Numerical Simulation of Flow-induced Vibration on Gates with Underflow

Flow-induced vibrations are an important issue in the design and operation of weir gates. Various mechanisms of vibration excitation are known. For underflow weir gates, the so-called press-shut mechanism is known to be a possible source of excitation. This mechanism works due to the inertia of flow through small gaps with a flow control point on the upstream end of the gap. Hereby, the movement of the gate affects the current in a way that a force is produced that acts in the direction of the gates movement. This self-excited process can lead to severe gate vibrations. Due to the coupled nature of self-excitation, it is insufficient to compare the distinct flow-induced forces and the natural frequency of the gate in a static model. A coupled approach is required in which both the flow and the gate movement are represented in a fully transient manner. In this paper a numerical approach will be presented, which combines transient flow-simulation and solid body dynamics.

The interDyMFoam solver of the open-source CFD-software package OpenFOAM provides an efficient tool for coupling solid body motion with free-surface flow. A radial gate with a box girder known for its tendency to vibrate when the gate opening is small was modelled. A deformable mesh was used to represent the movement of the solid body. The small openings that are typically prone for a press-shut effect require small mesh sizes. The deformation of these small cells affects the stability of the simulation which is counteracted with multiple iterations per time step.

Simulations were carried out in order to estimate the energy transfer from the flow field to the gate for various operational conditions. The magnitude of the transferred energy per vibration period was used to identify conditions with a significant risk of self-excitation. The results will be presented and the relevance in terms of practical gate operation will be discussed.