Application of LES to CFD simulation of Diesel combustion

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Application of LES to CFD simulation of Diesel
combustion
3604A058-2 Fumio KUWABARA
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Background
Diesel Combustion
Internal conditions
Turbulent flow etc.
CFD code
Prediction Method
RANS
Now
LES
Future ?
Calculation Results
Process
Ignition, Combustion, products
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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

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What is Eddy?
Small Eddies
Large Eddies

• 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

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

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Turbulence Simulation (comparison)
Reynolds -Averaged Numerical Simulation
More Computational Effort Precision
Large Eddy Simulation
More useful
Direct Numerical Simulation
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Navier - Stokes Equations
Navier - Stokes Equations
for an incompressible fluid
Unsteady
Advection
Pressure
Viscosity
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RANS What is RANS?
fluctuating parts
mean
Time
Decompose velocity into mean and fluctuating
parts
Reynolds -Average
RANS doesnt resolve any scales of turbulence at
all !
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RANS RANS equation
Reynolds -Averaged Navier -Stokes Equations
Additional term
Reynolds stresses
Closure Problem
Turbulence Model
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RANS Eddy viscosity model
RANS equations require closure for Reynolds
stresses
Mean velocity
Turbulent Viscosity
Dissipation Rate of Turbulent Kinetic Energy
Turbulent Kinetic Energy
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RANS k-emodel
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.
k equation
? equation
empirical constants
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RANS Result
Before
After
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LES What is LES?
This technique resolves the largest scales of
turbulence and models the smaller scales.
important
Large eddies
directly resolved
turbulent flow
not so important
Small eddies
modeled
Spatial filter
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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
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LES LES equation
The Filtered Equations
Additional term
Subgrid Scale (SGS)Stress
SGS Closure Problem
Smagorinsky model
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LES Smagorinsky model
LES equations require closure for SGS stresses.
SGS eddy Viscosity
empirical constants (theory value)
need for adjustment to turbulent flow !
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LES Result
Before
After
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A Study of application of LES
About Nishiwakis Study
Table 1 Calculation conditions
Cylinder borestroke (mm) 82.6114.3
Compression ration 8.0
Intake valve closure 146 deg.BTDC
Engine Speed (rpm) 600
Wall temp. (K) const. 460
equivalent ratio f 0.55
SGS model Cs 0.2
Fuel isooctane
Fig. 1 Computational grid system
Reactions29, Chemical species20
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Results
Temp.
RANS
LES
RHR
Fig. 2 Fields of Temp and RHR at TDC
calculated by RANS(Left) ,LES(Right)
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Criticism

• RANS??????????????????????????????????.
• ??????????????????????????????????,???????????????
  .
• LES??,???????????????????????.

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Future prospect on LES

• ?????????????????????????????????.????,???????????
  ???????????????????????.
• ??????,?????????????????????????.?????????????????
  ???????,??????????????????????.
• NOX ,?????????????????????,???????(??)????????.

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THE END