Abstract
When two open-channel flows merge in a three-branch subcritical junction, a mixing layer appears at the interface between the two inflows. If the width of the downstream channel is equal to the width of each inlet channel, this mixing layer is accelerated and is curved due to the junction geometry. The present work is dedicated to simplified geometries, considering a flat bed and a \(90^{\circ }\) angle where two configurations with different momentum ratios are tested. Due to the complex flow pattern in the junction, the so-called Serret–Frenet frame-axis based on the local direction of the velocity must be employed to characterize the flow pattern and the mixing layer as Cartesian and cylindrical frame-axes are not adapted. The analysis reveals that the centerline of the mixing layer, defined as the location of maximum Reynolds stress and velocity gradient, fairly fits the streamline separating at the upstream corner, even though a slight shift of the mixing layer towards the center of curvature is observed. The shape of the mixing layer appears to be strongly affected by the streamwise acceleration and the complex lateral confinement due to the side walls and the corners of the junction, leading to a streamwise increase of the mean velocity along the centerline and a decrease of the velocity difference. This results in a specific streamwise evolution of the mixing layer width, which reaches a plateau in the downstream region of the junction. Finally, the evaluation of the terms in the Reynolds-Averaged-Navier–Stokes equations reveals that the streamwise and normal acceleration and the pressure gradient remain dominant, which is typical of accelerated and rotational flows.
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Acknowledgments
The research was funded by the INSA-Lyon BQR Program, the French INSU EC2CO-Cytrix 2011 project No 231 and the French ANR-11-ECOT-007 project Mentor. The authors are very thankful to J.-N. Gence for his numerous scientific advices.
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Mignot, E., Vinkovic, I., Doppler, D. et al. Mixing layer in open-channel junction flows. Environ Fluid Mech 14, 1027–1041 (2014). https://doi.org/10.1007/s10652-013-9310-7
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DOI: https://doi.org/10.1007/s10652-013-9310-7