Strategies to Achieve Reliable and Accurate CFD Solutions UK/staticassets... · Strategies to...

© 2010 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary© 2010 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

Strategies to Achieve

Reliable and Accurate

CFD Solutions

Mark Keating

ANSYS UK

© 2010 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary

Agenda

• Why develop a CFD strategy?

• Pre-Processing Strategies

• Solver Strategies

• Post Processing Strategies

• Full Process Strategies

• Summary


Why Develop a CFD Strategy?

• Reliable results means a consistently accurate

result

• Using default settings is never optimised for each

application for speed/accuracy

• Process can be prone to user error

• Hone an optimised strategy for the application to

prevent deviation and maintain high quality

process

• Ensures repeatably accurate solution

• Allow full design space appreciation faster!


Pre-Processing Strategies


Pre Processing Considerations

• Think ahead about what you want to do and gain

from the CFD analysis

• What are the driving parameters?

• What zones need to be separate for constraints

or post processing?

• What fluids zones will be replaced?

• What level of geometric representation is

needed?

• Small changes can have large effects


Mesh Quality Affects Accurate

and Reliable Result

• Geometry problems

– Small edge

– Gaps

– Sharp angle

• Meshing parameters

– Sizing Function On / Off

– Min size too large

– Inflation parameters

• Total height

• Maximum angle

– Hard sizing

• Meshing methods

– Patch conformal or patch independent tetra

– Sweep or Multizone

– Cutcell

Geometry cleanup in Design Modeler

or

Virtual topology & pinch in Meshing

Mesh setting change

Mesh setting change

Direct meshing can be used to

minimize remeshing time



and Reliable Result


(max,avg)CSKEW=(0.912,0.291)

(max,avg)CAR=(62.731,7.402)

(max,avg)CSKEW=(0.801,0.287)

(max,avg)CAR=(8.153,1.298)

VzMIN≈-100ft/min

VzMAX≈400ft/min

VzMIN≈-90ft/min

VzMAX≈600ft/min

Large cell size

change

Mesh 2

Mesh 1


and Reliable Result


Parameterization in ANSYS

Meshing

Meshing controls can be parameterized

– Global controls and local controls

– Selection of parameter promotes the parameter to the

WB project page

– Geometry and Meshing parameters can be related using

expressions in the parameter manager


Parameterization project

example

• Number of divisions

on the outlet pipe

equal to two times its

length

• Number of divisons

on the inlet pipe equal

to its length + 4

8+

4=

12

div

isio

ns

Inlet

Outlet


Parameterization project

example


Pre-Processing Scripting


Systematic Errors

• Discrepancies remain, even if numerical and model errors are insignificant

• „Systematic errors‟:

– Approximations of:• Geometry

• Component vs. machine

• Boundary conditions (Turbulence, profiles, …)

• Unsteady-state flow behavior

• Fluid and material properties, …

• Try to „understand‟ application and physics

• Document and defend assumptions !


Poor quality mesh

Reducing Errors:

Some Best Practice Guidelines

• Grid quality

– Grid angles 90° for hex

affects truncation error

– Recommendation• Good: 20° < α < 160°

• Fair: 5° < α < 20° & 160° < α <

175°

• Poor: α < 5° & α > 175°

• Skewness = f(α)

High quality mesh

not scalable scalable

90

90,

90

90minmaxmax


• Use AMP, FLUENT and CFX to check mesh


and Reliable Result

+--------------------------------------------------------------------+

| Mesh Statistics |

+--------------------------------------------------------------------+

Domain Name: Air Duct

Minimum Orthogonality Angle [degrees] = 20.4 ok

Maximum Aspect Ratio = 13.5 OK

Maximum Mesh Expansion Factor = 700.4 !

Domain Name: Water Pipe




Global Mesh Quality Statistics :





Solver Strategies


Initialising

• Many options beyond simply by zone

• FMG-I (steady state single phase flows)

• Hybrid initialisation (poorer quality grids)

• Interpolation Files

– transfer data from one grid to any another (eg coarse

to fine mesh or different geometry)


Pre-Processing in ANSYS CFD

Workflow Parameters

• Input parameters

– Associate with multiple

boundaries

– Manage in a single panel

• Output parameters

– Quantitative values

– Report all at once

• ANSYS Workbench



ANSYS Fluent

• Automatic Solution

Initialization and Case

Modification

• Automatically executed user

specified solution strategies

• Journal setup

• Spatial interpolation

files for better start

• Gradually ramp up

conditions



ANSYS CFX and CCL


Choosing the solver

• Segregated solver remains default in FLUENT

• PBCS typically 5x faster, though can be orders of magnitude. Solves the continuity and momentum correction equations in a coupled implicit manner. Works by dampening out pressure-velocity decoupling errors inherent with segregated solver promoting faster convergence

• PBCS is much more stable on poor quality mesh (high skewness, high aspect ratio, jumps in cell size) and applicable for all flow regimes. Recommended for all but highly compressible flows.

