Experimental investigation of a fluidic oscillator for application to pulsed-jet propulsion

A fluid oscillator with no moving parts was manufactured based on a previous design by Tesař and modified with nozzles at outlets to experimentally investigate the pulsed jet propulsion aspect of it experimentally. The downfall with most of the pulsed-jet systems is the lack of thrust and the waste of energy in-between pulses. However, not having to deal with the complex designs of pistons and valves to generate the pulses, a fluid oscillator without any moving parts can potentially save the wasted energy and create continuous thrust by converting the steady input flow to continuous pulses exiting either of two nozzles.;Using nozzles in this study for outlet channels raises the question of how changing the downflow boundary condition, compared to older designs without nozzles, will affect the operation of device. In other words, for a given geometry, can we achieve desired flow pulsation downstream of the nozzles? And is passive feedback sufficient to create the short pulses beneficial for pulsed-jet propulsion, or is some active feedback required? The purpose of this study is to address these questions.;The operating frequency of the fluid oscillator constructed for this study was adjusted by both the flow rate and the feedback loop length and was varied through all the experiments. Pressure was measured at different locations inside the fluid oscillator to gather quantitative assessment of the internal flow behavior. As expected from prior studies, an increase in the feedback tube length (FBL) resulted in a decrease of the oscillation frequency and an increase of the flow rate resulted in an increase of the oscillation frequency. Shorter feedback tubes resulted in more irregular oscillation and blocking the feedback tube was directly effective to switching (on/off) the oscillation.;The vortical flow exiting by the oscillator was also investigated using digital particle image velocimetry (DPIV) near the exit nozzles to study the flow evolution. The formation of strong coherent vortices at the initiation of each jet pulse was not observed and the vortices followed a more irregular formation and breakup typical of steady jets, which was attributed to the large jet slug length to nozzle hydraulic diameter ratio (L/D h) obtained for the oscillation frequencies achieved in this investigation. The thrust developed by the flow exiting the oscillator was calculated and also measured experimentally to study the generation of continuous thrust. The thrust did not change significantly with frequency and it was similar to that expected for steady jets, consistent with the large L/Dh achieved for the pulses.