Particle image velocimetry

Context

Particle Image Velocimetry (PIV) is a non-intrusive measurement technique based on three principles:

• Seeding the flow with tracers. These tracers are very small particles, which must follow the flow precisely, rather than their own trajectory. To achieve this, it is necessary to check that their Stokes number in the flow is well below 1.
• Illuminating the zone with a laser sheet. The light is scattered by the tracer particles.
• The final step is the acquisition of successive images using a CCD camera, which, by auto-correlation, will enable us to identify the particles at each instant, determine their displacement and ultimately their velocity (hence the name velocimetry).

Ultimately, this measurement technique enables us to characterize a velocity field with high spatial and temporal resolution, calculating an average field and analyzing its fluctuations in a two-dimensional plane, using a non-intrusive method but still requiring optical access.

Objective

The development of new models systematically requires in-situ measurements. As part of the development of turbulence models, the Laboratoire de Physique Subatomique et de Cosmologie needed to carry out measurements of the mean velocity field in a classic academic flow case: “downward march”.

Results

Using a complete Particle Image Velocimetry (PIV) acquisition chain, we were able to provide the mean velocity field as an Eulerian representation of the experimental flow for comparison with the numerical mean field results obtained with the turbulence model under development.

The Particle Image Velocimetry method is also regularly used on physical models in hydroelectricity or for sprays characterization.