EGU21-13453, updated on 04 Mar 2021
https://doi.org/10.5194/egusphere-egu21-13453
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Vortex shedding by a circular cylinder in lock-exchange density current 

Ana M Ricardo1, Giovanni Di Lollo2, Moisés Brito3, Claudia Adduce2, and Rui M.L. Ferreira4
Ana M Ricardo et al.
  • 1CERIS, Lisboa, Portugal (ana.ricardo@tecnico.ulisboa.pt)
  • 2Department of Engineering, Roma Tre University, Italy
  • 3UNIDEMI, NOVA School of Science and Technology, NOVA University Lisbon, Portugal
  • 4CERIS, Instituto Superior Técnico, Universidade de Lisboa, Portugal

Flow around bluff bodies have been attracting the interest of the research community for more than a century. The physical mechanisms associated with the vortex shedding in the wake of bluff bodies is still of fundamental research interest. However, flow-structure interaction in density currents has not received enough attention. The transient nature of the interaction between the density driven flow and a stationary object constitutes the motivation for the present laboratory study aiming at investigating the vortex generation and fate on the wake of a circular cylinder in a density current.

The experiments were conducted in a horizontal and rectangular cross-section channel with 3.0 m long, 0.175 m wide and 0.4 m deep. The gravity current was generated using the classic lock-exchange configuration. A sliding stainless-steel gate with 1 mm thickness, sealed by PVC board glued in the sidewall, was positioned at 0.3 m from the left hand side of the channel. The experiment starts when the gate is suddenly removed, leaving the dense fluid to flow along the bottom of the channel, while the ambient fluid moves above in the opposite direction. The dense fluid consists in a mixture of fresh water and salt while the ambient fluid is a solution fresh water and ethanol (96%). The amount of salt and alcohol added in each mixture was determined in order to obtain a given density difference and to ensure the same refractive index in both fluids. Two different currents were tested with reduced gravity equal to 0.06 ms-2 and 0.24 ms-2. For each test ten repetitions were carried out. Instantaneous velocity maps were acquired with a Particle Image Velocimetry system at 15 Hz. Polyamide seeding particles of density equal to 1.03 were added in both dense and ambient fluids.

 The Reynolds number varied between 1500 and 4000. The results show that vortex shedding varies as the current reaches and overtakes the cylinder. Boundary layer detachment and shear instability is initiated shortly before the snout reaches the cylinder. A pattern of well-defined symmetrical vortexes is formed as a result of the initial shear instability. As the head of the current engulfs the cylinder, stronger turbulence diffusion contributes to reduce vortex coherence. Vortexes are smaller and detach sooner, while is not clear if shedding is alternate or simply random. The formation length is smaller than that of a steady flow with the same Re. When the back of the current passes, the formation length is increased and vortex shedding becomes periodical again. A striking feature is that the Von Kármán street is frequently symmetrical rather than exhibiting a pattern of alternate vortices.

This research was funded by national funds through Portuguese Foundation for Science and Technology (FCT) project PTDC/CTA-OHR/30561/2017 (WinTherface).

How to cite: Ricardo, A. M., Di Lollo, G., Brito, M., Adduce, C., and Ferreira, R. M. L.: Vortex shedding by a circular cylinder in lock-exchange density current , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13453, https://doi.org/10.5194/egusphere-egu21-13453, 2021.