Research On Turbulent Structure And Statistical Characteristics Of Flow Around A Hemisphere

Flow around a blunt body is widely used in nature and engineering applications.The study of its flow characteristics and mechanism has a wide application background and theoretical significance.In this paper,particle image velocimetry(PIV)experiment technique and large eddy simulation(LES)method are used to study the flow around a hemisphere.The structure evolution and statistical characteristics of the vortex structure are studied by using several methods of vortex identification and correlation analysis.The main work completed is summarized as follows:1.The turbulent structure and statistical characteristics of flow around a hemisphere under different Reynolds numbers are studied by experiments.Firstly,the reliability of the experimental data is verified by the mean velocity profile.Secondly,it is found that the probability density function(PDF)of the small-scale region in the middle is consistent with the Gaussian distribution,while the PDF of the large-scale region on both sides deviates from the Gaussian distribution.Through the study of joint probability density function(JPDF),it is found that with the increase of Reynolds number,the probability of ejection event Q2 increases.Finally,the proper orthogonal decomposition(POD)method is used to study the flow field under different Reynolds numbers.POD method combining?₍₈₄₎criterion is reconstructed flow field,which identifies the large-scale structure.A line connecting the hairpin vortex heads yields a slope of approximately 7^°.2.The turbulent structure characteristics of flow around a hemisphere are studied by numerical simulation under different Reynolds numbers.Firstly,the mean velocity distribution in the flow direction is compared with the experimental results and the grid convergence is verified.The hairpin vortex head is identified by the vortex identification method.In the process of the hairpin vortex head evolving downstream,it rotates to the wall and lifts upwards at the same time.The dynamic mode decomposition(DMD)method showed that mode 1 characterizes the hairpin vortex.The complete structure of hairpin vortex was captured by finite time Lyapunov exponents(FTLE).At the same time,the phenomenon of"kink"was found,which was the characteristic that caused the deformation of hairpin vortex.Through the above studies,it can be found that the lifting angle of the hairpin vortex head is about 7^°.This is consistent with the experimental results.3.The turbulent statistical characteristics of flow around a hemisphere are studied by numerical simulation under different Reynolds numbers.By studying the velocity spectrum under Eulerian framework and the FTLE spectrum under Lagrangian framework,it is found that in the same range of wave number k,both of them conform to Kolmogorov's-5/3 power law.By studying the scaling exponents of velocity and FTLE structure function,it is found that the scaling exponents of the first 5 order velocity structure function is basically consistent with the theory proposed by She and Leveque.The scaling exponent of the FTLE structure function is close to 0,which means that regardless of the order,the FTLE structure function may be independent of the scale.Under the Eulerian framework,the inclination angle and scale changes of the turbulent structure are analyzed through two-point correlation.By studying the correlation of the streamwise fluctuation velocities along several streamwise directions,it is found that the lateral scale of the turbulent structure increases significantly as the turbulent structure evolves downstream.Through a detailed comparison of the velocity correlation and FTLE correlation in the near-wall region,it can be found that the turbulence structure obtained by the two correlations has the same inclination angle,but it is obvious that the scales of the turbulence structure are different.