Self Test 6 - Aeroelasticity Flashcards
What are the characteristics of turbulent buffeting as a flow-induced vibration mechanism?
1) Broadband, relatively low amplitude random excitation
2) Amplitude more less proportional to flow velocity
3) Excitation independent of structural response (i.e. no feedback between structure and fluid)
4) Can be studied with a rigid model as problem is a forced response.
5) Cannot be completely eliminated, so must be managed with e.g. extra damping.
What are the characteristics of “fluid stiffness” in the context of flow induced vibration?
The fluid force that is in phase with the structure displacement is called the fluid stiffness.
No structural motion (vibration) is required, just deformation.
Can be measured with a rigid, statically displaced model
In the context of flow induced vibration, what is meant by dynamic stability.
A system is dynamically stable if any small perturbation from static equilibrium (e.g. from turbulent buffeting) results in an oscillatory response with decreasing amplitude. There may be negative fluid stiffness, the structural damping is higher and so the net system damping is positive. See lecture slides for diagram of response.
What are the characteristics of fluidelastic instability (or aero- or hydro-elastic instability) as a flow-induced vibration mechanism?
1) Structure exhibits “self excited” response. The fluid force does not exist unless there’s structural displacement (stiffness controlled) and/or structural motion (damping controlled)
2) Strong feedback path (i.e. strong coupling) means the fluid and strutural system are a single couple system
3) Models, simulation and wind tunnel testing are more complicated because as well as the fluid parameters, structural parameters like damping ratio and natural frequency have to be matched for dynamic similitude.
4) Prediction of onset of worrisome vibration usually requires semi-empirical model which may be linearized
5) Limited by non-linear behaviour (structure and/or fluid) or total failure.
In the context of flow induced vibration, what is meant by static divergence or static instability.
Static divergence occrs when any small pertubation froms static equilibirum (e.g. from turbulent buffeting) gives rise to a structural deflection that increases monotonically as time goes on. This is caused by a net negative stiffness (i.e. the positive structural stiffness is less than the negative fluid stiffness). An example of static instability is torsional divergence of aircraft wings (e.g. Gruman X29). See lecture slides for diagram of response.
What are the characteristics of “fluid damping” in the context of flow induced vibration?
The fluid force that is out of phase with the structure displacement is called the fluid damping.
Time delays (e.g. due to vorticity transport) in flow or changes in relative velocity due to structural motion can provide negative damping leading to dynamic instability.
What are the characteristics of periodic vortex shedding as a flow-induced vibration mechanism?
1) Narrow band, nearly sinusoidal excitation, with frequency proprotional to flow velocity (i.s. Strouhal number)
2) Only really an issue is vortex shedding frequency is close to natural frequency of structure.
3) weak feedback causes the vortex shedding frequency to “lock-on” to the natural frequency near conincidence
4) Response is still basically a forced response.
5) Useful information can be gathered from a rigid model - structural motion not necessary to assess susceptibility
In the context of flow induced vibration, what is meant by dynamic instability.
Dynamic instability is when any small perturbation from static equilibrium (e.g. from turbulent buffeting) gives rises to a vibration with increasing amplitude. This occurs due to a negative net damping in at least one mode of vibration. In a single degree of freedom system this can happen with galloping or classical flutter. See lecture slides for diagram of response.
