Kinematic tomography of oscillatory coherent structures through synchronized mode decomposition

Abstract

Coherent structures are generally three-dimensional (3D), and while this information may be readily available from numerical simulations, obtaining 3D time resolved measurements might be technically challenging. We present a methodology to produce 3D kinematic tomographies of spatially localized coherent structures from two-dimensional sequences of snapshots. The method operates by synchronizing modal decompositions of multiple independent Particle Image Velocimetry (PIV) scans of a given flow volume. The volume should be scanned in pairs of simultaneous planes, one of which is taken as sync reference. The case study is a rectangular channel partially obstructed by a permeable media showing a linear instability producing periodic waves. The volume of interest is the downstream-facing permeable step generated by the end of the permeable media. The method is first verified using 3D numerical simulation results produced with a lattice Boltzmann scheme, comparing the kinematic tomography against the complete 3D dataset. Then the method is applied to actual experimental measurements with excellent result, which reveals non-trivial 3D features of the dominating coherent structures.