Investigation of vortex flow fields around a circular cylinder using particle image velocimetry. II: Influence of strake cross-sectional geometry

Building on Paper I, which demonstrates that three-start semi-elliptical (D-type) helical strakes fundamentally suppress the Kármán vortex street and reduce near-wake turbulence by approximately 30%–40% at high Reynolds numbers (Re), the present study extends the analysis to compare wake modification and turbulence-suppression performance among semi-elliptical (D-), square (S-), and triangular (T-type) strakes at Re ≈ 1.32 × 105. All strake configurations fragment the dominant counter-rotating vortex system and restore wake symmetry without periodic vortex shedding. Geometry-specific effects are evident: sharper-edged S- and T-type strakes induce earlier separation and stronger streamwise vorticity, producing more aggressive wake disruption than the smoother D-type configuration. Therefore, triangular strakes yield the most diffuse wakes, with square strakes performing comparably effectively, whereas semi-elliptical strakes leave slightly more organized residual structures. Particle image velocimetry (PIV) measurements confirm these trends, showing that peak Reynolds shear stress decreases by 40%–60%, while the dominant in-plane turbulent kinetic energy (TKEip) in the principal wake region is reduced by approximately 50%–70%, with the local reduction varying depending on measurement location; the Q-criterion analysis further indicates that the organized rotational wake structures are weakened and redistributed by the strakes. At specific planes, triangular strakes reduce peak TKEip to one-third of the bare-cylinder value, with square strakes producing nearly equivalent results and semi-elliptical strakes exhibiting slightly higher but still markedly reduced residuals. Proper orthogonal decomposition of the PIV velocity fields is performed to gain a deeper understanding of the wake dynamics. This reveals the evolution of dominant energetic modes with varying strake geometries and illustrates how coherent structures are progressively fragmented along the span. The decisive mechanism, early three-dimensional and spanwise-decorrelated separation driven by the helical strakes, is common to all cases, breaking coherent rollers into smaller eddies and redistributing turbulence away from the centerline. Although sharper strake geometries accelerate wake breakdown (T ≳ S > D), the dominant mechanism for vortex-induced vibration suppression resides in the helical strake-induced, three-dimensional wake disruption, providing clear physical insight and practical guidance for optimizing passive flow-induced vibration control.