Study On Synthesis Of Inlet Turbulence In Large Eddy Simulation

Computational Fluid Mechanics(CFD)is a product of the combination of modern fluid mechanics,computer and numerical mathematics.It uses computer and discrete methods to solve practical problems by numerical simulation of various problems in convective mechanics.Now it has infiltrated into many fields such as aerospace,industrial design,and built environment.CFD research methods are mainly divided into RANS/Reynolds-Average Navier-Stokes and LES/Large-Eddy Simulation.RANS has been widely used because of its simple model and low consumption of computer resources.However,RANS regarded vortices as isotropic and averaged the calculated results,so the real flow field information could not be obtained.When dealing with unsteady flow problems,the simulated results were usually far from the actual results.With the development of CFD technology,a new simulation technique,LES,is proposed,which can simulate the instantaneous flow state with higher computational accuracy than RANS.However,LES is very sensitive to the inlet boundary conditions and needs to artificially add turbulent kinetic energy at the entrance,so how construct the inlet boundary conditions of LES has become a key problem.In recent years,turbulence field synthesized by turbulence energy spectrum method has been paid more attention and studied by many scholars because of its high calculation accuracy and easy realization.However,the method proposed by previous researchers changed the wave number distribution,which could not guarantee no divergence under large spatial gradient changes,and could not satisfy the continuity equation in the governing equation,which violated the principles of computational fluid dynamics,and the synthesized turbulent field was easy to diverge.To solve the above problems,a new anisotropic turbulence synthesis method is proposed in this paper,which is successfully transferred to Open FOAM open source computing software.Subsequently,numerical verification proves the effectiveness of this method.The main research contents and results are as follows:(1)in the synthesis of discrete random flow generated method(DSRFG)on the basis of adding a step projection,created a new unit vector of S^m,n and its direction is parallel to Reynolds stress direction,Sm through an Angle constraint at the same time,the direction of the n,synthetic velocity formula of q^m,n and P^m,n is random distribution,when the space Angle to the hour,S^m,n will make its and wave number k^m,nvertical to ensure no divergence requirements,thus can get new isotropy turbulent flow field.Then,the anisotropic turbulence field is generated by using the characteristic that the turbulence integral scale is different in x,y and z.The method is strictly non-divergent and does not change the wave number distribution,which can maintain the target turbulence spec-trum required by the LES velocity inlet.(2)The channel flow cases were simulated by the new method and DSRFG method at the same time,and the differences between the turbulence fields synthesized by the two methods were observed.The simulation results show that at Q=100,the turbulence field synthesized by the new method can observe more vortex structures,and the inlet turbulent kinetic energy can be reproduced at the position x=16?,while the DSRFG method needs to be at the position x=20?.(3)in order to validate the new method of numerical precision,select the standard in the association of Japanese architecture design manual2:1:1 flow around building model are simulated,and the main conclusion is:the new method is higher than the calculation precision of DSRFG method,the pulse velocity values more in line with the wind tunnel experiment,and the cost of computing time shorter.(4)using a new method of 2:1 standard mesh cross ventilation model and numerical simulation of the turbulent kinetic energy value of the calculated results and experimental values,and compares the result with the numerical simulation of past researchers,in order to evaluate the new method for the ability of simulating the complex turbulent flow,the results show that the new method in the center of the building openings,above,such as location,good prediction,the distribution of turbulent kinetic energy is the results of numerical simulation and the accuracy is higher than the previous researchers.(5)according to the rule of natural ventilation design of Hunan university in the 2:1 lee double opening model is simulated,the results show that the two open position of the instantaneous air volume change is the change rule of one large and one small and one of the opening of the instantaneous airflow when achieving the maximum,another opening of the instantaneous air volume to minimum,and near the opening of the vortex is the important factors influencing the ventilation rate.