The system according to claim 1 wherein said bluff body is a
cylinder defining a stagnation point and said roughness zone being
disposed between about 57.degree. and 85.degree. behind said
stagnation point when measured along an axis of said cylinder.
9. A system for enhancing vortex induced forces on a bluff body
disposed in a fluid, the fluid moving relative to the bluff body,
said system comprising: a cylindrical bluff body having a surface,
said cylindrical bluff body defining a bluff body diameter; and a
roughness zone disposed on said surface, said roughness zone
defining a roughness height extending above said surface that is
less than or equal to 5% of said bluff body diameter.
10. The system according to claim 9 wherein said roughness zone
comprises a base and a grit, said grit being disposed on said
base.
11. The system according to claim 9 wherein said roughness zone is
disposed on only a portion of said surface.
12. The system according to claim 1 wherein said roughness zone
comprises a member coupled to said bluff body.
13. The system according to claim 12 wherein said member comprises
sandpaper.
14. The system according to claim 9 wherein said roughness zone is
integrally formed on said surface of said bluff body.
15. The system according to claim 9 wherein said roughness zone
comprises an actively controllable roughness zone operable between
a first roughness state and a second roughness state, said first
roughness state being different than said second roughness
state.
16. The system according to claim 9 wherein said bluff body is a
cylinder defining a stagnation point and said roughness zone being
disposed between about 57.degree. and 85.degree. behind said
stagnation point when measured along an axis of said cylinder.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/931,957 filed on May 25, 2007. The disclosure of
the above application is incorporated herein by reference.
FIELD
[0003] The present disclosure relates to enhancement of vortex
induced forces and, more particularly, relates to enhancement of
vortex induced forces using surface roughness control.
BACKGROUND AND SUMMARY
[0004] The statements in this section merely provide background
information related to the present disclosure and may not
constitute prior art.
[0005] Roughness is added to the surface of a bluff body in a
relative motion with respect to a fluid. The amount, size, and
distribution of roughness on the body surface is controlled
passively or actively to modify the flow around the body and
subsequently the Vortex Induced Forces and Motion (VIFM). The added
roughness, when designed and implemented appropriately, affects in
a predetermined way the boundary layer, the separation of the
boundary layer, the level of turbulence, the wake, the drag and
lift forces, and consequently the Vortex Induced Motion (VIM), and
the fluid-structure interaction. The goal of surface roughness
control is to increase Vortex Induced Forces and Motion, which in
some applications can provide enormous benefits, such as in the
harnessing of clean and renewable energy from ocean/river currents
using the ocean energy converter VIVACE (Vortex Induced Vibration
for Aquatic Clean Energy). The name of the present teachings is
VIM-Enhance and is based on Surface Roughness Control (SRC). It is
hereafter referred to as VIM-Enhance+SRC
[0006] Further areas of applicability will become apparent from the
description provided herein. It should be understood that the
description and specific examples are intended for purposes of
illustration only and are not intended to limit the scope of the
present disclosure.
DRAWINGS
[0007] The drawings described herein are for illustration purposes
only and are not intended to limit the scope of the present
disclosure in any way.
[0008] FIG. 1 is a schematic drawing illustrating roughness in
terms of a protuberance on a body;
[0009] FIG. 2 is a schematic drawing illustrating vortex formation
and wake;
[0010] FIG. 3 is a schematic drawing illustrating a surface
roughness member, in the form of sandpaper, formed on a body;
[0011] FIG. 4 is an enlarged schematic drawing illustrating the
surface roughness member of FIG. 3;
[0012] FIG. 5 is a schematic perspective view illustrating an
enhancement of VIFM using SRC according to the principles of the
present teachings;
[0013] FIG. 6 is a graph illustrating the reduced velocity versus
the amplitude ratio (A/D) of a 5.0'' cylinder with and without
roughness (Case 2);
[0014] FIG. 7 is a graph illustrating the reduced velocity versus
the ratio of frequency of a 5.0'' cylinder with and without
roughness (Case 2);
[0015] FIG. 8 is a graph illustrating the reduced velocity versus
the amplitude ratio (A/D) of a 3.5'' cylinder with and without
roughness (Case 1);
[0016] FIG. 9 is a graph illustrating the reduced velocity versus
the ratio of frequency of a 3.5'' cylinder with and without
roughness (Case 1);
