1. Good Practice in CFD Dr Neil Bressloff 13th & 14th October 2005
Spalart-Allmaras, 1000 iterations, Turb. Visc. Ratio at inlet.
13th & 14th October 2005

2. Which of the above Cp variations is correct ?
Good Practice in CFD, 2005
Introduction
Which of the above Cp variations is correct ?
Is either of them correct ?
If so, how accurate are they ?
Do the associated solutions yield physically meaningful results ?

3. Preamble: Using Gambit and Fluent efficiently
Good Practice in CFD, 2005
Preamble: Using Gambit and Fluent efficiently
GUI
Precision
Input (journal) files
Batch mode
Running in parallel
Check memory usage and machine load
Monitor solution and auto-save

4. Preamble: Using Gambit and Fluent efficiently (2)
Good Practice in CFD, 2005
Preamble: Using Gambit and Fluent efficiently (2)
GUI (Unix, linux)
gambit &
fluent &
gambit –r2.0.4 –id rae2822 &
fluent –r ddp &
session name
version
version
Two-dimensional, double precision

5. Preamble: Using Gambit and Fluent efficiently (3)
Good Practice in CFD, 2005
Preamble: Using Gambit and Fluent efficiently (3)
Single or Double Precision?
28,000 quadrilaterals, Realizable k-epsilon turbulence model
Single precision requires 54 MBytes memory
Double precision requires 96 Mbytes memory
Results identical here (in terms of force coefficients)
BUT, this should always be checked
If memory is available, use double precision

6. Preamble: Using Gambit and Fluent efficiently (4)
Good Practice in CFD, 2005
Preamble: Using Gambit and Fluent efficiently (4)
Input (journal) files
all menu options (widgets, clicks etc) are available through command-line instructions.
store instructions/commands in input log files
alternatively, load and edit an existing dbs file (Gambit) or cas and dat file (Fluent)

7. Gambit input file example (1)
Good Practice in CFD, 2005
Gambit input file example (1)
vertex create "PT128" coordinates
vertex create "PT129" coordinates e-05 0
vertex create "PT130" coordinates 1 0 0 /
/ ***************************** /
/ *** Define aerofoil edges *** /
/ * suction side
edge create "upper" nurbs "PT1" "PT2" "PT3" "PT4" "PT5" "PT6" "PT7" "PT8“ interpolate
face create radius 10 xyplane circle
face create "foil" wireframe "upper" "lower" real
face subtract "face.1" faces "foil"

8. Gambit input file example (2)
Good Practice in CFD, 2005
Gambit input file example (2)
edge mesh "edge.4" "edge.5" successive ratio1 1.1 ratio2 1.1 intervals 40 /
blayer create first 1e-03 growth 1.4 total 0.1 transition 1 \
trows 0 wedge
blayer attach "b_layer.1" face "face.1" "face.1" edge "edge.4" "edge.5"
/* And generate mesh on the domain face
face mesh "face.1" triangle size 1
solver select "FLUENT 5/6"
/ * and then set the BCs
physics create "upper" btype "WALL" edge "edge.4"
physics create "lower" btype "WALL" edge "edge.5"
physics create "pressure-far" btype "PRESSURE_FAR_FIELD" edge "edge.3"
/ * we can save the mesh here as well
export fluent5 "acfd40a.msh" nozval

9. Fluent input file example (1)
Good Practice in CFD, 2005
Fluent input file example (1)
file read-case
"/home1/utp-10/nwb/Aerocfd/2006/r6_from2005_bl_10m3.msh"
grid check
define models solver coupled-implicit yes
define models energy yes
define models viscous spalart-allmaras yes
define models viscous sa-alt-prod yes
define materials
change-create air air yes ideal-gas yes constant no yes constant 1.983e-05 no no no no no no
quit
define operating-conditions operating-pressure 0.0
define boundary-conditions pressure-far-field pressure no no 0.73 no no no no no yes no 10
solve set courant-number 20
solve set discretization-scheme amg-c 1 nut 1

