Dynamic Response of Structures Flashcards
What are the 5 Aerodynamic excitation mechanisms?
Buffeting
Vortex Excitation
Galloping Excitation
Classical Flutter
Stall Flutter
What causes buffeting?
Turbulent free stream or shedding from upstream body
What is vortex shedding?
Bluff (rounded) bodies in wind shed alternate vortices, from each side
What is the Strouhal Number? What is the equation?
A non-dimensional factor describing oscillating flow
St = fD/V where D is diameter of bluff body
What does critical wind speed mean in terms of vortex shedding?
Vortex frequency is close to natural frequency of structure
What is galloping excitation?
Where motion of a structure causes aero-elastic instability and increasing oscillations
What can cause galloping excitation in cylindrical structures which otherwise d not experience it?
Ice build up
What is required for Classical Flutter?
At least two degrees of freedom at a single frequency
Typically a combined bending and torsional motion
Similar to stall of an aerofoil, what does stall flutter involve?
Separation and reattachment of flow from/to a body
How can natural frequency of a structure be changed?
Guy wires
Stiffening rings
Increase in non-structural mass
Damping
What are aerodynamic means of reducing oscillation?
Changing structure profile or adding devices which modify flow patterns
On a typical wind spectrum, what are the two peaks called?
Macrometeorological and Micrometeorological peaks, Macro is the larger
What is the space between the two peaks on a typical wind spectrum called?
Spectral Gap
On a typical wind spectrum, what is the gap after the second peak?
Frequency range of structure-generated turbulence
