SAE Paper 2007-01-0170
Automatic Mesh Motion with Topological Changes for Engine Simulation T. Lucchini, G. D’Errico Department of Energetics, Politecnico di Milano, Italy
H. Jasak WIKKI Ltd, London, England
Z. Tuković FSB, University of Zagreb, Croatia
TOPICS • INTRODUCTION • MESH MOTION FOR I.C. ENGINES Topological changes Automatic mesh motion • ENGINE MESH SETUP • VALIDATION
CFD SIMULATION OF I.C. ENGINES INTERACTING THERMO-FLUID DYNAMIC PROCESSES •
Turbulent, compressible flow;
•
Fuel spray;
•
Ignition and combustion;
•
Pollutants formation;
GEOMETRICAL CONSTRAINTS •
Complex geometry;
•
Moving boundaries;
NUMERICAL EFFICIENCY
STATE OF ART OF MESH MOTION GEOMETRY •COMPLEX COMPLEX GEOMETRY • Unstructured grids; • Moving piston and valves, ports; • High mesh quality required for the whole simulation; •MESH MESHMOTION MOTION REQUIRED REQUIRED • Pre-processing mesh tools for mesh motion; • Significant manual work required; • Mesh motion is not solution-dependent;
AIM OF THE WORK DEVELOPING A NEW APPROACH FOR MESH MOTION • No pre-processing. Mesh motion integrated in the solver, at any time step: Grid points moved; Mesh topology eventually changed; • Multiple-region decomposition: in each region, mesh motion is accommodated in different ways; • Combined use of different topological changes; • Polyhedral vertex based motion solver for mesh deformation based on Finite Element Method (FEM);
OpenFOAM (Field Operation and Manipulation) OVERVIEW OF THE CODE • C++ object-oriented; • New models easily developed and tested in isolation;
∂ρYtf + ∇ ⋅ ( ρUYtf ) + ∇ ⋅ ( µT ∇Ytf ) = 0 • ∂t solve ( fvm::ddt(rho, Ytf) + fvm::div(phi, Ytf) + fvm::laplacian(mut, Ytf) );
OpenFOAM (Field Operation and Manipulation) FINITE VOLUME METHOD • Polyhedral mesh support; • Numerical schemes available for temporal and spatial discretization; • Reliable and validated libraries for: Combustion (premixed and non-premixed); Complex chemistry; Lagrangian spray modelling; Turbulence (RANS and LES);
MESH MOTION FOR I.C. ENGINES • TOPOLOGICAL CHANGES Dynamic mesh layering Sliding interface Attach/detach boundary • AUTOMATIC MESH MOTION Polyhedral vertex based motion solver
DYNAMIC MESH LAYERING • Keep an optimum mesh size during the whole cycle; • User definition: maximum and minimum thicknesses, base surface; Addition/removal of cell layers in a moving cone
MESH MOTION FOR I.C. ENGINES • TOPOLOGICAL CHANGES Dynamic mesh layering Sliding interface Attach/detach boundary • AUTOMATIC MESH MOTION Polyhedral vertex based motion solver
SLIDING INTERFACE • Dynamically connecting different mesh regions by point projection algorithm; • Mesh quality is kept high, no distortions; • User definition of “master” and “slave” patches; Mixer vessel
Flow control device
MESH MOTION FOR I.C. ENGINES • TOPOLOGICAL CHANGES Dynamic mesh layering Sliding interface Attach/detach boundary • AUTOMATIC MESH MOTION Polyhedral vertex based motion solver