10. Fluent input file example (2)
Good Practice in CFD, 2005
Fluent input file example (2)
solve monitors residual n-save n-display check-convergence yes no no no no
solve monitors residual convergence-criteria plot no print yes
solve monitors residual scale-by-coefficient? yes
solve monitors force drag-coefficient yes lower upper , no yes "/home1/utp-10/nwb/Aerocfd/2006/cd_10m3" no no
report reference-values zone fluid
report reference-values compute pressure-far-field pressure
report reference-values area 1.0
solve initialize compute-defaults pressure-far-field pressure
solve initialize initialize-flow
solve iterate 1000
file
write-case
"/home1/utp-10/nwb/Aerocfd/2006/10m3_firstorder.cas"
write-data
"/home1/utp-10/nwb/Aerocfd/2006/10m3_firstorder.dat"
quit

11. Preamble: Using Gambit and Fluent efficiently (5)
Good Practice in CFD, 2005
Preamble: Using Gambit and Fluent efficiently (5)
Batch mode (Unix, linux)
gambit –r2.0.4 –id rae2822 –inp gambit_input1.jou &
fluent –r ddp < fluent_input1.log > fluent_output1.log &
Gambit input file
Fluent input file
Fluent output file

12. Preamble: Using Gambit and Fluent efficiently (6)
Good Practice in CFD, 2005
Preamble: Using Gambit and Fluent efficiently (6)
Running in parallel (Unix, linux)
fluent –r ddp –t4 –cnf=hostfile < fluent_input1.log > fluent_output1.log &
Gambit input file
Number of processes
Name of file containing list of processors to use

13. Preamble: Using Gambit and Fluent efficiently (7)
Good Practice in CFD, 2005
Preamble: Using Gambit and Fluent efficiently (7)
Check memory usage and machine load
Fluent will run slowly
if more physical memory is needed than is available
the computer is overloaded
Type top under Unix or Linux
Run task manager under Windows
Fluent requires approximately 1GB memory for each million grid cells (in single precision; double precision requires approximately twice as much memory).

14. Preamble: Using Gambit and Fluent efficiently (8)
Good Practice in CFD, 2005
Preamble: Using Gambit and Fluent efficiently (8)
Monitor solution and auto-save
don’t assume that a solution that appears to converge will continue to do so
periodically save the solution using the following command in the Fluent input journal file (or through the appropriate clicks in the menu structure).
file auto-save case-freq 1834 dat-freq 1834 root-name "./nwb_cab_3d_"

15. Standard aerofoil with available experimental data [1]
Good Practice in CFD, 2005
Example 1: RAE2822 aerofoil
Standard aerofoil with available experimental data [1]
Validation: How well can CFD predict pressure, lift and drag coefficients?
Mesh (resolution, dependence, boundary layer and turbulence model)
Boundary conditions (location and definition)
Solution (convergence, order of accuracy)
Flow physics (shock capture, separation)
[1] Cooke, P., McDonald, M. & Firmin, M. (1979), “Airfoil RAE2822 – Pressure Distribution and Boundary Layer Wake Measurements”, AGARD AR-138.

16. Example 1: RAE2822 aerofoil (2)
Good Practice in CFD, 2005
Example 1: RAE2822 aerofoil (2)
The problem under consideration involves flow over an RAE 2822 airfoil at a free-stream Mach number of The angle of attack is 3.19 degrees, which corresponds to case 9 in the experimental data of Cooke et al. Since the calculations were done in free-stream conditions, the angle of attack has been modified to account for the wind-tunnel wall effects. An angle of attack equal to 2.79 degrees was used in the calculations, as suggested by Coakley [2].
The domain extends 55 chord lengths from the airfoil, so that the presence of the airfoil is not felt at the outer boundary.
[2] Coakley., T. J., 1987, Numerical Simulation of Viscous Transonic Airfoil Flows, AIAA 25th Aerospace Sciences Meeting.

17. Good Practice in CFD, 2005
Mesh dependence
Note steepening of shock

18. Lack of convergence (very coarse mesh)
Good Practice in CFD, 2005
Lack of convergence (very coarse mesh)
Demonstrate lack of convergence (actually taken from 10m2 mesh results)

19. Better boundary layer resolution
Good Practice in CFD, 2005
Better boundary layer resolution
Spalart-Allmaras, 1000 iterations, Turb. Visc. Ratio at inlet., y+ too high for SA model. First point in BL at 0.001m. Same number of cells (3600) as previous mesh.

