CFD ACTIVITY AT DIEM DEPARTMENT - unibo.it PRESENTATI… · CFD ACTIVITY AT DIEM DEPARTMENT. DIEM...

1/73DIEM OVERVIEW

University of BolognaUniversity of Bologna

School School of of Engineering Engineering -- D.I.E.M.D.I.E.M.

CFD ACTIVITY AT DIEM DEPARTMENTCFD ACTIVITY AT DIEM DEPARTMENT

2/73DIEM OVERVIEW



Introducing D.I.E.M.

Review of main projects: Basic Research

Review of main projects: Design support

SI IGNITION MODEL

LES

WALL FILM MODEL

PRESENTATION OUTLINE

A Focus On:

3/73DIEM OVERVIEW



Human Resources

Hardware Facilities

CFD Codes

Research and Application Fields

Contracts

A Way to Work

INTRODUCING D.I.E.M.

4/73DIEM OVERVIEW



FULL PROFESSOR: Piero Pelloni

ASSOCIATED PROFESSOR: Gian Marco Bianchi

ASSISTANT PROFESSORS: Stefania Falfari,PhD

Giulio Cazzoli, PhD

PERMANENT ENGINEERS: Federico Brusiani, PhD

Claudio Forte

Ph.D. STUDENTS: Sergio Negro (2008->)

ENGINEER FELLOWS: Marco Costa

Rodolfo Piccioli Giacomo Bastia

INTRODUCING DIEM Human Resources

5/73DIEM OVERVIEW



DELL Blade 20 processor Linux Cluster

Linux Cluster (9 processors)

DELL-Linux Cluster (5 processors)

8 DELL Workstation Two-Processors

INTRODUCING DIEM Hardware Facilities

6/73DIEM OVERVIEW



INTRODUCING DIEM CFD Codes

ENGINE SIMULATIONS:

PREMIXED – NON-PREMIXED COMBUSTION KIVA3_UNIBO

PFI-GDI SIMULATION FIRE8.4

LES SIMULATION FLUENT 6.2

OpenFOAM 4.1

GENERAL FLUID-DYNAMICS: FLUENT 6.2

HYDRAULIC SIMULATION OF

FLUID-POWER/ INJECTION SYSTEMS: AMESIM

LIQUID JET ATOMIZATION: SURFER

7/73DIEM OVERVIEW



CFD ANALYSIS OF ICE INTAKE/EXHAUST SYSTEMS

CFD ANALYSIS OF ICE COMBUSTION SYSTEMS

CFD ANALYSIS OF VACUUM PUMP

ANALYSIS OF INJECTION SYSTEMS

ANALYSIS OF FLUID-POWER SYSTEMS

INTRODUCING DIEM Research and application Fields

8/73DIEM OVERVIEW



FERRARI RACING DE LONGHI

DUCATI RACING DVP VACUUM TECHNOLOGY

PIAGGIO & C S.p.A VARIAN VACUUM TECHNOLOGY

VM-MOTORI BUCHER HYDRAULICS

INTERNAL COMBUSTION ENGINES GENERAL CFD APPLICATIONS

INTRODUCING DIEM Contracts

Former Partner: Magneti Marelli, FIAT-GM Powertrain

9/73DIEM OVERVIEW



ADVANCED RESEARCH

DESIGN SUPPORT OR TECHNOLOGY TRANSFER TO COMPANYCFD is available: it must be used since it offers a unique tool of investigation

- New CFD methodologies or models development - Knowledge of model behaviour via implementation in our own codes- Test of models and numerics - Investigations of basics physics

INTRODUCING DIEM A way of work: To know and to apply

10/73DIEM OVERVIEW



Liquid Jet Atomization

3-Phase Nozzle Flow Simulation

OOP CFD Code

REVIEW OF MAIN PROJECTSBasic Research

11/73DIEM OVERVIEW



REVIEW OF MAIN PROJECTS Liquid Jet Atomization

ANALYSIS OF BASIC PROCESSES VIA QUASI-DIRECT SIMULATION

IMPROVEMENT OF ATOMIZATION MODELS

3D INVESTIGATION OF ATOMIZATION BASIC PHYSICS

RELATED MAIN PAPER: Bianchi, G.M., et al. ” 3D Large Scale Simulation of the High Speed Liquid Jet

