Good Practice in CFD. A rough guide.

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Good Practice in CFD

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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

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• Geometry > mesh > simulation > post-process

Introduction – the process

http://aaac.larc.nasa.gov/tsab/cfdlarc/aiaa-dpw/

Simulation Mesh

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Introduction

0 5 10 15 20 25 0 0.2 0.4 0.6 0.8 1 non-dimensional time fl o w r a te m 3 /s

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The issues - geometry

• Construct from scratch? OR

• Supplied geometry?

• Feature definition – wrapping • Outer domain (for external flow) • Parameterisation

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The issues – geometry software

Rhino Solidworks CATIA NX4 DesignModeler

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The issues - mesh

• Mesh tool? AND

• Mesh strategy?

• Boundary layer mesh? • Mesh dependence? • Computational cost?

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The issues – mesh software

Solidworks Harpoon ICEM CFD Starccm+ ANSYS mesher

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The issues - simulation

• Model definition? AND • Solution strategy? • Boundary conditions? • Initial conditions? • Monitoring? • Convergence?

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The issues – simulation software

Starccm+

OpenFOAM

Fluent

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Reynold’s number

• Why is Re important?

Laminar > Transition > Turbulent

Increasing Re

Boundary layer behaviour/representation

http://www.petrodanesh.com/Virtual%20Education/Mechanics/ANSYS-FLUENT/ANSYS%20CO/fluent12-lecture06-turbulence.ppsx

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Simulation – accuracy?

• Which of the above Cp variations is correct ? • Is either of them correct ?

• If so, how accurate are they ?

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The issues – post-process

• Need to show quantitative results • Explain the results

 Verification

 Validation

 Errors

 Significance

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Verification & Validation

• Verification

 Check for correct setup

• Validation

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Test case 1

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2D flow over a backward facing step – simulation

• 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

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Solver setup: default settings

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Solver setup: change to 1st _order

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Solver setup: pressure algorithm set to 2nd order

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

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

Mesh spacing = 0.5mm

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

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Solver setup: pressure based; Coupled; 2nd order (switch from SIMPLEC for finer mesh)

Coupled SIMPLEC

Coupled

Selecting Coupled from the Pressure-Velocity Coupling drop-down list indicates that you are using the

Switch to 1st _order

Test under-relaxation

Switch to 2nd _order

Switch to coupled solver Mesh spacing = 0.25mm

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Solver setup: The coupled solver

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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

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

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Boundary (layer) mesh or inflation layer

1mm spacing

1mm spacing 5 layers 0.1mm first layer Growth rate = 2.0

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Boundary (layer) mesh or inflation layer

0.5mm spacing

0.5mm spacing 5 layers 0.1mm first layer Growth rate = 1.5

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Boundary (layer) mesh or inflation layer

0.25mm spacing

0.25mm spacing 5 layers 0.1mm first layer Growth rate = 1.2

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The Lyceum cluster

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Logging into the Lyceum cluster

• You’ll need to request access to the Lyceum cluster from serviceline

• Read the web-pages (including the wiki pages) • Use secure shell to remotely login

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Fluent on the Lyceum cluster

• Type module load fluent/14.5.7

• Type fluent 2d –t4 &

– to run a 2d session of Fluent using 4 processes

– Note: only use this on the head node to perform quick tests

• A special script is needed to submit jobs to the scheduler in order to run

on the slave nodes

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Test case 2

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Drag prediction workshop

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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 C_REF=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.

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Grid guidelines – coarse grid

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Turbulence model selection

ALSO consider how to model

the near wall behaviour

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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

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Harpoon – second mesh

• Mesh settings

• BL settings

 Initial cell height = 2mm

• Volume mesh: 1,363,903 cells

• Including BL mesh: 2,238,970 cells • 112 seconds to create mesh

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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 • 148 seconds to create mesh

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DPW- 5 summary - drag

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DPW- 5 – drag (turbulence models)

• Scatter is still large for coarser grids

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Test case 3

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Coronary artery stent design

(pulsatile flow)

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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.

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Geometry construction

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

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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

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Simulation setup

• Governing Equations

∇.(v) = 0 (1)

ρ(∂v/∂t) + ρ(v.∇v ) = -∇P + μ∇2_v ₍₂₎

• Boundary conditions

– Numerical simulations are performed over a quarter stent to exploit symmetry

Inlet: velocity specified as a

Outlet:zero pressure

Plane2: Periodic/cyclic boundary condition

Stent & artery wall: No slip wall

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Mesh, time-step and pulse

Time step 10-3_s

Mesh size ~ 1 million cells

Mesh dependence test Time step independence

• Various time-step, mesh, and blood-pulse dependence tests help to

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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

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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

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Test case 4

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Sloshing in a LNG tank

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Test case 5

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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

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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

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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)

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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

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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”,

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Other resources: see articles and

papers in the Articles folder

• Software manuals

 Check recommended settings

• External aerodynamics best practice • Marine CFD best practice guidelines • Simulation versus reality

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Summary – learning outcomes

Good 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.

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And finally!

 http://www.soton.ac.uk/~nwb/lectures/GoodPracticeCFD

GoodPracticeCFD_2015.pdf

 CFD surgeries (Dr Zheng-Tong Xie) during semesters 1 & 2

 Blackboard (http://blackboard.soton.ac.uk/) CFD-SURG: enrolling code is

fluent

 ANSYS portal (use customer number 237419)