Aeroelastic

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Aeroelastic effects: 

Aeroelastic effects Wind loading and structural response Lecture 14 Dr. J.D. Holmes

Aeroelastic effects: 

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 effects: 

Aeroelastic effects Example : Tacoma Narrows Bridge WA - 1940 Example : ‘Galloping’ of iced-up transmission lines

Aeroelastic effects: 

Aeroelastic effects Aerodynamic damping (along wind) :

Aeroelastic effects: 

Aeroelastic effects Aerodynamic damping (along wind) : Drag force (per unit length) = along-wind aerodynamic damping is positive

Aeroelastic effects: 

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 effects: 

Aeroelastic effects Galloping : Aerodynamic force per unit length in z direction (body axes) : (Lecture 8) For  = 0 :

Aeroelastic effects: 

Aeroelastic effects Galloping : negative aerodynamic damping when transposed to left-hand side

Aeroelastic effects: 

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 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 effects

Aeroelastic effects: 

Aeroelastic effects Flutter : Can generate a cross-wind force and a moment Aerodynamic instabilities involving rotation are called ‘flutter’

Aeroelastic effects: 

Aeroelastic effects Flutter : General equations of motion for body free to rotate and translate : per unit mass per unit mass moment of inertia

Aeroelastic effects: 

Aeroelastic effects Flutter : Types of instabilities :

Aeroelastic effects: 

Flutter : Flutter derivatives for two bridge deck sections : Aeroelastic effects

Aeroelastic effects: 

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 effects: 

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 14 John Holmes 225-405-3789 JHolmes@lsu.edu