good practice in cfd -...
TRANSCRIPT
Good Practice in CFD.
A rough guide.
Prof. Neil W. BressloffMarch 2018
2
Material covered
Introduction External and internal flow
The CFD process Geometry, meshing, simulation, post-processing
The issues For each of the steps in the CFD process• The Reynold’s number• Verification and validation
Test case 1 Simulation of flow over a 2D backstep• Model selection• Order of accuracy• Mesh verification
Test case 2 Simulation of flow over an airplane• y+: the non-dimensional distance from the wall• Turbulence model selection• The drag prediction workshops (variation in CFD results)
Test case 3 Pulsatile (unsteady) blood flow • Verification of number of pulses, spatial and temporal spacing
Test cases 4 & 5 Verification in CFX (tank sloshing) and in OpenFOAM (rim-driven thruster)
Checklist The things to consider for setting-up, running and post-processing a CFD simulation
Other resources http://www.soton.ac.uk/~nwb/lectures/GoodPracticeCFD/Articles
Introduction – external flow
Good practice in CFD (Bressloff) 3
Introduction – external flow 2018
Introduction – external flow
Good practice in CFD (Bressloff) 5
Introduction – TOTALSIM
Good practice in CFD (Bressloff) 6
https://www.totalsimulation.co.uk/cfdsimulation/
Introduction – internal flow
Good practice in CFD (Bressloff) 7
Introduction – internal flow 2018
Introduction – biomedical flow 2018
• Geometry > mesh > simulation > post-process
10
Introduction – the process
Good practice in CFD (Bressloff)
http://aiaa-dpw.larc.nasa.gov/
The fourth drag prediction workshop
http://aaac.larc.nasa.gov/tsab/cfdlarc/aiaa-
dpw/Workshop4/presentations/DPW4_Presentations_files/
D1-9_DPW4-ANSYS-Marco-Oswald-new.pdf
Simulation
Mesh
Geometry
sixth
11
Introduction
Good practice in CFD (Bressloff)
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1
non-dimensional time
flow
ra
te m
3/s
• Geometry > mesh > simulation
Bressloff, N. W., 2007, Parametric geometry exploration in the carotid artery bifurcation, J. Biomech. , 40, 2483-2491.
12
The issues - geometry
• Construct from scratch?
OR
• Supplied geometry?
• Feature definition – wrapping
• Outer domain (for external flow)
• Parameterisation
Good practice in CFD (Bressloff)
13
The issues – geometry software
Good practice in CFD (Bressloff)
Rhino
Solidworks
CATIA
NX4
DesignModeler
14
The issues - mesh
• Mesh tool?
AND
• Mesh strategy?
• Boundary layer mesh?
• Mesh dependence?
• Computational cost?
Good practice in CFD (Bressloff)
15
The issues – mesh software
Good practice in CFD (Bressloff)
Solidworks
Harpoon
ICEM CFD
Starccm+
ANSYS mesher
16
The issues - simulation
• Model definition?
AND
• Solution strategy?
• Boundary conditions?
• Initial conditions?
• Monitoring?
• Convergence?
Good practice in CFD (Bressloff)
17
The issues – simulation software
Good practice in CFD (Bressloff)
Starccm+
OpenFOAM
Fluent
CFX
18
Reynold’s number
• Why is Re important?
Good practice in CFD (Bressloff)
➢Laminar > Transition > Turbulent
Increasing Re
➢Boundary layer behaviour/representation
http://www.princeton.edu/~gasdyn/
19
Simulation – accuracy?
Good practice in CFD (Bressloff)
• 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 ?
20
The issues – post-process
• Need to show quantitative results
• Explain the results
➢ Verification
➢ Validation
➢ Errors
➢ Significance
Good practice in CFD (Bressloff)
Has this flow separated?
21
Verification & Validation
• Verification
➢ Check for correct setup
• Validation
➢ Check accuracy of results (preferably against experimental data)
Good practice in CFD (Bressloff)
Test case 1.
