automotive aerodynamics-optimization---2013-07-17
DESCRIPTION
How do car companies optimize aerodynamic design?TRANSCRIPT
© 2011 ANSYS, Inc. April 11, 20231
Details of the Automotive Aerodynamics Optimization work featured in this Bloomberg Businessweek article in the March 11-17, 2013 issue, are presented here
© 2011 ANSYS, Inc. April 11, 20232
Automotive Aerodynamics Optimization
Sandeep Sovani, Ph.D.
Global Automotive Strategy and MarketingANSYS Inc, Detroit, USAJuly 17, 2012
Ashok KhondgeLead Technology SpecialistANSYS India Pvt Ltd, Pune India
© 2011 ANSYS, Inc. April 11, 20233
Aerodynamics Optimization Methods
Parametric Method – 50:50:50 Method
Gradient Based Method – Adjoint Method
Agenda
© 2011 ANSYS, Inc. April 11, 20234
• Method 1 – Parametric Method– Parameterize vehicle shape– Change shape parameters and run numerous simulations– Generate response surfaces and optimize vehicle shape
• Method 2 – Gradient Based Method– Solve for flowfield of baseline shape– Perturb shape and calculate derivative of drag w.r.t. to shape– Identify shape changes yield most improvement in drag
Aerodynamics Optimization Methods
© 2011 ANSYS, Inc. April 11, 20235
Aerodynamics Optimization Methods
Parametric Method – 50:50:50 Method
Gradient Based Method – Adjoint Method
Agenda
© 2011 ANSYS, Inc. April 11, 20236
• Aerodynamic Optimization via shape exploration– Parameterize vehicle shape– Change shape parameters and run numerous simulations– Generate response surfaces and optimize vehicle shape
• Ideal Aerodynamics Optimization Process– Ability (to explore a large design space)– Automatic (with least human effort)– Fast (Fits in the vehicle development process)– Accurate (High Fidelity Meshes, Physics Models)
Introduction
© 2011 ANSYS, Inc. April 11, 202377
ThreeEssentials
PAPER 2012-01-0174
50 M
illio
n
Cells
SPEED
HoursDays
Weeks
5 M
illio
n Ce
lls25
Mill
ion
Cells
EXTE
NT 50 Design Pts
25 Design Pts
5 Design PtsACCURACY
Introduction
© 2011 ANSYS, Inc. April 11, 20238
Introduction
The 50:50:50 Method
50 50 design points in the design space EXTENT
50 50 million cells used in CFD simulation of each design point ACCURACY
50 50 hours total elapsed time to simulate all the design points SPEED
“One – Click” – Entire design space is simulated and post-processed completely automatically after the initial baseline case setup
© 2011 ANSYS, Inc. April 11, 20239
MethodologyPrepare Meshed Model for
Baseline Vehicle Shape
CFD Solver Setup, Define Shape Parameters
Generate DOE using Input Shape Parameters
Collate Data,Perform Optimization
Morph Vehicle Shape
Run CFD Simulation
STEP 1
STEP 2
STEP 3
STEP 4
STEP 5
ANSY
S W
ORK
BEN
CH
© 2011 ANSYS, Inc. April 11, 202310
• To Demonstrate 50:50:50 Method– Volvo XC60 vehicle model– Four shape parameters– RBF Morph (Integrated within
FLUENT) to define shape parameters
• ANSYS WorkBench (Frame Work to Automate Process)– To drive shape parameters– To create DOE– To perform Goal Driven
Optimization
Test Case Description
© 2011 ANSYS, Inc. April 11, 202311
Methodology – ANSYS WB
STEP 3
STEP 5
STEP 2
STEP 4
© 2011 ANSYS, Inc. April 11, 202312
• Surface mesh – ANSA• Surface mesh size
– Front facia : 3.0 to 4.0 mm, – Windshield : 4.0 to 5.0 mm– Doors &windows : 5.0 to 6.0 mm– Roof : 6.0 to 8.0 mm – Rear : 4.0 to 5.0 mm– Underbody : 5.0 to 6.0 mm
Step #1 : Baseline Model Creation
© 2011 ANSYS, Inc. April 11, 20231313 PAPER 2012-01-0174
Prism Layer• 10 Layers (First Aspect Ratio 10, Growth 1.1)• 24.4 million cells (about half of total cells are in prism layer
Cell size needed if using cartesian cells with same cell count
Number of cartesian cells needed to achieve same near wall resolution 550 million!
