Shake-the-box measurements of vortex shedding and main coherent structures in a rectangular prism bluff body

This study presents a three-dimensional characterization of the wake behind a rectangular-cylinder bluff body using a time-resolved Tomographic Particle Image Velocimetry (Tomo-PIV) setup. The Shake-the-Box (STB) algorithm was used for Lagrangian particle tracking (LPT), enabling the reconstruction of volumetric velocity and pressure fields. Two chord-to-thickness ratio ( D / H ) orientations of the same geometry were tested: with its short side facing the flow ( D / H = 3 ) and with its long side facing the flow ( D / H = 0.33 ). Experiments were carried out in a shear layer transition regime, R e = 520 – 6 210 . The measurements address the mean wake topology (recirculation length, wake width, and flow separation), the primary spanwise vortex shedding, the secondary streamwise vortices, and the formation length and streamwise wavelength of the dominant POD modes. The D / H = 3 body yields a compact wake recirculation with secondary vortices of spanwise wavelength λ z / H ≈ 3 –4, whereas the D / H = 0.33 body yields a fully separated, expanded wake with λ z / H ≈ 1.1 –1.2 and comparatively stronger streamwise vorticity. These results provide data support for vortex-induced vibration prediction of offshore platform risers and rectangular-section marine structures. Existing experimental investigations of secondary streamwise structures in rectangular cylinder wakes are largely confined to planar and volumetric measurements in laminar regimes ( R e ≤ 500 ). Unlike previous studies, this work provides time-resolved 3D experimental flow and pressure fields well into the shear layer transition regime, where the wake is fully turbulent. Time-resolved volumetric fields allows for a comprehensive 3D characterization of primary and secondary coherent structures across two contrasting orientation scenarios.