• DBNS remains choice when there is a strong interdependence of momentum, energy and density


Turbulent Flow over a Backward

Facing Step

Problem Description:

•ReH = 37,400

•Inlet height = 8H

•Outlet/Inlet area ratio = 1.125

•Standard k-w model

•EWT

•Inlet profiles for u,v,k,w

•21,750 quad cells

•Mesh weighted towards

walls and backstep

Reference:

D. M. Driver and H. l. Seegmiller. Features of reattaching turbulent shear layer in

divergent channel flow. AIAA Journal, 23:163-171, 1985.


PBCS Solver

Settings:

•CFL = 200

•ERFs = 0.75

•PRESTO! for pressure

•MUSCL all other eqs

Contours of

Velocity

Magnitude

from PBCS

Skin Friction Coefficient (Cf*1000)

.vs.

Distance behind Step (X/H)

Pressure Coefficient (Cp)

.vs.

Distance behind Step (X/H)


Facing Step


Solver Memory (MB)Time per

Iteration (s)

Iterations to

Convergence

Time to

Convergence (s)

Segregated 73.2 0.288 2677 771

PBCS 80.7 0.500 494 247

Results from the different solvers

An accurate result can be obtained in a fraction of the time


Facing Step


Solver Options

to Improve Accuracy

• Steady state VOF. Use BGM instead for faster results but

comparable accuracy to geo-reconstruct.

• Transient Multiphase. Consider NITA or variable

extrapolation for faster transient results.

• High accuracy VOF solution maintained using new

compressive scheme and applied by zone or phase

• Conjugate heat transfer. Use W-cycle for energy with

BCGSTAB for stability and accuracy.

• Single Phase flows. Use F-cycle for flow and turbulence.

• Use NBG for high accuracy. More stable (reliable) than

cell based default gradient scheme

• DBNS has solution steering by regime


RBF Morpher – An Example of a

Fast, Reliable Process

• Radial Basis Function Morpher

• ANSYS Partner

• Designed by Marco Biancolini @ Rome Uni

• Embedded in FLUENT

• Morph in parallel on clusters

• Zero File I/O between designs

• Fast convergence from previous design

• High levels of control on boundaries moving or not

moving

• Easily scripted and connected to optimisation codes, e.g.

iSight, ModeFrontier, ANSYS Design Explorer, etc...


RBF Morpher


Solver Scripting

• The Fluent and CFX solvers have their own scripting that can be run interactively (open) or batch (closed)

• A journal file contains a sequence of TUI (Fluent) or CCL objects/commands (CFX), arranged as they would be typed interactively into the program or entered through the GUI.

• Fluent‟s GUI commands are recorded as Scheme code lines in journal files for re-play. FLUENT records everything you type on the command line or enter through the GUI.

• CFX, CFX-Pre, CFD-Post and TurboGrid commands invoked by Perl script.

• You can also create these scripts manually with a text editor. Comments can be included.

• Ensures to prevent any lost time due to incorrect setup

• Ramp up solver settings gradually


Solver Scripting

• Anything you normally do can be written as a script. Some

typical examples are:

– Running a batch job or RSF submission

– Setting up complex material properties (alternative to read

boundary conditions)

– Setting up a simulation

– Data analysis/post processing

– Transient data analysis

– Automating a known convergence strategy

• Wild card support at R13 (Fluent post operations)

report>surface integrals “*outlet*”

• Combined with batch solve, whole process can be run

“hidden” and is very repeatable & controlled


Employing CFD Strategy through

scripting

• Gradually ramp up conditions using staged process

• Rotating solid example process

– Start single phase with all fluid zones stationary and set

first order with conservative CFL

– Initialise using FMG-i and solve

– Switch on energy and enable thermal boundary conditions

then solve further

– Switch to second order/PRESTO/MUSCL and solve

– Change fluid to solids and solve further

– Change solid zone to MRF zone at N rpm and solve

– Switch to aggressive settings (CFL, AMG stabilisation)

and solve final section before reporting


Sources of Solver Error

• Round-off errors

• Iteration errors

– Difference between „converged‟ solution and solution at

iteration „n‟

• Solution errors

– Difference between converged solution on current grid

and „exact‟ solution of model equations

– „Exact‟ solution Solution on infinitely fine grid

• Model errors

– Difference between „exact‟ solution of model equations

and reality (data or analytic solution)


Iteration Error – Example

Residuals

Check for monotonic convergence

© Siemens PG


Iteration Error – Example

Res=10-2

Iteration 35

Res=10-3

Iteration 59

Res=10-4

Iteration 132

Relative error:

0.18% 0.01%

Ise

ntr

op

ic E

ffic

ien

cy

Convergence criterion

Iteration Number

Iteration errors:

Difference between

„converged‟ solution

and solution at

iteration „n‟


Discretisation Error – Example

• Compressor cascade

• Residual = 1 10-4

• 2nd order discretization scheme

Grid 1 Grid 3Grid 2


Model Errors - Example

• Inadequacies of (empirical)

mathematical models:

– Base equations (Euler vs.

