LAUSR

Abstract In this study, direct numerical simulations are performed to investigate the spatial evolution of a dual-plane jet flow with L d / L 0 = 6, where L d and L 0 are the separation distance between the two jets and the jet width, respectively. The formation mechanism of the turbulent kinetic energy along the centerline can be quite different. In the reversal flow region, the transport equation of turbulent kinetic energy can be seen as the combination of the transport, convection, production, pressure, and viscous dissipation terms, whereas in the downstream combined region, it can be further simplified as the combination of convection and viscous dissipation terms. The gain of the centerline TKE mainly occurs in the vertical direction through the turbulence transport effects. Concerning the strain product process, in all three regions the strain self-amplification term is the dominant source term, whereas the enstrophy production and viscous dissipation terms are main sink terms. Two important aspects of small-scale motions are considered, one being the correlation between the vorticity vector and the corresponding eigenvectors of strain rate tensor and the other being the characteristics of invariant of the velocity gradient. The joint PDFs of the strain self-amplification and enstrophy production are highly squeezed in the vertical direction in the converging region. In contrast, in the reversal flow region, where the flow is still highly intermittent, the contour lines already take similar albeit slightly squeezed shapes as those in HIT. The joint PDFs of the second and third invariants in the reversal and highly intermittent flow region and the subsequent fully turbulent regions acquire the well-known ‘teardrop’ shape. It is demonstrated that even in the highly intermittent region, the characteristics of small-scale motions are already close to the fully developed flow. These above findings contribute to a better understanding of the dual-plane jet flows.