An experimental investigation of flow-induced vibrations of gates with multiple degrees-of-freedom will be presented: An underflown vertical gate plate was allowed to oscillate both in the cross-flow (z-) and in the streamwise (x-) direction. The two purposes of the investigation were to further the insight into the hydrodynamic coupling mechanisms of the two vibration modes and to determine the interaction of the unsteady lift and drag forces. Self-excited vibration tests were run with reduced velocities Vrz and Vrx from 0.8 to 14 covering a range in which the instability-induced excitation (IIE) due to impinging-leading-edge-vortices (ILEV) as well as the transition to galloping (MIE) occurred. The ratio of the natural frequencies of the two vibration modes fx0/fz0, the gate opening ratio s/d, and the submergence of the gate plate were varied. Depending on the ranges of reduced velocities and frequency ratios, a complex interaction of two different kind of instability induced excitation was detected. Furthermore, it was found that streamwise IIE-excitation and cross-flow galloping coexist simultaneously. To assess the relevant fluid dynamic amplification and attenuation mechanisms, simultaneous body response and flow velocity measurements were carried out.

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