[0017] FIG. 10 is a graph illustrating the reduced velocity versus
the ratio of frequency for different roughness distribution (Cases
1, 2, and 3; less stiff spring K=424N/m and stiffer spring K=872
N/m);
[0018] FIG. 11 is a preliminary visualization of wake in Case 1
showing four vortices in half cycle, with cylinder marked as a
clear white circle;
[0019] FIG. 12 is a graph illustrating Reynolds number versus
amplitude ratio (A/D) of Cases 1 and 2 versus Re.sub.k+p(less stiff
spring K=424 N/m and stiffer spring K=872 N/m);
[0020] FIG. 13 is a graph illustrating Reynolds number versus
amplitude ratio (A/D) of Cases 1 and 2 versus Re.sub.k (less stiff
spring K=424N/m and stiffer spring K=872 N/m);
[0021] FIG. 14 is a graph illustrating ratio of roughness
thickness, and BL thickness versus amplitude ratio (A/D) versus
k/.delta. (less stiff spring K=424N/m and stiffer spring K=872
N/m);
[0022] FIG. 15 is a graph illustrating ratio of roughness
thickness, and BL thickness versus amplitude ratio (A/D) versus
(k+P)/.delta. (less stiff spring K=424N/m and stiffer spring K=872
N/m);
[0023] FIG. 16 is a graph illustrating ratio of roughness
thickness, and BL thickness versus amplitude ratio (A/D) versus
k/.delta.* (less stiff spring K=424N/m and stiffer spring K=872
N/m); and
[0024] FIG. 17 is a graph illustrating ratio of roughness
thickness, and BL thickness versus amplitude ratio (A/D) versus
(k+P)/.delta.* (less stiff spring K=424N/m and stiffer spring K=872
N/m).
DETAILED DESCRIPTION
[0025] The following description is merely exemplary in nature and
is not intended to limit the present disclosure, application, or
uses.
1.1 General Principles
[0026] There are three types of fluid induced loading on a
structure which may result in structural vibration: (a)
Extraneously Induced Excitation (EIE), (b) Instability-induced
Excitation (IIE), and (c) Movement Induced Excitation (MIE). In
each case, the fluid relative flow initiates excitation. For bluff
bodies in relative flows, shedding of large vortices occur
following flow separation at the end of the boundary layer and
coalescence of vorticity generated at the boundary layer and along
the shear layer into large vortices. The latter are called von
Karman vortices and have a core diameter on the order of the bluff
body linear body dimension transverse/perpendicular to the flow.
Hereafter, vortex shedding refers to von Karman vortices.
[0027] Control of vortex dampening behind a bluff body and control
of vortex induced motion of a bluff elastic body or bluff rigid
body on elastic support have been topics of research and patenting
for over a hundred years. Applications appear in several
engineering disciplines such as offshore engineering, aerospace,
mechanical, civil, nuclear, and power transmission. In ocean
engineering, suppression of vortex shedding is important because of
the destructive effect of vortex induced vibration on marine
risers, underwater pipelines, SPAR offshore platforms, etc. In
other engineering disciplines, Vortex Induced Vibration (VIV) of
cylindrical structures, such as tubes in heat exchangers, cooling
towers, nuclear fuel rods, and smoke stacks can be destructive and
must be suppressed. On the contrary, in marine renewable energy
conversion, for example, using the VIVACE converter, vortex
shedding and Vortex Induced Forces and Motion (VIFM) are enhanced.
Such control of vortex shedding for enhancement of VIFM can be
achieved by active control, passive control, or combination
thereof.
[0028] Hereafter the present teachings are referred to as
VIM-Enhance+SRC. In accordance with these teachings, VIM control by
introducing Surface Roughness Control (SRC) on the structure can be
achieved. The goal is to enhance VIFM using SRC.
[0029] Recently, enhancement of VIFM has become important in the
case of utilization of VIM to harness hydrokinetic energy from
ocean/river currents, using devices such as the VIVACE converter.
It should be appreciated, however, that the present teachings have
a wide variety of applicability in other applications and
environments.
[0030] S2.1. The Underlying Concepts
[0031] The underlying principles for the present teachings
(VIM-Enhance+SRC) are the following two:
[0032] Principle #1: Increasing the Correlation Length Using
Surface Roughness
[0033] Surface roughness of appropriate size and distribution can
increase the spanwise correlation of vortex shedding along a bluff
body. Increasing the correlation length results in increased lift
forces and subsequently increased VIFM.
[0034] Principle # 2: Controlling the Boundary Layer Turbulence
Using Surface Roughness
[0035] Surface roughness of appropriate size and distribution can
increase turbulence at the boundary layer scale which feeds the
shear layer along a bluff body and in turn affects the momentum of
the separating shear layer. VIM-Enhance+SRC uses these two
principles to increase VIFM.
