| Turbulent jet is a hightly complex turbulent flow, with application in a vast array of engineering problems. It has been studied both analytically and experimentally for many decades, not only because of their wide application in technology but also because of their fundamental significance as a basic flow to the scientific research community. The large-scale coherent structures in turbulent jets play leading role in the process of turbulence generation, entrainment, and fluid mixing, therefore it is very important to understand the dynamical evolution process, such as the generation, development, collsion and breakup of the large-scale coherent structures, meanwhile it is also important to the control of turbulent jets.In this thesis, the method of large-eddy simulation of turbulence, the coherent structure theory, POD method and the statistical theory of turbulence are applied to analyze the dynamical evolution process of large-scale coherent structures in the turbulent jets. We have done the following research work:The current situation of research of turbulent free jets, turbulent jets in crossflow and control of turbulent jets are investigated in detail, the basic understand of the large-scale coherent structures of turbulent jets are summarized, and based on these understandings the problems needed further study are proposed.The numerical method of large-eddy simulation, the computational method of compressible turbulent flow, parallel cluster, POD method and the statistical analysis method of turbulence are studied, and the parallel program of large-eddy simulation of turbulent jet are obtained.Using the parallel code of large-eddy simulation, three turbulence databases of turbulence elliptic jet are obtained, with Reynolds number is 15000 and grid numbers are 258×125×125 and the total volume size is 552GB. Based on the analysis of the spatio-temporal evolution of large-scale coherent structures of free jet, turbulent jets in crossflow with stream-wise and transverse elliptic nozzles, the basic vortical structures of turbulent elliptic jet are revealed. The method of Snapshots-POD is applied to the turbulent jetsin crossflow, and the analysis of time series, spectrum, PDF and the statistical average properties on the spatio-temporal sections of the fluctuation velocity field are examed in detail.The results show that the most significant feature of turbulent free jet is the intermittently generation, shed of elliptic vortex rings, and once they generate they shed immediately. The model of 3-dimensional stretching vortex ring of turbulent elliptic jet in crossflow are discovered for the first time. It is pointed out that in the evolution of turbulent jet in crossflow the 3D vortex rings, which stretching, distorting, swing in transverse direction and twisting around the stream-wise, play the leading role in the large-scale coherent structures. The counter-rotating vortex pair observed in the literature is in fact the head of the 3-dimensional vortex ring. The POD method is applied for the first time to study the above turbulence database, to obtain the energy distribution of coherent structures of turbulent elliptic jet in crossflow and the evolution process of POD bases. The statistic theory of turbulence is also used to study the turbulence database, and the very complex statistical behavior of the fluctuation velocity field of the turbulent jet in crossflow are revealed. These results are deepening the comprehension of the nonlinear dynamical mechanism of the basic physics process of turbulent elliptic jets.In the future, some issues need to be investigated further, which include: establish and study the low-dimensional dynamical system of turbulent jet, with the POD bases, the control of turbulent jet based on the low-dimensional dynamical system of turbulent jet, investigate the physics mechanism of intermittence shed-off of the vortex ring in turbulent jets in crossflow.
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