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On flowing soap films as experimental models of 2D Navier–Stokes flows

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Abstract

We show the validity of using a flowing soap film system as a two-dimensional laboratory model of flow past a circular cylinder at low Reynolds numbers through a novel combination of qualitative wake visualizations and quantitative velocity measurements and through a new quantitative method for determining the relative film thickness. We verify the correlation between interference fringe patterns and underlying flow structures by simultaneously obtaining digital particle image velocimetry (DPIV) data and interferograms. We introduce a quantitative soap film thickness measurement method using background-oriented schlieren (BOS) to provide a spatially resolved (relative) thickness field. Vortex cores in the cylinder wake appear as low thickness zones, and the minimum thickness regions identified with BOS are shown to coincide with the vortex centers identified in phase-matched interferograms. The vortex positions and circulations in the soap film correlate well with those reported in the literature for the case of low-Reynolds-number flow past a circular cylinder.

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Notes

  1. A number of other definitions of vortex formation length have been introduced; see, e.g., Yang, Masroor & Stremler (in preparation).

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Acknowledgements

The authors thank Emad Masroor for his assistance in reviewing the historical works on the wake of cylinders.

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Javad Eshraghi was involved in methodology, software, formal analysis, data curation, investigation, visualization, and writing—original draft. Lalit K. Rajendran was involved in methodology, software, investigation, and writing—original draft. Wenchao Yang was involved in methodology and writing—review and editing. Mark A. Stremler was involved in conceptualization, methodology, investigation, supervision, and writing—original draft, review, and editing. Pavlos P. Vlachos was involved in conceptualization, methodology, investigation, supervision, project administration, funding acquisition, and writing—original draft, review, and editing.

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Correspondence to Pavlos P. Vlachos.

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Eshraghi, J., Rajendran, L.K., Yang, W. et al. On flowing soap films as experimental models of 2D Navier–Stokes flows. Exp Fluids 62, 162 (2021). https://doi.org/10.1007/s00348-021-03238-z

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