Turbulence Characteristics Of A Submerged Semi-confined Impinging Jet Issuing From A Long Circular Pipe

At present,the research on the impinging jet flow of the circular tube under constrained conditions mainly focuses on the heat transfer and noise properties,and the research on the flow characteristics is not comprehensive.In order to improve the understanding of the submerged semi-confined impinging jet issuing from a long circular pipe under the influence of multiple key control factors,This paper is based on a turbulent jet experimental device independently developed by our research group,and the PIV high-speed particle velocimeter is used to conduct experiments to statistically analyze the time-average characteristics and turbulence characteristics of the corresponding jet under different working conditions.The blending and diffusion of the jet itself during its development will offer the way for further research on the material diffusion and energy dissipation of this type of jet,and provide a mechanism explanation and hydraulic structure design reference for engineering applications.The main research work and results of this paper are listed as follows:(1)The flow diagram of the flow field under different working conditions is drawn.The recirculation zone in the flow field moves in the outflow direction with the increase of Reynolds number and impact distance.In the analysis of the wall half-value width and velocity attenuation,the influence of the recirculation zone on the self-similarity was found.(2)The study on the change law of the entrainment coefficient of the jet main section shows that the length of the jet's potential flow core increases with the increase of the impingement distance.The entrainment coefficient along the main section of the jet shows a linear function change trend.The entrainment expansion rate increases with the increase of Reynolds number and impact distance,but its increase gradually decreases with the increase of impact distance.(3)The half-value width expansion rate of the self-similar zone of the wall jet is statistically calculated,and the expansion rate of the wall jet in this section shows a linear function growth.Under the same impingement distance condition,the expansion rate gradually decreases with the increase of Reynolds number,which is similar to the research results of tapered circular impinging jet.With the increase of the Reynolds number,the expansion rate of the wall jet zone gradually changes the same as that of the free impinging jet,and the expansion rate gradually increases with the increase of the impingement distance.(4)The velocity attenuation and distribution of the wall jet have been studied,and the results show that the velocity attenuation in the self-similar zone along the maximum radial velocity of the cross-section is characterized as a power function change trend.In the working condition where the self-similarity is not affected by the recirculation zone,the attenuation index increases with the increase of the impact distance at the same Reynolds number,and the attenuation index increases with the increase of the Reynolds number at the same impact distance as a whole.The change trend of velocity distribution in the jet zone on the wall is roughly the same.When the impact distance condition is constant,as the Reynolds number increases,the radial velocity distribution curve gradually moves closer to the wall,and the curvature radius of the velocity curve at the peak gradually decreases.The radial velocity distribution of the wall jet has self-similarity,and the influence range of the recirculation zone on the self-similarity is basically the same as the wall jet expansion rate and half-value width.(5)The statistical analysis of the distribution law of relative turbulence intensity shows that the axial relative turbulence intensity on the jet centerline increases in a power function along the jet direction.There is self-similarity in the axial relative turbulence intensity change on the center line.At the same Reynolds number,the axial relative turbulence strength growth index increases with the increase of impact distance;at the same impact distance,as the Reynolds number increases as a whole,the growth index tends to increase accordingly;in the growth range from Re=11040 to Re=13920,the growth index under the condition of partial impact distance does not change significantly,and has a slight downward trend.(6)The statistical analysis method is used to study and analyze the probability density distribution and quasi-periodicity of the instantaneous velocity on the jet center line.The statistical results show that the time-averaged flow velocity has the shortest recurring sample size interval during the instantaneous velocity change.With the increase of the instantaneous pulsating flow rate,the corresponding recurring sample size interval also increases,and the increase of the pulsating velocity increase and the recurring sample size are very close to each other.At the same time,it can be found that the larger the statistical sample size,the quasi-periodicity of the turbulence is more obvious.(7)The skewness coefficients and kurtosis coefficients of the velocity on the jet centerline are studied,and the variation rules under different working conditions are analyzed.The research results show that: on the whole,the radial velocity deviates from the normal distribution to a greater extent near the nozzle and the impact wall,and the pulsation is stronger;the axial velocity deviates from the normal distribution to a greater degree near the wall,and the skewness coefficient and kurtosis coefficient gradually increase with the increase of the impact distance.The research results of this paper can not only provide data reference for the mathematical model of turbulent jet,to evaluate the predictive ability of the turbulence model,but also improve the understanding of the jet flow mechanism under the influence of multiple key control factors.In addition,it lays a good foundation for explaining the jet flow energy,energy transfer and dissipation mechanism under the boundary conditions of the semi-confined space.The research results are expected to provide technical reserves for the hydraulic structure design of the ship lock filling/emptying system.