An Investigation into Hydrodynamic Effects on Vortex Drop Structure liners using Fluid-Structure Interaction Techniques

Drop structures are designed to safely transfer water to lower elevations through hydraulic energy dissipation. However, structural deterioration due to fluid-material interaction is a significant and recurring issue within hydraulic ‘drop’ structures. Despite this, there is a lack of information available on the response of the structure to debris laden fluids, possibly due to the complex processes involved.

This study analyses the dynamics of flow and debris impact within a classic tangential vortex drop shaft structure. A scaled physical model of the vortex drop shaft is investigated to observe flow streamline characteristics, pressures and monitor debris impact behaviour at various flow conditions. Results from a validated computational fluid dynamics model on the structure are transferred to a finite element analysis to identify the critical pressure and stress points within the structure. Three materials (Stainless Steel, Concrete and Glass Reinforces Plastic (GRP)) are evaluated for their effectiveness by analysing surface wear, corrosion and fatigue. The study proposes a new parameter - the “critical spiral height” - which denotes the position in the drop where centrifugal forces begin to diminish their effects on material surface. Observation of the critical spiral height below the inlet is used to determine the potential to realise significant cost savings by improving the material selection process.

The study comprises two phases; (i) scaled physical modelling and (ii) three-dimensional numerical modelling. The physical modelling stage is undertaken on a 1/9.5 scaled model of a tangential type vortex drop shaft (the model is scaled using Froude similitude). Flow streamline characteristics and debris impact behaviour are examined using imaging techniques. The second phase involves numerical modelling by transferring force and stress information from a computational fluid dynamics study into a finite element analysis model of the drop shaft using ANSYS for both fluid and structural models. Through this, critical pressures and stress points within the structure are determined for three different material types. Material selection is based on the materials resistance to surface wear due to scour and debris impact resistance under the predicted fluid behaviour.

Key findings:

• A relationship between the streamline angle and length along the drop shaft.
• Position and magnitude of likely debris impacts.
• Critical pressure and stress points and their effects on material selection.
• Identification of a new parameter “critical spiral height” which denotes the position where material changes can be implemented to reduce costs.
• Analysis of potential uncertainty introduced as a result of scale effects.