Flow Induced vibration fundamentals present

Flow induced vibration

1. 13.42 Lecture:
Vortex Induced Vibrations
Prof. A. H. Techet
18 March 2004

2. Classic VIV Catastrophe
If ignored, these vibrations can prove catastrophic to
structures, as they did in the case of the Tacoma Narrows
Bridge in 1940.

3. Potential Flow
U(q) = 2U
sinq
P(q) = 1/2 rU(q)2 = P
+ 1/2 r U

2
Cp = {P(q) -P
}/{1/2 rU

2
}= 1 -4sin
2
q

4. Axial Pressure Force
i) Potential flow:
-p/w < q< p/2
ii) P ~ P
B
p/2 q3p/2
(for LAMINAR flow)
Base
pressure
(i) (ii)

5. Reynolds Number Dependency
R
d< 5
5-15 < R
d< 40
40 < R
d< 150
150 < R
d< 300
300 < R
d< 3*10
5
3*10
5
< R
d< 3.5*10
6
3.5*10
6
< R
d
Transition to turbulence

6. Shear layer instability causes
vortex roll-up
•Flow speed outside wake is much higher than inside
•Vorticity gathers at downcrossing points in upper layer
•Vorticity gathers at upcrossings in lower layer
•Induced velocities (due to vortices) causes this
perturbation to amplify

7. Wake Instability

8. Classical Vortex Shedding
Von Karman Vortex Street
l
h
Alternately shed opposite signed vortices

9. Vortex shedding dictated by
the Strouhal number
S
t=f
sd/U
f
sis the shedding frequency, dis diameter and Uinflow speed

10. •Reynolds Number
–subcritical (Re<10
5
) (laminar boundary)
•Reduced Velocity
•Vortex Shedding Frequency
–S0.2 for subcritical flow
Additional VIV ParametersD
SU
fs effects viscous
effects inertial
Re 
v
UD Df
U
V
n
rn

11. Strouhal Number vs. Reynolds
Number
St = 0.2

12. Vortex Shedding Generates
forces on Cylinder
F
D(t)
F
L(t)
U
o
Both Lift and Drag forces persist
on a cylinder in cross flow. Lift
is perpendicular to the inflow
velocity and drag is parallel.
Due to the alternating vortex wake (“Karman street”) the
oscillations in lift force occur at the vortex shedding frequency
and oscillations in drag force occur at twice the vortex
shedding frequency.

13. Vortex Induced Forces
Due to unsteady flow, forces, X(t) and Y(t), vary with time.
Force coefficients:
C
x= C
y=
D(t)
1
/
2 rU
2
d
L(t)
1
/
2 rU
2
d

14. Force Time Trace
C
x
C
y
DRAG
LIFT
Avg. Drag ≠ 0
Avg. Lift = 0

15. Alternate Vortex shedding causes
oscillatory forces which induce
structural vibrations
Rigid cylinder is now similar
to a spring-mass system with
a harmonic forcing term.
LIFT = L(t) = Lo cos (w
st+)
w
s= 2pf
s
DRAG = D(t) = Do cos (2w
st+ )
Heave Motion z(t)2
( ) cos
( ) sin
( ) cos
o
o
o
z t z t
z t z t
z t z t
w
ww
ww




16. “Lock-in”
A cylinder is said to be “locked in” when the frequency of
oscillation is equal to the frequency of vortex shedding. In this
region the largest amplitude oscillations occur.
w
v= 2p f
v= 2p S
t(U/d)
w
n=
k
m + m
a
Shedding
frequency
Natural frequency
of oscillation

17. Equation of Cylinder Heave due
to Vortex shedding
Added mass term
Damping If L
v> b system is
UNSTABLE
kb
m
z(t)()mz bz kz L t   ( ) ( ) ( )
av
L t L z t L z t   ( ) ( ) ( ) ( ) ( )
av
mz t bz t kz t L z t L z t     ( ) ( ) ( ) ( ) ( ) 0
av
m L z t b L z t kz t    
Restoring force

18. LIFT FORCE:
Lift Force on a Cylinder( ) cos( )
oo
L t L tw v
ifww ( ) cos cos sin sin
o o o o
L t L t L tw  w  2
cos sin
( ) ( ) ( )
o o o o
oo
LL
L t z t z t
zz

ww


where w
vis the frequency of vortex shedding
Lift force is sinusoidal component and residual force. Filtering
the recorded lift data will give the sinusoidal term which can
be subtracted from the total force.

