Pollak-Reibenwein C., Drobir H.

Flow-induced vibrations are often a concern in the design and operation of trashracks. Trashracks are hydraulic structures, consisting of an array of bars. They are typical inlet devices of river power plants used to prevent large obstacles from hitting the turbines. This paper experimentally investigates the oscillation of trashrack models with two degrees of freedom in the streamwise and the crossflow directions. The rack construction was made up of several rectangular shaped bars with a chord-to-thickness ratio of 10. The mutual interaction of the formation of single vortices, the bar spacing, the angle of incidence and the free-stream turbulence affected the rack vibration characteristics. The hydraulic experiments covered a wide range of different bar spacings and flow incidence angles extending from 5° to 45°. The qualitative response of the trashrack models due to flow-induced excitation was determined by plotting the entire rack motion appropriate to the directions of the main vibration modes. Similar to measurements by Kerenyi (1997), the oscillation of the trashrack bars due to vortex-induced excitation was able to be suppressed by restricting the bar spacing to a value smaller than the length of a single bar. Besides the variation of flow incidence angles and bar spacings, the influence of free-stream turbulence on the excitation of the trashrack models was object of the investigations. In order to prove a possible turbulence effect on the vibration behaviour of the rack, the turbulence intensity of the approaching flow was varied between 3.5% and 5.5%. It will be demonstrated, that with decreasing turbulence level, excitation due to turbulence occurs. The hydraulic model tests were carried out in the water channel of the Laboratory of Hydraulic Engineering at Vienna University of Technology.