Wakes of Three Cylinders in a Falling Soap Film: Linking Lagrangian Coherent Structures from PIV to Interferograms

The falling soap film tunnel offers an inexpensive and convenient experimental solution for studying cross-sectional flows at low Reynolds numbers. Interferometry and Particle Image Velocimetry (PIV) are the primary measurement techniques for analyzing such flows. Interferometry visualizes vortical patterns through the variation in the film thickness, while PIV enables both flow visualization and quantification using particles seeded in the soap film. Existing works compare PIV measurements with interferometry either visually from the Eulerian properties like velocity and vorticity or by quantitatively extracting local vortical features such as the vortex center from these fields. This however does not compare how a vortex stretches downstream and how the surrounding fluid gets entrained. Thus, we aim to establish a connection between the instantaneous Lagrangian Coherent Structures (LCS) from the PIV measurements and interferograms. A flow of Re = 280 across nine configurations of three cylinders forming a triangle is considered, by varying height and base w.r.t. cylinder size. The LCS is calculated from the Finite Time Lyapunov Exponents (FTLE). Noise effects in the PIV results can alter the particle trajectory and corrupt the FTLE field, particularly in regions of slow flow. To address this, we explore a noise reduction technique based on principal orthogonal decomposition. This approach aims to enhance the quality of the FTLE field, thereby enabling a more accurate comparison.