Visualization of several vortex shedding mechanisms in jets from a two-dimensional nozzle equipped with cutoff airfoils

This study examines the flow characteristics and vortical dynamics of plane jets issued from two-dimensional converging nozzles equipped with cutoff NACA (National Advisory Committee for Aeronautics) 0036 airfoils using particle image velocimetry (PIV). Building upon previous hot-wire measurements that demonstrated centerline velocity enhancements, this work provides direct spatial visualization of the boost effect [centerline velocity enhancement relative to the baseline jet (BJ)] and elucidates its underlying mechanisms. Three configurations were investigated: a BJ without an airfoil, a complete NACA0036 airfoil (AF), and a cutoff airfoil (CA) truncated at 95% of the chord length. PIV results show that, at a low-to-moderate Reynolds number, the CA configuration achieved centerline velocity enhancements of up to 40% at x/D = 2, compared with 30% for AF, with both configurations substantially outperforming the BJ. Spatial visualization of Reynolds shear stress revealed that truncated airfoils generate inner stress regions near the jet centerline, forming dual inward momentum fluxes that converge at the centerline. Instantaneous vorticity field visualization revealed fundamentally distinct enhancement mechanisms among the configurations: the BJ exhibited gradual vortex roll-up and pairing; the AF demonstrated cross-stream vortex merging between same-signed vortices from the inner and outer shear layers; and the CA produced high-intensity individual vortices without merging. Vortex strength and scale play a more dominant role in jet enhancement in the momentum transfer process. The PIV measurements offer spatial visualization of the complete momentum transfer process, confirming that high-intensity vortical structures govern the inward momentum redistribution responsible for the boost effect.