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CFD Advice, Tips and Tricks, Best Practices

Computational Fluid Dynamics is often said to be mishmash between science and art. Navier-Stokes equations can be elegantly derived, precisely discretized and efficiently coded. However, once a scientist or an engineer is faced with unexpected, persistent and inexplicable convergence problems associated with numerical, iterative methods of CFD, one need not only help of exact science but also extensive experience, subtle intuition and a dash of fine art of fluid flow modeling. The right advice, tip or trick from fellow modeller can save hours and days of frustration and futile drudgery. So if you have any CFD tips or tricks to contribute - please share with others.

CFD Best Practices Guide?

Errors and uncertainties in modelling the physics of fluid flow can be divided in few groupes:

1. Assumptions and approximations at model building stage (pre-processing):

model simplifications
available experimental data (quality, applicability, ...)
geometric boundaries (proper resolution of important small scale details)
boundary conditions (proper location of inlets and exits, turbulence intensity assumptions, ...)
proper selection of physical submodels (RANS, LES, steady, transient, ...)

2. Errors and uncertainties related to model mesh building stage (pre_processing):

total number of cells, local mesh sizing relative to physics of the problem
boundary layers resolution (first cell thickness, expansion ratio)
cells skewness, volume expansion ratio

3. Numerical errors/uncertainties at model running stage (solution)

programming errors (bugs embedded in code)
physical models assumptions (turbulence, combustion, radiation, ...)
computer round-off (single precisions, double precision)
spatial and time discretisation (first or second order, blended, ...)
iterative convergence criteria (RMS of residuals for mass and momentum equations, ...)

4. Post-processing and Interpretation or Results

numerical errors associated with calculation, spatial interpolation of results
verification and validation, comparison with available experimental data

Links to CFD Best Practices Guide/Guildelines:
ercoftac.org/publications/ercoftac_best_practice_guidelines/single-phase_flows_spf/
http://www.grc.nasa.gov/WWW/wind/valid/tutorial/tutorial.html
http://www.oecd-nea.org/nsd/csni/cfd/workshops/XCFD4NRS/summaries-gama-wg/Mahaffy.pdf
http://files.asme.org/Divisions/FED/16302.pdf
pronet.wsatkins.co.uk/marnet/guidelines/guide.html
cfd-online.com/Wiki/Best_practice_guidelines
ansys.com/customer/webinars/cfdbest.html
nafems.org/resources/cfd_guidance
http://my.fit.edu/itresources/manuals/fluent13.0/cfx_intr.pdf

How to run OpenFOAM on Windows XP/Win7?

Recently, OpenFoam folks put very nice and easy tutorial how to run OpenFoam on Windows on their website
http://openfoam.com/resources/windows.php

How to search for latest patents?

Search or browse

FAQs.org

by inventor name or other search pattern
e.g., AIRFLOW DISTRIBUTION TO A LOW EMISSIONS COMBUSTOR

http://www.faqs.org/patents/inv/658945
http://www.faqs.org/patents/app/20090222187

Copy the number 20090222187, go to

FreePatentsOnline.com

and search or just replace #.pdf with the correct defining number:

http://www.freepatentsonline.com/20090222187.pdf

Enjoy full pdf document that can be viewd and/or printed.

How to view patents on uspto.gov?

Yes, it is puzzling to have US Patent Office website (http://uspto.gov/) use plugins that do not work on most computers. Anyway, there is a relatively simple workaround, just use Apple web-browser Safari.

How to run OpenFOAM in parallel?

If you have multi-core processor, and most likely you do, you can first try to run in parallel on one PC.

The steps are as follows:

1. Copy decomposer file into your working directory

let's say your working directory is

$HOME/OpenFOAM/userid-1.6/run/tutorials/incompressible/simpleFoam/pitzDaily

then copy decomposeParDict, decomposition parameter file, to your working folder

cp $HOME/OpenFOAM/userid-1.6/run/tutorials/inco
mpressible/simpleFoam/pitzDailyExptInlet/system/decomposeParDict
$HOME/OpenFOAM/userid-1.6/run/tutorials/incompressible/simpleFoam/pitzDaily/system/.

and edit decomposeParDict (use nedit or gedit or vi depending on Linux Distros)

$ nedit system/decomposeParDict

and set parameters (number of processors 2, method simple)

numberOfSubdomains 2;
method          simple;
simpleCoeffs
{
    n               ( 2 1 1 );
    delta           0.001;
}

2. Prepare mesh

$ blockMesh

and run decomposition script

$ decomposePar

and if need to run again

$ decomposePar -force

(-force to overwrite previous folders and files)

3. Now you are ready to run mpirun script and save output data in case.log. You might want to add & at the end command line to run it in background.

$ mpirun -np 2 simpleFoam -parallel >case.log &

4. Reconstruct final results (stored in subfolder processor0, processor1 etc)

$ reconstructPar

5. Postprocess results using your favorite postprocessing software eg ParaView on Windows or Linux

$ foamToVTK

creates VTK folder result ready to be used by ParaView