19. Lift Force Components:
Lift in phase with acceleration (added mass):
Lift in-phase with velocity:2
( , ) cos
o
ao
L
Ma
a
w
w
 sin
o
vo
L
L
a

w

Two components of lift can be analyzed:
(a = z
ois cylinder heave amplitude)
Total lift:( ) ((, ()() ,))
a v
L t z t LaM za tww  

20. Total Force:
•If C
Lv> 0then the fluid force amplifies the motion
instead of opposing it. This is self-excited
oscillation.
•C
ma, C
Lvare dependent on wand a.( ) ((, ()() ,))
a v
L t z t LaM za tww    
 
2
4
2
1
2
( , ) ( )
( , ) ( )
ma
Lv
d C a z t
dU C a z t
p
rw
rw



21. Coefficient of Lift in Phase with
Velocity
Vortex Induced Vibrations are
SELF LIMITED
In air: r
air~ small, z
max~ 0.2 diameter
In water: r
water~ large, z
max~ 1 diameter

22. Lift in phase with velocity
Gopalkrishnan (1993)

23. Amplitude Estimation
z=
b
2 k(m+m
a
*
)
m
a
*
= r V C
ma; where C
ma= 1.0
Blevins (1990)
a
/
d
=
1.29
/[1+0.43 S
G]
3.35~
S
G=2 pf
n
2
2m (2pz
r d
2
; f
n= f
n/f
s; m = m + m
a
*
^
^
_
_

24. Drag Amplification
Gopalkrishnan (1993)
C
d= 1.2 + 1.1(
a
/
d)
VIV tends to increase the effective drag coefficient. This increase
has been investigated experimentally.
Mean drag: Fluctuating Drag:
C
doccurs at twice the
shedding frequency.
~
3
2
1
C
d |C
d|
~
0.1 0.2 0.3
fd
U
a
d
= 0.75

25. Single Rigid Cylinder Results
a)One-tenth highest
transverse
oscillation amplitude
ratio
b)Mean drag
coefficient
c)Fluctuating drag
coefficient
d)Ratio of transverse
oscillation frequency
to natural frequency
of cylinder
1.0
1.0

26. Flexible Cylinders
Mooring lines and towing
cables act in similar fashion to
rigid cylinders except that
their motion is not spanwise
uniform.
t
Tension in the cable must be considered
when determining equations of motion

27. Flexible Cylinder Motion Trajectories
Long flexible cylinders can move in two directions and
tend to trace a figure-8 motion. The motion is dictated by
the tension in the cable and the speed of towing.

28. •Shedding patterns in the wake of oscillating
cylinders are distinct and exist for a certain range
of heave frequencies and amplitudes.
•The different modes have a great impact on
structural loading.
Wake Patterns Behind
Heaving Cylinders
‘2S’ ‘2P’f , A
f , A
U
U

29. Transition in Shedding Patterns
Williamson and Roshko (1988)
A/d
f* = fd/UVr = U/fd

30. Formation of ‘2P’ shedding pattern

31. End Force Correlation
Uniform Cylinder
Tapered Cylinder
Hover, Techet, Triantafyllou (JFM 1998)

32. VIV in the Ocean
•Non-uniform currents
effect the spanwise vortex
shedding on a cable or
riser.
•The frequency of shedding
can be different along
length.
•This leads to “cells” of
vortex shedding with
some length, l
c.

33. Strouhal Number for the tapered
cylinder:
S
t= fd / U
where dis the average
cylinder diameter.
Oscillating Tapered Cylinder
x
d(x)
U(x) = Uo

34. Spanwise Vortex Shedding from
40:1 Tapered Cylinder
Techet, et al (JFM 1998)
d
max
R
d= 400;
St = 0.198; A/d = 0.5
R
d= 1500;
St = 0.198; A/d = 0.5
R
d= 1500;
St = 0.198; A/d = 1.0
d
min
No Split: ‘2P’

35. Flow Visualization Reveals:
AHybridShedding Mode
•‘2P’ pattern results at
the smaller end
•‘2S’ pattern at the
larger end
•This mode is seen to
be repeatable over
multiple cycles
Techet, et al (JFM 1998)

36. DPIV of Tapered Cylinder Wake
‘2S’
‘2P’
Digital particle image
velocimetry (DPIV)
in the horizontal plane
leads to a clear
picture of two distinct
sheddingmodes along
the cylinder.
Rd = 1500; St = 0.198; A/d = 0.5
z/d = 22.9
z/d = 7.9

