An experimental investigation of the approach flow conditions for a non-rotating, very low head water-turbine model

• Coupling of turbine angles and spacings affects vortical structures.
• Spanwise vortical structures subsided at larger turbine spacings.
• Rotating-turbine model shares similar trends with non-rotating turbine model.
• Non-rotating turbine models useful for first-order modeling and optimization.

The adverse in-flow conditions for a very low head (VLH) water turbine have been investigated using a non-rotating turbine model. The aim of the study was to develop a better understanding of the three-dimensional flow generated upstream of the turbine by a step-like obstacle for varying turbine angles and spacings. Through flow-rate measurements, velocity-profile measurements and dye visualizations, the coupled effects between the turbine model and the varied in-flow geometry could be quantified. It was observed that the flow generated by the backward-facing step produced vortical structures that were found to corkscrew from the lower channel walls towards the mid-plane of the flow. These spanwise vortical structures were present through the interaction of the turbine model with the backward-facing step geometry for shorter cavity lengths. The coupled effects between model and step were found to be most pronounced for small turbine spacings on the order of one turbine diameter or less. Also significant velocity variations in the spanwise direction were observed in the cavity-like geometry. Finally, some preliminary results obtained from a rotating model are provided for comparison with the non-rotating model. Similar trends in terms of optimal spacing were observed, suggesting that a non-rotating model can be used for the preliminary experimental optimization of such three-dimensional cavity-like flows.