Near-wake investigations of cylinder and normal flat plate: effect of seeding on PIV measurements

Particle Image Velocimetry (PIV) relies on the assumption that tracer particles faithfully follow the fluid. While a small Stokes number (Stk ≪ 1) is the standard criterion for faithful tracking, it is not sufficient in flows containing organised coherent vortices: particles denser than the fluid are centrifugally expelled from vortex cores and accumulate in straindominated regions – a phenomenon known as preferential concentration – which scales with the density ratio ρ_p/ρ_f independently of particle size. This work investigates whether this distinction affects PIV measurements in the near wake of a circular cylinder and a normal flat plate, both of aspect ratio AR ≃ 6.67, at Re = 37500 and 57300, measured at the off-midspan plane z/L = 0.25. Two tracers of widely different density ratio are compared under identical flow conditions: air-filled soap bubbles (ρ_p/ρ_f ≈ 30) and oil droplets (ρ_p/ρ_f ≈ 800), both with Stk ≪ 0.01. The comparison is structured through scalar integral metrics applied to transverse profiles of mean velocity, turbulence intensities and Reynolds shear stress, and through a Proper Orthogonal Decomposition (POD) of the velocity fluctuation fields. In the cylinder wake, the two tracers yield broadly equivalent results, consistent with the absence of compact organised vortices within the measurement domain at this off-midspan location. In the flat plate wake, measurable differences emerge in the POD modal energy distribution, suggesting that preferential concentration becomes relevant where an organised vortex street forms within the field of view. The implications of these findings for the choice of seeding in bluff-body PIV experiments are discussed, together with the limitations of the present dataset.