Concise - Self Test 6

Question 1

What are the characteristics of turbulent buffeting as a flow-induced vibration mechanism?

Answer 1

• Broadband, low amplitude random excitation
• Amplitude proportional to flow velocity
• Excitation independent of structural response (no fluid-structure feedback)
• Can be studied with rigid models (forced response)
• Cannot be fully eliminated; managed with extra damping

Question 2

What are the characteristics of "fluid stiffness" in the context of flow induced vibration?

Answer 2

• Fluid stiffness: fluid force in phase with structure displacement
• Requires only structural deformation, no vibration
• Measurable with rigid, statically displaced models

Question 3

In the context of flow induced vibration, what is meant by dynamic stability.

Answer 3

• Dynamic stability: small perturbations lead to oscillations with decreasing amplitude
• May involve negative fluid stiffness and higher structural damping
• Net system damping remains positive

Question 4

What are the characteristics of fluidelastic instability (or aero- or hydro-elastic instability) as a flow-induced vibration mechanism?

Answer 4

• Self-excited structural response; fluid force depends on structural displacement/motion
• Strong feedback path; fluid and structural systems are coupled
• Complex models and testing; must match fluid and structural parameters (damping ratio, natural frequency)
• Onset prediction requires semi-empirical or linearized models
• Limited by non-linear behavior or potential for total failure

Question 5

In the context of flow induced vibration, what is meant by static divergence or static instability.

Answer 5

• Static divergence: small perturbations cause structural deflection to increase monotonically over time
• Caused by net negative stiffness (positive structural stiffness < negative fluid stiffness)
• Example: torsional divergence of aircraft wings (e.g., Gruman X29)

Question 6

What are the characteristics of "fluid damping" in the context of flow induced vibration?

Answer 6

• Fluid damping: fluid force out of phase with structure displacement
• Time delays (vorticity transport) or relative velocity changes can cause negative damping
• Negative damping can lead to dynamic instability

Question 7

What are the characteristics of periodic vortex shedding as a flow-induced vibration mechanism?

Answer 7

• Narrow band, nearly sinusoidal excitation; frequency proportional to flow velocity (Strouhal number)
• Problematic when vortex shedding frequency matches structure’s natural frequency
• Weak feedback causes vortex shedding to lock-on to natural frequency near coincidence
• Response remains a forced response
• Susceptibility can be assessed using rigid models; structural motion not required

Question 8

In the context of flow induced vibration, what is meant by dynamic instability.

Answer 8

• Dynamic instability: small perturbations lead to vibrations with increasing amplitude
• Caused by negative net damping in at least one vibration mode
• Examples: galloping, classical flutter in single degree of freedom systems