37. NEKTAR-ALESimulations
Objectives:
•Confirm numerically the existence of a stable,
periodic hybrid shedding mode 2S~2P in the
wake of a straight, rigid, oscillating cylinder
Principal Investigator:
•Prof. George Em Karniadakis, Division of Applied
Mathematics, Brown University
Approach:
•DNS -Similar conditions as the MIT experiment
(Triantafyllou et al.)
•Harmonically forced oscillating straight rigid
cylinder in linear shear inflow
•Average Reynolds number is 400
Vortex Dislocations, Vortex Splits & Force
Distribution in Flows past Bluff Bodies
D. Lucor & G. E. Karniadakis
Results:
•Existence and periodicity of hybrid mode
confirmed by near wake visualizations and spectral
analysis of flow velocity in the cylinder wake and
of hydrodynamic forces
Methodology:
•Parallel simulations using spectral/hp methods
implemented in the incompressible Navier-Stokes
solver NEKTAR
VORTEX SPLIT
Techet, Hover and Triantafyllou (JFM 1998)

38. VIV Suppression
•Helical strake
•Shroud
•Axial slats
•Streamlined fairing
•Splitter plate
•Ribboned cable
•Pivoted guiding vane
•Spoiler plates

39. VIV Suppression by Helical Strakes
Helical strakes are a
common VIV suppresion
device.

40. Oscillating Cylinders
d
y(t)
y(t) = a cos wt
Parameters:
Re = V
md / n
V
m= a w
y(t) = -awsin(wt)
.
b = d
2/nT
KC = V
mT / d
St = f
vd / V
m
n  m/ r ; T  2p/w
Reduced
frequency
Keulegan-
Carpenter #
Strouhal #
Reynolds #

41. Reynolds # vs. KC #
b = d
2/nT
KC = V
mT / d = 2p
a
/
d
Re = V
md / n 
wad
/
n
 2p
a
/
d
d
/
nT
2
)(( )
Re = KC * b
Also effected by roughness and ambient turbulence

42. Forced Oscillation in a Current
q
U
y(t) = a cos wt
w= 2 pf = 2p / T
Parameters:a/d, r, n, q
Reduced velocity: U
r= U/fd
Max. Velocity: V
m= U + awcos q
Reynolds #: Re = V
md / n
Roughness and ambient turbulence

43. Wall Proximity
e + d/2
At e/d > 1 the wall effects are reduced.
Cd, Cm increase as e/d < 0.5
Vortex shedding is significantly effected by the wall presence.
In the absence of viscosity these effects are effectively non-existent.

44. Galloping
Galloping is a result of a wake instability.
m
y(t), y(t)
.
Y(t)
U
-y(t)
.
V
a
Resultant velocityis a combination of the
heave velocityand horizontal inflow.
If w
n<< 2pf
vthen the wake is quasi-static.

45. Lift Force, Y(a)
Y(t)
V
a
C
y=
Y(t)
1
/
2 rU
2
A
p
a
C
y
Stable
Unstable

46. Galloping motion
m
z(t), z(t)
.
L(t)
U
-z(t)
.
V
a
b k
a mz + bz + kz = L(t)
...
L(t) =
1
/
2rU
2
a C
lv-m
ay(t)
..
C
l(a) = C
l(0) +
C
l (0)
a
+ ...
Assuming small angles, a:
a~ tan a= -
z
U
.
b=
C
l (0)
a
V ~ U

47. Instability Criterion
(m+m
a)z + (b +
1
/
2r U
2
a )z + kz = 0
.. .
b
U
~
b +
1
/
2r U
2
a
b
U
< 0If
Then the motion is unstable!
This is the criterion for galloping.

48. bis shape dependent
U
1
1
1
2
1
2
1
4
Shape C
l (0)
a
-2.7
0
-3.0
-10
-0.66

49. b
1
/
2ra ( )
Instability:
b=
C
l (0)
a
<
b
1
/
2rU a
Critical speed for galloping:
U >
C
l (0)
a

50. Torsional Galloping
Both torsional and lateral galloping are possible.
FLUTTER occurs when the frequency of the torsional
and lateral vibrations are very close.

51. Galloping vs. VIV
•Galloping is low frequency
•Galloping is NOT self-limiting
•Once U > U
critical then the instability occurs
irregardless of frequencies.

52. References
•Blevins, (1990) Flow Induced Vibrations,
Krieger Publishing Co., Florida.