RANS, steady-state vs.

unsteady-state, …)

– Turbulence models

– Combustion models

– Multi-phase flow models

• Discrepancies between data

and calculations remain,

even after all numerical

errors have become

insignificant


Model Error - Example

Model error: k-


Solver Side Changes

ANSYS Fluent

• You need not always revert back to the geometry

or meshing to change the grid

• Extrude domain (3D)

• Separate face or cell zones

• Adapting grids (grid independence)

• Deactivate/Activate cell zones

• Delete/Append cell zones

• Mesh swapping in parallel (R13)

• Append case/data in parallel (R13)


Post Processing Strategies


Post-Processing in ANSYS CFD

Expressions, State & Session Files

• CFDPost Expressions (user defined outputs eg

pressure co-efficients) can be pre-defined and

called via CCL, session or state file for quick

analysis (like Custom Functions)

• CFDPost state files can be written and read to

allow same objects to be used on different

results file ensuring consistency

• CFDPost can record and replay a session file to

repeat repetitive operations and reduce user

error

• Fluent Post Journals



Post object definition



Case Comparison

Click to

activate

Select two of the loaded

cases or two timesteps

• Objects can be

locked across

models


Workbench Integration of

CFDPost

• No need to worry about files

• No need to save/load state (auto-saved on

close)

• All project files saved in one shot, including CFD-

Post state

• Automatic refresh of files when they change

• Integration with ANSYS DX for Optimisation

• Automatic Report creation


Full Process Strategies



Workbench Integration

• All in one simulation – huge saving of effort!

• Project schematic can be stored for re-use

• Customised schematics can be generated

(ensuring process consistency for quality and

reducing user deviation/error)


Design Updates in ANSYS Workbench

1. Change geometry dimensions and/or

boundary conditions

2. Generate updated results with the click

of a button.

1. Change the geometry in the CAD

system

2. Export a STEP, Parasolid, or ACIS file

from the CAD system

3. Import the STEP or other file into

geometry tool

4. Reclean/re-simplify the geometry, often

from scratch

5. Recreate the mesh, often from scratch

6. Export the mesh

7. Import the new mesh into CFD solver

8. Re-apply the physics setup

9. Calculate the new CFD solution

10.Redo post-processing

ANSYS Workbench Workflow Traditional CFD Workflow

This is an enormous time

savings for even the most

trivial geometry changes!!!


Save Project as Custom System



Scripting Overview

• ANSYS 12.1 fully supports Workbench journaling and scripting – Project concepts & operations

– Parameter management

– Native applications• Project Schematic, Design Exploration, Engineering Data

– File management and data models

• Python-based scripting language– Object-oriented

– Platform-independent

• Fully documented & supported

• Works “hand-in-hand” with application-level scripting– DesignModeler, Meshing,

Mechanical, Mechanical APDL,FLUENT, CFX, etc.



Workbench Journaling

• Workbench operations are recorded in a journal file

• Each session creates a new journal file

• Playing back the journal recreates the session

• Two types of Workbench journals

– Automatically recorded session journals

– Manually recorded journals

• Tools -> Options… -> Journals and Logs



Plane Creation in CFD-Post


Spreadsheet Controller

A simple spreadsheet can be used to control or set up

workbench workflows thanks to IronPython. (Iron Python is

the journaling language of workbench but also allows you

to program and link to other applications through the .NET

framework.).


Data Entry Tab

Data entry

including analyst

and simulation ID

describes the

simulation

Status/progress is

is reported here

Numerical reports

retrieved and

displayed on

completion

Do the stuff buttons


Recorded simulations

On completion of the run the background script will

record all your settings and the results on the next

worksheet

Hyperlinks takes you to a

detailed automatically

generated HTML detailed

report with graphics


Summary: CFD Strategy

• Walk before you can run

• Think about what you want to gain in advance of

setting up the simulation

• Is process repeatable?

• Do I need to have a constrained process?

• What extensibility tools can I use? (UDF‟s, CCL)

• Look to combine strategies

• Look beyond the defaults

• Construct a reliable and accurate process for

your application


Summary: Reducing Errors

• Representative mesh

• Define target variables:

– Pressure loss

– Efficiency

– Mass flow rate

– …

• Select convergence criterion (e.g. residual)

• Plot target variables as a function of convergence criterion

• Set convergence criterion such that value of target variable

becomes „independent„ of convergence criterion

• Check for monotonic convergence

• Check convergence of global balances


Summary

• Quality assurance is essential for industrial use of CFD

• Ensure all details captured suitably

• Accept and understand the sources of error

• Quantify and reduce numerical errors by deterministic and

rational procedures

• Quantify model and systematic errors by validation work

• Resources:

– ERCOFTAC SIG: „Quantification of Uncertainty in CFD‟

– CFD Best Practice Guidelines for CFD Code Validation for Reactor-

Safety Applications

– ANSYS CFD Best Practice Guidelines

– Your helpful ANSYS support office

http://www.ercoftac.org/publications/ercoftac_best_practice_guidelines/

http://domino.grs.de/ecora/ecora.nsf/16edc336e8a342f580256502004d0e8b/684c0ed5f389add8c1256fcd002e7cb8/$FILE/D01_WP1.pdf



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