les of internal combustion engine flows using cartesian...
TRANSCRIPT
LES of Internal Combustion Engine Flows Using Cartesian Overset Grids
T. Falkenstein1, S. Kang2, M. Davidovic1, M. Bode1, H. Pitsch1 1 Institute for Combustion Technology RWTH Aachen University 2 Sogang University, South Korea
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Cartesian Grids for Complex Geometries?
• High-Quality LES Solution • Insignificant Meshing Effort
• Uncertainty in LES Grid Quality
Reduced to Choosing Appropriate
Resolution
• Computational Performance
Academic Interest Industrial Application
Challenges
• Local Grid Refinement
• Wall Treatment
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Contents
• Motivation
• Numerical Framework
o Baseline Code
o Recent Enhancements
• Fundamental Verification
• Validation on Steady Engine Port Flow
• Sensitivities
• Conclusions
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Spatial Discretization
• Momentum Eq.: Central Difference
• Scalar Eq.: WENO This Study: 2nd Order Accuracy
Features of CIAO Flow Solver
Incompressible
Low-Mach Number
Compressible
Governing Equations
Physical Models
Representation of Turbulence
DNS
RANS
LES
Multiphase
Combustion
SFS Model
• Dyn. Smagorinsky
• Coherent Structure
• Sigma
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Spatial Discretization
• Momentum Eq.: Central Difference
• Scalar Eq.: WENO This Study: 2nd Order Accuracy
Features of CIAO Flow Solver
Governing Equations
Incompressible
Low-Mach Number
Compressible
Physical Models
Representation of Turbulence
DNS
RANS
LES
Multiphase
Combustion
SFS Model
• Dyn. Smagorinsky
• Coherent Structure
• Sigma
Numerical Schemes
• 2nd-order low-storage RK
• 6th-order spatial filtering to
remove spurious wiggles
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Enhanced Numerical Framework
Fluid
Kang, S., Iaccarino, G., Ham, F., Moin, P., Prediction of wall-pressure fluctuation in turbulent flows with an immersed boundary method, Journal of Computational Physics 228
(2009) 6753–6772
Gaitonde, D. V., Visbal, M. R., Padé-Type Higher-Order Boundary Filters for the Navier–Stokes Equations, AIAA Journal Vol. 38, No. 11, November 2000
Roman, F.; Armenio, V.; Fröhlich, J., A simple wall-layer model for large eddy simulation with immersed boundary method, Physics of Fluids, Volume 21, Issue 10, pp. 101701-
101701-4 (2009)
Immersed Boundary Method Wall Model
Velo
city
Err
or
y-location
Prev. Revised
Velo
city
Err
or
y-location
Overset Grid Method Higher-Order Filtering Near Walls
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Overset Grid Method
• Patches with different grid spacing can be flexibly
combined during mesh generation (no AMR)
• Hole Cutting:
o Equations are solved on the finest available patch &
small overlap region
• Currently, all patches are integrated on the same
time step
• Same RK scheme as baseline solver, with additional
coupling procedures after each solution variable has
been updated
x x x x x x x x x x x x x x x x x x x x x x x x x
Overlap
grid1
grid2 Masked cells on grid1,
active (fine) cells on grid2
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Contents
• Motivation
• Numerical Framework
o Baseline Code
o Recent Enhancements
• Fundamental Verification
• Validation on Steady Engine Port Flow
• Sensitivities
• Conclusions
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Overset Grid Method Verification
• Method has 2nd-order accuracy
• Can be effectively used to reduce
computational cost
Inviscid Vortex Convection (1)
Base grid
Overset
2x finer peri
odic
U0
peri
odic
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Overset Grid Method Validation
• Inviscid Vortex Convection (2)
Inviscid Vortex Convection (2)
Base grid
Overset
2x finer peri
odic
U0
peri
odic
• Mass and kin. energy are well conserved
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Overset Grid Method Validation
• Inviscid Vortex Convection (2)
Inviscid Vortex Convection (2)
Base grid
Overset
2x finer peri
odic
U0
peri
odic
• Vortex well preserved after 1 cyle
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
1) Wall Treatment: Approx. Immersed Boundary Method
• Exact solution prescribed at IB
• Nearly 2nd-order in both cases
Grid Convergence Study on Taylor-Green Vortex Problem
Case A
Case B
IB
IB
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
2) Wall Treatment: Wall Function
• Here, a combination of two models for
the shear stress is used:
o Eddy-Viscosity-based model for IBs
(Roman et al.)
o Body-Fitted approach based on modified
shear stress (Lee at al.), if BC location is
close to actual IB wall location
• Both models assume LOG-Law
o Velocity BC and shear stress / eddy
viscosity are evaluated from time-
averaged velocity field
Wall Model
IB
BC
IP
IB : Actual Wall Location
BC: Velocity Boundary Condition
IP : Interpolation of Avg. LES Solution
𝑦𝐵𝐶
Roman, F., Armenio, V., Froehlich, J.: „A Simple Wall-Layer Model for Large-Eddy Simulations with Immersed Boundary Method“, Physics of
Fluids Vol. 21, 101701, 2009
Lee, J., Cho, M., Choi, H.: „Large-Eddy Simulations of Turbulent Channel Flows at High Reynolds Number with Mean Wall Shear Stress
Boundary Condition“, Physics of Fluids Vol. 25, 110808, 2013
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
2) Wall Treatment: Wall Function
• Here, a combination of two models for
the shear stress is used:
o Eddy-Viscosity-based model for IBs
(Roman et al.)
