Research And Application On LES Grid Scale Based On Energy Ratio Coefficient

With the sustainable development and further study of turbulent flow,engineering raises more and higher requirements for flow analysis and prediction.Large Eddy Simulation(LES)is one of the most effective ways to solve complex turbulent flows,which can obtained more turbulence characteristics than Reynolds Averaged Numerical Simulation(RANS).LES is based on coherent vortex structures,whose analysis technology of flow field is similar to Direct Numerical Simulation(DNS),but whose grid scale is much less than that of DNS and may be ten times more than RANS.So it is limited when we use this method into complex flows such as flow passing circular cylinder arrays and turbine cascade flow.Now there isn't any reliable method on grid generation according to flow field structures.Energy ratio coefficient is defined by the ratio of inertial and dissipative turbulence kinetic energy of total turbulence kinetic energy,which is derived from LES governing equation and Kármán–Howarth equation.Based on the scope of application of energy ratio coefficient in engineering flow,the LES grid scale is studied in different turbulence flow fields.Then LES results are compared with unsteady RANS(URANS)results and experimental data,and turbulent vortex structures are analyzed.The main work is as follow:Firstly,energy ratio coefficient is analyzed in theory and the variable law is obtained combined with Navier-Stokes equation.Secondly,the flow passing circular cylinder arrays is solved,which are investigated under the condition of Re=3000&20000,then the results are compared with experimental data at the downstream flow field of circular cylinders.The results show that the LES results are in good agreement with the experimental data when energy ratio coefficient reaches to 30%-40%.Then the convection,diffusion and dissipation mechanisms are analyzed under the different eddy structures and scales.The achievements of flow passing circular cylinder arrays are applied to the research of radical compressor passage.The grid ratio coefficient is determined by unsteady RANS results,and then a suitable mesh refinement is carried out.The vortex structure of different positions in flow passage is analyzed.All these research achievements lay a foundation for proposing extending stability technology.By analyzing of energy ratio coefficient and application of corresponding engineering problems,it can be confirmed that the precise flow information can be obtained by increasing grid scale,which also can reduces total grid number and computing time largely.