Influence of the secondary motions on pollutant mixing in a meandering open channel flow

Abstract

This paper presents large eddy simulation of turbulent flow in a meandering open channel with smooth wall and rectangular cross-section. The Reynolds number based on the channel height is 40,000 and the aspect ratio of the cross-section is 4.48. The depth-averaged mean stream-wise velocity agree well to experimental measurements. In this specific case, two interacting cells are formed that swap from one bend to the other. Transport and mixing of a pollutant is analysed using three different positions of release, e.g. on the inner bank, on the outer bank and on the centre of the cross section. The obtained depth-average mean concentration profiles are reasonably consistent with available experimental data. The role of the secondary motions in the mixing processes is the main focus of the discussion. It is found that the mixing when the scalar is released on the centre of the cross-section is stronger and faster than the mixing of the scalar released on the sides. When the position of release is close to a bank side, the mixing is weaker and a clear concentration of scalar close to the corresponding side-wall can be observed in both cases. Acknowledgements: The simulation was carried out using the supercomputing facilities of the Steinbuch Centre for Computing (SCC) of the Karlsruhe Institute of Technology. The authors would like to thank Clemens Chan-Braun for his valuable and constructive suggestions during the development of this research. MGV acknowledges the financial support of the Spanish Ministry of Education through the program Jose Castillejo.

A numerical study of the complex flow structure in a compound meandering channel

“…The subgrid-scale (SGS) stresses, resulting from the unresolved motions, are modelled using the Smagorinsky subgrid-scale model (Smagorinsky, 1963) with a model constant Cs of 0.1. Such an approach has been used successfully for similar flows by Hinterberger et al (2007) and Moncho-Esteve et al (2017). For flows without homogeneous directions, as the present one, the Smagorinsky model is more robust than other models like the dynamic Smagorinsky which require some kind of smoothing of the model parameter (like time relaxation).…”

“…With respect to modelling, in the literature there are several Reynolds-averaged Navier-Stokes (RANS) studies of flow in compound meandering channels (Morvan et al, 2002;De Marchis and Napoli, 2008;Shukla and Shiono, 2008;Jing et al, 2009). However, Van Balen et al (2010), Constantinescu et al (2013) and Moncho-Esteve et al (2017) have shown that, generally, Large Eddy Simulation (LES) provides better results than RANS in curved open-channel turbulent flows. An additional advantage is that using LES it is possible to obtain further physical insights from the computed data.…”

Procesos de Mezcla en Flujos Turbulentos con Técnicas de la Mecánica de Fluidos Computacional (CFD)

“…The subgrid-scale (SGS) stresses, resulting from the unresolved motions, are modelled using the Smagorinsky subgrid-scale model (Smagorinsky, 1963) with a model constant C s of 0.1. Such an approach has been used successfully for similar flows by Hinterberger et al (2007) and Moncho-Esteve et al (2017). For flows without homogeneous directions, as the present one, the Smagorinsky model is more robust than other models like the dynamic Smagorinsky which require some kind of smoothing of the model parameter (like time relaxation).…”

“…Capítulo 5. A Numerical Study of the Complex Flow Structure in a Compound Meandering Channel secondary motions and its influence on the streamwise velocity distribution as well as on mixing are summarised in Blanckaert and Graf (2004), Blanckaert (2009) and Moncho-Esteve et al (2017). Moreover, river flows in a compound channel often inundate the adjacent plains, which generate more complicated flow structures between the in-bank and the floodplain flows.…”

“…Due to the effort to handle and regulate rivers, e.g. river restoration, navigation, water quality, production of energy, it is of great importance to engineers in understanding the various governing mechanisms, including secondary motions within the meander channel(Moncho-Esteve et al, 2017).…”