Atomization”, SAE 2007-01-0244 Accepted for -SAE Journal of Engines

CFD Grid

1 mm0

10

20

30

40

50

60

70

80

90

100

Caso

turbolento

Caso

laminare

Nucleo

Struttureliquide

Turbulent LaminarTurbulent Laminar

Core

Detached

liquid

12/73DIEM OVERVIEW



REVIEW OF MAIN PROJECTS Liquid Jet Atomization

DEVELOPMENT OF ATOMIZATION MODEL FOR LIQUID JET

BASED ON SIMPLE 2D SIMULATIONS

0

2 104

4 104

6 104

8 104

1 105

0 5 10 15 20 25 30 35 40

100 m/s200 m/s400 m/s

pd

f

Droplets Diameter (µm)

DROPLET SIZE PDF

21

22

2

2

2

)()2()2(

1)(

χχχχ

χχχχχχχχ−−−−−−−−

ΓΓΓΓ⋅⋅⋅⋅==== e

np

n

nn

500

300

200

100

400

150

250

Injection velocity

[m/s]

500

300

200

100

400

150

250

Injection velocity

[m/s]

30

1

15

Back density

[kg/m3]

30

1

15

Back density

[kg/m3]

30

10

20

Mean fluctuation level

[% of vinj]

30

10

20

Mean fluctuation level

[% of vinj]

130

200

Orifice diameter

[µm]

130

200

Orifice diameter

[µm]

126 CASES

ILASS 2004HTCE 2004

RELATED MAIN PAPERS: Bianchi, G.M., et al. (ASME PAPER No. ICEF2004-848 and Proceeding ILASS 2004)

13/73DIEM OVERVIEW



REVIEW OF MAIN PROJECTS 3-Phase Nozzle Flow RANS Simulation

Internal Flow

Simulation

Generations of

Nozzle FilesSpray

Simulation

FROM NOZZLES TO SPRAYS

Internal Flow

Simulation

Generations of

Nozzle FilesSpray

Simulation

FROM NOZZLES TO SPRAYS

COUPLED NOZZLE FLOW – LIQUID JET ATOMIZATION

RELATED MAIN PAPERS: AVL AST UGM 2005

14/73DIEM OVERVIEW



REVIEW OF MAIN PROJECTS OOP Code

DEVELOPMENT OF A NEW PARALLEL OBJECT ORIENTED

CODE FOR COMPUTATIONAL CONTINUUM MECHANICS

(CCM) IN FORTRAN 95.

NEMO -Numerical Engine for MultiphysicsOperators

Contributors:

- University of Bologna – Main Driver (with fundamental contribution by Dr. Stefano Toninel)

- University of Rome ”Tor Vergata” (Prof. Bella & Dr. Filippone): parallel solvers.

- University of Massachusetts at Amherst (Prof. Schmidt): node smoothing algorithm

based on cell quality

http://www.ce.uniroma2.it/nemo/

15/73DIEM OVERVIEW



ICE Non-Reactive Flow Simulation

SI Engines Combustion Optimization

HSDI Diesel Engine Modeling

Port-Fuel Optimization in SI Engines

Injection System Development

REVIEW OF MAIN PROJECTSDesign Support

16/73DIEM OVERVIEW



APPLICATION OF BOUNDARY LAYER SOLUTION IN RANS STEADY FLOW SIMULATION

REVIEW OF MAIN PROJECTS ICE Non-Reactive Flow Simulation

Recirculation

NORecirculation

Valve lift

Discharge coeff.

W.F.

T.B.L

EXP

RELATED MAIN PAPERS: SAE Papers: 2002-01-1118, 2003-01-0003 (Both in Journal of Engines)

Two-layer Wall-functions

17/73DIEM OVERVIEW



13.480 bar

13.490 bar

13.500 bar

13.510 bar

13.520 bar

13.530 bar

13.540 bar

13.550 bar

13.560 bar

13.570 bar

13.580 bar

13.590 bar

64.00 °

[BTDC]

64.50 °

[BTDC]

65.00 °

[BTDC]

65.50 °

[BTDC]

66.00 °

[BTDC]

66.50 °

[BTDC]

67.00 °

[BTDC]

67.50 °

[BTDC]

68.00 °

[BTDC]

68.50 °

[BTDC]

69.00 °

[BTDC]

