Simultaneous measurement of surface topography and subsurface velocity field in free-surface turbulent flow

TL;DR

This paper introduces a combined optical method using fluorescent dye to simultaneously measure surface topography and subsurface velocity in turbulent flow, overcoming conflicting requirements of traditional techniques.

Contribution

It presents a novel dye-based approach enabling concurrent PIV and FPP measurements in turbulent free-surface flows, with validated accuracy and practical demonstration.

Findings

Achieved accurate surface elevation measurements with 20 micrometres error.

Demonstrated the method on vortex street generated by a cylinder.

Validated the technique's effectiveness at specific dye concentrations.

Abstract

This work presents a novel combination of two well-established techniques: particle image velocimetry (PIV) and fringe projection profilometry (FPP). Despite seemingly conflicting requirements -- PIV requires a transparent fluid, while FPP requires an opaque surface to project onto -- both requirements are met by adding fluorescein disodium salt, a fluorescent dye, to the water. This dye strongly absorbs the blue light projected onto the surface for FPP while interacting weakly with the green laser light used for PIV, achieving simultaneous opacity and transparency depending on colour. However, this approach presents several challenges, which we are able to solve with a combination of optical filters on the projector and cameras. A series of validation experiments were performed to assess the accuracy of surface elevation measurements at various dye concentrations. The technique was then demonstrated for the case of a vortex street generated by a cylinder interacting with surface waves. Our results show that a dye concentration of 12\,mg/L, although insufficient to make the water opaque to the projected light, yields a mean absolute error in surface elevation of only 20 micrometres.