The influence of mesh characteristics on OpenFOAM simulations of the DrivAer model

Transcription

1 The influence of mesh characteristics on OpenFOAM simulations of the DrivAer model Vangelis Skaperdas, Aristotelis Iordanidis, Grigoris Fotiadis BETA CAE Systems S.A. 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 1

2 BETA CAE Systems S.A. R e s e a r c h & d e v e l o p m e n t o f C A E s o f t w a r e a n d e n g i n e e r i n g p r o d u c t s s t u d i e s private company headquarters: Thessaloniki, Greece established in 1999 more than 20 years on the field leader in multi-disciplinary CAE pre- & post-processing innovative, high performance software high quality services 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 2

3 pre- & post- processing suite 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 3

4 multi-disciplinary solutions Impact & Safety Durability & Fatigue NVH CFD 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 4

5 ANSA and META integration with CFD ANSA has been used in pre-processing for CFD since the mid 90s for geometry clean up and surface mesh generation. Currently, ANSA and META provide complete support for the most robust and high quality use of CFD in industrial scale and complexity simulations 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 5

6 The DrivAer model of the Technical University of Munich Experimental setup: 1:2.5 scale wind tunnel model Re = 4.87x10 6 L = 1.84 m U = 40 m/sec Free stream turbulence = 0.4% Acknowledgments to: Institute of Fluid Mechanics and Aerodynamics of the Technical University of Munich for providing the model geometries in IGES and STEP formats Sources: 1) Introduction of a New Realistic Generic Car Model for Aerodynamic Investigations, A. Heft, T. Indinger and N. Adams, SAE ) Experimental and Numerical Investigation of the DrivAer Model, A. Heft, T. Indinger and N. Adams, ASME 2012 Fluids Engineering Summer Meeting, July 8-12, 2012, Rio Grande, Puerto Rico 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 6

7 Previous related work of BETA CAE Studies with Fluent and OpenFOAM simulations were presented at: ANSYS Automotive Simulation Congress Group, Frankfurt, October 2013 International Open Source CFD Conference, Hambourg, October 2013 Model was scaled up to full size L = m Domain size 50 x 20 x 11.5 m blockage ratio= 1% domain sides set to symmetry Steady State RANS simulations Re = 4.87x10 6 Turbulence model: k-omega SST Cases with and without moving ground simulation with MRF modeling of rotating wheels Presence of model support seems to decelerate the flow locally 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 7

8 Software and hardware used - ANSA v for pre-processing - OpenFOAM v2.3 for solving on 16 nodes - ETA v for post-processing 2 Linux Centos 6.2 PCs each one with 8 core i at 3.6GHz 64 Gb RAM 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 8

9 Geometry preparation: STEP file input and property assignment on certain areas Geometries that included detailed underbody and mirrors were selected 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 9

10 Geometry preparation: Construction of wind tunnel geometry 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 10

11 Geometry preparation: Construction of wind tunnel geometry Blockage ratio 8% 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 11

12 Model management in ANSA Managing CFD model properties while tracking Part and Property from PDM system CFD model properties Part structure assembly 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 12

13 Configurations management in ANSA handling three variants in one file 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 13

14 Flexible Size Boxes controlling mesh refinement aligned to the flow 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 14

15 Batch Meshing setup: automation and consistency in meshing Model Properties (OpenFOAM BCs) Batch Mesh provides: - Automation - Consistency - Mesh spec traceability Surface batch mesh sessions 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 15

16 Batch mesh generated surface mesh Automatic curvature and sharp edge refinement, in combination with the use of Size Boxes ensure the efficient and accurate capturing of all details of the model. Quality according to Fluent skewness < nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 16

17 Batch mesh generated surface mesh Automatic generation of models with variable resolution using batch meshing Fine Medium Coarse million k trias trias on vehicle on vehicle million in in total 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 17

18 Re-using the same mesh for other variants Batch mesh generated (medium size) 2.3 million tria elements in 16 minutes Using configurations management the model is updated to the new variant Size boxes are also updated respectively Original mesh of common body is retained unaffected Batch mesh tool is then used to mesh only the updated unmeshed area at a fraction of the time 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 18

19 Boundary layer generation First height 0.8 mm Growth rate = layers +3 layers in aspect mode Last aspect ratio 40% of length Total layer height 12 mm 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 19

20 Boundary layer generation : local squeezing at proximities 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 20

21 Boundary layer generation: local exclusion of layers at problematic areas Very confined area leading to excessive squeezing Boundary layer scoop area with sharp leading edge Trailing edge of aerofoil shaped model support 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 21

22 Batch mesh generated volume mesh Copyright 2014 BETA CAE Systems S.A. All rights reserved Automatic generation of layers and volume mesh for all variants and mesh densities (12 combinations) Image below of medium size mesh with layers (43.5 million cells) generated in under 40 minutes 2nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 22

23 Indicative mesh quality statistics : Notchback tetra medium with layers Max Non orthogonality = Maximum skewness = nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 23

24 Mesh refinement study for tetra with layers case Automatic generation of models with variable resolution using batch meshing Copyright 2014 BETA CAE Systems S.A. All rights reserved Coarse mesh Fine mesh Medium mesh million trias million cells 2nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 24

25 Mesh refinement study for HexaPoly with layers case Automatic generation of models with variable resolution using batch meshing Fine Coarse Medium mesh mesh million trias million cells 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 25

26 Generation of Polyhedral mesh from hybrid mesh conversion Coarse mesh 1.4 million polys 13.9 million cells 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 26

27 Overview of final volume mesh Copyright 2014 BETA CAE Systems S.A. All rights reserved Medium tetra with layers model 2nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 27

