Files and links (1)
Free to read and download, Open Access
Hydropower is the most important renewable energy source on the planet. Though it provides abundant benefits to society, it also has environmental and ecological consequences. Dam construction significantly alters natural flow conditions. Fish numbers decline and other aquatic life may be adversely affected, especially during migration and reproduction cycles, due to degradation of their natural habitat. High summer water temperatures in hydropower reservoirs and elevated total dissolved gas (TDG) concentrations in downstream tailrace regions can increase mortality rates of fish passing through the dam.
This study proposes to develop a numerical model to improve the prediction of hydrodynamics and water-quality parameters in hydropower flows. The main focus is to simulate temperature dynamics and TDG distribution in the McNary Dam forebay and tailrace. Existing numerical temperature and TDG models, developed by Politano et al. (2008, 2009c), were improved and implemented into the open-source CFD code OpenFOAM. These newly developed models can be used to evaluate the efficiency of operational changes or structural modifications to reduce the negative environmental impacts of hydropower facilities.
The forebay temperature model was based on the incompressible Reynolds-Averaged Navier-Stokes (RANS) equations with the Boussinesq approximation. Turbulence was modeled with an improved realizable model taking into account wind turbulence generation at the free surface. A thermal model incorporating solar radiation and convective heat transfer at the free surface was employed. The model was validated against field data collected on August 18th, 2004 at McNary Dam. Observed vertical and lateral temperature distributions and dynamics in the forebay were captured by the model. The incorporation of the atmospheric heat flux, solar radiation, and wind-induced turbulence improved the temperature predictions near the free surface.
The multi-phase TDG model utilized the Volume of Fluid (VOF) method combined with a Detached Eddy Simulation (DES) approach to calculate hydrodynamics. A one-way coupling approach was used to incorporate a TDG model, which includes the transport and dissolution of bubbles entrained in the spillway and takes into account bubble size change caused by dissolution and compression. The capability of the present model to predict spillway flow regimes was evaluated against observations in a reduced scale laboratory model. Simulation results demonstrated that flow regimes downstream of a spillway can be adequately reproduced by the numerical model. The capability of the model to quantify dissolved gas exchanges and TDG distribution was evaluated using a tailrace sectional model. The model captured TDG production and observed longitudinal TDG reduction under different flow regimes. Disparities between predicted and measured average TDG values fell within 4%. The model developed in this study is an effective predictive numerical tool to identify flow regimes and quantify TDG production under various flow conditions in near dam regions when lateral flows are not important.