Analysis of the role of turbulence in curved open-channel flow at different water depths by means of experiments, LES and RANS

Abstract

In order to unravel the main flow and secondary flow characteristics and the role of turbulence in a curved single-bend open-channel flow, large-eddy simulations (LES) and Reynolds-averaged numerical simulations (RANS) were carried out and compared with experiments of the flow through a strongly bent laboratory flume. Turbulence was found to play an important role with respect to processes that are important in natural rivers. The strength of the curvature-induced secondary flow in the core of the flow domain, which is the most typical feature of curved open-channel flow, depends on the turbulence. Turbulence is especially important in the flow regions near the banks. Only the LES model is able to resolve accurately the boundary layer detachment and the formation of an internal shear layer at the inner bank as well as the outer-bank cell of secondary flow, whereas the RANS model is unable to reproduce these processes. Turbulence also conditions the magnitude of the bed shear stress, as indicated by the considerable overestimations of the bed shear stress by the RANS model as compared with the LES model. As a result, only LES properly reproduces the experimentally measured overall friction losses over the bend, whereas RANS overestimates these losses. The large scales of turbulence play an essential role in generating these flow processes by means of their interaction with the mean secondary flow structures, whereas small-scale turbulence is merely dissipative and does not play a dynamic role with respect to the time-averaged flow pattern. This implies that the simulated flow field is rather insensitive to the sub-grid model in the LES computation.

Keywords:

• large-eddy simulation (LES)
• Reynolds-averaged numerical simulation (RANS)
• river flow
• secondary flow
• open-channel flow
• curvature
• flow separation
• outer-bank cell

Acknowledgement

The numerical work is supported by the Dutch Technology Foundation (STW), Applied Science Division of NWO and the Technology Programme of the Ministry of Economic Affairs. The experimental work was supported by the Swiss National Science Foundation under grants 2100-052257.97 and 2000-059392.99.