Simultaneous free-surface profilometry and subsurface velocimetry with fringe projection and PIV

This work presents a novel combination of two well-established techniques: fringe projection profilometry (FPP) and particle image velocimetry (PIV). Despite seemingly conflicting requirements-FPP requires an opaque surface to project onto, while PIV requires a transparent fluid-both requirements are met by adding low concentrations (4-25 mg/L) of fluorescein dye to the water. This dye strongly absorbs the blue light projected onto the surface for FPP while remaining nearly transparent to the green PIV laser, achieving simultaneous opacity and transparency depending on wavelength. A set of three optical filters suppresses fluorescence-induced noise in the PIV images and specular reflections in the profilometry images, enabling clean simultaneous acquisition. Validated against point laser-induced fluorescence measurements of the surface, the method achieves a mean absolute error in surface elevation of 18  $\mu \text{m}$ at a dye concentration of 12 mg/L, above which further increases in concentration yield little improvement. PIV correlation values remain robust up to 20 mg/L. The technique resolves surface features differing in amplitude by two orders of magnitude and is demonstrated on flow past a cylinder interacting with surface waves and on droplet impacts on a quiescent surface.

The online version contains supplementary material available at 10.1007/s00348-026-04242-x.