ATTACH-DETACH BOUNDARY • Splits the mesh in two separate regions from a list of internal faces; • Simulates valve closure; • “Minimum lift” to impose valve closure; • No geometry modifications or “inert cells” required;
Valve closure time
MESH MOTION FOR I.C. ENGINES • TOPOLOGICAL CHANGES Dynamic mesh layering Sliding interface Attach/detach boundary • AUTOMATIC MESH MOTION Polyhedral vertex based motion solver
POLYHEDRAL VERTEX-BASED MOTION SOLVER • Grid is deformed and mesh refined around TDC; • Solve the Laplace equation of motion; • Tetrahedral decomposition of the polyhedral mesh to keep the mesh valid; FEM decomposition of a polyhedral cell
POLYHEDRAL VERTEX-BASED MOTION SOLVER Mesh deformation around TDC for a pent-roof, SI engine during the intake stroke (lift from 1 mm to 5 mm) 1) MOTION EQUATION
∇i( γ∇u ) = 0 2) NEW POINT POSITION
x new = x old + u∆t
ENGINE MESH SETUP: TWO-STROKE ENGINES
TWO-STROKE ENGINES: MESH SETUP THREE REGIONS
Cylinder
Exhaust port
Intake ports
TWO-STROKE ENGINES: MESH MOTION COMBINED USE OF MULTIPLE TOPOLOGICAL CHANGES 1) PISTON MOTION • dynamic layering • deformation 2) PORTS-CYLINDER CONNECTION • sliding-interface
ENGINE MESH SETUP: FOUR-STROKE ENGINES
FOUR-STROKE ENGINES: MESH SETUP Remainder of the cylinder
FIVE REGIONS Intake and exhaust ports
Valve curtains
FOUR-STROKE ENGINES: MESH MOTION COMBINED USE OF MULTIPLE TOPOLOGICAL CHANGES 1) PISTON MOTION • dynamic layering • deformation 2) VALVE MOTION • dynamic layering • deformation 3) VALVE CLOSURE • attach-detach boundary 4) CYLINDER-VALVE CURTAINS • sliding-interface
VALIDATION
• SCAVENGING IN A TWO-STROKE ENGINE • COMBUSTION IN A SIDE-VALVE ENGINE • DIESEL ENGINE COMBUSTION • INTAKE STROKE IN A FOUR-STROKE ENGINE
SCAVENGING IN A TWO-STROKE ENGINE ENGINE GEOMETRY Bore Stroke Comp. Ratio Speed Boost pressure
COMPUTATIONAL MESH
66.5 mm 57 mm 10.8 2500 rpm 1.05 bar
PHYSICAL MODELS AND BOUNDARY CONDITIONS • k-ε turbulence model; • No slip at walls; • Total pressure at intake; • Fixed temperature at walls;
Axial-symmetric No. of cells at BDC
25000
No. of cells at TDC
8000
SCAVENGING IN A TWO-STROKE ENGINE EVOLUTION OF EGR AND IN-CYLINDER FLOW FIELD
SCAVENGING IN A TWO-STROKE ENGINE Radial velocity at different locations
• High velocities close to the scavenging ports • Tumble created by the piston motion
SCAVENGING IN A TWO-STROKE ENGINE Radial velocity at different locations Turbulence intensity at different locations
• High u’ produced by the incoming air jet • Turbulence decay at the end of compression
VALIDATION
• SCAVENGING IN A TWO-STROKE ENGINE • COMBUSTION IN A SIDE-VALVE ENGINE • DIESEL ENGINE COMBUSTION • INTAKE STROKE IN A FOUR-STROKE ENGINE
COMBUSTION IN A SIDE-VALVE ENGINE ENGINE GEOMETRY
COMPUTATIONAL MESH
MV-CAGIVA GABBIANO NV 2 Bore Stroke
98 mm 73.2 mm
Comp. Ratio Speed
• Sliding interface models the partial side head covering by the piston
8 1000 rpm
