Keywords: rain-wind induced vibrations, self-excited systems, non-linear dynamics, stability, aeroelastic instabilities, subcritical and critical range of flow, mode switching.

Rain-wind induced vibrations are generally described in the range of low wind speeds (5-15m/s) [1,2]. However, further experiments in the wind tunnel [3], show that after these vibrations stop, a new type of rain-wind induced vibrations occurs in the range of higher wind speeds (9-30 m/s). These observations indicate that there are two types of rain-wind induced vibrations. Typical for both vibration types is a critical onset velocity and an upper limit of critical velocity. It is not possible that both vibration types occur at the same time as their wind ranges do not overlap.

Comprehensive experimental investigations of the authors in the wind tunnel in order to determine the equilibrium positions of the rivulets on the cable surfaces in combination with numerical investigations show that the observed vibrations are vibrations in different flow conditions. The vibrations in the range of lower wind speeds correspond to the subcritical flow condition whereas the vibrations in the range of higher wind speeds correspond to the critical flow condition. This connection could be proven by a measurement of the rivulets' positions for the cable configurations specified in [3].

A possible excitation mechanism of rain-wind induced vibrations in the subcritical flow is derived in [4]. This mechanism is based on the phenomenon of the Prandtl tripwire and considers the rivulets as movable disturbances. This is valid for rain-wind induced vibrations in subcritical and critical flow. A mechanical model developed on this basis as well as a model for a numerical calculation of the aerodynamic coefficients is adequate to describe the observed vibrations as well as phenomena like a possible change of the vibration mode from one flow condition to the other or the enlarged unstable ranges of oncoming flow for the critical flow condition.

When mode switching appears, the rivulets move away from the equilibrium position of the subcritical flow to the equilibrium position of the critical flow. This phenomenon effects a fundamental modification of the drag and lift coefficients. Numerical investigations based on the mechanical model confirm the experimental results.

1

Hikami Y., Shiraishi N., "Rain-Wind Induced Vibrations of Cables in Cable Stayed Bridges", Journal of Wind Engineering and Industrial Aerodynamics, 29, 409-418, 1988. doi:10.1016/0167-6105(88)90179-1

2

Matsumoto M., Shiraishi N., Shirato H., "Rain-Wind Induced Vibration of Cables of Cable-Stayed Bridges", Journal of Wind Engineering and Industrial Aerodynamics, 41-44, 2011-2022, 1992. doi:10.1016/0167-6105(92)90628-N

3

Verwiebe C., "Erregermechanismen von Regen-Wind induzierten Schwingungen", Baukonstruktionen unter Windeinwirkung (TU Braunschweig 1997), Aachen: Windtechnologische Gesellschaft e.V. 1998.

4

Seidel C., Dinkler D., "Rain-wind induced vibrations - phenomenology, mechanical modelling and numerical analysis", Computers & Structures, 84, 1584-1595, 2006. doi:10.1016/j.compstruc.2006.01.033

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