Using an eddy‐resolving technique to assess the effects of reservoir operation on the bend flow in Yangtze River, China

Abstract

With increasing computing power over the last decade, eddy‐resolving methods in hydraulics engineering have been widely used to investigate the turbulence structure in natural rivers, where accurate field measurements were difficult to conduct and investigate. A detached eddy simulation (DES) model was developed on the basis of the k‐ω‐SST (shear stress transport) eddy‐viscosity model within the framework of an unstructured‐mesh Semi‐implicit Eulerian–Lagrangian Finite Element (SELFE) hydrodynamic model. The DES model was then applied to simulate the bend flow in the upper Yangtze River, and the calculation efficiency was enhanced through parallelized computation to mitigate the high computation costs involved in the DES modelling. The time‐averaged velocity, turbulent intensity and Reynolds stress were compared, using DES modelling and acoustic Doppler current profiler (ADCP) measuring results, indicating good agreement, especially in the near‐bed region. The coupled relationship between the turbulent flow and riverbed terrain was also investigated through extracting and analysing the high‐order statistical variables reflecting the turbulence structure, including non‐hydrostatic pressure, vorticity and helicity, indicating the fluctuated amplitude of non‐hydrostatic pressure can reach to 100 Pa, and the stripe distribution of non‐hydrostatic pressure and vorticity had an orthogonality relation. At the same time, the streamwise velocity at the middle of the HLM bend was found to be decreased at least 0.7 m/s because of unsteady process smoothing and backwater effects attributable to water releases and storage of the reservoirs located upstream and downstream of the HLM bend. The developed unstructured‐mesh DES model will provide a new tool to study turbulent flows in a natural river and assess the effect of reservoir operations on river flows.