# Role of the shear layer instability in the near wake behavior of two side-by-side circular cylinders

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## Abstract

Wakes, and their interaction behind two parallel cylinders lying in a plane perpendicular to the flow, have been investigated experimentally in the sub-critical Reynolds number regime. The experiments were performed in a water channel using laser Doppler velocimetry. The gap between the two cylinders was less than the cylinder diameter, a geometry referred to as strong interaction configuration. In this case the blockage is strong and a gap-jet appears between the cylinders. Two flow regimes of the near wake region have been identified: one below a critical Reynolds number* Re* _{c} ∈]1000;1700[, where the gap jet is stably deflected to one side and the double near-wake becomes asymmetric; the other, above* Re* _{c}, where the gap-jet deflection is unstable and a random flopping phenomenon takes place. When* Re*<*Re* _{c}, two different Strouhal numbers are identified, related to the Kármán vortex shedding behind each cylinder. When* Re*>*Re* _{c}, a third frequency appears in the near wake, related to the development of Kelvin-Helmholtz vortices in the separated shear layer of the cylinders [Prasad A, Williamson CHK (1997) J Fluid Mech 333:375]. The observed flopping behavior is attributed to the birth of these Kelvin-Helmholtz instabilities and their intermittent nature. Further downstream, beyond about five cylinder diameters, the random flopping flow phenomena disappear while a slightly asymmetric single wake persists. It is characterized by a Strouhal number* St*=0.13, a value that one would normally measure behind a single cylinder of twice its diameter.

## Keywords

Vortex Reynolds Number Shear Layer Strouhal Number Laser Doppler Velocimetry## Notes

### Acknowledgements

The authors wish to thank the Comissariat à l’Energie Atomique for providing the experimental support for the study. The first author also thanks Jan Dusek for interesting discussions and advices.

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