o Body-Fitted approach based on modified
shear stress (Lee at al.), if BC location is
close to actual IB wall location
• Both models assume LOG-Law
o Velocity BC and shear stress / eddy
viscosity are evaluated from time-
averaged velocity field
Wall Model Channel Flow (Re = 2000)
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Contents
• Motivation
• Numerical Framework
o Baseline Code
o Recent Enhancements
• Fundamental Verification
• Validation on Steady Engine Port Flow
• Sensitivities
• Conclusions
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Validation Case: Compressible Flowbench
• Production SI Engine Geometry with High Tumble
• Valve Lift: 8 mm
• Air as Working Fluid
• Inlet & Outlet Pressures Prescribed
• Resulting Mass Flow is Similar to Max. at 3000rpm
• PIV Measurements in Planes Perpendicular
to Cylinder Axis
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Simulation Setup
• Non-Reflective Boundary Conditons at
Inlet and Outlet
• Dyn. Smagorinsky Model with
Lagrangian Averaging
Name # of Cells Lift / x
Port 1.7 Mio. 16
Cyl. 1 39.4 Mio. 32
Cyl. 2 5.4 Mio. 16
𝑃𝑡𝑜𝑡
𝑃𝑠𝑡𝑎𝑡
Wall Model
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Characteristic Flow Quantities
• Mach Numbers Up to 0.4
• Reynolds Numbers on the Order of 100,000
• Pressure Loss Occurs Mostly as Flow
Passes Valves
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Integral Flow Measures
• Overprediction of Mass Flow by ~3 %
• Good Agreemen in Tumble Intensity
Mass Flow Tumble Ratio
Numerical Wall Treatment is Suitable
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
In-Cylinder Flow Field
Simulation PIV
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Comparison with PIV Data: Vertical Velocity
• Avg. Vertical Velociy Component in Good
Agreement with Experimental Data
• RMS Profile Seems Shifted, But Similar
Magnitudes
𝑥𝑚𝑖𝑛
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Comparison with PIV Data: Horizontal Velocity
• Avg. x-Velociy Component Overall in
Good Agreement, but Some Deviation
Near Cylinder Axis
• RMS Profile Agrees Well with PIV
𝑥𝑚𝑖𝑛
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Contents
• Motivation
• Numerical Framework
o Baseline Code
o Recent Enhancements
• Fundamental Verification
• Validation on Steady Engine Port Flow
• Sensitivities
• Conclusions
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
• Similar Results, But Statistics Not Yet Converged
Sensitivity of Vertical Velocity to SGS Model
Dyn. Smag. PIV CSM
Menevau, C.; Lund, T.S.; Cabot, W.H.: “A Lagrangian dynamic subgrid-scale model of turbulence” Journal of Fluid Mechanics 319, pp. 353–385, 1996
Kobayashi, H.: „The subgrid-scale models based on coherent structures for rotating homogeneous turbulence and turbulent channel flow “,
Phys. Fluids, Vol. 17, pp. 045104-045104-12 (2005)
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Effect of Applied Methods on Integral Flow Quantities
• Mass Flow (=Pressure Loss) is Very
Sensitive to Wall-Numerical Method
• Tumble is Very Sensitive to Wall Model
Mass Flow Tumble Ratio
Case 1 Single Grid (SG); Prev. Code
Case 2 SG; IB; Higher Order Filter
Case 3 Overset Grids; Refined Near Valves
Case 4 OG; Wallfunction in Port
Case 5 OG; Refined Down to z=-D
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Contents
• Motivation
• Numerical Framework
o Baseline Code
o Recent Enhancements
• Fundamental Verification
• Validation on Steady Engine Port Flow
• Sensitivities
• Conclusions
Institute for Combustion Technology | Prof. Dr.-Ing. H. Pitsch
Tobias Falkenstein
Conclusions
• Methods to Reduce Drawbacks of Cartesian Approach in Complex Wall-
Bounded Flows have been Implemented
• 2nd-order accuracy has been demonstrated
• Pressure Loss in Port Flow is Governed by Singular/Local Losses in the Valve
Region
o Additional Friction from Wall Model has Minor Effect
o Accuracy of Upstream Velocity Field is Most Important
• Successful Validation of Extended Framework in Steady SI Engine Port Flow
• Next steps
o Finalize Developments for Moving Geometries
Acknowledgements
Honda R & D
Gauss Centre for Supercomputing e.V.
Leibniz Supercomputing Centre
Thank you for your attention
Tobias Falkenstein Institute for Combustion Technology RWTH Aachen University
http://www.itv.rwth-aachen.de