Meandering jets in shallow rectangular reservoirs: POD analysis and identification of coherent structures

• Published in: Experiments in fluids Vol. 55; no. 6; pp. 1 - 16
• Main Authors: Peltier, Y., Erpicum, S., Archambeau, P., Pirotton, M., Dewals, B.
• Format: Journal Article Web Resource
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01-06-2014; Springer; Springer Science & Business Media B.V
• Subjects: Civil engineering; Coherence; coherent structures; Engineering; Engineering Fluid Dynamics; Engineering Thermodynamics; Engineering, computing & technology; Exact sciences and technology; Far fields; flow topology; Fluctuation; Fluid dynamics; Fluid flow; Fluid- and Aerodynamics; Friction; Fundamental areas of phenomenology (including applications); Heat and Mass Transfer; Ingénierie civile; Ingénierie, informatique & technologie; Instrumentation for fluid dynamics; Jets; meandering jet; Physics; Proper Orthogonal Decomposition; Research Article; Reservoirs; Shallow rectangular reservoir; Trajectories; Turbulent flows, convection, and heat transfer; Spatial Mode; Coherent Structure; Proper Orthogonal Decomposition; Proper Orthogonal Decomposition Mode; Flow Case; Test facilities; Orthogonal function; Periodic oscillation; Eigenfunctions; Jets; Coherent structures; Particle image velocimetry; Experimental study; Velocity measurement; Shallow water; Sudden enlargement; Meander
• ISSN: 0723-4864; 1432-1114; 1432-1114
• DOI: 10.1007/s00348-014-1740-6

The effect of the shallowness on meandering jets in a shallow rectangular reservoir is investigated. Four meandering flows were investigated in an experimental shallow rectangular reservoir. Their boundary conditions were chosen to cover a large range of friction numbers (defined with the sudden expansion width). Due to the unsteady characteristics of the flows, a proper orthogonal decomposition (POD) of the fluctuating part of the surface velocity fields measured using Large-Scale Particle Image Velocity was used for discriminating the flow structures responsible for the meandering of the jet. Less than 1 % of the calculated POD modes significantly contribute to the meandering of the jet, and two types of instability are in competition in such a flow configuration. The sinuous mode is the dominant mode in the flow, and it induces the meandering of the flow, while the varicose mode is a source of local mixing and weakly participates to the flow. The fluctuating velocity fields were then reconstructed using the POD modes corresponding to 80 % of the total mean fluctuating kinetic energy, and the coherent structures were identified using the residual vorticity, their centres being localised using a topology algorithm. The trajectories of the structures centres emphasise that at high friction number the coherent structures are small and laterally paired in the near, middle and far fields of the jet, while with decreasing friction number, the structures merge into large horizontal vortices in the far field of the jet, their trajectories showing more variability in space and time. The analysis of the stability regime finally reveals that the sinuous mode is convectively unstable and may become absolutely unstable at the end of the reservoir when the friction number is small.