Anticipo Ottimale

p583

p583cmd052a

p681

p681cmdStudio2

p681cdm053EStep4

Studio1

Studio2

Studio2cmdp583

Studio2cmdp681

p747cdm053EStep4

p747cdm053EStep4.2

p747cdmp681

13.480 bar

13.490 bar

13.500 bar

13.510 bar

13.520 bar

13.530 bar

13.540 bar

13.550 bar

13.560 bar

13.570 bar

13.580 bar

13.590 bar

64.00 °

[BTDC]

64.50 °

[BTDC]

65.00 °

[BTDC]

65.50 °

[BTDC]

66.00 °

[BTDC]

66.50 °

[BTDC]

67.00 °

[BTDC]

67.50 °

[BTDC]

68.00 °

[BTDC]

68.50 °

[BTDC]

69.00 °

[BTDC]

Anticipo Ottimale

p583

p583cmd052a

p681

p681cmdStudio2

p681cdm053EStep4

Studio1

Studio2

Studio2cmdp583

Studio2cmdp681

p747cdm053EStep4

p747cdm053EStep4.2

p747cdmp681

+0.59% PMI

-2.78%ANT.

START CONFIGURATION

IME

P

SPARK ADVANCE

REVIEW OF MAIN PROJECTS SI Engines Combustion System Optimization

OPTIMIZATION OF RACING ENGINE COMBUSTION EFFICIENCY: CFD IN DESIGN STEPS FERRARI F1 (1996-2005) AND DUCATI MotoGp (2003->)

18/73DIEM OVERVIEW



DEVELOPMENT OF NON-EQUILIBRIUM TURBULENCE CORRECTION

ADVANCED COUPLING BETWEEN INJECTOR FLOWAND ATOMIZATION MODEL

DEVELOPMENT OF SUITABLE ATOMIZATION MODELS

APPLICATION TO HSDI DIESEL ENGINE DEVELOPMENT

LOW COMBUSTION TEMPERATURE CONCEPTDEVELOPMENT BASED ON HIGHLY COOLED EGR RATE

REVIEW OF MAIN PROJECTS HSDI Diesel Engine Modeling

RELATED MAIN PAPERS: SAE Papers: 2000-01-1179, 2001-01-1068, 2002-01-1115

IN COOPERATION WITH VM-Motori (1996->) AND FIAT-GM (2003-2004)

19/73DIEM OVERVIEW



REVIEW OF MAIN PROJECTS Port-Fuel Optimization in SI Engines

MIXTURE FORMATION ANALYSIS

WALL-FILM ANALYSIS

EFFECT OF LUBRICANT CONTAMINATION WITH FUEL

DEVELOPMENT OF SUITABLE METHODOLOGIES FOR SPRAY COMPUTATIONS

PFI TWO-PHASE SIMULATION

20/73DIEM OVERVIEW




0

0,025

0,05

0,075

0,1

0

0,5

1

1,5

2

360 420 480 540 600 660

Advanced injection

Retarded injection

Standard injection

Perc

en

tag

e o

f to

tal

ma

ss

in

jecte

d [

%]

Pe

rce

nta

ge

of to

tal m

ass

inje

cte

d (s

tan

dard

cas

e) [%

]

Angle [°]

CFD PROVIDED A ZERO COST SOLUTION TO REDUCE LUBRICANT CONTAMINATION WITH FUEL

RELATED MAIN PAPERS: SAE Papers 2006-32-0022 (In Journal of Engines) and 2007-24-0041

21/73DIEM OVERVIEW




Racing Application

RELATED MAIN PAPERS: BEST PAPER AWARD AVL – UGM 2005

22/73DIEM OVERVIEW



DEVELOPMENT OF ELECTRO-MAGNETIC MODEL

DEVELOPMENT OF ADVANCED DRIVING CIRCUIT

1D-3D SIMULATION OF INJECTOR AND SYSTEM DYNAMICS FOR DESIGN

SIMULATION OF TWO-PHASE FLOW

COUPLING BETWEEN INTERNAL NOZZLE FLOWAND ATOMIZATION MODEL

REVIEW OF MAIN PROJECTS Injection System Development

RELATED MAIN PAPER: SAE Papers: 2000-01-2042, 2003-01-0006, 2004-01-0019, 2005-01-1236

(all in SAE Journals), ASME PAPER No. ICEF2004-847

A PROJECT IN COOPERATION WITH MAGNETI MARELLI (2001-2005)