PHYSICAL MODELS AND BOUNDARY CONDITIONS • k-ε turbulence model; • Weller b-Ξ combustion model; • Fixed temperature and no-slip at walls;
Compression-Combustion No. of cells at BDC
60000
No. of cells at TDC
30000
COMBUSTION IN A SIDE-VALVE ENGINE EVOLUTION OF THE REGRESS VARIABLE b (b=0: fully burnt, b=1: fully unburnt) DURING THE COMBUSTION PROCESS
Predicted cylinder pressure trace
• Quasi constant-volume combustion correctly predicted by the model;
VALIDATION
• SCAVENGING IN A TWO-STROKE ENGINE • COMBUSTION IN A SIDE-VALVE ENGINE • DIESEL ENGINE COMBUSTION • INTAKE STROKE IN A FOUR-STROKE ENGINE
DIESEL ENGINE COMBUSTION ENGINE GEOMETRY
COMPUTATIONAL MESH
SEATEK 850 PLUS Bore
127 mm
Stroke
134 mm
Comp. Ratio Speed
14 3100 rpm
Swirl Ratio Boost pressure
2.5 4 bar
PHYSICAL MODELS AND BOUNDARY CONDITIONS • Lagrangian spray modelling; • Complex chemistry and ISAT to model combustion; • RNG k-ε turbulence model;
Compression-Combustion No. of cells at BDC
45000
No. of cells at TDC
7000
DIESEL ENGINE COMBUSTION FUEL INJECTION AND COMBUSTION
Predicted cylinder pressure
• Agreement with experimental cylinder pressure data, Lagrangian particles correctly tracked also with dynamic-layering;
VALIDATION
• SCAVENGING IN A TWO-STROKE ENGINE • COMBUSTION IN A SIDE-VALVE ENGINE • DIESEL ENGINE COMBUSTION • INTAKE STROKE IN A FOUR-STROKE ENGINE
INTAKE STROKE IN A FOUR-STROKE ENGINE ENGINE GEOMETRY Bore Stroke
COMPUTATIONAL MESH 100 mm 92 mm
Comp. Ratio Speed
14 3000 rpm
PHYSICAL MODELS AND BOUNDARY CONDITIONS • k-ε turbulence model; • Fixed temperature and no-slip at walls;
No. of cells at BDC
200000
No. of cells at TDC
40000
INTAKE STROKE IN A FOUR-STROKE ENGINE PREDICTED FLOW-FIELD AND TURBULENCE INTENSITY – INTAKE STROKE
• Large ring vortex below the valve moving downward
INTAKE STROKE IN A FOUR-STROKE ENGINE PREDICTED FLOW-FIELD AND TURBULENCE INTENSITY – INTAKE STROKE
• Turbulence generated by high velocity and flow deflection
INTAKE STROKE IN A FOUR-STROKE ENGINE Velocity and turbulence intensity distribution across the valve lift
Typical values of
U/sp ≈ 10 correctly predicted
INTAKE STROKE IN A FOUR-STROKE ENGINE In-cylinder turbulence intensity evolution
u 'TDC ≅ 0.5sp
CONCLUSIONS
CONCLUSIONS PROPOSED MESH MOTION APPROACH SUCCESSFULLY TESTED • Engine geometries; • Compressible flow equations; • Turbulence; • Scalar transport (EGR); • Premixed combustion; • Lagrangian particle tracking; • Complex chemistry;
CONCLUSIONS GENERALITY OF THE APPROACH • Finite Volume Method + Moving Meshes; DEVELOPMENTS • Engines with canted valves; • Rotary machines: Wankel engines; Automotive CFD (superchargers, fuel pumps,...);
ACKNOWLEDGMENTS
• Dr. Hrvoje Jasak • Dr. Zeljko Tukovic • Dr. Gianluca D’Errico • Mr. Mario Mazuran and Mr. Luciano Spaggiari • MV Agusta S.p.A. and SEATEK S.p.A.
THANKS FOR YOUR ATTENTION! Tommaso Lucchini Politecnico di Milano, Dipartimento di Energetica Via La Masa, 34 20158 Milano (Italy) +39 02 23 99 86 36
[email protected] www.engines.polimi.it