Step #1 : Baseline Model Creation
© 2011 ANSYS, Inc. April 11, 202314
• Volume Mesh – TGrid• Cell Count : 50.2 Million Cells• Prism Layers : 10 (First Aspect Ratio 10,
Growth 1.1)• Prism Count : 24.4 Million Cells• Skewness < 0.9
Step #1 : Baseline Model Creation
Prism Layers
Cut Plane Y=0
Cut Plane Z = 1.4 m
© 2011 ANSYS, Inc. April 11, 202315
• Boundary Conditions– Inlet : Velocity Inlet 100 kmph– Outlet : Pressure Outlet, 0 Pa (Gage)– Side walls : Wall, no-slip– Top wall : Wall, no-slip
• Solver Settings– Steady, PBCS, Green Gauss Node
Based Gradient– Fluid : Incompressible air, – Density = 1.225 kg/m3
– Turbulence : Realizable K-epsilon, Non-equilibrium wall treatment
– Discretization : • Pressure – Standard • Momentum, TKE, TDR – 2nd Order
Step #2 : CFD Setup
• Solution Controls– Courant Number = 200– ERF
Momentum, Pressure = 0.7
– URFs Density = 1.0, Body Forces = 1.0TKE, TDR = 0.8TR = 1.0
© 2011 ANSYS, Inc. April 11, 202316
Step #2 : RBF Morph
• RBF Morph : Add-On Module– Fully Integrated within ANSYS
FLUENT with GUI/TUI
• Uses Radial Basis Function Technique for Mesh Morphing– System of radial basis function is
used to produce solution for mesh movement
– List of source points and their displacements are used as input
– Valid for both surface shape change and volume mesh smoothing
• Developed by RBF Morph http://www.rbf-morph.com/ RBF GUI
© 2011 ANSYS, Inc. April 11, 202317
Step #2 : Shape Parameters - Definition
1. Boat Tail Angle (P2)Constraint :
Point “B” to move upto 20 mm in
+ve /-ve Y-direction about the Pivot axis
2. Long Roof Drop Angle (P3)Constraint :
Rear edge to move
Upto 30 mm in +ve Z-direction
Upto 45 mm in -ve Z-direction about the Pivot axis
© 2011 ANSYS, Inc. April 11, 202318
Step #2 : Shape Parameters - Definition
3. Green House (P4)Constraint:
Point “A” to move 20 mm in
+ve /-ve Y-direction about the Pivot axis
4. Front Spoiler Angle (P5)Constraint:
Point “C” to move 30 mm in +ve Z direction
© 2011 ANSYS, Inc. April 11, 202319
Axis about which selected surface set gets morphedEncapsulation RegionSurface set selection
Step #2 : Setup – Boat tail angle (P2)
© 2011 ANSYS, Inc. April 11, 202320
Step #2 : Setup – Long roof drop angle (P3)
Encapsulation RegionSurface set selection Axis about which selected surface set gets morphed
© 2011 ANSYS, Inc. April 11, 202321
P2 – Boat Tail Angle
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P3 – Long Roof Drop Angle
© 2011 ANSYS, Inc. April 11, 202323
P4 – Green House Angle
© 2011 ANSYS, Inc. April 11, 202324
P5 – Front Spoiler Angle
© 2011 ANSYS, Inc. April 11, 202325
• RBF Morph shape parameters– Define in FLUENT – Available in WB for
• Input Shape Parameters– P2 : Boat tail angle– P3 : Long roof drop angle– P4 : Green house – P5 : Front spoiler
• Output Parameter– P1: Drag Force on vehicle
• DOE Algorithm– Central Composite Design– Design Type : Face Centered with
Enhanced Template– 49 DOE Points Generated
Step #3 : Design Space
Design Space BoundsParameter Min Baseline Max
Boat tail angle - 1.85 0.0 + 1.85
Long roof drop angle - 2.30 0.0 + 1.50
Green House Angle - 0.70 0.0 + 0.70
Front Spoiler Angle 0.0 + 3.80
© 2011 ANSYS, Inc. April 11, 202326
DOE Table
© 2011 ANSYS, Inc. April 11, 202327
Step #4 : Running Simulations
• Current Study– Simulations were run outside of
WorkBench using journal file in batch mode
– Output the drag force – DOE table updated by
importing output parameters
• Five Runs Using – 768, 384, 288, 240, 144 Cores
• Convergence Monitors– Drag force– Pressure /velocity at few points
in wake
Design point # 1
Design point # 2
Design point # 50
© 2011 ANSYS, Inc. April 11, 202328
Step #4 : Running Simulations
768 Cores 384 Cores 288 Cores 240 Cores 144 Cores
Task Time (Seconds) Time (Seconds) Time (Seconds)