23/73DIEM OVERVIEW



FEATURES

BACKGROUND

BASIC TEST CASE

ENGINE APPLICATION

SI IGNITION MODEL

24/73DIEM OVERVIEW



FLAME KERNEL FORMATION IS A CRITICAL ISSUE IN ICE SINCE:

ITS FEATURES AFFECT THE COMBUSTION DURATION

CYCLE-TO-CYCLE VARIABILITY AFFECTS RIGHT EARLY BURNING RATE

THE 5%MFB TAKES 35-40% OF TOTAL COMBUSTION DURATION

PHYSICS OF SPARK DISCHARGE

FLOW CONVECTIVE EFFECTS (ARC ELONGATION)

TURBULENCE

PHYSICAL CHARACTERISTICS OF THE MIXTURE (p, T, φφφφ or pdf of φφφφ)

PHYSICAL PROCESS TO BE CONSIDERED

SI IGNITION MODEL Background

RELATED MAIN PAPERS: SAE Papers 2007-01-0148 (In Journal of Engines)

25/73DIEM OVERVIEW



Ignition time and lenght scales are too small to be resolved during

kernel formation

Electrical circuit too complex to be modelled in detail

Coupling with main combustion model

Grid dependency

Many 1D models decoupled from CFD solver have

been presented and referenced.

They would be considered all Eulerian except AKTIM AND DPKI.

SI IGNITION MODEL Background

WHY ARE CFD IGNITION MODELS NOT COMPLETELY RELIABLE ?

26/73DIEM OVERVIEW



Electrical sub-model for Spark plug

Lump model based on the main available specifications

Lagrangian kernel submodel

Reignition submodel (Simple first step)

Mass and energy conservation solved in a variable control volume defined by mean flame surface

Coupling with main solver enforced with particular emphasis on flame surface

SI IGNITION MODEL Features

27/73DIEM OVERVIEW



LUMP/EASY MODELLING (NO WEAK POINT)

PLASMA FORMATION NEGLECTED (time and length scales too short)

gapbd

bdbd

dC

VE

⋅=

2

2

unb

unb

bi T

T

T

kT

+

−= 11

1

2/1

0

,

1

1

−

⋅−

=

πi

unbgap

bdik

T

Tdp

E

k

kr

),( tEfE spglow =&

Energy released during breakdown and glow phases:

Initial kernel conditions after plasma formation (Song and Sunwoo, 2000) :

(Duclos and Colin, 2001)

SI IGNITION MODEL Features: Spark Model

28/73DIEM OVERVIEW



AFTER THE PLASMA PHASE:

One flame kernel is deposited and initialized

Flame Kernel is discretized by a set of triangular elements which expand radially

Each of these elements varies its area surface because of expansion and wrinkling by turbulence

It contributes to reaction rate in its own reference fluid cells I-th

I-th cell

SI IGNITION MODEL Features: Kernel Model

29/73DIEM OVERVIEW



−−Ξ⋅=

dt

dp

pdt

dT

TA

Vs

dt

dr k

kk

kklam

k

unbk 11,

ρ

ρ

Mass Conservation for a lagrangian system

)(, Ξ⋅⋅⋅= kklamunb

kAs

dt

dmρ

Rearranged -> An expression for the mean kernel expansion rate

Turbulence wrinkling


Energy Conservation for open system

30/73DIEM OVERVIEW



MASS SOURCE TERMS IN I-TH CELL FROM FUEL CONSUMPTION RATE

thiklamunbthik s −− Σ⋅⋅= ,, ρω&

ENERGY SOURCE TERMS IN I-TH CELL FROM:

1. Fuel oxidation2. Spark discharge during breakdown and glow phase

(Equivalent to ECFM)


31/73DIEM OVERVIEW



FLAME SURFACE DENSITY IN I-TH CELL CONTINUOSLY UPDATED

thi

thithi

V

S

−

−−

Ξ⋅=Σ ∑

=−− =

M

i

thjkthi AS1

,j-th kernel element

surface


IGNITION MODEL IS SWITCHED OFF ONCE THE KERNEL RADIUS IS 2.00 mm -> Reasonable flow lenght resolved in RANS (grid size 0.5 -> 1 mm)

MASS SOURCE TERMS IN I-TH CELL FROM FUEL CONSUMPTION RATE

thiklamunbthik s −− Σ⋅⋅= ,, ρω& (Equivalent to ECFM)

32/73DIEM OVERVIEW



Main Combustion model ECFM – Flamelet

Mono-component Fuel CnHm according to actual gasoline fuel used

Laminar flame speed

( )EGRLL Xp

p

T

Tss 21

00

0 −⋅

⋅

⋅=

βα

Metchalghi & Keck

Meintjes and MorganPost-flame chemistry

Flame front chemistry 1-step chemistry

SI IGNITION MODEL Features: Combustion Model

33/73DIEM OVERVIEW



Herweg and Maly (SAE paper 922243).

