| The self-excited oscillation pusled jet(SOPJ) has the advantages of simple structure, high efficiency, low cost, etc. Continuous jet can be turned into pusled one just through the modulation of Helmholtz cavity. Along with the wide application of water jet in all walks of life in recent years, if the SOPJ nozzles that have all the characteristics of traditional nozzles replace the latter and apply to these fields, it will have broad application prospects. However, engineering use is mostly based on experience and the mechanism of the SOPJ is to be improved. Artificial control and internal flow mechanism still remain to be researched. The SOPJ refers to shear cavitation, vortex wall effect, fluid elastic and other complex theory, affected by many factors. Therefore, the flow characteristics of Helmholtz SOPJ is investigated in details based on large eddy simulation(LES), in combination with high-speed photography. The mechanism of SOPJ is improved acccording to the study results. The results and conclusions are described briefly as follows:(1) In order to verify the reliability of the large eddy simulation, the cavitation flow in the chamber was measured by high-speed photography, and compared with the corresponding numerical simulation results. The results show that the peak value of the self-excited oscillation jet matches well with the experimental results. Compared with the RANS models, the LES model describes the complex turbulent characteristics of the high-speed pulsed jet more precisely. Also, and the flow characteristics obtained by LES are closer to real conditions.(2) The cavitation inception appeared near the upper nozzles and formed the annular cavitation, which evolved into the braid-vortex with the jet development towards downstream. Then the braid-vortex collapsed rapidly. As the working pressure increased, the cavitation in the shear layer became more and more intense. The interaction between the cloud and the collision wall could be observed clearly. The oscillation frequency is mainly affected by the frequency of the upstream vortex shedding. The final purpose of the chamber modulation is to enlarge the vortex ring. In this study, with the increase of the Reynolds number, the natural frequency becomes higher. With the increase of the cavity length, the pulse frequency decreases. And the cavity diameter has little effect on the pulse frequency.(3) The self-oscillating mechanism of Helmholtz-type is described as follows: the jet contains the inherent ordered perturbation, and the Kelvin-Helmholtz vortex shed periodically by the amplification of the shear layer near the upper nozzle. The flow impinged in the exit of the narrow orifice of the plane, causing the periodic backflow in the cavity, as well as the back to the periodic flow and the exit plane changes in the resistance of the pressure pulsation The pressure fluctuation of the periodic backflow and the outlet plane change in the cavity; the ordered vortex ring of the upstream changes periodically in the chamber; the cavitation caused, by the low pressure in the vortex center, has effects on the cycle of the energy in the chamber. At this time, the chamber is equivalent to the accumulator. the mass flow in the exit section of was regulated periodically by the chamber. This pulsating pressure field resulted in pulsation of the inlet flow to produce an effective jet drive of the same frequency as the inlet flow. The jet shear layer responded to the jet driving force in its natural frequency range and amplifies it to form a closed loop in the chamber. And the self-excited oscillating jet with periodic pulsations is formed.(4) As for the external flow field of the self-vibration jet, the cavitation cloud only concentrated near the nozzle outlet when the working pressure is low. And the incipient cavitation appeared in the shear layer. As the working pressure increases, the entrainment ability of the jet was enhanced, and large vortex was formed on the two sides of the nozzle outlet. The cavitation cloud became longer with the working pressure increasing, and the cavitation intensified. As the cavity length increases, the space used for storing energy increases, and the instability of the downstream flow-field decreases. The vortices at the outlet were relatively more ordered at the same time. The influence of the cavity diameter on the self-oscillating jet is mainly reflected by the feedback modulation of the jet in the cavity. Large-scale cavitation appeared in the outer flow field, and the vacuole developed along the axial and radial direction. Cloud cavitation was formed and evolved downstream. Then the cloud cavitation were broken up into small bubbles, and dissipated in the ambient fluid Eventually.(5) As the jet working pressure increases, the pressure pulsation peak increases. And the impact performance is also improved. As the cavity length increases, the peak pressure will gradually decrease. When the dimensionless cavity length changes from 1 to 4, the amplitude of the pulsation increases with the cavity length. The higher the working pressure, the higher the peak of the same structure under the pulsation, since the cavity always play a role in amplification of the jet. When the cavity length is constant, the cavity diameter mainly affects the vortex in the downstream collision wall. When the cavity diameter is too small, the jet entrains the surrounding fluid in the chamber to form a backflow vortex. When the cavity diameter is too large, although the vortex is fully developed, the vortices fed back from the collision wall could not modulate and disturb the jet effectively. So the peak pressure can reach maximum value under certain conditions.(6) When the vortex shedding frequency is close to the natural frequency, the chamber pulse is amplified. And based on this theory, the relationship between the St number and the parameters of the Helmholtz cavity was deduced, which is important for the design and modulation of the self-oscillating nozzle.(7) The experiments of eroding the aluminum plate were performed. The large vacuoles are broken into small vacuoles, and the small vacuoles are broken instantaneously, resulting in great destructiveness. A large amount of the honeycomb-shaped holes were formed on the surface of the aluminum plates. When the target distance exceeds the length of the bubble cloud, the vacuoles had completely collapsed before reaching the specimen, and the effect on the specimen is very small. As the increase of the cavity length, the length of cavitation cloud decreases while the diffusion angle of the jet increases. The erosion area increases as a result. And the erosion depth decreases with the cavity length decreasing. The cavitation strength of the outer flow-field increases with the increase of the cavity diameter. The erosion strength is improved as a result. While the cavity diameter has little effect on the erosion area, the erosion depth increases with the increase of the cavity diameter. |
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