Time (Seconds)
Time (Seconds)
Baseline Case (i.e. Design Point 1)
Read volume mesh of baseline case into the CFD solver and apply solver settings
225 340 365 481 228
CFD Solution 6979 11153 14409 17256 27246
Writing CFD data file 681 538 558 600 532
Each Subsequent Design Point
Morph vehicle shape 84 59 65 69 100
CFD Solution 1284 1754 2208 2630 4100
Writing CFD data file 734 559 572 621 532
Total Run Time (Wall Clock) Needed for All 50 Design Points (Hours)
30.80 35.63 42.98 50.28 72.19
If data file is not written at each data point, then 50 hours target is achieved in less than 200 cores
Same study was repeated with newer hardware. 50 hours target is achieved in roughly 150 cores
© 2011 ANSYS, Inc. April 11, 202329
Step #4 : Running Simulations
Compute Cluster Details
1. Intel’s Endeavor Cluster
2. Intel Xeon X5670 (dual socket)
3. Clock speed 2.93 GHz
4. Six cores per socket (12 cores per node)
5. 24 GB RAM @ 1333 MHz, SMT ON, Turbo ON
6. QDR Infiniband
7. RHEL Server Release 6.1
© 2011 ANSYS, Inc. April 11, 202330
Updated DOE Table
© 2011 ANSYS, Inc. April 11, 202331
• Response Surface Analysis– Non Parametric Regression (NPR) Algorithm– Plots
• Goodness of fit• 2d / 3d Response surface plots• Sensitivity plots• Parallel Co-ordinates (Pareto) Plots• Trade-Off plots
• Optimization Study– Goal driven optimization – Screening algorithm (no of samples = 5000)– NLPQL (Non-Linear Programming by Quadratic Lagrangian)
• Flow Results
Results
© 2011 ANSYS, Inc. April 11, 202332
Results : Response Surface Plots
© 2011 ANSYS, Inc. April 11, 202333
Results : Response Surface Plots
© 2011 ANSYS, Inc. April 11, 202334
Results : Response Surface Plots
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Results : Sensitivity Plots
Local Sensitivity Global Sensitivity
© 2011 ANSYS, Inc. April 11, 202336
Results : Parallel Co-ordinates (Pareto) Plot
© 2011 ANSYS, Inc. April 11, 202337
Results : Trade-off Plots
© 2011 ANSYS, Inc. April 11, 202338
Results : Goal Driven Optimization
© 2011 ANSYS, Inc. April 11, 202339
Results : Goal Driven Optimization
© 2011 ANSYS, Inc. April 11, 202340
Flow Results
Design Points
Boat Tail Angle(P2)
Long Roof Angle(P3)
Green House (P4)
Front Spoiler Angle (P5)
Drag Force (N) (P1)
1 0.000 0.000 0.000 0.000 388.01
9 0.000 1.500 0.000 1.900 393.01
19 1.850 -2.300 -0.700 0.000 372.30
25 -1.850 1.500 -0.700 0.000 397.33
• Flow Results Discussion– Design point 1, 9, 19 & 25– Velocity contours– Iso-surface of total pressure = 0.0
© 2011 ANSYS, Inc. April 11, 202341
Summary of 50:50:50 Method
The 50:50:50 Method
50 50 design points in the design space EXTENT
50 50 million cells used in CFD simulation of each design point ACCURACY
50 50 hours total elapsed time to simulate all the design points SPEED
“One – Click” – Entire design space is simulated and post-processed completely automatically after the initial baseline case setup
© 2011 ANSYS, Inc. April 11, 202342
• The 50:50:50 Method– An extensive, fast, and accurate DOE method– In case study: 4% drag reduction achieved in 1 week of work
• Fully automated workflow (after baseline case setup) using industry leading technologies– FLUENT Solver (With High Performance Computing)– RBF Morph (Fast, User-Friendly, Accurate Mesh Morpher)– ANSYS WorkBench (Integration Platform)– Design Xplorer (Optimization)– CFD Post (Flow Visualization)
Summary of 50:50:50 Method
© 2011 ANSYS, Inc. April 11, 202343
Aerodynamics Optimization Methods
Parametric Method – 50:50:50 Method
Gradient Based Method – Adjoint Method
Agenda
© 2011 ANSYS, Inc. April 11, 202344
Overview Application Areas and Associated
Technologies R14.5 New Features Solved Examples using R14.5 Summary
Agenda
© 2011 ANSYS, Inc. April 11, 202345
What is It ?