Side chamber to promote a swirl motion in the incoming charge.

The effect of different parameters on the kernel development has been investigated.

Spark main characteristics: CDI (6mJ) and TCI (60 mJ) Systems

Mixture properties (λλλλ): 1.00 to 1.30

Turbulence: 0.44 to 1.09 m/s

Flow velocity at spark plug: 7.50 to 31.10 m/s

SI IGNITION MODEL Basic Test Case

34/73DIEM OVERVIEW



0

2

4

6

8

10

0 200 400 600 800 1000 1200 1400 1600

COARSE

MEDIUM

FINE

Fla

me

su

rfa

ce [

cm

2]

Time [µs]

GRID SIZE SENSITIVITY

0

1

2

3

4

5

6

0 200 400 600 800 1000

10002500500010000

Fla

me s

urf

ace [

cm

2]

Time [µs]

TRIANGULAR ELEMENT SENSITIVITY

To ECFM

COARSE: 2.0 mmMEDIUM: 1.0 mmFINE: 0.5 mm

100025005000

10000

To ECFM


35/73DIEM OVERVIEW



0

1 104

2 104

3 104

4 104

5 104

6 104

7 104

0 50 100 150 200 250

1.0

1.3

1.5

Kern

el te

mpe

ratu

re [K

]

Time [µs]

0

0.5

1

1.5

2

0 50 100 150 200 250

1.0

1.3

1.5

La

gra

ng

ian

ke

rnel ra

diu

s [

mm

]

Time [µs]

EFFECT OF MIXTURE AIR INDEX @ 1250 rev/min


36/73DIEM OVERVIEW



0

0.5

1

1.5

2

0 0.2 0.4 0.6 0.8 1 1.2

1.0 Exp1.0 Sim1.3 Exp1.3 Sim1.5 Exp1.5 Sim

Tim

e a

fte

r spark

onse

t [m

s]

Flame kernel volume [cm3]

0

0.5

1

1.5

2

2.5

3

3.5

4

0 100 200 300 400 500 600

1.0

1.3

1.5

Fla

me s

urf

ace [

cm

2]

Time [µs]

Predictions only

Models slightly under predicts expansion rate (No tuning done)


EFFECT OF MIXTURE AIR INDEX @ 1250 rev/min

37/73DIEM OVERVIEW



Effect of flow convection - turbulence – λλλλ=1300 rpm 700 rpm 1250 rpm

100 µµµµs

250 µµµµs

500 µµµµs

625 µµµµs

SI IGNITION MODEL

38/73DIEM OVERVIEW



λλλλ=1 λλλλ=1 .3 λλλλ=1.5

TCI[60 mJ]

CDI [6 mJ]

EFFECT OF AIR INDEX VARIATION CAPTURED

FLAME HOLDER EFFECT MODELLED


39/73DIEM OVERVIEW



SI IGNITION MODEL Engine Case

APPLIED TO SI ENGINE TO RECOVERC CYCLIC VARIABILITY

EXPERIMENTAL DATA (DUCATI CORSE SRL) ON REAL ENGINE CONFIGURATIONS

SHOW THE CLOSE RELATIONSHIP BETWEEN THE MEAN LAMBDA CYCLE BY CYCLE

VARIATION) OF THE ENGINE

THE IGNITION MODEL HAS BEEN MODIFIED IN ORDER TO ANALYSE IN DETAIL THE

INFLUENCE OF MIXTURE UNIFORMITY ON CYCLIC VARIATION

40/73DIEM OVERVIEW




Racing Application

41/73DIEM OVERVIEW




Racing Application

42/73DIEM OVERVIEW



HOW? ICE STEADY FLOW

HOW? BASIC TEST-CASE AND MERIT INDEX

WHEN? ENGINE CASES

LES

OPEN QUESTIONS: HOW AND WHEN ?

43/73DIEM OVERVIEW



LES OPEN QUESTIONS: HOW AND WHEN?