2D flow over a backward facing step
23
2D flow over a backward facing step – validation
Good practice in CFD (Bressloff)
24
2D flow over a backward facing step – the experiment
Good practice in CFD (Bressloff)
25
2D flow over a backward facing step – flow settings
Good practice in CFD (Bressloff)
26
2D flow over a backward facing step - results
Good practice in CFD (Bressloff)
27
2D flow over a backward facing step – 2D versus 3D
Good practice in CFD (Bressloff)
28
2D flow over a backward facing step – simulation
Good practice in CFD (Bressloff)
29
2D flow over a backward facing step – simulation
Good practice in CFD (Bressloff)
• Model definition?
AND
• Solution strategy?
• Boundary conditions?
• Initial conditions?
• Monitoring?
• Convergence?
• Solver settings
• Mesh dependence
➢ Resolution
➢ Type
➢ Boundary layer mesh
➢ Memory
• Convergence
• Simulation time
➢ Hardware
➢ Parallel simulation
Solver setup: default settings
30Good practice in CFD (Bressloff)
Mesh spacing = 1mm
Solver setup: change to 1st order
31Good practice in CFD (Bressloff)
Mesh spacing = 1mm
Solver setup: pressure algorithm set to 2nd order
32Good practice in CFD (Bressloff)
Mesh spacing = 1mm
Solver setup: pressure based; SIMPLE; 2nd order(finer mesh)
33Good practice in CFD (Bressloff)
Mesh spacing = 0.5mm
Solver setup: switch to SIMPLEC and use higher under-relaxation factors
34Note: 0.8 for momentum didn’t converge
Mesh spacing = 0.5mm
Higher under-relaxation factors
Default values
So SIMPLEC converges well with high under-relaxation factors.BUT….do we trust the solution?
35Good practice in CFD (Bressloff)
Mesh spacing = 0.5mm
Solver setup: pressure based; Coupled; 2nd order(switch from SIMPLEC for finer mesh)
CoupledSIMPLEC
Coupled
Selecting Coupled from the Pressure-Velocity Coupling drop-down list indicates that you are using the
pressure-based coupled algorithm, described in this section in the separate Theory Guide. This solver
offers some advantages over the pressure-based segregated algorithm. The pressure-based coupled
algorithm obtains a more robust and efficient single phase implementation for steady-state flows. It
is not available for cases using the Eulerian multiphase, NITA, and periodic mass-flow boundary conditions.
Switch to 1st
order
Test under-relaxation
Switch to 2nd
order
Switch to coupled solver
36Good practice in CFD (Bressloff)
Mesh spacing = 0.25mm
Solver setup: The coupled solver
37Good practice in CFD (Bressloff)
Solver setup: The coupled solver
38Good practice in CFD (Bressloff)
Mesh spacing = 0.5mm
Solver setup for mesh dependence
39Good practice in CFD (Bressloff)
• Coupled solver is more robust and is recommended for steady-state solutions
➢ N.B. only incompressible flow considered here
• Use at least 2nd order discretisation schemes
• Check convergence
➢ N.B. aim for at least three orders of magnitude
• Mesh dependence
➢ Consider at least four mesh resolutions
➢ Halve the mesh spacing each time
Mesh dependence: spacing of 1mm to 0.125mm
40Good practice in CFD (Bressloff)
Mesh dependence – x-component of shear stress on bottom wall.
41Good practice in CFD (Bressloff)
42
2D flow over a backward facing step - results
Good practice in CFD (Bressloff)
Boundary (layer) mesh or inflation layer1mm spacing
43Good practice in CFD (Bressloff)
1mm spacing
5 layers
0.1mm first layer
Growth rate = 2.0
Cell count increased from 6,000 to 10,069
Boundary (layer) mesh or inflation layer0.5mm spacing
44Good practice in CFD (Bressloff)
0.5mm spacing
5 layers
0.1mm first layer
Growth rate = 1.5
Cell count increased from 24,353 to 31,366
Boundary (layer) mesh or inflation layer0.25mm spacing
45Good practice in CFD (Bressloff)
0.25mm spacing
5 layers
0.1mm first layer
Growth rate = 1.2
Cell count increased from 99,844 to 105,407
Triangular cells – spacing = 0.25mm
46Good practice in CFD (Bressloff)
Cell count increased from 99,844 to 211,589
The Lyceum cluster
47
http://www.southampton.ac.uk/isolutions/staff/lyceum.page
https://cmg.soton.ac.uk/community/wiki/lyceum
Logging into the Lyceum cluster
48
• Normally, your supervisor will need to request
access to the Lyceum cluster for you through
serviceline
• Read the web-pages (including the wiki pages)
• Use secure shell to remotely login
Three scripts!
