INVESTIGATION ON THE BEHAVIOUR OF HIGH REYNOLDS ROUND IMPINGING ZERO-NET-MASS-FLUX JETS

The purpose of this research is to experimentally study the influence of the Strouhal number and the orifice-to-plate distance on the flow field of an impinging zero-net-mass-flux (ZNMF) jet at high Reynolds number by using PIV technique. A zero-net-mass-flux (ZNMF) jet is a fluid stream with non-zero mean streamwise momentum formed oscillatory flow through an orifice (Cater and Soria, 2002). The ZNMF jet is generated within the fluid in which the generator is embedded without the net injection of additional fluid. Due to a periodic movement the fluid is entrained into the cavity and consequently expelled through the orifice completing the cycle. The literature on ZNMF jets is very wide and includes several fields of applications such as: flow control, heat transfer, enhancement of mixing between fluid currents and generation of microthrust for propulsion. Focusing our attention on the heat transfer field, it is possible to note that a wide literature is present describing and evaluating the heat transfer capabilities of ZNMF jets (Greco et al., 2014; and Valiourge et al., 2009). Instead not so wide is the literature on the characterization of the impinging flow field of such jets. For this reason, the aim of this paper is to provide a detailed insight on the impinging ZNMF jet flow field features. Experiments are undertaken in the water tank used by Cater and Soria (2002). Differently from that experimental apparatus, the reciprocating piston is connected to an AC motor and an impinging plate has been added with the possibility of varying the distance between the orifice plate and the impinging plate itself. ZNMF jets are generated by setting up the half piston stroke (2, 4 and 8 mm), the piston frequency (16, 8 and 4 Hz) obtaining the desired Reynolds number (35000) and Strouhal number (0.044, 0.022 and 0.011). Three different values of the orifice-to-plate distance (equal to 2, 4 and 6 diameters) are investigated. Vector fields were obtained processing images with a multigrid cross-correlation digital particle velocimetry (MCCD-PIV) analysis which is described in Soria (1996 and 1998). Than, the obtained data are analysed by using a triple decomposition (Hussain and Reynolds, 1970). The behaviour of the impinging ZNMF jets varying the Strouhal number and the orifice-to-plate distance is analysed and discussed . The evolution of the velocity and its turbulent components, during one actuator cycle, have been described through the performed phase-average measurements. Indeed, as the orifice-to-plate distance increases a different axial velocity profile approaches the impinging plate. At high orifice-to-plate distance a bell-shape profile is visible while, at lower distance, a minimum on the jet axis is observed. The Strouhal number influences the position along the jet axis of the maximum axial velocity, as also reported by McGuinn et al. (2013). A region of low turbulence has been detected, as also shown in Greco et al. (2013). The extension of such a region is influenced by the Strouhal number. High turbulence is located, above all, in the vortex ring core and along the shear layer. Such a shear layer has an influence on the flow field which is lower, than the vortex ring one, as the Strouhal number increases. Furthermore the turbulent velocity value near the vortex core increases as the Strouhal number increases. The Strouhal number also influences the vortex ring trajectory and the saddle point behaviour.