TWO OPEN ISSUES:

HOW (DOES LES HAVE TO BE APPLIED) ?

WHEN (CAN LES BE REASONABLY APPLIED) ?

Focus: simulation of complex case

We do not present only results but the way we got them

Be aware of LES quality at least to know that it is V-LES

RELATED MAIN PAPERS: SAE Papers 2007-01-4145, Best Paper Award Fluent Italia UGM 2006

POLICY

44/73DIEM OVERVIEW




HOW ? These is the minimum check list before performing LES

LOW KIN. ENERGY DISSIPATION SCHEME REQUIRED (2nd, 3rd)

CFL NUMBERS REQUIRED BELOW ONE

CHECK ON CELL SHAPE INFLUENCE

CHECK ON TIME WINDOW LENGTH

PROPER BOUNDARY CONDITIONS

45/73DIEM OVERVIEW




HOW ? These is the minimum check list before performing LES

MERIT INDEX TO KNOW THE REAL FRACTION OF ENERGY RESOLVED

(NOBODY SHOWS …)

FILTER SIZE ADAPTED TO LOCAL FLOW CONDITIONS

WALL-FUNCTIONS ?

GRID REFINEMENT OR COARSENING EFFECT ON EN. SPECTRA

(Mov. Boundary)

46/73DIEM OVERVIEW



Bounded flow;

Easy 3D geometry;

Flow detachment/reattachment.

Numerical setup in Fluent 6.2

Segregated flow solver together with a fully implicit second-order scheme;

Diffusive fluxes are discretized by using central differencing scheme;

Convective fluxes are discretized by using bounded central differencing scheme;

LES model: WALE and Localized Dynamic 1-EQ. Model (integration at wall)

TEST CASE: Flow over a backward facing step

(Eaton, Johnston, Westphal, Ames Research Center NASA, 1986)

1 2 3

Mean axial velocity profile

4 5 6Flow direction

LES HOW? BASIC TEST-CASE AND MERIT INDEX

Most suitable commercial code for LES

47/73DIEM OVERVIEW



A LES simulation is accurate when (one needs to know the energy

fraction really resolved (see Pope):

2.0),(),(

),(),( ≤

+=

txktxK

txktxM

sgs

sgs

vrr ),( tx

r∆


A refinement procedure has to be used1>+

y15≥Rν

Seven refinement steps

(from 0.39 MCells to 2.0 MCells)

An auto-adaptive LES simulation is complete

48/73DIEM OVERVIEW



0

2

4

6

8

10

12

14

-0,4 0 0,4 0,8 1,2

Y [

cm

]

U/Uo

Experimental

LDKEM

0

2

4

6

8

10

12

14

-0,4 0 0,4 0,8 1,2

Y [

cm

]

U/Uo

0

2

4

6

8

10

12

14

-0,4 0 0,4 0,8 1,2

Y [

cm

]

U/Uo

0

2

4

6

8

10

12

14

-0,4 0 0,4 0,8 1,2

Y [

cm

]

U/Uo

Secti

on

1

Secti

on

2

Secti

on

3

Secti

on

5

(2 M. of cells)


49/73DIEM OVERVIEW



M parameter has been used as final merit index.


0

2

4

6

8

10

12

14

0 0,2 0,4 0,6 0,8 1

Y [

cm

]

M

0

2

4

6

8

10

12

14

0 0,2 0,4 0,6 0,8 1

Y [

cm

]

M

0

2

4

6

8

10

12

14

0 0,2 0,4 0,6 0,8 1

Y [

cm

]

M

Secti

on

2

Secti

on

3

Secti

on

5

0.39 Mcells

2.0 Mcells

50/73DIEM OVERVIEW



10Valve lift [mm]

132L2 [mm]

300L1 [mm]

34D3 [mm]

16D2 [mm]

120D1 [mm]

ICE STEADY (NON-REACTIVE) FLOWThobois, Rymer, Souleres, Poinsot, SAE Paper, 2004

LDA velocity profiles are available on two planes at 20mm and 70mm from cylinder head.

20m

m

70m

m

LES HOW? BASIC ICE TEST-CASE

Numerical Settings: same as previous using Fluent 6.2

Ckecking: WALE with and w/o wall function

LDKEM -> Merit index for evaluating LES

51/73DIEM OVERVIEW




Questions:

1. How far is LES from providing results in ICE case

(complex geometry, complex wall-bounded effects, ecc) ?