28 Summary of mesh models for different variants Layers Coarse Medium Fine Notchback No Tetra (22.2 million) Yes Yes Yes Tetra (30.8 million) Hexa Poly (20.8 million) Polyhedral (13.9 million) Tetra (32.0 million) Tetra (43.5 million) Hexa Poly (29.5 million) - - Fastback Yes - Tetra (43.7 million) Tetra (47.8 million) Tetra (65.1 million) Tetra (42.9 million) - Estate Yes - Tetra (54.6 million) - 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 28

29 Setting up the OpenFOAM case in ANSA 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 29

30 OpenFOAM simulations: setup U inlet 6.4 m/sec p fixed value 0 BL scoop suction Numerical settings U= 40 m/sec LinearUpwindV scheme for velocity Upwind scheme for turbulence GAMG solver for pressure, tolerance 10-10, reltol 0.05 smoothsolver for velocity and turbulence, tolerance 10-10, reltol 0.1 All walls zero velocity Steady State simulations simplefoam Turbulence model: k-omega SST Stationary ground All runs started from potentialfoam initialization Transient simulations pisofoam time step 10-4 sec run for 1.5 sec real time Turbulence model: DES Spalart Almaras model for near wall All runs starting from converged steady state solution 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 30

31 OpenFOAM simulations: Steady state simplefoam convergence Indicative convergence history of residuals and drag and lift coefficients for Notchback HexaPoly coarse with layers model Drag coefficient Lift coefficient +/- 5% +/- 50% 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 31

32 Post-processing in ETA: y+ results Without layers 20<y+<300 Post-processing was performed manually for one CFD run and then META run in batch mode for the other 14 simulations producing automatically the same images With layers 20<y+<50 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 32

33 Effect of layers on skin friction coefficient and wall flow (tetra medium meshes) No layers With layers The case without layers does not predict any vortex formation and recirculation around the A-pillar and the rear window 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 33

34 Velocity field at symmetry plane of notchback Tetra with layers medium mesh Transient DES (55msec animation) RANS 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 34

35 Velocity field at symmetry plane of notchback (tetra with layers medium) RANS Transient DES (55 msec animation) 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 35

36 Cut-plane of velocity magnitude Steady RANS Transient DES (55msec animation) 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 36

37 Pressure loss regions: Iso-surface of total pressure = 0 Tetra with layers medium mesh 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 37

38 Open section wind tunnel corrections Correction is applied on U ref based on the Plenum Method described by B. Nijhof, G. Wickern SAE P inlet P plenum U average =40m/sec k P inlet P plenum = 1 ρ U2 2 P inlet P plenum U ref = 2 k P inlet P plenum ρ 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 38

39 Averaging of fluctuating forces: Open domain case of tetra medium notchback 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 39

40 Mesh refinement study for Tetra and HexaPoly meshes: C D convergence Tetra with layers HexaPoly with layers coarse medium fine coarse medium fine Coefficients calculated based on notchback projected frontal area = m 2 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 40

41 Comparison with experimental C D value of for notchback model Open Domain with layers Wind tunnel without layers Wind tunnel with layers Wind tunnel with layers Run Coarse Medium Fine RANS k-omega RANS k-omega RANS k-omega RANS k-omega RANS k-omega DES S-A DES S-A - Tetra (-0.3%) Tetra (-4%) Tetra (-5%) Hexa Poly (-9%) Polyhedral (-3%) Tetra (-2%) Tetra (-7%) Hexa Poly (-10%) Tetra (-2%) - Hexa Poly (-3%) - - Tetra (-7%) HexaPoly (-10%) nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 41

42 C L values for notchback model (wheels not considered) Open Domain with layers Wind tunnel without layers Wind tunnel with layers Wind tunnel with layers Run Coarse Medium Fine RANS - Tetra RANS RANS RANS RANS Tetra Tetra Hexa Poly Polyhedral Tetra Tetra Hexa Poly DES - Tetra DES - Hexa Poly Tetra HexaPoly nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 42

43 Summary of C D and C L values for three variants Tetra with layers medium meshes RANS simulations C D Experiment C D CFD C L CFD Notchback (-7%) Fastback (-5%) Estate (-11%) nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 43

44 Comparison with experiment: C P along upper symmetry line Notchback medium tetra with layers 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 44

45 Concluding remarks - ANSA and µeta pre and post-processing for OpenFOAM was demonstrated with key points like: - High quality automated surface and volume meshing allowing quick mesh alternatives - Effective management of a CFD model with multiple variants of a vehicle geometry in one file - Fully automated post-processing for multiple simulation results - Mesh refinement study showed that acceptable mesh independence can be reached at medium size. All medium refinement size meshes gave similar C D values the fine meshes - The k-omega turbulence model produces highly transient realistic flow features even in the steady steate simplefoam runs. The DES runs show very good transient flow characteristics. - Interpretation of results is of outmost importance. Averaging of forces must be performed with great caution and should consider several thousands of iterations - Pressure coefficient distributions were very well captured by the simulations that included the wind tunnel. However drag coefficient values did deviated by 2 to 10% depending on the mesh - Lift estimations showed great oscillations and scatter between different meshes. No experimental data are available to make any conclusions - Nobody trusts a computer simulation except the guy who did it, and everybody trusts experimental data except the guy who did it!!!! - In order to extract real conclusions from this and from future studies we need to have the exact experimental setup specifications, like, velocity correction method, k factor, reference pressure measurement and of course accurate geometry of the problem. - Why not get together and start the fist Automotive Drag and Lift prediction workshop!! 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 45

46 stay connected: 2 nd Northern Germany OpenFOAM User Meeting, 24 September 2014, Braunschweig, Germany 46