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

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.