Development of a wake and backwater prediction approach for hydrokinetic turbines

Abstract

Hydrokinetic turbine deployment in inland water reticulation systems holds untapped potential for future development in renewable energy. However, prior to implementation, it is crucial to understand the hydrodynamic effects associated with these devices. In particular, the flow fields effects prevalent in bounded subcritical flow regimes such as wake propagation and possible backwater effects. While a few analytical approximations for wake determination have been developed, most of them do not account for operational conditions in confined flow. Moreover, there is a lack of usable approaches for backwater determination in the existing literature. This limitation complicates the design and deployment process, leading to problematic installations and issues with regulatory procedures due to the numerous unknowns surrounding turbine deployment. This study focuses on developing a new semi-empirical model for the prediction of the wake generation and flow recovery which includes a study on metrics found to affect wake generation. Once the flow behaviour is well understood a generic and simplified method for calculating the backwater effect of HK turbines is tested. In this dissertation, data obtained from experimentally validated computational fluid dynamics (CFD) simulations provides a basis for the new simplified wake and backwater prediction approach. Among the available commercial software capabilities, Reynolds-averaged Navier-Stokes (RANS) models showed a strong correlation with turbine performance. A virtual disk model utilising the blade element momentum theory and employing Reynolds’s stress closure models was found to give the best representation of the wake and surrounding flow behaviour. The developed semi-empirical wake model performed well across various performance conditions (linked to the specific turbine thrust), ambient turbulence conditions, and blockage ratios. This model facilitates a reasonably accurate estimation of wake behaviour, enabling effective planning of turbine placement and spatial requirements for inland hydrokinetic schemes. The analytical backwater model developed in this study also demonstrated good correlation with experimental results. Its energy-based approach offers a simplified tool that can be easily incorporated into backwater approximations, also allowing for the inclusion of retaining structures as additional blockages. All models utilise only the flow characteristics and the turbine thrust coefficient, making them valuable tools for the initial analysis of wake and backwater effects resulting from the deployment of inland turbine system