The influences of the modification of turbulent coherent structures on temperature field and heat transfer in turbulent channel flow are studied using large eddy simulation(LES) of compressible turbulent channel flows with spanwise wall oscillation(SWO).The reliability of the LES on such problems is proved by the comparisons of the drag reduction data with those of other researches.The high consistency of coherent velocity structures and temperature structures is found based on the analyses of the turbulent flow field.When the coherent velocity structures are suppressed,the transportations of momentum and heat are reduced simultaneously,demonstrating the same trend.This shows that the turbulent coherent structures have the same effects on the transportations of momentum and heat.The averaged wall heat flux can be reduced with appropriate oscillating parameters.
The application of large-eddy simulation(LES)to particle-laden turbulence raises such a fundamental ques-tion as whether the LES with a subgrid scale(SGS)model cancorrectly predict Lagrangian time correlations(LTCs).Mostof the currently existing SGS models are constructed basedon the energy budget equations.Therefore,they are able tocorrectly predict energy spectra,but they may not ensure thecorrect prediction on the LTCs.Previous researches investi-gated the effect of the SGS modeling on the Eulerian timecorrelations.This paper is devoted to study the LTCs in LES.A direct numerical simulation(DNS)and the LES with aspectral eddy viscosity model are performed for isotropicturbulence and the LTCs are calculated using the passivevector method.Both a priori and a posteriori tests are car-ried out.It is observed that the subgrid-scale contributions tothe LTCs cannot be simply ignored and the LES overpredictsthe LTCs than the DNS.It is concluded from the straininghypothesis that an accurate prediction of enstrophy spectrais most critical to the prediction of the LTCs.
Hybrid molecule/cluster statistical thermodynamics (HMCST) method is an efficient tool to simulate nano-scale systems under quasi-static loading at finite temperature.In this paper,a self-adaptive algorithm is developed for this method.Explicit refinement criterion based on the gradient of slip shear deformation and a switching criterion based on generalized Einstein approximation is proposed respectively.Results show that this self-adaptive method can accurately find clusters to be refined or transferred to molecules,and efficiently refine or transfer the clusters.Furthermore,compared with fully atomistic simulation,the high computational efficiency of the self-adaptive method appears very attractive.
The Arrhenius law implies that reaction rate is a continuous function of temperature. However,the steady laminar flamelet model(SLFM) does not explicitly give this functional relationship. The present study addresses this relation in the SLFM.It is found that reaction rate is not continuous in the mixture-fraction space.As a result,the SLFM is unable to predict local extinction and reignition.Furthermore,we use the unstable branch of the"S-curve"to fill the gap between steady burning branch and extinctio...
Jian Zhang,~(a)) Guowei He,~(b)) and Guodong Jin LNM,Institute of Mechanics,Chinese Academy of Sciences,Beijing 100190,China
Recently,people are confused with two opposite variations of elastic modulus with decreasing size of nano scale sample:elastic modulus either decreases or increases with decreasing sample size.In this paper,based on intermolecular potentials and a one dimensional model,we provide a unified understanding of the two opposite size effects.Firstly,we analyzed the microstructural variation near the surface of an fcc nanofilm based on the Lennard-Jones potential.It is found that the atomic lattice near the surface becomes looser in comparison with the bulk,indicating that atoms in the bulk are located at the balance of repulsive forces,and the elastic moduli decrease with the decreasing thickness of the film accordingly.In addition,the decrease in moduli should be attributed to both the looser surface layer and smaller coordination number of surface atoms.Furthermore,it is found that both looser and tighter lattice near the surface can appear for a general pair potential and the governing mechanism should be attributed to the surplus of the nearest force to all other long range interactions in the pair potential.Surprisingly,the surplus can be simply expressed by a sum of the long range interactions and the sum being positive or negative determines the looser or tighter lattice near surface respectively.To justify this concept,we examined ZnO in terms of Buckingham potential with long range Coulomb interactions.It is found that compared to its bulk lattice,the ZnO lattice near the surface becomes tighter,indicating the atoms in the bulk are located at the balance of attractive forces,owing to the long range Coulomb interaction.Correspondingly,the elastic modulus of one-dimensional ZnO chain increases with decreasing size.Finally,a kind of many-body potential for Cu was examined.In this case,the surface layer becomes tighter than the bulk and the modulus increases with deceasing size,owing to the long range repulsive pair interaction,as well as the cohesive many-body interaction caused by the electron redistribution.
The numerical simulation of flows past flapping foils at moderate Reynolds numbers presents two challenges to computational fluid dynamics: turbulent flows and moving boundaries. The direct forcing immersed boundary(IB) method has been developed to simulate laminar flows. However,its performance in simulating turbulent flows and transitional flows with moving boundaries has not been fully evaluated. In the present work,we use the IB method to simulate fully developed turbulent channel flows and transitional flows past a stationary/plunging SD7003 airfoil. To suppress the non-physical force oscillations in the plunging case,we use the smoothed discrete delta function for interpolation in the IB method. The results of the present work demonstrate that the IB method can be used to simulate turbulent flows and transitional flows with moving boundaries.
YANG XiaoLei,HE GuoWei & ZHANG Xing The State Key Laboratory of Nonlinear Mechanics,Institute of Mechanics,Chinese Academy of Sciences,Beijing 100190,China