49Good practice in CFD (Bressloff)
• First script requests a job to be run on the cluster
• Second script is the actual file that is run when the job is allocated to a compute node
– This will contain a command to run a simulation
• The third script will contain the commands needed by the simulation
– For example, read a particular mesh file and setup the solver, BCs etc
200 iterations of the 1mm mesh on the Lyceum cluster
50Good practice in CFD (Bressloff)
Test case 2.
3D flow around a transonic aeroplane
Fluent
52
Drag prediction workshop
Good practice in CFD (Bressloff)
http://aaac.larc.nasa.gov/tsab/cfdlarc/aiaa-
dpw/Workshop4/presentations/DPW4_Presentations_files/
D1-9_DPW4-ANSYS-Marco-Oswald-new.pdf
53
DPW-4 - grid guidelines
• Grid Convergence Case – NASA Common Research Model:
– Coarse (3.5M), Medium (10M), and Fine (35M) grids are required;The Extra-fine (100M) grid is optional
– Total grid size to grow ~3X between each grid level for grid convergence cases
– Initial spacing normal to all viscous walls (RE=5e+6 Based on CREF=275.80):
• coarse: y+ ~ 1.0 dy = 0.001478
• medium: y+ ~ 2/3 dy = 0.000985
• fine: y+ ~ 4/9 dy = 0.000657
• extra-fine: y+ ~ 8/27 dy = 0.000438
– Recommended: generate grids with 2 cell layers of constant spacing normal to viscous walls
– Grid convergence cases must maintain the same grid family between grid levels, i.e. maintain the same stretching factors, same topology, etc.
– Growth rate of cell sizes in the viscous layer should be < 1.25.
– Farfield located at ~100 CREF’s for all grid levels.
Good practice in CFD (Bressloff)
~ 0.04mm
54
DPW-5 - overview
Good practice in CFD (Bressloff)
55
Grid guidelines – coarse grid
Good practice in CFD (Bressloff)
..\Articles\DPW4-ANSYS-Marco-Oswald-new_2009.pdf
56
Solver setup
Good practice in CFD (Bressloff)
57
Turbulence model selection
Good practice in CFD (Bressloff)
ALSO consider how
to model
the near wall
behaviour
➢ Is y+
in the
correct range?
58
RANS models descriptions
Good practice in CFD (Bressloff)
59
RANS models behaviour and usage
Good practice in CFD (Bressloff)
60
Near-wall treatment (y+)
Good practice in CFD (Bressloff)pyy
61
Harpoon – first mesh
Good practice in CFD (Bressloff)
• Mesh settings
➢ Surface cell size = 138mm
• BL settings
➢ Initial cell height = 20mm
➢ No. of layers = 3
➢ Expansion rate = 1.3
• Volume mesh: 389,585 cells
• Including BL mesh: 553,566 cells
• 39 seconds to create mesh
62
Harpoon-Fluent - first mesh y+
Good practice in CFD (Bressloff)
63
Harpoon – second mesh
Good practice in CFD (Bressloff)
• Mesh settings
➢ Surface cell size = 69mm
• BL settings
➢ Initial cell height = 2mm
➢ No. of layers = 4
➢ Expansion rate = 1.5
• Volume mesh: 1,363,903 cells
• Including BL mesh: 2,238,970 cells
• 112 seconds to create mesh
64
Harpoon-Fluent – second mesh y+
Good practice in CFD (Bressloff)
65
Harpoon – third mesh
Good practice in CFD (Bressloff)
• Mesh settings
➢ Surface cell size = 69mm
• BL settings
➢ Initial cell height = 0.5mm
➢ No. of layers = 10
➢ Expansion rate = 2.0
• Volume mesh: 1,363,903 cells
• Including BL mesh: 3,521,225 cells
• 148 seconds to create mesh
66
Harpoon-Fluent – third mesh y+
Good practice in CFD (Bressloff)
67
DPW- 5 summary - drag
Good practice in CFD (Bressloff)
68
DPW- 5 – drag (turbulence models)
Good practice in CFD (Bressloff)
• Scatter is still large for coarser grids
• Best results for hex-based grids (even if unstructured)
•Discretisation and turbulence modelling major contributors to scatter
69
DPW4 summary - separation
Good practice in CFD (Bressloff)
70
DPW4 summary – no separation
Good practice in CFD (Bressloff)
Test case 3.