20. Finer mesh (Realizable k-epsilon)
Good Practice in CFD, 2005
Finer mesh (Realizable k-epsilon)
Realizable k-eps, y+~20, Turb. Visc. Ratio.

21. Finer mesh (Spalart-Allmaras)
Good Practice in CFD, 2005
Finer mesh (Spalart-Allmaras)
Spalart-Allmaras, y+ < 2ish, Turb. Visc. Ratio.

22. Good Practice in CFD, 2005
Order of accuracy
Ran from 10m3 RKE NWF

23. Gradient based adaption
Good Practice in CFD, 2005
Gradient based adaption
Static pressure gradient, Experiment with adaption thresholds and dynamic adaption
1) Initial refinement; 2) After 100 iterations; 3) Coarsening as well and another 100 iterations; 4) Another 500 iterations.

24. Good Practice in CFD, 2005
Adaption (2)
Grid and X-Y plots not quite at converged state of residual plot

25. Separation (reverse flow)
Good Practice in CFD, 2005
Separation (reverse flow)
Adapted 10m3 mesh look at Mach number and separation.
Spalart-Allmaras, y+ < 2ish,

26. Separation (reverse flow)
Good Practice in CFD, 2005
Separation (reverse flow)
Adapted 10m3 mesh look at Mach number and separation. Angle of attack increased to 5 degrees
Spalart-Allmaras, y+ too large, Shock further forward than lower angle of attack.

27. Increased angle of attack
Good Practice in CFD, 2005
Increased angle of attack
Angle of attack increased to 5 degrees
Spalart-Allmaras, y+ too large, Shock further forward than lower angle of attack.

28. Mesh dependence (revisited 1)
Good Practice in CFD, 2005
Mesh dependence (revisited 1)
50, 100, 200 cells on upper and lower surfaces
Outer boundaries 50 chords away
Cd experiment =
No. of cells Cd 50
100
200

29. Mesh dependence (revisited 2)
Good Practice in CFD, 2005
Mesh dependence (revisited 2)
No. of chords Cd 7 20 50
200 cells on foil surfaces
Outer boundaries 7, 20, 50 chords away
Cd experiment =

30. Final checklist (1) Grid design Geometry Boundary conditions
Good Practice in CFD, 2005
Final checklist (1)
Grid design
Geometry
Boundary conditions
Boundary layer (Turbulence model)
y+ of first grid point
how many points in the boundary layer?
structured BL or size functions?
Avoid skew cells
Local resolution (adaption)
Run grid check in Fluent
Fluent defaults to SI units (i.e. will assume metres)

31. Good Practice in CFD, 2005
Size functions

32. Final checklist (2) Validation Compare to experimental data
Good Practice in CFD, 2005
Final checklist (2)
Validation
Compare to experimental data
Compare with other simulations
Grid dependence
At least 3 different grid resolutions
8 times 8?
Time dependence
At least 3 significantly different time step sizes
Use engineering judgement and a sensible Courant number.

33. Final checklist (3) Solution scheme Segregated or coupled solver ?
Good Practice in CFD, 2005
Final checklist (3)
Solution scheme
Segregated or coupled solver ?
Implicit or explicit ?
At least 2nd order accuracy (in space and time)
Set high under-relaxation parameters
Monitor residuals, derived variables, point data
Flow physics
Post-process (Fluent, Fieldview, TecPlot, Ensight)
How meaningful ?
Discuss results using graphical evidence
Label all axes and figures

34. Final checklist (4) Convergence problems Mesh quality (errors)
Good Practice in CFD, 2005
Final checklist (4)
Convergence problems
Mesh quality (errors)
Boundary conditions
Under-relaxation
First order and then switch to second order
Slowness due to problem size
Check memory and CPU power
Consider running in parallel
speed-up from multiple processors
avoid paging through distributed memory

35. Final checklist (5) Research the literature
Good Practice in CFD, 2005
Final checklist (5)
Research the literature
Journal and conference papers, reports etc
Read the Fluent manual
Casey, M. & Wintergerste, T., 2000, Special Interest Group on “Quality and Trust in Industrial CFD”, Best Practice Guidelines, Version 1, ERCOFTAC.