2.2. Terminology
[0036] Terms that are used in describing the present teaching of
enhancing VIM using Surface Roughness Control (SRC), as well as the
physics behind it, are defined below:
[0037] Structure refers to a body in a relative fluid flow. The
body can be elastic, elastically mounted, rigid, or a combination
of structural parts thereof. Vortex shedding behind the structure
(typically a bluff body) is expected. Shed vortices may induce
forcing and motion.
[0038] A bluff body has a non-streamlined shape that produces
considerable resistance when immersed in a moving fluid. A region
of separated flow occurs over a large portion of the surface of a
bluff body, which results in a high pressure drag force and a large
wake region. The flow often exhibits unsteadiness in the form of
periodic vortex formation and shedding, which may result in
periodic forces transverse (lift forces) to the fluid flow. Bluff
bodies are widely encountered in many engineering applications and
design problems, including bridges, stacks, towers, offshore
pipelines, offshore structures, heat exchangers, mooring lines,
flagpoles, car antennas, and any circular or cylindrical body
having a size ranging from about 0.1 mm or larger.
[0039] In some embodiments, surface roughness can be defined as any
two or three-dimensional excrescence whose dimension perpendicular
to the body surface, k, is on the order of the boundary layer
thickness. However, in some embodiments, surface roughness can be
defined as any two or three-dimensional excrescence whose dimension
perpendicular to the body surface, k, is no more than about 5% of
the largest linear dimension, D, of the cross section of the bluff
body in the plane of the flow. For example, a plane perpendicular
to an axis of a cylindrical member (i.e. a circle) defines a plane
of the fluid flow. Such elements can be closely or sparsely packed.
Depending on the application, roughness may cover the entire
structure or any part thereof. It should be appreciated that
"smoothly curved protuberances", strakes and wires (two dimensional
protuberances) do not constitute roughness as defined in the
present teachings. According to the present teachings,
three-dimensional roughness elements are used. Roughness textures
can contain irregular size and shape of excrescences--uniformly or
non-uniformly distributed. Examples include: pyramidal, grooves,
brickwall type, and wire gauze. Roughness can be hard or soft. It
should also be appreciated that such surface roughness can be in
the form of affix members, such as sandpaper or other friction
member; can be machined or otherwise formed on the bluff body; can
be an acitive configurable member(s); and the like.
[0040] Passive/active control refers to the way of applying surface
roughness to control turbulence generated in the boundary layer.
Passive control implies that the added roughness is fixed on the
surface of the structure and is not adjustable to meet flow
fluctuations. Active control implies that distribution and/or size
of applied surface roughness are altered during operation depending
on flow conditions.
[0041] Boundary layer is the layer of fluid in the immediate
vicinity of the structure. A measure of its thickness, .delta., is
the distance perpendicular to the surface of the structure where
the flow velocity has reached 99% of the outer flow velocity. The
relative flow velocity on the surface of an impermeable/nonporous
structure is zero.
[0042] Separation point is the point on the surface of the
structure where the gradient of the relative velocity tangential to
the surface of the structure with respect to the direction
perpendicular to the surface of the body is zero.
[0043] Flow Turbulence refers to the three dimensional, unsteady
motions of fluid particles in a practically chaotic manner.
[0044] Wake is the region of turbulence immediately to the rear of
a solid body caused by the flow of fluid around the body.
[0045] Von Karman vortices are the vortices formed behind a bluff
body, such as a cylinder. By coalescence of vorticity generated at
the boundary layer and the shear layer on each side of the bluff
body.
[0046] Drag is the force that resists the movement of a body
through a fluid or the movement of the fluid around the body. Drag
is the sum of frictional forces, which act tangentially to the body
surface, and the component of the pressure forces parallel to the
fluid flow. For a body, the drag is the sum of fluid dynamic forces
in the direction parallel to the fluid flow.
[0047] Lift is the sum of all the fluid dynamic forces on a body in
the direction perpendicular to the direction of the relative fluid
flow.
[0048] Fluid-structure interaction is the phenomenon where the
fluid forces exerted on the structure move or deform the structure
whose motion in turn affects the fluid forces exerted on the
structure. Thus, the dynamics of the structure and the fluid are
interdependent.
[0049] Vortex Induced Motion (VIM) is a fluid-structure interaction
phenomenon where the motion of a bluff structure is induced
primarily by the vortices shed into the wake of the structure due
to the relative flow between the fluid and the structure.
[0050] Vortex Induced Vibration (VIV) is a special case of VIM
where forcing is predominantly periodic. A well known VIV
phenomenon may occur when a flexible circular cylinder or a rigid
circular cylinder on elastic support is placed in a steady flow
with its axis perpendicular to the direction to the flow. In VIV,
synchronization of vortex shedding and cylinder oscillation occurs
over a broad range of flow velocities. FIG. 2 shows a typical
periodic vortex formation and wake for a circular cylinder in
VIV.
