underhood thermal modeling using ansys fluent – · pdf fileunderhood thermal modeling...

© 2014 ANSYS, Inc. October 6, 2014 1

Underhood Thermal Modeling Using ANSYS Fluent – Models and Latest Advances

Hamid Ghazialam Principal Technical Services


Aerodynamics Aeroacoustics

Underhood Thermal

Management

Cabin, HVAC Powertrain

ANSYS CFD

Complete Automotive CFD


Underhood Simulation Using ANSYS Fluent Introduction Model Overview Advances in Solver Capabilities Advances in Automatic Mesh Generation Summary and Conclusions Appendix

Outline


Introduction

Type of UTM simulation • Front-End Air Flow (FEAF) • Full Thermal • Loal Thermal

Industries • Automotive • Off-Highway • Truck and Bus • Aircraft • Military

Challenges • Complex geometry • Dirty CAD • Large models

Road conditions simulated • Vehicle moving at constant speed

– Steady-state • Constant speed Idle

– Steady-state • Constant speed Soak

– Transient • Warm up Grade Soak

– Transient • Warm up Grade Idle

– Transient

• City-drive – Transient


Underhood Simulation Using ANSYS Fluent Introduction Challenges Model Overview Advances in Solver Capabilities Advances in Automatic Mesh Generation Summary and Conclusions Appendix

Outline


Steady-state as well as efficient transient flow solvers

• Pressure-Based Coupled Solver

Ability to automate the case setup and solution process

Robust parallel processing and outstanding scalability

Porous media (for heat exchangers and grills)

Dual Cell and Macro based Heat Exchanger models

Fan models (MRF, plane fan, and sliding mesh)

Fast View-Factor Based Radiation Models o 45 minutes on 80 million cell using 256 cores!

Exhaust Skin Temperatures

Automated Post-Processing

Models ANSYS Fluent Offers



Outline


Multi-Layer Shell Conduction Models

Allows conduction in planar direction

Thin shield is meshed as a single-surface

Accounts conduction at junctions

User Inputs: material, thickness, heat generation (if any)

• Thermal conductivity can be bi-axial

Two Models

Single Layer Shell (SLS) New: Multi-Layer Shell (MLS)


Fluid-Solid Explicit Mapping

• With the mapping technique we can totally avoid conformal mesh, which can significantly reduce the time it takes to generate the mesh.

Trad

h,Tcell

Twall

Fluid Solid

Mapping Conformal

Trad

h,Tcell

Twall

Fluid Shell Fluid/solid

Fluid/Shell Mapping Fluid/Solid Mapping Conformal

Mapping


Fast and Accurate Modeling Full Vehicle Transient Soaking and Other Road Conditions

CPU Effort: 30 Hrs. wall-clock, 48 CPUs, 12 million cells, 60 minutes physical time

• All-in-One Approach •Entire Simulation Process is Fully Automated

•Max Temperatures on All Solids are Automatically Generated in XLS Format

Animations Generated in CFD Post



Outline


Introduction to the Wrapper Scripts

• The entire wrapping process is fully automated using scripts

• Three scripts • AdvWrapNPrisms_R15R16_v500_Main.bin

• This is the main scheme that defines all the functions

• AdvWrapNPrisms_R15R16_v500_UserInputs.scm • This is the user input scheme. The user only modifies this file.

• AdvWrapNPrisms_R15R16_v500_Run.scm • This is the run scheme that call the functions defined in above


Case 1: Application of Script on a Dirty Engine

Step 1: Import geometry

Step 2: create Box • It will create object

Step 3: define object for engine • Note: Automatic hole detection (skinning)

will be applied on the engine, thus need to assign it to a separate object.



Step 5: extract features for object engine

Step 6: define material point for the main fluid region



Step 7: Define curvature, proximity, and soft SF and save as engine.msh.gz

• Note: Max allowable tet size = global Max


(define input_geometry “engine") (define compute_size_field "yes") (define extract_intersected_feature_edges “no") (define dirty_objects '(("engine" 1 16 skin))) (define live_regions '("main_fluid")) (define additional_coarsening_factor_and_max_size '((1.1 1000) (1.5 24))) (define prism_layer_method "aspect_ratio") (define PrismLayers 3) (define PrismAspectRatio 5) (define PrismLayerGrothRate 1.2) (define volume_fill_type "tet") (define tet_resolution_factor 1.5)


Step 8: Edit user input scheme as shown on the right

Step 9: Run AdvWrapNPrisms_R15R16_v500_Run.scm

It will produce engine-final.msh.gz and print out total meshing time (8.6 minutes):



High quality tet + prism generated from all faces, all conformal, fully automated. There are no pyramids or non-conformal interfaces!

