Aeroelastic interference effects between slender structures

Slender structures close together may be excited to self-sustained oscillation, if the spacing ratio a/d is low and the wind direction is in a critical range. This motion-induced-excitation is associated with hysteretic flow switching called “interference galloping”, because it is characterized by an onset flow velocity which depends on the natural frequency and the diameter of the structure, the square root of the mass-damping parameter, the square root of the spacing ratio and the interference galloping criterion (−∂cy/∂β)·sin θ. Contracy to classical galloping, the gradient ∂cy/∂β must be positive for instability because the hysteresis lag angle θ normally is negative. Model tests with elastic stack models with different mass damping parameters confirm theoretical considerations and show that the effects of vortex resonance and interference galloping coexist.