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.
We compare the space-time correlations calculated from direct numerical simulation(DNS) and large-eddy simulation(LES) of turbulent channel flows.It is found from the comparisons that the LES with an eddy-viscosity subgrid scale(SGS) model over-predicts the space-time correlations than the DNS.The overpredictions are further quantified by the integral scales of directional correlations and convection velocities.A physical argument for the overprediction is provided that the eddy-viscosity SGS model alone does not includes the backscatter effects although it correctly represents the energy dissipations of SGS motions.This argument is confirmed by the recently developed elliptic model for space-time correlations in turbulent shear flows.It suggests that enstrophy is crucial to the LES prediction of spacetime correlations.The random forcing models and stochastic SGS models are proposed to overcome the overpredictions on space-time correlations.
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
