This technique is called ‘Particle-shadow-tracking’. It is not 3D and it works with very low concentration of seeding particles, but it is very cute, simple and really elegant. Recommended, for reading and thinking. It is only on arxiv, but seems to be submitted to Experiments in Fluids. Let’s wait and see.
The abstract is:
Near-wall velocity measurements by Particle-Shadow-Tracking
Authors: Pierre Lancien (IPGP), Eric Lajeunesse (IPGP), François Métivier (IPGP)
Subj-class: Fluid Dynamics; Classical Physics
We report a new method to measure the velocity of a fluid in the vicinity of a wall. The method, that we call Particle-Shadow Tracking (PST), simply consists in seeding the fluid with a small number of fine tracer particles of density close to that of the fluid. The position of each particle and of its shadow on the wall are then tracked simultaneously, allowing one to accurately determine the distance separating tracers from the wall and therefore to extract the velocity field. We present an application of the method to the determination of the velocity profile inside a laminar density current flowing along an inclined plane.