good practice in cfd -...

Good Practice in CFD.

A rough guide.

Prof. Neil W. BressloffMarch 2018

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

Introduction – TOTALSIM

https://www.totalsimulation.co.uk/cfdsimulation/

Introduction – internal flow

Introduction – internal flow 2018

Introduction – biomedical flow 2018

• Geometry > mesh > simulation > post-process

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

Geometry

Introduction

0 0.2 0.4 0.6 0.8 1

non-dimensional time

• Geometry > mesh > simulation

Bressloff, N. W., 2007, Parametric geometry exploration in the carotid artery bifurcation, J. Biomech. , 40, 2483-2491.

The issues - geometry

• Construct from scratch?

• Supplied geometry?

• Feature definition – wrapping

• Outer domain (for external flow)

• Parameterisation

The issues – geometry software

Solidworks

DesignModeler

The issues - mesh

• Mesh tool?

• Mesh strategy?

• Boundary layer mesh?

• Mesh dependence?

• Computational cost?

The issues – mesh software

Solidworks

Harpoon

ICEM CFD

Starccm+

ANSYS mesher

The issues - simulation

• Model definition?

• Solution strategy?

• Boundary conditions?

• Initial conditions?

• Monitoring?

• Convergence?

The issues – simulation software

Starccm+

OpenFOAM

Fluent

Reynold’s number

• Why is Re important?

➢Laminar > Transition > Turbulent

Increasing Re

➢Boundary layer behaviour/representation

http://www.princeton.edu/~gasdyn/

Simulation – accuracy?

• 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 ?

The issues – post-process

• Need to show quantitative results

• Explain the results

➢ Verification

➢ Validation

➢ Errors

➢ Significance

Has this flow separated?

Verification & Validation

• Verification

➢ Check for correct setup

• Validation

➢ Check accuracy of results (preferably against experimental data)

Test case 1.

2D flow over a backward facing step

2D flow over a backward facing step – validation

2D flow over a backward facing step – the experiment

2D flow over a backward facing step – flow settings

2D flow over a backward facing step - results

2D flow over a backward facing step – 2D versus 3D

2D flow over a backward facing step – simulation

• Model definition?

• 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

Mesh spacing = 1mm

Solver setup: pressure algorithm set to 2nd order

Mesh spacing = 1mm

Solver setup: pressure based; SIMPLE; 2nd order(finer mesh)

Mesh spacing = 0.5mm

Solver setup: switch to SIMPLEC and use higher under-relaxation factors

34Note: 0.8 for momentum didn’t converge

Higher under-relaxation factors

Default values

So SIMPLEC converges well with high under-relaxation factors.BUT….do we trust the solution?

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

Test under-relaxation

Switch to 2nd

Switch to coupled solver

Solver setup: The coupled solver

Solver setup for mesh dependence

• 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

Mesh dependence – x-component of shear stress on bottom wall.

2D flow over a backward facing step - results

Boundary (layer) mesh or inflation layer1mm spacing

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

0.5mm spacing

5 layers

0.1mm first layer

Growth rate = 1.5

Boundary (layer) mesh or inflation layer0.25mm spacing

0.25mm spacing

5 layers

0.1mm first layer

Growth rate = 1.2

Triangular cells – spacing = 0.25mm

The Lyceum cluster

http://www.southampton.ac.uk/isolutions/staff/lyceum.page

https://cmg.soton.ac.uk/community/wiki/lyceum

Logging into the Lyceum cluster

• 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!

• 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

Test case 2.

3D flow around a transonic aeroplane

Fluent

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

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.

~ 0.04mm

DPW-5 - overview

Grid guidelines – coarse grid

..\Articles\DPW4-ANSYS-Marco-Oswald-new_2009.pdf

Solver setup

Turbulence model selection

ALSO consider how

to model

the near wall

behaviour

➢ Is y+

in the

correct range?

RANS models descriptions

RANS models behaviour and usage

Near-wall treatment (y+)

Good practice in CFD (Bressloff)pyy

Harpoon – first mesh

• 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

Harpoon-Fluent - first mesh y+

Harpoon – second mesh

• Mesh settings

• BL settings

➢ Initial cell height = 2mm

• Volume mesh: 1,363,903 cells

• Including BL mesh: 2,238,970 cells

Harpoon-Fluent – second mesh y+

Harpoon – third mesh

• Mesh settings

• BL settings

➢ Initial cell height = 0.5mm

• Volume mesh: 1,363,903 cells

• Including BL mesh: 3,521,225 cells

Harpoon-Fluent – third mesh y+

DPW- 5 summary - drag

DPW- 5 – drag (turbulence models)

• Scatter is still large for coarser grids

• Best results for hex-based grids (even if unstructured)

•Discretisation and turbulence modelling major contributors to scatter

DPW4 summary - separation

DPW4 summary – no separation

Test case 3.

Coronary artery stent design

(pulsatile flow)

Starccm+

Coronary artery disease

• 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.

Stents

(1) National heart lung and blood institute (nlhbi).http://www.nhlbi.nih.gov/health/dci/Diseases/Angioplasty/Angioplasty All.html.

Geometry construction

• Representative models of the ART stent and Bx VELOCITY are constructed using Rhinoceros 4.0

ART stent – Flat model Bx VELOCITY – Flat model

Problem formulation

• Blood flow in coronary arteries

• Inlet velocity profile

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)

Simulation setup

• 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

Mesh, time-step and pulse

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.

Meshing

• 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

Results – wall shear stress

• 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

Flow differences: ART vs Bx-VELOCITY

Test case 4.

Sloshing in a LNG tank

(oscillatory free surface flow)

Sloshing of LNG

Sloshing verification & validation

Test case 5.

Rim driven thruster

OpenFOAM

Mesh verification of open propeller flow

Validation of open propeller flow

Validation Against Experimental Data for the Wageningen B4-70 Pro-

peller Using k-omega SST Turbulence Model

Validation of rim driven thruster

Validation Against Experimental Data for the 70mm Rim Driven Thruster

Checklist (1)

• 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

Checklist (2)

• 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.

Checklist (3)

• 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

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

o speed-up from multiple processors

o avoid paging through distributed memory

Checklist (5)

• 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.

http://www.ercoftac.org/ercoftac_news/wiki1/

Resources

➢ 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

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.

And finally!

http://bramblecfd.com/

good practice in cfd -...

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