• Different methods for computing derivatives – sometimes referred to as sensitivities.
• Consider a high-level view of a fluid system
ADJOINT METHOD
Inputs c Quantities qFlow Solver Flow variables
Integral quantities
MeshBoundary conditionsMaterial properties
Transfer matrix j
i
c
q
Tangent method
Adjoint method
© 2011 ANSYS, Inc. April 11, 202346
Overview of the adjoint method
Workflow
• Solve the flow equations and post-process the results as usual.• Pick an observation that is of engineering interest.
• Lift, drag, total pressure drop?
• Set up and solve the adjoint problem for this observation• Define solution advancement controls• Set convergence criteria• Initialize• Iterate to convergence
• Post-process the adjoint solution to view• Shape sensitivity• Sensitivity to boundary condition settings• Contour & vector plots
© 2011 ANSYS, Inc. April 11, 202347
Overview of the adjoint methodShape sensitivity: Sensitivity of the observed value with respect to (boundary) grid node locations
mesh
nn xwDrag .)(
Shape sensitivity coefficients:Vector field definedon mesh nodes
Node displacement
Visualization of shape sensitivity
• Uses vector field visualization.
• Identifies regions of high and low sensitivity.
• These are the places where changes to the shape can have a big impact on the quantity of interest.
• The guidance is specific to the quantity of interest, and the current flow state.
Drag sensitivity for NACA0012
© 2011 ANSYS, Inc. April 11, 202348
Associated TechnologiesMesh Morphing
• Use a Bernstein polynomial-based morphing scheme for freeform mesh deformation.
• Select portions of the geometry to be modified by specifying a rectilinear deformation region.
• Define a uniform distribution of control points in space in each coordinate direction.
• Movement of any control point leads to a smooth deformation field throughout the deformation region.
• Can be driven by non-gradient based algorithm – e.g. Simplex.
© 2011 ANSYS, Inc. April 11, 202349
Increase the downforce on the vehicle
Look for regions of high sensitivity of downforce to shape
Downforce enhancement for a generic race car
© 2011 ANSYS, Inc. April 11, 202350
Front wing redesign to generate more downforce
Downforce enhancement for a generic race car
Downforce (N)
Geometry Predicted Result
Original --- 425.7
Modified 447.4 (+5.1%) 451.1 (+6.0%)
© 2011 ANSYS, Inc. April 11, 202351
Rear wing redesign to generate more downforce
Lift enhancement for a generic race car
Downforce (N)
Geometry Predicted Result
Original --- 425.7
Modified 481.3 (+13.1%) 492.5 (+15.7%)
© 2011 ANSYS, Inc. April 11, 202352
• What are the major factors affecting the uniformity in the mass flow rates at the 4 outlets?
• Material is air
• Solve the flow problem.
• Set up and solve the adjoint problem with the variance in mass flow rates as the quantity of interest.
• Post-process the field to identify important influences.
Robust Design Example: Internal Flow
Total pressure (pascal)
60 cm
12
34
m1: 0.0020 Kg/sm2: 0.0023 Kg/sm3: 0.0028 Kg/sm4: 0.0025 Kg/svar: 7.52e-08
4
14
1
iimm
24
14
1var
i
i mm
inlet
© 2011 ANSYS, Inc. April 11, 202353
• Variance computed to be 7.52e-08 (Kg/s)^2
• Plot the displacements of the surface that, based on linear extrapolation, would drive the variance to zero.
• Geometry far upstream is dictating the flow split.
• Manufacturing variances of the order of 3mm in the inlet region can cause O(1) flow variance variations.
Robust Design Example: Internal Flow
Surface normal displacements thatInduce O(1) change in variance
© 2011 ANSYS, Inc. April 11, 202354
Appendix
© 2011 ANSYS, Inc. April 11, 202355
Design Point #1
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Design Point #9
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Design Point #19
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Design Point #25