2. Is LES ready for being applied with accuracy or

V-LES is likely performed ?

3. What LES can bring more with respect to RANS ?

52/73DIEM OVERVIEW



Mesh Generation

It provides a good solution level for the turbulent scales in the flow core.

2 Million of cells.

5030 ÷≈+y

WALE

4.3 Million of cells.

1≈+y

LDKEM

It provides a quasi-complete LES simulation on all the flow domain execept in turbulent boundary layer.


53/73DIEM OVERVIEW



-0,6

-0,4

-0,2

0

0,2

0,4

0,6

0,8

1

1,2

-1 -0,5 0 0,5 1

LES_LDKEM

LES_WALE

LDA

U(t

)/U

o

r/R

-0,6

-0,4

-0,2

0

0,2

0,4

0,6

0,8

1

1,2

-1 -0,5 0 0,5 1

LES_LDKEM

LES_WALE

LDA

U(t

)/U

o

r/R

20mm

70mm

�

�

On the 20mm plane, both LES models allow to obtain a reasonable good agreement between numerical and experimental profiles.

On the 70mm plane, both LES models fit the overallexperimental trend except for the flow under the valve.


54/73DIEM OVERVIEW



0

0,1

0,2

0,3

0,4

0,5

-1 -0,5 0 0,5 1

LES_LDKEM

LES_WALE

LDA

u'/U

o

r/R

0

0,1

0,2

0,3

0,4

0,5

-1 -0,5 0 0,5 1

LES_LDKEM

LES_WALE

LDA

u'/U

o

r/R

20mm

70mm

�

�

On the 20mm plane, both WALE and LDKEM sgs models match with good agreement the experimental trend. The dynamic model provides better predictions in the evaluation of peak positions and magnitudes.

On 70mm plane, both LES models fit the overallexperimental trend on all the section.


55/73DIEM OVERVIEW



LDKEM Model: M Parameter

0

0,1

0,2

0,3

0,4

-1 -0,5 0 0,5 1

M on the plane at 20mm

M

r/R

0

0,1

0,2

0,3

0,4

-1 -0,5 0 0,5 1

M on the plane at 70mm

M

r/R


20mm

70mm

56/73DIEM OVERVIEW



LES WHEN? ENGINE CASES

Answer (steady flow condition, fixed grid):

1. LES (V-LES) is close to provide answer every time investigationof large-scale unsteadiness is required (lack of experiment)

2. LES might provide effective information of flow separation by adverse pressure gradient accepting a compromise in turbulentboundary layer solution (RANS does it better ?)

3. Still issues in wall-bounded flow solution -> friction prediction

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WHEN ?

LARGE SCALE UNSTEADINESS (I.E., MIXING, DROPLET CONVECTION)

LARGE SCALE UNSTEADINESS INDUCED BY FLOW SEPARATION

BENCHMARK FOR IMPROVING RANS SIMULATIONS

ACOUSTICS

CYCLE-TO-CYCLE VARIABILITY EFFECT IN ICE FLOWS (IFP, …)

LIQUID JET ATOMIZATION

COMBUSTION ? -> Through RANS model (Flame thickness too thin)

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IMPROVING RANS MODELING (ICE TEST-CASE)


LES

RANS 1

NORANS 2

YES

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IMPROVING RANS MODELING

MEAN FLOW PEDICTIONS IN RANS LES OFFERS MORE FOR (LARGE) SCALEFLOW UNSTEADINESS


Non-dimensional meax axial velocity Non-dimensional rms axial velocity fluctuation

Non-dimensional Radial Distance Non-dimensional Radial Distance

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Racing Application

61/73DIEM OVERVIEW




Racing Application

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Racing Application

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TIPS TO MAKE IT WORK

BASIC EQUATION

BENCHMARK

WALL FILM MODEL

PHYSICAL ISSUES

Related Main Papers: ICES2005-1063, SAE 2007-24-0087

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INTERACTION

WALL

SPRAY

GAS

Two-dimensional flux over three-dimensional surfaces

�Impinging Spray

�Heat transfer with wall and gas

�Fuel evaporation

�Gravity and other body forces

�Shear forces at the interface with gas and wall

WALL FILM PHYSICAL ISSUES

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HYPOTHESIS

Boundary-layer approximation

Laminar Flow

Incompressible Flow

Newtonian FluidThe equations of mass, momentum and energyare written and integrated following a finite volume

approach and ALE time advancement:

