Study On Flow Characteristics Of Free Round Turbulent Jet

As a basic flow form, free round turbulent jet has been used in various industry fields. During the development and improvement of some technics, such as water jet cutting, water cleaning, fuel spray, etc., it is apparently more and more important to recognize the instability and complicated interface structure of the jets.To learn more about the development of the jet and some of its underlying characteristics, the instability of the jet has been analyzed in this paper, which is funded by the national natural science fund ‘Research on mesoscopic ultra-high pressure jet flow interface and instability mechanism(51176065)’. Based on the experimental results, the simplified nozzle model and the appropriate simulation model were designed. With the help of numerical simulation, the unstable surface structure and large scale coherent structure in the flow field were captured. And the effects of inflow conditions on the instability of the jet were discussed. More details and major conclusions are as follows:Liquid ejection process at low velocity and the formation of droplet were simulated by applying numerical approach. By changing contact angle and systematically varying the inlet boundary conditions, contrast experiments were conducted. The obtained results show that the interactive dimensionless numbers Re、CH、ZF could significantly affect the liquid ejection characteristics in computational domain. The droplet shows different angles in neck region with different wettability nozzles. And the droplet through a hydrophobic surface breaks up later than one through a hydrophilic surface. Within the range of Re=27~63, the uniform and steady droplets could be obtained. By the comparison between numerical results and experiment results and the analysis of interface characteristics, the effectiveness of volume of fluid(VOF) to capture the interface of two phases has been verified.The free round turbulent jets of the moderate Reynolds number, within the range of Re=10 000 ~ 80 000, were simulated by using the VOF technique and applying large eddy simulation(LES) algorithm. The evolution characteristics of the micro-scale interface structures were captured. Swirling strength iso-surface was used to visually describe the large scale coherent structure in the flow field. Special attention was focused on how the instability in the near field of round jet was affected by the imposed inlet velocity and disturbance. Results reveal that the inflow conditions play an important role in the interface structure and stability of the jets: The unperturbed liquid core length and the scale of surface wave are decreased, and a wide variety of vortexes are produced in the near field of jet, as the velocity and disturbance intensity increasing. Under different Reynolds numbers at the same disturbance intensity, the positive entrainment is enhanced and the radial spread of the jet along the streamwise becomes more obvious with the increase of Reynolds number.Additionally, the effects of LES algorithm on capturing the micro-scale interface structures and discrete vortexes containing high energy were investigated based on the open source code OpenFOAM-2.3.0. The results reveal that the filter methods could significantly affect the surface wave and the discrete high energy vortexes while the effects using different SGS model are not obvious. And the resolved high energy vortexes above a certain scale are directly related to the surface wave.