Effect of pulsation on the near flow field of a submerged water jet

摘要

The current study investigates the effect of pulsation frequency on the near field characteristics of a submerged water jet using the technique of dye visualization. Flow visualization was performed in water over the range: Reynolds number 540-1540, Strouhal number 0.16-1.75, and at constant amplitude of pulsation of 18%. The results show that the mixing and entrainment process at lower Reynolds number occurs due to diffusion process owing to relatively stable shear layer for the case of a steady jet, whereas the external pulsation promotes an early instability in the shear layer where irregular structures promotes mixing between the jet and surrounding fluids. Images of streaklines show that initial mixing and entrainment processes in the potential core of the jet is due to the development of large vortical structures. While beyond the potential core, mixing and entrainment are governed by the small-scale structures. Further results show that the initiation and growth of vortices in the shear layer depends on the pulse frequency. For a given Reynolds number and amplitude, the number of vortical structures and their size changes with frequency. With an increase in the pulsation frequency, there is an increase in the spreading of the jet along with stretching of the vortical structures. An optimum pulsating frequency at which the effect of pulsation on the flow is maximum occurs at St = 0.44, independent of Reynolds number. These results should eventually lead to a better understanding of the physical phenomena responsible for enhanced mixing and entrainment processes in the presence of pulsating jets.