Investigation On Compressibility Effects And Coupling Consistency In LES-Scalar PDF Method

Transported probability density function method is investigated in present work.This thesis studys compressibility effects amd enegy consistency in LES-scalar PDF method,compares and analyzes the mixing performance of Micro-mixing model in incompressible turbulent jet flame and compressible mixing layer as well.The compressible source term of enthalpy equation is modeled preliminarily,and the compressible LES-scalar PDF method is futher extended to keep the energy consistentency.The model is implemented through the approximation of the resolved quantities and neglecting the influence of subgrid effect.The method is validated in a Sod shock tube case,based on which the effect of compression source term is analyzed.The results showed that both the pressure time derivative and the flow work by pressure in the source term has an important effect on enthalpy equation.The influence of different grid particle number on the results of PDF is tested,found that with the increase of the particle number in the grid,the statistical error is reduced.And when the particle number is too few,the accumulation of particle statistical error is very obvious,consequently it will seriously affect the accuracy of the solution of PDF.A 2D temporally mixing layer case without reaction is numerically simulated using the compressible LES-scalar PDF method,and the results are compared with LES and DNS.The compressible LES-PDF method correctly predicts the development process of the mixed layer and obtains more detail information than the LES method.Especially,the PDF method is in good agreement with the DNS results in the simulation of subgrid fluctuation terms,which shows the advantages of PDF method,comparing to the conventional large eddy simulation method.At the same time,the influence of particle number is also studied.It is found that when the particle number reaches a certain extent,the influence on the solved quantities will gradually weaken,but the influence on the subgrid quantities is always obvious.The mixing characteristics of micro-mixing model in incompressible and compressible flow are all investigted.The Sandia Flame D methane / air jet flame is simulated using NGA-HPDF program for incompressible condition.The mixing effects under different mixing frequency constants are compared.The results show that the larger the mixing frequency constant is,the lower the predicted combustion temperature.The mixing properties of IEM and Curl are analyzed,and it is found that both of them do not maintain the local characteristics in the component space,and the randomly mixed model Curl predict a more scatter result than deterministic IEM.For compressible conditions,the compressible LES-scalar PDF method is used to simulate the temporally mixing layer flow,and the peformance of three mixing models,IEM,Curl and EMST,are compared.The subgrid fluctuation terms predicted by the three methods are all higher than the actual results,and with the increase of the mixing frequency constant,the predicted values decrease gradually.The results of randomly mixed models Curl and EMST are basically identical.In the absence of reaction,the results of deterministic IEM model are closer to the DNS results than the randomly mixed model Curl and EMST.