Near-Wake Investigations of Cylinder and Normal Flat Plate: Effect of Seeding on PIV Measurements

Particle Image Velocimetry relies on the assumption that tracer particles faithfully follow the fluid. While a small Stokes number 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 strain-dominated regions — a phenomenon known as preferential concentration — whose magnitude scales with the particle-to-fluid density ratio independently of particle size. This work investigates whether this distinction affects PIV measurements in the near wake of a finite circular cylinder and a normal flat plate, both of aspect ratio 6.67, at Reynolds numbers of 37500 and 57300, measured at an off-midspan plane close to the free end. Two tracers of widely different density ratio are compared under identical flow conditions: air-filled soap bubbles and oil droplets, both with Stokes number well below 0.01. The comparison is structured through integral metrics applied to transverse profiles of mean velocity, turbulence intensities and Reynolds shear stress, and through a Proper Orthogonal Decomposition 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 and in the dominant shedding frequency, confirming that preferential concentration becomes relevant wherever an organised vortex street forms within the field of view.