Coronary artery stent design
(pulsatile flow)
Starccm+
72
Coronary artery disease
Good practice in CFD (Bressloff)
• Coronary Artery Disease (CAD) is a condition caused by the accumulation of
plaque (usually atheromatous or fibrous plaque) on the inner walls of the
artery.
(1) GNU Free Documentation License - http://commons.wikimedia.org/wiki/Commons:GNU_Free_Documentation_License(2) Creative Commons License - http://creativecommons.org/licenses/by-sa/2.5/(3) Antonio Colombo and Goran Stankovic. Colombo’s Tips & Tricks with Drug-Eluting Stents. Taylor and Francis Group, 2005.
73
Stents
Good practice in CFD (Bressloff)
(1) National heart lung and blood institute (nlhbi).http://www.nhlbi.nih.gov/health/dci/Diseases/Angioplasty/Angioplasty All.html.
74
Geometry construction
Good practice in CFD (Bressloff)
• Representative models of the ART stent and Bx VELOCITY are constructed using Rhinoceros 4.0
ART stent – Flat model Bx VELOCITY – Flat model
75
Problem formulation
Good practice in CFD (Bressloff)
• Blood flow in coronary arteries
• Inlet velocity profile
75
Flow type Unsteady, Newtonian, Incompressible and laminar
- Unsteady due to the pulsatile nature of blood flow
-Blood behaves as a Newtonian fluid for shear rates higher than 100 s-1 (1)
- Incompressible laminar flow for Reynolds numbers lower than 200
Dynamic Viscosity(μ) 3.7x10-3 Pa-s
Density (ρ) 1.06 x 103 kg/m3
Peak and mean blood velocities 8.99 cm/s & 5.04 cm/s
Peak and mean Reynolds number 77 & 44
1. Fung Y C 1993 Biomechanics: Mechanical Properties of Living Tissues vol 18 2nd edn (New York: Springer)2. K. Perktold,M. Hofer, G. Rappitsch,M. Loew, B.D. Kuban, and M.H. Friedman. Validated computation of physiologic flow in a realistic coronary artery artery branch.
”Journal of Biomechanics”, 31:217–28, 1998.
Inlet velocity profile(2)
76
Simulation setup
Good practice in CFD (Bressloff)
• Governing Equations
∇.(v) = 0 (1)
ρ(∂v/∂t) + ρ(v.∇v ) = -∇P + μ∇2v (2)
• Boundary conditions
– Numerical simulations are performed over a quarter stent to exploit symmetry
Inlet: velocity specified as a
fourier series representing
pulsatile blood flow
Outlet: zero pressure
Plane1: Periodic/cyclic
boundary condition
Plane2: Periodic/cyclic
boundary condition
Stent & artery wall: No slip wall
77
Mesh, time-step and pulse
Good practice in CFD (Bressloff)
Time step 10-3 s
Mesh size ~ 1 million cells
Blood pulses 2
Mesh dependence test Time step independence
Pulse dependence test
Final parameters
• Various time-step, mesh, and blood-pulse dependence tests help to
determine the final parameters for CFD simulations.
