报告摘要：All turbulent flowsare unsteady by nature. Even if the mean flow can be regarded as steady (ande.g. two-dimensional) the turbulence is always unsteady (andthree-dimensional). In some simple attached flows, the mean flow andcorresponding turbulence structure can be correctly captured by usingconventional models employed in (steady/unsteady) RANS (Reynolds-AveragedNavier Stokes) framework. However, in configurations featured by flow separatedfrom curved continuous walls (characterized by intermittent separation region)the fluctuating turbulence associated with the separated shear layer has to beappropriately resolved in order to capture even the mean flow properties. Atthis point, the hybrid LES/RANS methods come into play. Their aim is to combinethe advantages of both RANS and LES methods in order to provide a computationalprocedure that is capable to affordably capture the unsteadiness of the flow.
The basic of any Hybrid RANS/LES method is a RANS-basedmodel formulation describing the modelled fraction of turbulence. This model isappropriately “sensitized” to account for turbulence unsteadiness (fluctuatingturbulence) by introducing either
• a grid-spacing-dependent filter parameter (mostly in thelength-scale determining equation – PANS/PITM or νt -equation/expression –DES-related schemes/VLES) or
• the von Karman length scale Lνk=κS/|▽2U| – nominally agrid-spacing-free model formulation (SAS-related models).
Accordingly, the model equations (formulated and validatedwithin the Steady RANS framework, describing the fully-modeled turbulence),adopt automatically (by interplaying with the grid resolution) to thehighly-unsteady (unresolved, residual) sub-scale turbulence. Followingeddy-resolving methods are presently in focus:
VLES (Very Large-Eddy Simulation) and PANS(Partially-Averaged Navier Stokes) - seamless, variable-resolution hybridLES/RANS models. In both methods a four-equation, anisotropy-resolvingeddy-viscosity model, based on the elliptic-relaxation method was employed tomimic the sub-scale model seamlessly in the entire flow domain. Whereas thedestruction term in the equation governing the scale-supplying variable isappropriately modelled in the PANS framework, the VLES method is concerned withappropriate suppression of the turbulent viscosity in the equation of motiondirectly. Such actions cause turbulence level to be suppressed towards the‘sub-scale’ (‘sub-filter’) level. Herewith, the development of the structuralcharacteristics of the flow and associated turbulence is enabled.
An instability-sensitive Second-Moment Closure model forunsteady flow computations. The model scheme adopted, functioning as a‘sub-scale’ model in the Unsteady RANS framework, represents a differentialnear-wall Reynolds stress model formulated in conjunction with thescale-supplying equation governing the homogeneous part of the inverseturbulent time scale. The model capability to account for the vortex length andtime scales variability was enabled through a selective enhancement of the productionof the dissipation rate in line with the SAS proposal (Scale-AdaptiveSimulation, Menter and Egorov, 2010) pertinent particularly to the highlyunsteady separated shear layer region. In all cases considered the fluctuatingvelocity field was obtained started from the steady RANS results. The modelproposed does not comprise any parameter depending explicitly on the gridspacing.
The predictive performances of the proposed models areintensively validated in numerous aerodynamic- type flows of differentcomplexity featured by 2D and 3D separation (see the list of references). Theseas well as the results obtained by the consequent models application to someconfigurations relevant to car aerodynamics and IC engines will be presented.
报告人简介：Prof.Jakirlic graduated from the University of Sarajevo in Mechanical Engineering(Dipl.-Ing., 1983), followed by a Masters degree in Computational FluidMechanics (M.Sc., 1991). He has received his PhD degree at the University ofErlangen/Nuremberg in 1997 and his Habilitation in Fluid Mechanics at theUniversity of Darmstadt in 2004. Since 1997 he has been heading the group forModelling and Simulation of Turbulent Flows at the Institute of Fluid Mechanicsand Aerodynamics, Technische Universität Darmstadt, Germany. He isEditor-in-Chief of the Int. Journal of Heat and Fluid Flow (Elsevier SciencePublisher) and Coordinator of the ERCOFTAC (European Research Community onFlow, Turbulence and Combustion) Special Interest Group on Refined TurbulenceModelling. He is furthermore the Organizing Committee Member of the ConferenceSeries on Turbulence, Heat and Mass Transfer (THMT). His field of interest isthe Computational Fluid Dynamics focusing on the RANS (with special focus onthe near-wall second-moment closure models) and hybrid LES/RANS modelling ofturbulent single and two-phase flows and heat transfer.