Exploring very-large-scale motions (VLSMs) in open channel flows (OCFs) is crucial for comprehensively understanding material transport and energy exchange. While previous studies focused on OCFs in ideal flumes with simple boundaries, this paper presents large eddy simulation (LES) results on the existence and scale characteristics of VLSMs in complex river morphologies with the Minjiang River in southwestern China as a case study. This research demonstrates that the OpenFOAM-based LES model is capable of accurately reconstructing the time-averaged flow field and providing instantaneous velocity data that capture VLSMs with sufficient resolution. Spectrum analysis of the streamwise fluctuating velocity shows that VLSMs are present in the straight segment of the river, with streamwise wavelengths approximately (16–22) times the water depth but are absent in the bend and confluence segments due to the suppression by secondary currents. This article validates the effectiveness of LES in examining the characteristics of VLSMs in natural rivers, thereby laying a good foundation for further studies on the impacts of such structures on sediment transport and pollutant dispersion.

The process of pollutant mixing is significantly influenced by secondary flow and turbulence in meandering rivers. To investigate the influence of different point source release positions on the pollutant mixing process in sinuous open channel flows, a 3D large-eddy simulation (LES) model based on OpenFOAM was established to simulate the process of passive scalar transport in a sinuous channel with a rectangular cross-section. After verification by a flume experiment, two sets of cases in which the point sources were arranged at identical intervals in spanwise and streamwise directions were configured to evaluate the mixing efficiency. The effect of flow structure, secondary motion, and the turbulent viscosity on the scalar transport and mixing was discussed. The distribution of scalar as well as the scalar flux was analyzed in detail, and the fluctuation characteristics were also described. The results demonstrate that due to the existence of secondary flow in the sinuous channel, different transverse and streamwise release positions of the point source have significant influence on mixing efficiency and spatial distribution of the pollutant. The point source placed near the center of the cross-section in transverse or near the apex of the bend in streamwise result in higher mixing efficiency. Mixing efficiency calculated by different indices can be different, which requires comprehensive assessment.