[0051] Vortex Induced Forces and Motion (VIFM) refers to both the
forces and motion induced by vortex shedding.
2.3. Method of Control of Vortex Induced Forces and Motion
(VIFM)
[0052] The method implemented according to the present teachings,
in order to control the VIFM of the structure, is based on
Principles #1 and #2 above. Specifically, surface roughness is
added, to modify passively or actively, the strength and
three-dimensional distribution of turbulence which in turn affects
vortex shedding, and subsequently vortex induced motion of the
structure. The three elements of control of the method implemented
according to the present teachings are surface roughness control,
turbulence control, and control of vortex induced forces and
motion, which are described next.
Surface Roughness Control:
[0053] An objective of surface roughness is to alter vortex
shedding and its effects, including but not limited to vortex
induced forces and vortex induced motion. To this end, part or all
of the surface of the structure may be covered by roughness
elements.
[0054] Distribution of surface roughness depends on the objective
of decreasing or increasing vortex induced forces and motion. FIGS.
3, 4 and 5 depict one method of distributing roughness to enhance
vortex shedding and amplify vortex induced forces and motion as
required in hydrokinetic energy harnessing, such as those
implemented in the VIVACE converter, according to the principles of
the present invention.
[0055] Passive roughness control consists of distributing roughness
elements on the surface of the structure permanently without the
possibility of adjusting their configuration during the flow.
[0056] Active roughness control consists of altering size and
distribution of the roughness on the surface of the structure based
on relative flow characteristics such as direction and magnitude of
velocity, which affect properties of the boundary layer such as
thickness and separation.
Turbulence Control:
[0057] The present teachings, VIM-Enhance+SRC, control the amount
and distribution of turbulence in a flow past a structure by
distributing roughness on the surface of the body as discussed
herein. Some specific ways in which surface roughness affects
turbulence and consequently the flow past the structure are
described herein.
Control of Flow Correlation Using Roughness:
[0058] Spanwise vortex shedding correlation behind a bluff body is
typically limited. For example, for a stationary cylinder in a
steady flow perpendicular to its axis, the correlation length
l.sub.c is 2-3 cylinder diameters. Theoretically, VIV induces
infinite correlation length resulting in increased VIFM. In
practice, the correlation length in VIV is large but finite. A way
of controlling VIFM is by controlling the correlation length.
Increase in the correlation length results in increased Vortex
Induced Forces and Motion.
[0059] FIG. 5 shows use of a straight roughness strip or zone of
length L.sub.f equal to the structural length L.sub.s. Experimental
results show that roughness strip increases correlation length.
This strip is more effective than a trip-wire because of the
inherent oscillatory nature of the separation point. The roughness
strips accommodate the oscillatory nature of the separation points
because of their depth d.sub.r as shown in FIG. 4. Further, the
roughness elements on the strip act like vortex generators or
turbulators, thus generating vorticity of the boundary layer scale,
which further enhances shed vortices and induced VIFM.
[0060] Another application of control of flow correlation using
this surface roughness control (SRC) is in the regime of transition
of flow from laminar to turbulent (critical regime). Surface
roughness restores vortex shedding and establishes a spanwise
correlation even in the critical regime, where those don't exist
without surface roughness. This enables sustaining and enhancing
VIV even in the critical regime.
Control of Flow Separation Using Roughness:
[0061] A flow past a structure typically separates at two
separation points, one on each side of any cross section of the
structure. Using the roughness strips before the regular separation
point determines the nature of the flow downstream. The flow can be
laminar, or in transition between laminar and turbulent, or
turbulent. In each case, control of separation using roughness may
have different effect on the flow and consequently VIFM.
[0062] The most profound effect of separation point control appears
in the critical flow regime. Transition from laminar to turbulent
flow can be controlled using roughness strip/s. This exploits the
concept of tripping the boundary layer and energizing the boundary
layer with eddies that are shed from the roughness elements in the
roughness strip/s. Depending on the size, width, height of the
strips and the location of the roughness strip/s, the flow can be
controlled to reattach in a laminar or turbulent manner forming a
separation bubble. The size of the separation bubble can be
controlled changing the roughness configuration. The size of the
separation bubble is linked to the pressure loss; the larger the
bubble, the larger the loss of pressure, and the larger the loss in
lift.
