Volumetric reconstruction of the 3D boundary of stream tubes with general topology using tracer particles
A general method is proposed to reconstruct the volumetric interface between two fluid flows using tracer particles and 3D particle tracking techniques. The method relies on the fact that a homogeneous dispersion of tracer particles introduced in a stream tube remains confined in that tube so that the cloud of particles can be used to reconstruct the boundary of the flow covered by the stream tube. Thus it becomes possible to quantitatively determine the interface between laminar and turbulent flow regions in boundary or shear layers as well as the interface between two mixing fluids. Tracer particles, as opposed to dye tracers, have negligible diffusion and their position in the measurement volume can be precisely localized by means of 3D particle tracking methods. On the other hand, they provide a discrete representation of a continuous volume and the reconstruction of the interface cannot be implemented in a straight forward fashion. In this work, the problem of interface reconstruction, from a randomly scattered particle cloud, is addressed and two different reconstruction algorithms are proposed: one based on numeric diffusion and one based on Delaunay triangulation. The two methods are qualified and compared by means of numerical simulations using the Monte Carlo method. The simulations are used to estimate the accuracy of the method and to provide guidelines for the choice of parameter settings. Finally, results on the interface between two mixing fluids in a microfluidic mixer are shown. A resolution of 2.5 µm in the optical-axis direction, with a maximum estimated error of 5.5 µm in the three directions, was obtained.