Wake Structure Analysis via Modal Decomposition in Undulated Cylinder Arrays

An experimental investigation is conducted on undulated cylinder arrays inspired by seal whisker (phocid pinniped vibrissae) geometry, to compare the structure of their downstream wakes. The experiments are performed in the Portland State University wind tunnel, which features a test section measuring 5 m in length, 1.2 m in width, and 0.8 m in height. The undulated cylinders are 3D-printed and scaled to a mean chord length, C, of 33 mm, a mean thickness of 17.53 mm, and an axial length of 720 mm. The arrays consist of nine cylinders arranged in a 3×3 grid, oriented vertically with a spacing of 3C in both the streamwise and spanwise directions. Particle image velocimetry (PIV) measurements are taken directly downstream of the array using a 6C×6C PIV window (streamwise × spanwise) with an inflow velocity of 3.7 m/s, corresponding to a Reynolds number based on C, of 2400. Proper Orthogonal Decomposition (POD) is applied to identify dominant energetic modes and vortex shedding characteristics. The undulated cylinder array is compared to a corresponding array of smooth elliptical cylinders, as well as to a single undulated cylinder and a single smooth ellipse. Reconstruction of the flow field is incrementally performed to provide insight into the dominant structures responsible for momentum transport within the wake. Initial results indicate that spanwise undulations influence local flow separation, broadening the vortex shedding frequency spectrum and reducing wake coherence. In array configurations, the undulated geometries alter wake interactions, potentially affecting drag and turbulence intensity. Compared to the whisker geometry, both the single and arrayed ellipse configurations exhibit higher energy concentration in the dominant POD modes, suggesting more coherent, two-dimensional flow structures. Conversely, the whisker-inspired geometry generates a more complex, three-dimensional wake with increased spatial variability.