Control of Coanda Effect on the Body Near Free Surface:
[0063] Using roughness strips of appropriate size and roughness
distribution we have enhanced VIFM in the lab, bringing a cylinder
in VIV closer to the free surface. These roughness strips permit
postponement of the Coanda effect and maintain a strong vortex
street.
Control of Vortex Induced Forces and Motion:
[0064] In some embodiments, the goal of the present teachings,
VIM-Enhance+SRC, is to increase Vortex Induced Forces and Motion.
This is achieved by controlling turbulence as described herein,
such as through roughness control. Thereby, enhancement is possible
in such applications as harnessing of clean and renewable energy
from ocean/river currents using devices, such as the ocean/river
energy converter VIVACE.
2.4. New Elements of the Present Teachings
[0065] The present teachings, specifically VIM-Enhance+SRC, are
composed of simple and readily available components, which are
described below, but define an innovative design based on many of
the newly applied principles. Specifically, the present teachings
may include one or more of the following attributes:
[0066] It can enhance Vortex Induced Forces and Motion of the
structure in a relative flow as shown in FIG. 6 and FIG. 8. As an
example, this is to improve performance of the converter which
extracts hydrokinetic energy from fluid flows using vortex induced
vibrations.
[0067] It can increases the spanwise flow correlation length to a
high value by appropriate design of size and distribution of
roughness on the surface of the body as shown in the example in
FIG. 5.
[0068] It can increase the range of synchronization of VIFM of the
structure in a relative flow as shown in FIG. 6 and FIG. 8.
[0069] It can affect the point of separation by appropriate design
of size and distribution of roughness on the surface of the
body.
[0070] It can affect the turbulence shed into the wake (see FIG.
11) by appropriate design of size and distribution of roughness on
the surface of the body.
2.5. Description of the Present Teachings
Thickness of Roughness
[0071] In some embodiments, the size of the roughness for VIFM
enhancement should be on the order of the boundary layer thickness
so that the turbulent eddies created behind the roughness elements
are of boundary layer size. This efficiently energizes the boundary
layer.
Density of Roughness
[0072] In some embodiments, the density of roughness elements
attached to the base has an impact on the amount of turbulence
generated which subsequently determines whether VIFM will be
enhanced.
Distribution of Roughness on the Surface
[0073] From the model tests on cylinder in VIV, conducted in Low
Turbulence Free Surface Water Channel of the Marine Hydrodynamics
Lab at the University of Michigan, Ann Arbor, it was found that for
enhancement of VIV, roughness should be distributed as shown in
FIG. 5. Specifically, the roughness strip or zone should cover the
cylinder surface from about 57.degree. to about 85.degree. behind
the mean position of the forward stagnation point. However, it
should be appreciate that these angles may vary depending upon the
exact design criteria and environment.
Base of Roughness Elements
[0074] In some embodiments, the thickness of the base is a critical
element in VIFM control. For enhancement, the base supporting the
roughness correlates the spanwise separation and the transition of
the boundary layer. The base aids in the transition of the boundary
layer which is enhanced downstream by the roughness elements.
3.1. Working Models
[0075] Six different models of the invention have been built and
tested in the Low Turbulence Free Surface Water Channel of the
Marine Hydrodynamics Laboratory at the University of Michigan, Ann
Arbor. In our model tests, six different cylinders with diameters
1'', 2.5'', 3'', 3.5'', 5'', 6'' were used as a generic form of
bluff body to demonstrate the concept.
[0076] Increase of amplitude was achieved depending on the
orientation and size of the roughness elements. Increase of range
of synchronization was achieved by optimal orientation and size of
the roughness elements.
Experimental Results
[0077] The following observations can be made relative to the
amplitude ratio (A/D), the range of synchronization, the frequency
of oscillation, the wake structure, and the critical roughness
height. Please refer to Table 1 herebelow:
TABLE-US-00001 Grit size k Sandpaper thickness k + P Diameter No:
of Circumferential Case Sandpaper (10.sup.-6 m) (10.sup.-6 m) D
(inch) k/D k + P/D strips angle 1 P120 125 508 3.5 0.0014 0.0057 2
.+-.64.degree.-.+-.80.degree. 2 P80 201 711 5.0 0.0016 0.0056 2
.+-.57.degree.-.+-.80.degree. 3 P120 125 508 3.5 0.0014 0.0057 4
.+-.47.degree.-.+-.80.degree. .+-.102.degree.-.+-.135.degree.
4.1. Amplitude of oscillation and synchronization range:
[0078] A/D and range of synchronization for Cases 2 and 1 are shown
in FIG. 6 and FIG. 8, respectively. For Case 2, A/D for the rough
and smooth cylinders was nearly the same until the smooth cylinder
VIV started reducing. An earlier reduction in A/D with respect to
reduced velocity is observed in Case 2 in comparison to Case 1.
