On the behaviour of impinging zero-net-mass-flux jets

This paper reports on an experimental study of the influence of the Strouhal number (0.011, 0.022 and 0.044) and orifice-to-plate distances (2, 4 and 6 orifice diameters) on the flow field of an impinging zero-net-mass-flux jet at a Reynolds number equal to 35 000. These jets are generated by a reciprocating piston that oscillates in a cavity behind a circular orifice. Instantaneous two-dimensional in-plane velocity fields are measured in a plane containing the orifice axis using multigrid/multipass cross-correlation digital particle image velocimetry. These measurements have been used to investigate the mean flow quantities and turbulent statistics of the impinging zero-net-mass-flux jets. In addition, the vortex ring behaviour is analysed via its trajectory and azimuthal vorticity as well as the saddle point excursion, the flow rate and entrainment. The behaviour of all these quantities depends on the Strouhal number and the orifice-to-plate distance because the former governs the presence and the relative importance of the vortex ring and the trailing jet on the flow field and the latter delimits the downstream evolution of these structures.