Establishment And Application Of The Dual-Eddy Mesoscale Turbulence Model Based On EMMS Principle

Turbulence is a century-old problem in classical physics.How to accurately pre-dict the transition process from laminar to turbulent flow is still one of major challenges in computational fluid dynamics.The physical understanding of turbulent multi-scale structures is contained in the statistical theory of turbulence and hierarchical structure theory.The experimental and numerical studies also show the importance of structures for turbulence generation and transition processes.The local turbulent structural physi-cal variables can be solved by utilizing turbulence stability condition,kinetic equations of turbulent eddies,momentum and mass conservation equations in the single-eddy En-ergy Minimization Multi-scale(EMMS)based turbulence model.The model regards single-phase turbulence as composed of laminar fluids and turbulent eddies,where het-erogenous structure of turbulence is characterized by the volume fraction of turbulent eddies,is used to simulate turbulence more accurately by improving transitional turbu-lence models based on Reynolds-Averaged Navier-Stokes(RANS)methods.Based on the single-eddy EMMS-based turbulence model,a dual-eddy EMMS-based turbulence model for laminar-turbulent transition prediction,is proposed with consideration of the domination role of large eddies and small eddies in turbulence.The main contents of this paper are as follows:(1)The relevant work of EMMS principle to explore turbulence is reviewed and summarized.The theoretical,experimental and numerical methods of laminar-turbulent transition research,as well as frequently-used laminar-turbulent transition models for engineering prediction are summarized.The multi-scale structure decomposition and hierarchy concept contained by the turbulent statistical theory and hierarchical struc-ture theory are expounded.In addition,the phenomenological understanding based on the turbulent coherent structure and vortex structure in the transition process is elab-orated.The inspiration raised by the recognition of turbulent multi-scale structure for establishing turbulence models based meso-scale eddy structure is underlined.(2)Single-phase turbulence is regarded as the laminar components and two kinds characteristic turbulent eddies according to the analysis of multi-scale concept.On the basis of kinetic equations and the expressions of energy dissipation rate,the dual-eddy EMMS-based turbulence model is established.Under the principle of the competition-in-compromise between two single extreme conditions of the energy dissipation rate correspondingly under the laminar and turbulent states,the extreme condition for pre-dicting transition is constructed to close model.(3)The heterogamous structure variables solved by the dual-eddy EMMS-based turbulence model are analyzed.By comparing the results under different extreme condi-tions,it is shown that the proposed extreme condition is effective for describing laminar-turbulent transition.The physical meaning of the parameters of the dual-eddy EMMS-based turbulence model is expanded and explored.(4)Based on the fitting algebraic expression of the solved volume fraction of turbu-lent eddies,a new transition model modified by k-? SST model according to laminar-turbulent transition mechanism for engineering practice is constructed.Various cases of laminar-turbulent transition are used to verify the new turbulent transition model.Besides,some improvement directions of the model in the future are prospected.