Aeroelastic effects
This presentation is the property of its rightful owner.
Sponsored Links
1 / 17

Aeroelastic effects PowerPoint PPT Presentation


  • 92 Views
  • Uploaded on
  • Presentation posted in: General

Aeroelastic effects. Wind loading and structural response Lecture 14 Dr. J.D. Holmes. Aeroelastic effects. Very flexible dynamically wind-sensitive structures . Motion of the structure generates aerodynamic forces. Positive aerodynamic damping : reduces vibrations - steel lattice towers .

Download Presentation

Aeroelastic effects

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Aeroelastic effects

Aeroelastic effects

Wind loading and structural response

Lecture 14 Dr. J.D. Holmes


Aeroelastic effects1

Aeroelastic effects

  • Very flexible dynamically wind-sensitive structures

  • Motion of the structure generates aerodynamic forces

  • Positive aerodynamic damping : reduces vibrations - steel lattice towers

  • if forces act in direction to increase the motion : aerodynamic instability


Aeroelastic effects2

Aeroelastic effects

  • Example : Tacoma Narrows Bridge WA - 1940

  • Example : ‘Galloping’ of iced-up transmission lines


Aeroelastic effects3

Consider a body moving with velocity in a flow of speed U

Relative velocity of air with respect to body =

Aeroelastic effects

  • Aerodynamic damping (along wind) :


Aeroelastic effects4

transfer to left hand side of equation of motion :

for small

total damping term :

aerodynamic damping term

Aeroelastic effects

  • Aerodynamic damping (along wind) :

Drag force (per unit length) =

along-wind aerodynamic damping is positive


Aeroelastic effects5

From vector diagram :

Aeroelastic effects

  • Galloping :

galloping is a form of aerodynamic instability caused by negative aerodynamic damping in the cross wind direction

Motion of body in z direction will generate an apparent reduction in angle of attack, 


Aeroelastic effects6

Fz = D sin  + L cos  =

Aeroelastic effects

  • Galloping :

Aerodynamic force per unit length in z direction (body axes) :

(Lecture 8)

For  = 0 :


Aeroelastic effects7

Substituting,

For , Fz is positive - acts in same direction as

Aeroelastic effects

  • Galloping :

negative aerodynamic damping when transposed to left-hand side


Aeroelastic effects8

Aeroelastic effects

  • Galloping :

den Hartog’s Criterion

critical wind speed for galloping,Ucrit , occurs when total damping is zero

Since c = 2(mk)=4mn1 (Figure 5.5 in book)

m = mass per unit length n1 = first mode natural frequency


Aeroelastic effects9

Aeroelastic effects

  • Galloping :

Cross sections prone to galloping :

Square section (zero angle of attack)

D-shaped cross section

iced-up transmission line or guy cable


Aeroelastic effects10

Consider a two dimensional body rotating with angular velocity

Apparent change in angle of attack :

Vertical velocity at leading edge :

Aeroelastic effects

  • Flutter :

Can generate a cross-wind force and a moment

Aerodynamic instabilities involving rotation are called ‘flutter’


Aeroelastic effects11

Flutter derivatives

Aeroelastic effects

  • Flutter :

General equations of motion for body free to rotate and translate :

per unit mass

per unit mass moment of inertia


Aeroelastic effects12

Aeroelastic effects

  • Flutter :

Types of instabilities :


Aeroelastic effects13

U/nd

U/nd

unstable

2

4

6

0

10

8

12

A1*

3

0

stable

stable

2

2

-2

1

1

-4

1

1

2

0

H1*

-6

0

10

4

6

2

8

12

H2*

0.4

A2*

8

2

0.3

6

1

0.2

4

2

2

2

0.1

0

0

A

-2

-0.1

-0.2

1

Aeroelastic effects

  • Flutter :

Flutter derivatives for two bridge deck sections :


Aeroelastic effects14

Aeroelastic effects

  • Flutter :

Determination of critical flutter speed for long-span bridges:

  • Empirical formula (e.g. Selberg)

  • Experimental determination (wind-tunnel model)

  • Theoretical analysis using flutter derivatives obtained experimentally


Aeroelastic effects15

Aeroelastic effects

  • Lock - in :

Motion-induced forces during vibration caused by vortex shedding

Frequency ‘locks-in’ to frequency of vibration

Strength of forces and correlation length increased


End of lecture 14 john holmes 225 405 3789 jholmes@lsu edu

End of Lecture 14John Holmes225-405-3789 [email protected]


  • Login