1.6 million cells


• Change to hexcore in the user input on the right and re-run.


1.6 million cells

(define volume_fill_type “hexcore")


Case 1: Application of Script on a Dirty Engine (BOI)

• Create 2 boxes for body-of-influence (BOI) • No need to create objects • Surfaces can also be imported

• Add “boi” size functions for those boxes • Save • Re-run script

Tet region refined


Extra layers can be generated on select zones (e.g. __p-*)

Case 1: Application of Script on a Dirty Engine – Additional Prism Layer on Select Zones

Extra layers

2 layers

Stair-step to 7 layers

;====local prism layer settings (define list_of_zones_with_extra_layers "__p-*") (define local_prism_layer_method "aspect_ratio") (define local_prism_layers 7) (define local_last_ratio_percentage 40) (define local_prism_layer_first_height 0.2592) (define local_prism_aspect_ratio 20) (define local_prism_layer_growth_rate 1.2)


Feature resolution factor can be used to determine the degree in which the geometry is cleaned.

Case 1: Application of Script on a Dirty Engine – Feature Resolution Factor

Faceted Geom

(define dirty_objects '(("engine" 1 16 skin))) (define dirty_objects '(("engine" 4 16 skin)))


Case 2: Application of Script on a Dirty FEAF

Step 1: Import / read faceted geometry

Step 2: Create box

Step 3: Create Heat Exchanger volume mesh and copy + triangulate its quad sides.

Step 4: Create cylinder around the fan

Step 5: Create objects

eng

Needs skinning

Note: the triangulated quad side is part of the object.



Step 6: extract features

Step 7: define SF

Step 8: define material points • One for main fluid region and • One for the MRF region Step 9: Edit user input file Step 10: Execute run script

(define input_geometry "feaf") (define compute_size_field "yes") (define extract_intersected_feature_edges "yes") (define dirty_objects '(("eng" 1 32 skin))) (define live_regions '("main_fluid" "fan_fluid")) (define coarsening_method "standard_coarsening") (define additional_coarsening_factor_and_max_size '((1.1 1000) (1.5 24))) (define prism_layer_method "aspect_ratio") (define PrismLayers 3) (define prism_layer_first_height 1) (define PrismAspectRatio 10) (define PrismLayerGrothRate 1.2) (define volume_fill_type "tet") (define tet_resolution_factor 1.5)



main_fluid

fan_fluid Non-conformal

Non-conformal

6.6 million cells


(define objects_with_gaps '(("ext" 12 0.375)))

Case 3: Automatic Gap Closure

Unwanted gaps are common in complex geometry. The script will automatically close gaps before the main wrap.

Object: ext

Max size to cover all gaps

~ 4 times smaller than min gap to be closed

Gaps in external aero surfaces


Automated Meshing with Explicit Mapping

1. Import CAD in SCDM

2. (Automated) Export STL into Fluent Meshing

3. (Automated) Setup and run script to obtain main fluid region with 2 prism layers

4. (Automated) In parallel create solid meshes

5. (Automated) Merge main fluid with solid

6. (Automated) Generate contact pairs

7. (Automated) Setup solid2solid NCI and fluid2solid mapping

8. Run case with radiation and CHT

Run Script in batch (automatic) – 1 CPU-hr

Over 200 objects

208 solids

Solid Meshing (automatic) – ~2.5 CPU-hrs

Main fluid

2nd-order Solid temperatures

Rapid prism2tet transition

Fluid/Solid Mapping Interface

Solid/Solid NCI


Automated Meshing with Explicit Mapping

To speed up solid meshing, we can run multiple sessions simultanously, meshing chunks of solids per core (this will be scripted).

