Investigation of unsteady engulfment flows in a cross-shaped mixer by particle image velocimetry

An experimental study was conducted to characterize flow fields in a cross-shaped mixer by particle image velocimetry technique (PIV). Instantaneous flow fields describe the formation and development of secondary flows at 20 <= Re <= 400, with focus on the dynamic evolution of vortices for periodic unsteady engulfment flows. Three stages are detected during one flow period, i.e., vortex merging, growing, and splitting. Vortex characteristics including populations, distributions, and strengths, are evaluated by a vortex identification and vorticity distributions. Reversal flow regions triggered by vortex breakdown are identified in merging and splitting vortices, which periodically shed downstream, thus causing an axial oscillation. The statistical properties and proper orthogonal decomposition (POD) modes of unsteady engulfment flows are analyzed. The peak of turbulent kinetic energy appears near the chamber corners where two co-rotating vortices are observed, and it decreases with Re. The second and third-order POD modes describe large-scale coherent structures in unsteady engulfment flows.