Soap Film Patterns Versus Flow Structures: Do they match?

Flowing soap films are widely used to visualize two-dimensional hydrodynamics. In that regard, interference fringe visualization is one of the most common methods. In this method, soap film fringes appear based on the thickness of each grid point in the flow field, while the thickness itself is a function of the velocity at that point. Contrary to this qualitative visualization, Particle Image Velocimetry (PIV) is applied to obtain a quantitative description of the velocity field and subsequently the flow structures. However, correlation between fringe patterns and underlying flow structures, which is crucial in understanding of complex wakes, has remained unexplored. Here, the challenge is a simultaneous comparison between the identified flow structures and visualized soap film patterns. In this study, we conduct a well-controlled experiment and characterize the flow field behind a stationary cylinder by applying simultaneous PIV measurement and interference fringe visualization in a vertical flowing soap film. This helps us obtain the flow structures from PIV and compare them with visualized fringe patterns.