"Vortices created by the imbalance of a centrifugal force and a radial pressure gradient in a curved pipe move fluid from the inner wall (left side) towards the outer wall. These so called 'Dean vortices' (referring to the two symmetric structures in the rightmost figure) are common features of flows in curved geometries and occur in nature, such as in rivers, and technical applications, such as in heat exchangers and piping systems, where they are sometimes specifically generated for particle separation or filtration purposes.
By measuring the speed of tracer particles it is possible to obtain snapshots of the three dimensional velocity field in a cross plane and thereby capture the unsteady behavior of the vortices under turbulent flow conditions. Surrounded by an irregular and chaotic environment, the couple of 'undetermined' vortices appears to 'dance.' Sometimes they dance together while at other times one of them is taking the lead by swinging its partner (referring to the three snapshots to the left). Nonetheless, on average, they successfully manage to keep up an ordered, laminar-like, appearance as a harmonic couple."