Cagney, N;
Balabani, S;
(2015)
Lagrangian coherent structures in the wake of a streamwise cylinder.
In:
Bulletin of the American Physical Society 2016.
American Physical Society
(In press).
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Abstract
Lagrangian analysis is capable of revealing the underlying structure and complex phenomena in unsteady flows. We present Particle-Image Velocimetry measurements of the wake of a cylinder undergoing streamwise vortex-induced vibrations and calculate the Finite-Time Lyapunov Exponents (FTLE) in backward- and forward-time. The FTLE fields are compared to the phase-averaged vorticity fields for the four di↵erent wake modes observed while the cylinder experiences streamwise vortex-induced vibrations. The backward-time FTLE fields characterise the formation of vortices, with the roll up of spiral-shaped ridges coinciding with the roll up of the shear layers to form the vortices. Ridges in the forward-time fields tend to lie perpendicular to the flow direction and separate nearby vortices. The shedding of vortices coincides with a ‘peel o↵’ process in the forward-time FTLE fields, in which a ridge connected to the cylinder splits into two strips, one of which moves downstream. Particular attention is given to the ‘wake breathing’ process, in which the streamwise motion of the cylinder causes both shear layers to roll up simultaneously and two vortices of opposite sign to be shed into the wake. In this case, the ridges in forward-time FTLE fields are shown to define ‘vortex cells’, in which the new vortices form, and the FTLE fields allow the wake to be decomposed into three distinct regions. Finally, the mixing associated with each wake mode is examined, and it is shown that crosswake mixing is significantly enhanced when the vibration amplitude is large and the vortices are shed alternately. However, while the symmetric shedding induces large amplitude vibrations, no increase in mixing is observed relative to the von K´arm´an vortex street observed behind near-stationary bodies.
| Type: | Proceedings paper |
|---|---|
| Title: | Lagrangian coherent structures in the wake of a streamwise cylinder |
| Event: | 68th Annual APS DFD Meeting |
| Location: | Boston, United States |
| Dates: | 22 November 2015 - 24 November 2015 |
| Open access status: | An open access version is available from UCL Discovery |
| Publisher version: | http://www.aps.org/meetings/baps/archives-2016.cfm |
| Language: | English |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Mechanical Engineering |
| URI: | https://discovery.ucl.ac.uk/id/eprint/1522475 |
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