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13.42 Lecture: Vortex Induced Vibrations. Prof. A. H. Techet 18 March 2004. 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. Potential Flow. U( q ) = 2U  sin q.

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13.42 Lecture: Vortex Induced Vibrations

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## 13.42 Lecture:Vortex Induced Vibrations

Prof. A. H. Techet

18 March 2004

### 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.

### Potential Flow

U(q) = 2Usinq

P(q) = 1/2 r U(q)2 = P + 1/2 r U2

Cp = {P(q) - P }/{1/2 r U2}= 1 - 4sin2q

### Axial Pressure Force

(i)

(ii)

Base

pressure

i) Potential flow:

-p/w < q < p/2

ii) P ~ PB

p/2  q  3p/2

(for LAMINAR flow)

### Reynolds Number Dependency

Rd < 5

5-15 < Rd < 40

40 < Rd < 150

150 < Rd < 300

Transition to turbulence

300 < Rd < 3*105

3*105 < Rd < 3.5*106

3.5*106 < Rd

### 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

### Classical Vortex Shedding

l

h

Von Karman Vortex Street

Alternately shed opposite signed vortices

### Vortex shedding dictated by the Strouhal number

St=fsd/U

fs is the shedding frequency, d is diameter and U inflow speed

### Additional VIV Parameters

• Reynolds Number

• subcritical (Re<105) (laminar boundary)

• Reduced Velocity

• Vortex Shedding Frequency

• S0.2 for subcritical flow

St = 0.2

### Vortex Shedding Generates forces on Cylinder

Uo

Both Lift and Drag forces persist on a cylinder in cross flow. Lift is perpendicular to the inflow velocity and drag is parallel.

FL(t)

FD(t)

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.

L(t)

D(t)

1/2 r U2 d

1/2 r U2 d

### Vortex Induced Forces

Due to unsteady flow, forces, X(t) and Y(t), vary with time.

Force coefficients:

Cx =

Cy =

DRAG

Cx

Avg. Drag ≠ 0

LIFT

Cy

Avg. Lift = 0

### Alternate Vortex shedding causes oscillatory forces which induce structural vibrations

Heave Motion z(t)

LIFT = L(t) = Lo cos (wst+)

DRAG = D(t) = Do cos (2wst+ )

Rigid cylinder is now similar to a spring-mass system with a harmonic forcing term.

ws = 2p fs

k

wn =

m + ma

### “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.

wv = 2p fv = 2p St (U/d)

Shedding

frequency

Natural frequency

of oscillation

### Equation of Cylinder Heave due to Vortex shedding

z(t)

m

b

k

Restoring force

If Lv > b system is

UNSTABLE

Damping

### Lift Force on a Cylinder

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.

LIFT FORCE:

where wvis the frequency of vortex shedding

### Lift Force Components:

Two components of lift can be analyzed:

Lift in phase with acceleration (added mass):

Lift in-phase with velocity:

Total lift:

(a = zois cylinder heave amplitude)

### Total Force:

• If CLv > 0 then the fluid force amplifies the motion instead of opposing it. This is self-excited oscillation.

• Cma, CLvare dependent on w and a.

### Coefficient of Lift in Phase with Velocity

Vortex Induced Vibrations are

SELF LIMITED

In air: rair ~ small, zmax ~ 0.2 diameter

In water: rwater ~ large, zmax ~ 1 diameter

### Lift in phase with velocity

Gopalkrishnan (1993)

b

2 k(m+ma*)

z =

ma* = r V Cma; where Cma = 1.0

a/d = 1.29/[1+0.43 SG]3.35

~

### Amplitude Estimation

Blevins (1990)

_

_

2m (2pz)

r d2

^

^

; fn = fn/fs; m = m + ma*

SG=2 p fn2

~

Cd

|Cd|

3

2

a

d

= 0.75

1

fd

U

0.1

0.2

0.3

### Drag Amplification

VIV tends to increase the effective drag coefficient. This increase has been investigated experimentally.

Gopalkrishnan (1993)

Fluctuating Drag:

Mean drag:

~

Cd = 1.2 + 1.1(a/d)

Cd occurs at twice the

shedding frequency.

Single Rigid Cylinder Results

1.0

• One-tenth highest transverse oscillation amplitude ratio

• Mean drag coefficient

• Fluctuating drag coefficient

• Ratio of transverse oscillation frequency to natural frequency of cylinder

1.0

### 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

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.

f , A

U

U

f , A

‘2P’

‘2S’

### Wake Patterns Behind Heaving Cylinders

• 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.