78
Meshing
Good practice in CFD (Bressloff)
• Tool used for meshing and CFD runs: Star CCM+ 3.06.006
Cells 1,097,951
Interior faces 6,023,874
Vertices 4,850,151
Cells 1,076,793
Interior faces 6,177,303
Vertices 5,010,556
79
Results – wall shear stress
Good practice in CFD (Bressloff)
• Axial WSS patterns at point 3 of the cardiac pulse – areas of low WSS are localised around the struts and the connectors.
• In earlier studies low WSS areas are reported to correlate with sites of more intimal thickening
80
Flow differences: ART vs Bx-VELOCITY
Good practice in CFD (Bressloff)
Test case 4.
Sloshing in a LNG tank
(oscillatory free surface flow)
CFX
82
Sloshing of LNG
Good practice in CFD (Bressloff)
83
Sloshing verification & validation
Good practice in CFD (Bressloff)
Test case 5.
Rim driven thruster
OpenFOAM
85
Mesh verification of open propeller flow
Good practice in CFD (Bressloff)
86
Validation of open propeller flow
Good practice in CFD (Bressloff)
Validation Against Experimental Data for the Wageningen B4-70 Pro-
peller Using k-omega SST Turbulence Model
87
Validation of rim driven thruster
Good practice in CFD (Bressloff)
Validation Against Experimental Data for the 70mm Rim Driven Thruster
88
Checklist (1)
Good practice in CFD (Bressloff)
• Grid design
➢ Geometry (check/fix CAD model)
➢ Boundary conditions
➢ Boundary layer (Turbulence model)
➢ y+ of first layer of grid points
➢ how many points in the boundary layer?
➢ structured BL or size functions or refinement?
➢ Avoid skew cells
➢ Local resolution (adaption)
➢ Check/improve the grid
➢ Check units, scaling, reference values
89
Checklist (2)
Good practice in CFD (Bressloff)
• Validation
➢ Compare to experimental data
➢ Compare with other simulations
• Grid dependence
➢ At least 3 (preferably 4) different grid resolutions
➢ Select a sensible range of grids
➢ 8 times 8?
• Time dependence
➢ At least 3 significantly different time step sizes
➢ Use engineering judgement and a sensible Courant number.
90
Checklist (3)
Good practice in CFD (Bressloff)
• Solution scheme
➢ Pressure based (segregated) or density based (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
91
Checklist (4)
Good practice in CFD (Bressloff)
• 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
o speed-up from multiple processors
o avoid paging through distributed memory
92
Checklist (5)
Good practice in CFD (Bressloff)
• Research the literature
• Journal and conference papers, reports etc
• Read the software manuals
Casey, M. & Wintergerste, T., 2000,
Special Interest Group on “Quality and
Trust in Industrial CFD”,
Best Practice Guidelines,
Version 1, ERCOFTAC.
93Good practice in CFD (Bressloff)
http://www.ercoftac.org/ercoftac_news/wiki1/
94
Resources
Good practice in CFD (Bressloff)
➢ GoodPracticeCFD_2018.pdf
http://www.soton.ac.uk/~nwb/lectures/GoodPracticeCFD
➢ Applications of CFD (FEEG6005)
https://www.southampton.ac.uk/courses/modules/feeg6005.page
➢ Computational Modelling Group: CFD
http://cmg.soton.ac.uk/research/categories/physical-systems-and-
engineering-simulation/cfd/
➢ Online help with ANSYS
http://www.ansys.com/en-GB/Products/Academic/Support-Resources
95
Summary – learning outcomesGood Practice in CFD
Understand the key steps in setting-up, running and post-processing a CFD simulation.
Knowledge about the issues relating to each of these steps.
Appreciate the importance of verification (particularly with respect to mesh resolution and the effect this has on results).
Understand the significance of the Reynold’s number.
Knowledge about turbulence model selection and the impact of mesh resolution close to solid boundaries.
Appreciation of the critical need to read the CFD manuals (theory and user guides) and other supporting literature.