( )∑ =⋅+∆

∆ Nside

i w

diiif

w A

SlnV

At ρδ

δˆ

1 r

( )wp

H

wp

wNside

i wp

g

iiifi

w Ac

S

Ac

J

Ac

JlnVT

At

T

ρρρδ ++=⋅+

∆

∆∑ ˆ

1 r

( )( ) ( ) ( )tan

1 1 1 1ˆ ˆ

NedgeNside Nsidef

f f i i i i i ii

i i iw w w w

VV V n l pn l g M A

t A A A A

δδ δ δ τ

ρ ρ ρ

∆+ ⋅ = − + + +

∆∑ ∑ ∑

rr r rr r

WALL FILM BASIC EQUATIONS

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A relative system of coordinates is created on each face

WALL FILM TIPS TO MAKE IT WORK

x’

y’

z’

pgp

p

The different film heights between neighbour cells must be taken into account by appling

the film pressure over the common contact area, while the gas pressure is applied over

the remaining boundary area

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The approximation of constant

film height over the control

volume does not permit to

reconstruct the characteristics

of the interface: high numerical

diffusion and overextimation of

evaporation,

WALL FILM TIPS TO MAKE IT WORK

In order to reproduce the effect of

surface tension a fitting minimum

threshold value is used to impose a

minimum film height into a control

volume the area for surface

evaporation is accordingly

evaluated

Real gas-film interface of boundary edge

Computed gas-film interface of boundary

edge

Real gas-film interface of boundary edge

Computed gas-film interface of boundary

edge

Real interface

Computed

with

threshold

Min Height

Yes No

Real interface

Computed

with

threshold

Min Height

Real interface

Computed

with

threshold

Min Height

Yes No

68/73DIEM OVERVIEW



174 barInjection pressure

19 C.A.Duration of injection

-19 ATDCStart of injection

12.3 mg/cycleInjected mass

2.94 barInlet air pressure

673 KWall temperature

DieselFuel

14Compression ratio

750 rpmEngine speed

8 Number of cylinders

225 mmStroke

150 mmBore

Two-Stroke DI Diesel Engine

Injection and Combustion simulation

Simulation between IVC and EVO

Small angle between spray direction and bowl wall

Prevalent impinging regime is Stick

WALL FILM VALIDATION: DIESEL

Test Case:

Stanton, D., and Rutland, C. J., 1998. “Multi-dimensional modeling of heat and mass transfer of fuel films resulting from impinging sprays”. SAE Paper 980132

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Sensor 1

Sensors position

Sensor 3

WALL FILM VALIDATION: DIESEL

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0.42m3/minAir flow rate

21.5mm3Injected quantity per shot

40HzInjection frequency

45 °Angle injector/tube

6.7msInjection Duration

30°Spray Cone Angle

16m/sDroplet Velocity

90µmSauter Mean Diameter

Reproduces injection and liquid-film

conditions similar to those occuring in PFI gasoline engines

Prevalent impinging regime is Stick

Characterized by low Weber number

Pulsed injection with 8 injection events

WALL FILM VALIDATION: PFI

Test case:

Le Coz, J. F., Catalano, C., and Baritaud, T., 1994. “Application of laser induced fluorescence for measuring the thickness of liquid films on transparent wall”. In 7th International Symposium on Application of Laser Techniques to Fluid Mechanics.

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Sensor 2Sensor 1

WALL FILM VALIDATION: PFI

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WALL FILM VALIDATION: PFIF

ilm

He

igh

t

Sensor 1

Comparison with Fluent and Fire 8.4

FIREFLUENT

KIVA_UNIBO

EXPERIMENTS

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WALL FILM MODEL INTERFACE

INTERFACE

Wall Film

Model

GeneralCFD CODE

The integration of the equation of the model is fully explicitand it is implemented in Fortran 77

In order to make the model totally indipendent from the CFD code it has been created an interface for the comunication of the variables of interest

INPUT DATA• Geometry information of wall cells- compatibilty with 4 edge-faces tested- mixed 3edge- 4edge faces to be tested

• Velocity and mass of the impinging droplets• Thermodynamic information of the surrounding gas• Temperature of the wall• Shear stress of gas on the wall

OUTPUT DATA• Wall film height evolution• Source terms because of film evaporation

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