208 solids:


An-Isotropic Y+ Adaption

1. Solve for flow with 3 layers

2. (Automatic) Perform an-isotropic adaption based on y+ (~2)

3. Improve volume mesh if needed

4. Perform final solution with radiation, mapping, etc.

3 layers (5.80 million total cells)

Adapted (max 23 layers!) (7.96 million total cells)

Excellent Convergence on Adapted Mesh


adapted

Un-adapted


Imprinted Surface Creation



Outline


Summary ANSYS CFD is fully capable to accurately predict fluid flow and heat

transfer in automotive underhood application for different road conditions

We are working to improve and reduce the turnaround time:

• We introduced schemes that will – starting from CAD - generate high-quality meshes faster, and more automated from extremely complex and dirty geometry.

• New mapping technique in R16 allows us to include many solids for CHT highly automated.

Automatic continuous prism layer generation and being able to adapt the mesh to obtain mesh-independent result on such complex and dirty geometry with many contributing solids is state-of-the-art.



Outline


• In spite of the fact that nuts-n-bolts are not resolved enough, all though conduction path maybe broken, but solution still converges.

• As is generally the case, for accurate CFD predicitons, geometry has to be accurately represented and enough resolution to be given.

215 solids

Mapping Goal: To be Able to Converge with Poor Geometry Resolution

Since the bolt is not important (but was included by mistake), the geometry is purposely not well resolved.


Mapping Goal: To be Able to Converge with Poor Geometry Representation

Solid is not important but was included by mistake!

Solid-solid conduction takes place only if the two solids “touching” within a tolerance. If a solid overlaps beyond that tolerance, conduction will be broken. But it will still converge!

© 2014 ANSYS, Inc. October 6, 2014 37 Fluid/Solid Mapping Fluid/Shell Mapping

Trad

h,Tcell

Twall

Fluid Shell Fluid/solid

Fluid/Shell Mapping

Accuracy of a Single Layer Tet for Thin Baffles

• Coarse tet mesh can be used to mesh the baffles as solids • What is the accuracy level?

Baffle is meshed as a single layer tet

Baffle is not meshed but shell conduction is applied on one side


k=16 W/mK

1-layer Shell on Baffle

E: 1st-order

k=16 W/mK

4-layer Shell on Baffle Tet on Baffle

k=16 W/mK

Accuracy of a Single Layer Tet for Thin Baffles

For this steel baffle structgure, a single tet layer compares well with multi-layer shell


• Remove large gaps between solids

• Create missing geometry

Main and Secondary Fluid with Prism Layers

Solid Chunck 1

Solid Chunck 2

Solid Chunck 3

Solid Chunck N

Prepare: • Naming objects and zones • May merge solids of same material • Extract Features • Define Size Function • Define Material Points

N: number of sessions

Fluent Meshing: Parallel Solid Meshing

Fluent Meshing SCDM

STL

Fluent Meshing Scheme (Batch)

• Extract Contact Pairs for Mapping and NCIs

Fluent Meshing

• Define Models, Materials, BC, Solver • Read scheme to setup mapping and

NCIs • Compute View Factors • Init and run

Fluent Solver

Automated Meshing and Mapping


Appendix – Additional Utilities For UTM

*Source code will be shared.

Quick Case setup • UH_Wall_Panel_Scheme_Utility_CAF.scm

‒ Write/Read wall boundary condition in EXCEL format

• Cellzone_Panel_Scheme_Utility_1.scm ‒ Write/Read settings for MRF cell zones in EXCEL format

• Export-Material-Properties.scm ‒ Write material library in EXCEL format

• Read-Material-Setup.scm ‒ Read material settings for cell zones in EXCEL format


Appendix – Additional Utilities For UTM

Automatic Post-processing • Postprocessing_complete_tool_ver7.scm

‒ Write out information such as Min. Max. and Average Temperature on fluid cell zones, solid cell zones and wall face zones in EXCEL format

‒ Create and export temperature contour plots on solid zones

• plot-t-on-shells.scm ‒ Create and export temperature contour plots on shell conduction zones

• take-plane-cut.scm (Edit and give inputs before read in) ‒ Create and export temperature and velocity magnitude contour plots, velocity vector plot on selective x,y and z plane cuts

• get-wall-adjacent-cell.scm (Edit and give inputs before read in) ‒ Create and export static temperature and HTC contour plots on selective components

• export-wall-temperature.scm ‒ Write Max. Temperature on solid walls and shell conduction walls VS. flow time during transient simulation

• get-flow-adj-cell.scm (Edit and give inputs before read in) ‒ Report mass and energy flow rate on selective face zones in normal and opposite normal direction.

underhood thermal modeling using ansys fluent – · pdf fileunderhood thermal modeling...

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