### Transition in Shedding Patterns

A/d

Williamson and Roshko (1988)

f* = fd/U

Vr = U/fd

Formation of ‘2P’ shedding pattern

### End Force Correlation

Uniform Cylinder

Hover, Techet, Triantafyllou (JFM 1998)

Tapered Cylinder

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, lc.

x

d(x)

### Oscillating Tapered Cylinder

Strouhal Number for the tapered cylinder:

St = fd / U

where d is the average

cylinder diameter.

U(x) = Uo

### Spanwise Vortex Shedding from 40:1 Tapered Cylinder

Rd = 400;

St = 0.198; A/d = 0.5

Rd = 1500;

St = 0.198; A/d = 0.5

Rd = 1500;

St = 0.198; A/d = 1.0

dmax

Techet, et al (JFM 1998)

No Split: ‘2P’

dmin

### Flow Visualization Reveals: A HybridShedding 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)

### DPIV of Tapered Cylinder Wake

Digital particle image

velocimetry (DPIV) in the horizontal plane leads to a clear picture of two distinct shedding modes along the cylinder.

‘2S’

z/d = 22.9

‘2P’

z/d = 7.9

Rd = 1500; St = 0.198; A/d = 0.5

Techet, Hover and Triantafyllou (JFM 1998)

Vortex Dislocations, Vortex Splits & Force Distribution in Flows past Bluff BodiesD. Lucor & G. E. Karniadakis

• Objectives:

• Confirm numerically the existence of a stable, periodic hybrid shedding mode 2S~2P in the wake of a straight, rigid, oscillating cylinder

• 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 SPLIT

• Methodology:

• Parallel simulations using spectral/hp methods implemented in the incompressible Navier- Stokes solver NEKTAR

NEKTAR-ALE Simulations

• 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

• Principal Investigator:

• Prof. George Em Karniadakis, Division of Applied Mathematics, Brown University

### VIV Suppression

• Helical strake

• Shroud

• Axial slats

• Streamlined fairing

• Splitter plate

• Ribboned cable

• Pivoted guiding vane

• Spoiler plates

### VIV Suppression by Helical Strakes

Helical strakes are a

common VIV suppresion

device.

.

y(t) = -aw sin(wt)

### Oscillating Cylinders

Parameters:

y(t)

Re = Vm d / n

Reynolds #

d

b = d2/nT

Reduced

frequency

y(t) = a cos wt

Keulegan-

Carpenter #

KC = Vm T / d

Vm = a w

St = fv d / Vm

Strouhal #

n = m/ r ; T = 2p/w

Re = Vm d / n = wad/n = 2p a/d d /nT

(

)(

)

2

### Reynolds # vs. KC #

KC = Vm T / d = 2pa/d

Re = KC * b

b = d2/nT

Also effected by roughness and ambient turbulence

Parameters:

a/d, r, n, q

### Forced Oscillation in a Current

y(t) = a cos wt

w = 2 p f = 2p / T

q

U

Reduced velocity: Ur = U/fd

Max. Velocity: Vm = U + aw cos q

Reynolds #: Re = Vm d / n

Roughness and ambient turbulence

### 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.

Y(t)

.

y(t), y(t)

U

.

-y(t)

m

a

V

### Galloping

Galloping is a result of a wake instability.

Resultant velocity is a combination of the heave velocity and horizontal inflow.

If wn << 2p fv then the wake is quasi-static.

Y(t)

1/2 r U2 Ap

### Lift Force, Y(a)

Y(t)

V

a

Stable

Cy =

Cy

a

Unstable

L(t)

.

z(t), z(t)

U

..

L(t) = 1/2r U2 a Clv - ma y(t)

.

-z(t)

m

a

V

 Cl (0)

 a

 Cl (0)

 a

..

.

mz + bz + kz = L(t)

.

a ~ tan a = -

b =

z

U

### Galloping motion

a

b

k

Cl(a) = Cl(0) +

+ ...

Assuming small angles, a:

V ~ U

b

U

b

U

### Instability Criterion

..

.

~

(m+ma)z + (b + 1/2r U2 a )z + kz = 0

b + 1/2r U2 a

< 0

If

Then the motion is unstable!

This is the criterion for galloping.

1

1

1

2

2

 Cl (0)

 a

1

4

1

Shape

-2.7

0

U

-3.0

-10

-0.66

 Cl (0)

 a

 Cl (0)

 a

b

1/2r U a

### Instability:

<

b =

Critical speed for galloping:

b

1/2r a

U >

( )

### Torsional Galloping

Both torsional and lateral galloping are possible.

FLUTTER occurs when the frequency of the torsional

and lateral vibrations are very close.

### Galloping vs. VIV

• Galloping is low frequency

• Galloping is NOT self-limiting

• Once U > Ucritical then the instability occurs irregardless of frequencies.

### References

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