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Turbulent entrainment into a cylinder wake from a turbulent background

Published online by Cambridge University Press:  04 November 2020

Krishna S. Kankanwadi Open the ORCID record for Krishna S. Kankanwadi [Opens in a new window]  and
Oliver R. H. Buxton Open the ORCID record for Oliver R. H. Buxton [Opens in a new window]
Krishna S. Kankanwadi*
Affiliation:
Department of Aeronautics, Imperial College London, South Kensington Campus, LondonSW7 2AZ, UK
Oliver R. H. Buxton
Affiliation:
Department of Aeronautics, Imperial College London, South Kensington Campus, LondonSW7 2AZ, UK
*
†Email address for correspondence: krishna.kankanwadi12@imperial.ac.uk
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Abstract

The effects of background turbulence on the entrainment process, as well as the nature of the interfacial region between two bodies of turbulent fluid, were examined through an investigation of the far wake of a circular cylinder that is subjected to free-stream turbulence. Simultaneous particle image velocimetry and planar laser induced fluorescence measurements were conducted 40 diameters downstream of the cylinder. Despite the availability of turbulent, rotational fluid in the background, the outer interface between the wake and the ambient fluid exhibits an enstrophy jump akin to the classical result of a turbulent/non-turbulent interface. This jump at the wake boundary persists even when the intensity of the background turbulence is greater than the turbulence intensity of the wake itself. Analysis on the structure of the wake boundary reveals that an increase in background turbulence intensity results in an increased interfacial surface area relative to the non-turbulent case. However, instead of the intuitive result of increased entrainment as a result of increased surface area, a reduction in mean entrainment mass flux is observed with increased background turbulence intensity. Through the analysis of the flux probability density functions, the reduction in mean entrainment can be attributed to a tip in balance of extreme entrainment and detrainment events to the detrainment side in the presence of background turbulence. Lastly, a scale by scale analysis of entrainment behaviour revealed that free-stream turbulence affects entrainment behaviour across all length scales and is not just limited to the energy containing scales.

JFM classification

Mixing: Turbulent mixing Wakes/Jets: Wakes
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 905 , 25 December 2020 , A35
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Copyright
© The Author(s), 2020. Published by Cambridge University Press

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References

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Supplementary material: File

Kankanwadi and Buxton supplementary movie 1

Video of run 1a (no-grid case) highlighting entrainment behaviour.

Download Kankanwadi and Buxton supplementary movie 1(File)
File 9.7 MB
Supplementary material: File

Kankanwadi and Buxton supplementary movie 2

Video of run 1b highlighting entrainment behaviour.

Download Kankanwadi and Buxton supplementary movie 2(File)
File 9.4 MB
Supplementary material: File

Kankanwadi and Buxton supplementary movie 3

Video of run 2a highlighting entrainment behaviour.

Download Kankanwadi and Buxton supplementary movie 3(File)
File 8.5 MB
Supplementary material: File

Kankanwadi and Buxton supplementary movie 4

Video of run 2b highlighting entrainment behaviour.

Download Kankanwadi and Buxton supplementary movie 4(File)
File 9.7 MB
Supplementary material: File

Kankanwadi and Buxton supplementary movie 5

Video of run 2c highlighting entrainment behaviour.

Download Kankanwadi and Buxton supplementary movie 5(File)
File 9.3 MB
Supplementary material: File

Kankanwadi and Buxton supplementary movie 6

Video of run 2d highlighting entrainment behaviour.

Download Kankanwadi and Buxton supplementary movie 6(File)
File 7.7 MB
Supplementary material: File

Kankanwadi and Buxton supplementary movie 7

Video of run 2e highlighting entrainment behaviour.

Download Kankanwadi and Buxton supplementary movie 7(File)
File 8.2 MB
Supplementary material: File

Kankanwadi and Buxton supplementary movie 8

Video of run 3a highlighting entrainment behaviour.

Download Kankanwadi and Buxton supplementary movie 8(File)
File 9 MB
Supplementary material: File

Kankanwadi and Buxton supplementary movie 9

Video of run 3b highlighting entrainment behaviour.

Download Kankanwadi and Buxton supplementary movie 9(File)
File 8.3 MB
Supplementary material: File

Kankanwadi and Buxton supplementary movie 10

Video of run 3c highlighting entrainment behaviour.

Download Kankanwadi and Buxton supplementary movie 10(File)
File 8.1 MB