Application of LES to CFD simulation of Diesel combustion 3604A058-2 Fumio KUWABARA

Background Ignition, Combustion, products LES RANS Prediction Method CFD code Turbulent flow etc. Internal conditions Diesel Combustion Future ? Calculation Results Process Now

Key aspects of turbulence Unsteady, aperiodic motion Turbulence is characterized by eddies or instabilities Largest eddies are the same scale as the flow and are often anisotropic Smaller eddies form off the larger eddies and become more isotropic at smaller scales

What is Eddy?  Large eddies : anisotropic Large eddies extract energy from the flow Large eddies are and carry most of the turbulent energy Directly affecting the mean fields  Small eddies : isotropic Smaller eddies extract energy from larger eddies The smaller scales act mainly as a sink for the turbulent energy Small Eddies Large Eddies

What is Turbulence Model? turbulent flow resolved flow not resolved flow Turbulence Model Operation: Turbulence Simulation Separate the flow field

Turbulence Simulation Direct Numerical Simulation (DNS) –Resolves the whole spectrum of scales –No modeling is required Large Eddy Simulation (LES) –Large eddies are directly resolved –Smaller eddies are modeled Reynolds -Averaged Numerical Simulation （ RANS ） –Solves “averaged” Navier-Stokes equations –The most widely used approach for industrial flows

Turbulence Simulation （ comparison ） Large Eddy Simulation Direct Numerical Simulation Reynolds -Averaged Numerical Simulation More useful More Computational Effort & Precision

Navier - Stokes Equations Unsteady AdvectionPressureViscosity Navier - Stokes Equations for an incompressible fluid:

RANS : What is RANS? Time Decompose velocity into mean and fluctuating parts: Reynolds -Average mean fluctuating parts RANS doesn’t resolve any scales of turbulence at all !

RANS : RANS equation Reynolds stresses Reynolds -Averaged Navier -Stokes Equations Additional term Closure Problem Turbulence Model

RANS : Eddy viscosity model RANS equations require closure for Reynolds stresses: Turbulent Viscosity: Turbulent Kinetic Energy: Dissipation Rate of Turbulent Kinetic Energy: Mean velocity

RANS : k-εmodel k equation  equation empirical constants Turbulent viscosity is determined from Transport equations for turbulent kinetic energy and dissipation rate are solved so that turbulent viscosity can be computed for RANS equations.

RANS : Result Before After

LES : What is LES? turbulent flow Large eddies Small eddies Spatial filter directly resolved modeled important not so important This technique resolves the largest scales of turbulence and models the smaller scales.

LES : Spatial filter Select a spatial filter function G Define the resolved-scale (large-eddy): Find the unresolved-scale (small-eddy ): GridScale SubGridScale All Scale

LES : LES equation Subgrid Scale （ SGS ） Stress SGS Closure Problem Smagorinsky model Additional term The Filtered Equations

LES : Smagorinsky model LES equations require closure for SGS stresses. empirical constants （ theory value ） SGS eddy Viscosity need for adjustment to turbulent flow !

LES : Result Before After

A Study of application of LES Fig. 1 Computational grid system Cylinder bore×stroke (mm)82.6×114.3 Compression ration8.0 Intake valve closure146 deg.BTDC Engine Speed (rpm)600 Wall temp. (K) const.460 equivalent ratio φ 0.55 Table 1 Calculation conditions Reactions:29, Chemical species20 SGS model C s =0.2 About Nishiwaki’s Study Fuel : isooctane

Results RHR Temp. Fig. 2 Fields of Temp and RHR at TDC calculated by RANS （ Left ）,LES （ Right ） RANSLES

Criticism RANS モデルでは捕らることができない自着火 空間分布を予測できる可能性がある． モデル定数の補正が必要となるスマゴリンス キーモデルを導入しているため，モデルの変更 を考える必要がある． LES では，噴流の濃度・空間的変化について把 握することが重要．

Future prospect on LES エンジン内流れのサイクル平均ではない非定常流れ として直接解析できる．そのため，ノッキングなど のサイクル変動に起因する現象メカニズムの解明に つながる． 乱流中の噴霧，燃焼過程を普遍性のある物理モデル で表すことができる．流れパターンなどに一貫した モデルを使用することで，新しい機構･代替燃料の導 入に際しても適用可能． NO X ，すすなどの微量有害物質の生成予測に対して は，瞬時・局所の温度（濃度）分布の予測が可能．

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