Time-Resolved Flow Field Measurements of Momentum-Driven Pulsed Transient Jets

摘要

The flow physics of consecutively pulsed, transient jets were studied where two successive volumes of fluid were ejected from a nozzle into a quiescent fluid. Previous work has shown that the flows from double pulsed jets persist for much longer times than the equivalent single pulsed jets, suggesting powerful fluid dynamic interactions between the two ejection events. The flow fields of double pulsed transient jets were, therefore, compared in detail to both a steady jet and to a single pulsed jet. The transient jets had an exit Reynolds number of 1,500 based on the nozzle diameter. Time-resolved particle image velocimetry (PIV) was used to characterize the behavior of these interactions, focusing on the velocity and vorticity fields as well as the mechanisms of flow entrainment. The time-resolved PIV system used two synchronized high-speed CMOS cameras and a dual-head, high-speed YLF laser. The interactions between the jets was found to change with the time separation between the two ejections. The instantaneous velocity distributions showed that for shorter time intervals between the two ejections, the second starting vortex was completely formed, which was a primary difference to the individually pulsed jet. However, the increased momentum flux and turbulent wake of the first ejection stabilized the near flow field of the double pulsed jet at longer pulse intervals by eliminating the starting vortex structure. A greater coorelation between the jet eddies and entrainment was shown at long downstream distances than at the farthest field.