This earlier reduction in amplitude of oscillation in Case 2 is
attributed to the proximity of its operational Reynolds number to
the critical regime. The roughness strips start taking effect at
this point when the cylinder approaches the critical regime. The
strips sustain and actually increase VIV. The roughness strips also
increase the range of synchronization. This consequence can be
attributed to straightening of the separation line by the roughness
strip in the critical regime where the separation line for a smooth
cylinder loses vortex shedding correlation. At the end of
synchronization, the force correlation is nearly zero. In Cases 1
and 2, the roughness strips increase the force correlation in this
regime by straightening out the separation line and energizing the
separated fluid with velocity fluctuations which are higher
harmonics of the fundamental oscillation. These resulted in
synchronizing the vibration with the shedding of vortices. In Case
2, the amplitude was limited due to close proximity to the free
surface at these high amplitudes. The proximity of a smooth
cylinder to a free surface results in the Coanda effect suppressing
VIV; but with the roughness strip VIV is sustained.
[0079] In Case 1, synchronization starts earlier for the rough
cylinder. The initial A/D for the cylinder with roughness is lower
than A/D for the smooth cylinder. This is observed in other cases
too with different roughness strip configurations which are
discussed later in relation to the critical Reynolds number. The
amplitude ratio of oscillation reached values of 2.7 and the range
of synchronization extended to reduced velocity of 13. In a few
cases, the synchronization range extended to reduced velocity as
high as 16. In Cases 1 and 2 with two strips, the range of
synchronization started at an earlier reduced velocity and a jump
in the frequency of oscillation was observed at a reduced velocity
equal to four. In Case 3 with four strips, the range of
synchronization started at an earlier reduced velocity and a jump
in the frequency of oscillation was observed at a reduced velocity
equal to 4.4. In all the roughness configurations used, the end of
synchronization was not observed within the lab capabilities.
4.2. Frequency of Oscillation:
[0080] In Cases 1 and 2, where the end of the roughness strip is
located at 80.degree. and the front edge is located between
47.degree.-64.degree., the frequency of oscillation locks to the
natural frequency in water (added mass calculated using potential
theory). In Cases 1 and 3, the frequency of oscillation initially
increases and then curves downward to lock onto the natural
frequency in water as the reduced velocity increases (FIG. 9 and
FIG. 10). As the frequency of oscillation curves downward toward
the natural frequency in water, the amplitude of vibration starts
increasing from the plateau of lesser amplitude vibration as shown
in FIG. 8. FIG. 7 compares the frequency ratio response of a 5''
cylinder with and without roughness strips. In Case 2 of the 5'' of
the cylinder with roughness strips, lock on to the natural
frequency is perfect over a large range of reduced velocity. For
the smooth cylinder, the frequency of oscillation with the natural
frequency of the system in water curves up which is attributed to
the variation of the added mass with the reduced velocity and with
A/D. In the case of smooth cylinder VIV in air, perfect lock on to
the natural frequency of cylinder in air is observed due to the
negligible added mass in comparison to the mass of the
cylinder.
4.3. Wake Structure:
[0081] The roughness strips affect the wake mode of shedding, as
evidenced by the higher harmonics of vortex shedding in the
displacement spectrum. In the present experiments using roughness
strips the corresponding amplitude and the reduced velocity are
plotted on the Williamson-Roshko map. It is noticed that the plot
passes through the desynchronization region in the map and reaches
the 2P+2S region at higher reduced velocities. Further
investigation was performed by using flow visualization and it was
noticed that the number of vortices shed in half-period increased
to four or five (FIG. 11) as the velocity of the flow increased.
Limitations of the LTFSW Channel made it impossible to proceed
further; VIV was so vigorous that could damage the Channel.
