On the existence and evolution of a spanwise vortex in laminar shallow water dipoles

• Published in: Physics of fluids (1994) 2011-08, Vol.23 (8), p.086601-086601-11
• Main Authors: Albagnac, J., Lacaze, L., Brancher, P., Eiff, O.
• Format: Article
• Language: English
• Subjects: Earth, ocean, space; Engineering Sciences; Exact sciences and technology; External geophysics; Fluids mechanics; Geophysics. Techniques, methods, instrumentation and models; Mechanics
• ISSN: 1070-6631; 1089-7666
• DOI: 10.1063/1.3611419

The present work investigates the existence and evolution of a spanwise vortex at the front of shallow dipolar vortices. The vortex dipoles are experimentally generated using a double flap apparatus. Particle image velocimetry measurements are performed in a horizontal plane and in the vertical symmetry plane of the flow. The dynamics of such vortical structures is investigated through a parametric study in which both the Reynolds number Re = U 0 D 0 / ν ∈ [ 90 , 470 ] and the aspect ratio α = h / D 0 ∈ [ 0 . 075 , 0 . 7 ] , associated with the shallowness of the flow, are varied, where U 0 is the initial velocity of the vortex dipole, D 0 is the initial diameter, h is the water depth, and ν is the kinematic viscosity of the fluid. The present experiments confirm the numerical results obtained in a companion paper by Duran-Matute [Phys. Fluids 22 , 116606 (2010)], namely that the flow remains quasi parallel with negligible vertical motions below a critical value of the parameter α 2 Re . By contrast, for large values of α 2 Re and α ≲ 0 . 6 , a three-dimensional regime is observed in the shape of an intense spanwise vortex generated at the front of the dipole. The present study reveals that the early-time motion and dynamics of the spanwise vortex do not scale on the unique parameter α 2 Re but is strongly influenced by both the aspect ratio and the Reynolds number. A mechanism for the generation of the spanwise vortex is proposed. For α ≳ 0 . 6 , a third regime is observed, where the spanwise vortex is replaced by a vorticity tongue.