The effect of the shallowness on meandering jets in a shallow rectangular reservoir isinvestigated. Four meandering flows were investigated in an experimental shallowrectangular reservoir. Their boundary conditions were chosen to cover a large range offriction numbers (defined with the sudden expansion width). Due to the unsteadycharacteristics of the flows, a Proper Orthogonal Decomposition of the fluctu-ating partof the surface velocity fields measured using LSPIV was used for discriminating theflow struc-tures responsible for the meandering of the jet. Less than 1 % of thecalculated POD modes significantly contribute to the meandering of the jet and twotypes of instability are in competition in such a flow con-figuration. The sinuous mode isthe dominant mode in the flow and it induces the meandering of the flow, while thevaricose mode is a source of local mixing and weakly participates to the flow. Thefluctuating velocity fields were then reconstructed using the POD modes correspondingto 80% of the total mean fluctuating kinetic energy and the coherent structures wereidentified using the residual vorticity, their centres being localised using a topologyalgorithm. The trajectories of the structures centres emphasize that at high frictionnumber the coherent structures are small and laterally paired in the near, middle andfar fields of the jet, while with decreasing friction number the structures merge intolarge horizontal vortices in the far-field of the jet, their trajectories showing morevariability in space and time. The analysis of the stability regime finally reveals that thesinuous mode is convectively unstable and may become absolutely unstable at the endof the reservoir when the friction number is small.
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