4.4. Critical Roughness Height and Reynolds Number:
[0082] The boundary layer transition induced by surface roughness
is a complex phenomenon. The effect of the roughness elements on
the boundary layer depends on the size of the roughness elements
relative to the boundary layer thickness. If the roughness elements
lie completely within the laminar layer it is argued that the
roughness has least effect on the flow. The surface is regarded as
hydraulically smooth or rough based on whether the roughness
elements are completely embedded or not in the laminar boundary
layer. When the height of the roughness elements is on the order of
the laminar boundary layer thickness, horseshoe eddies are shed
around the roughness element and aid in transition of the boundary
layer from laminar to turbulent. In the case of distributed
roughness, turbulent "spots" are formed behind the roughness
elements above a critical Reynolds number. Past experiments tried
to explain partially what might be happening behind the rough
particles in sandpaper. In the case of sandpaper, an agreement has
never been reached as to the value of k or Re.sub.k at which
transition occurs, due to the random distribution of the sandpaper
particles and statistical non-uniformities. Transition for a single
three-dimensional roughness element occurs by the formation of
hairpin eddies behind the roughness particle. For sandpaper
however, transition becomes complex because elements are closely
packed. In FIG. 12 and FIG. 13, A/D is plotted versus Re.sub.k, and
Re.sub.k+P. Re.sub.k is the Reynolds number pertinent to the
roughness element size (k) and Re.sub.k+P, is the Reynolds number
pertinent to thickness of roughness strip (k+P) where P is the
backing-paper thickness. In FIG. 12 in Cases 1 and 2 with different
operational Reynolds number (Re) the results collapse at a critical
value for the Re.sub.k+P.apprxeq.600-700. Above this critical
value, jump in A/D to a higher value is observed (FIG. 12). In FIG.
13, the VIV response for both Cases 1 and 2 with different
operational Reynolds number (Re) is plotted versus Re.sub.k and the
results do not collapse as well as in FIG. 12.
[0083] In FIG. 13, for Case 1 with different operational Reynolds
number (Re), the plots collapse. The jump in the amplitude of
oscillation occurs above a critical value for Re.sub.k
.apprxeq.120, which coincides with Re.sub.k+P.apprxeq.600-700. In
Case 2, the jump to high amplitude occurs at Re.sub.k .apprxeq.180
and coincides with Rek+P.apprxeq.600-700. Re.sub.k .apprxeq.120 is
achieved right after the early jump from initial to upper branch,
Point A to Point B in FIG. 6. It is hypothesized that this is the
reason why the amplitude is not affected. In Case 2, the jump to
the upper branch occurs at Re.sub.k+P.apprxeq.600-700 even though
Re.sub.k>120 is achieved earlier. In Case 1, with softer
springs, Re.sub.k.apprxeq.120 is achieved at the end of
synchronization of the smooth cylinder and the amplitude reduces by
nearly half till Re.sub.k+P.apprxeq.600-700 and
Re.sub.k.apprxeq.120 is reached. In Case 1 with stiffer springs,
the synchronization range shifts to higher velocity/Reynolds number
because of the shift in natural frequency. This implies that
Re.sub.k .apprxeq.120 is achieved at a lower reduced velocity and
the amplitude plot is not less affected than in Case 1 with softer
springs. In all the 3.5'' cylinder cases with stiff and soft
springs, the jump from the upper branch to higher amplitude of
oscillation occurrs at Re.sub.k+P.apprxeq.600-700. The critical
Reynolds number seen in the above cases Re.sub.k+P.apprxeq.600-700
and Re.sub.k .apprxeq.120 is close to the critical Reynolds number
observed for three dimensional roughness elements and two
dimensional roughness elements. In the cases analyzed above, the
synchronization range occurs for Re.sub.D>5.times.104.
[0084] For the high amplitude VIV in Cases 1 and 2, the
experimental results are presented in FIG. 14 through FIG. 17 based
on the ratio of roughness height to the boundary layer thickness
(.delta.) and the ratio of roughness height to the boundary layer
displacement thickness (.delta.*). The effect of roughness can be
analyzed using the roughness height in comparison to the
displacement and boundary layer thickness. In FIG. 16 and FIG. 17,
the displacement thickness for Case 2 is recalculated at 640 in
order to compare to Case 1. In our experiments, transition to high
A/D VIV is observed when k/.delta.*=1 and (k+P)/.delta.*=1.6-1.7.
When k/.delta.*<1, roughness elements are completely submerged
underneath the displacement thickness and lower amplitude of
oscillation is observed in the case of 3.5'' cylinder with
roughness strips in comparison to the smooth cylinder (FIG. 16).
When the roughness element is larger than the displacement
thickness it results in higher amplitude of oscillation (FIG. 16).
The reason for the above observation can be hypothesized to be the
following: When k/.delta.*=1, then Re.sub.k.apprxeq.120 and this is
the Reynolds number value around which transition in wake occurs
behind a circular element. Therefore when k/.delta.*=1, small
eddies/vortices are formed behind the roughness elements. Those
energize the flow and result in delayed separation. To confirm the
above statement, visualization of the details behind roughness
elements is needed.
