The trapezoidal mixing tab has attracted growing attention due to its ability to generate hairpin vortices to enhance cross-stream mixing (Gretta & Smith, 1993; Elavarasan & Meng, 2000). In spite of several previous experimental studies, the physics pertaining to the topological, dynamical and statistical characteristics of the trapezoidal-tab wake is still poorly understood. A major difficulty is the highly three-dimensional nature of the tab wake. The objective of this work is to provide a comprehensive physical picture of the topological, dynamical and statistical features of the tab flow by using direct numerical simulations (DNS), and to elucidate several unresolved fundamental questions about the trapezoidal tab wake.; The current work considers a trapezoidal tab mounted on a flat plate. Simulations are conducted for three tab inclination angles α = 12.25°, 24.5° and 49° with the Reynolds number Re = 600 based on the free-stream velocity and the tab height. A finite-volume discretization scheme involving about 2.6 × 106 control volumes is employed for the simulation and the results are compared with PIV experimental data. The simulation results are shown to reproduce all the flow features as observed in experiments and reveal several new phenomena.; It is shown that the hairpin vortex comprises clustered flat-plate boundary-layer vortex lines from various streamwise locations. The hairpin vortex is found to be capable of lifting up and entraining new vortex lines from the local boundary layer, thereby increasing its strength to counter vorticity diffusion. This characteristic provides a self-sustaining mechanism for the hairpin structure. It is also shown that the turbulence production is mostly accomplished by the hairpin heads/arches, while the highest turbulent kinetic energy is associated with the hairpin legs. The topological characteristics of the vortex structures, the statistical characteristics and the mixing mechanisms of the flow are discussed in detail. Simulations also show that a larger inclination angle results in more enhanced mixing in the tab wake; however, with the penalty of a larger pressure drop across the tab.
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