5. Main Findings
[0085] To increase the power harnessed by the Converter, lift and
amplitude of oscillation of the cylinder in high damping VIV need
to be enhanced. Higher vorticity and resulting circulation are
required. Increased range of VIV synchronization increases the
robustness of the converter. The experimental results have shown
that all requirements can be achieved by designing and distributing
surface roughness based on the three Principles defined in the
present disclosure. The results of the cylinder with roughness
strips, undergoing VIV in the TrSL3 regime with a high
(m*+Ca).zeta. are summarized below:
[0086] 1. Roughness strips were effective when placed in the range
(57.degree.-80.degree.), which is the range of oscillation of the
separation point. That resulted in increased synchronization range
and A/D.
[0087] 2. When roughness strips were attached to the cylinder in
that range, the frequency of synchronization at high A/D or higher
reduced velocity was found to be f.sub.o3c=f.sub.n,water. For a
smooth cylinder, the f.sub.o3c detunes away from f.sub.n,water.
[0088] 3. When the roughness strips were attached to the cylinder
aft of 80.degree., the range of synchronization increased but A/D
decreased.
[0089] 4. In the present experiments, an amplitude ratio of 2.7 was
achieved and synchronization passed reduced velocity of 13. The end
of synchronization was not observed within the lab's
capabilities.
[0090] 5. In general, roughness strips induced earlier start of
synchronization by increasing the spanwise correlation length of
vortex shedding.
[0091] 6. A critical Reynolds number based on the roughness element
size and the paper backing was determined, above which the
roughness strip was effective for enhancing VIV. Below this
critical Reynolds number (Re.sub.k+P<600, Re.sub.k<120) the
roughness strip reduces the amplitude ratio in the original
synchronization region.
[0092] 7. An optimal designed roughness can enhance VIV without
affecting the original synchronization range, as observed in Case
2.
[0093] 8. When the roughness element was on the order of the
boundary layer the flow around the cylinder was modified and the
separation point was moved downstream.
[0094] 9. In preliminary visualization, the wake constituted four
vortices shed per half-cycle. Strategically arranged roughness can
affect lock-in (synchronization) frequency.
[0095] 10. When k>.delta.* it brings considerable momentum from
the outer flow into the boundary layer. Eddies that are already
generated at the edge of the roughness strips by tripping the
boundary layer interact more vigorously with the roughness elements
when k>.delta.*.
[0096] 11. For k=.delta.* the scale of the vorticity generated by
roughness is on the order of the boundary layer vorticity scale.
This enhances the strength of the Karman vortices because the
boundary layer scale vorticity is absorbed into the Karman
vortices.
[0097] 12. In FIG. 6, point B is the same for smooth and rough
cylinders. Concurrently at B (Re.sub.k .apprxeq.120), first, k
introduces friction resulting in pressure loss which would cause
reduction in A/D; and second, since at this Re.sub.k k=.delta.*, it
brings in turbulence from the outer higher momentum flow thus
generating vorticity still at the boundary layer scale. That is
absorbable by Karman vortices, which counterbalances the friction
change in A/D that we hypothesized.
3.2. Alternative Implementations
[0098] Several variations of the present teachings of
VIM-Enhance+SRC or components thereof may be equally effective in
achieving VIFM control using surface roughness control.
Specifically:
[0099] Control of VIFM through roughness maybe passive or active.
Passive control was described above. Active control, however, can
be achieved by raising or by lowering surface roughness or
components thereof in response to flow variations. This can be
achieved through mechanically actuated excrescences, electrically
actuated excrescences, and the like. In other words, the roughness
zone of the present teachings can be an actively controllable
roughness zone operable between a first roughness state and a
second roughness state, said first roughness state being different
than said second roughness state. Such differences could include
roughness size, roughness density, roughness configuration, or any
other parameter effect fluid flow thereby.
[0100] The type of material used to fabricate surface roughness can
be any material which satisfies the following requirements: Be
rigid or flexible; have rough or smooth individual roughness
elements; roughness elements can be metallic, composite, plastic or
any other natural or manmade product.
[0101] The configuration of the surface roughness can have any form
that can be modeled using its size, amount, distribution, and
density as described in this disclosure. One of the possible
configurations is shown in FIG. 3 through FIG. 5.
Unique Benefits
[0102] The disclosed teachings of VIM-Enhance+SRC can be used to
enhance VIFM. We have implemented SRC in VIVACE converter models
and enhanced its VIFM and improved its efficiency in extracting
energy from fluid flows drastically. Ocean or fresh moving water
provides clean and renewable energy. The total energy flux due to
surface and underwater currents of the world has been estimated at
280 trillion watt-hours. The converter can be improved by
implementing the present teachings thus, making it possible to
harness more efficiently some of this abundant clean and renewable
ocean/river energy. Implementation of VIM-Enhance+SRC is
simple.
* * * * *