\[ \]
\[ \]
\[ \]
Issue No 63, 16 September 2024
By: Anthony O. Ives
CFD stands for Computational Fluid Dynamics a more accurate title might be Numerical Fluid Dynamics as CFD uses numerical methods to solve the complex equations of fluid dynamics via computing. CFD is useful in aircraft design as it allows you to calculate more accurately things like lift and drag however, it also good at determining more detail about aerodynamic behaviour in general. This blog article hopes to introduce and discuss CFD including it's pros and cons as well as the fact sometimes it can be overkill and that simpler aerodynamic calculations can give just as good results.
I do not particularly like the use of computational in CFD because it implies that a computer can do something special. Computers are really only good at carrying huge numbers of simple arithmetic operations which would be tedious for a human to do but that make CFD practical. An important point to remember about CFD is that it still a theoretical method that gives a lot of information which may not be correct and needs to be validated, etc.
The theoretical behaviour of fluid flow can be described by a set equations mainly the continity equation, the momentum equations and the energy equation. A fluid is really anything that flows but is typical a gas or liquid, air is considered a gas and water is considered a liquid. Most of my blog content just needs to consider air as that is what a aircraft flies in. My occasional sailboat content will obviously need to consider water.
The number of fluid equations that you need to solve for your specific problem varies depending whether you are looking flow in pipe which is 1D (1 dimensional) or external flow such as airflow around an aircraft wing, etc which needs to be 2D (2 dimensional) or 3D (3 dimensional). 1D flow can also be used to do simple calculations for gas turbine engines. External flow can sometimes be simpflied to 2D such as wings, generally if there is symmetry. For 1D flow you only need one momentum equation, for 2D you need two momentum equations and as expected for 3D you need three momentum equations. Hence if you can solve your flow problem using a 1D or 2D equations it makes life easier. The continity and energy equations can also become more complicated with the dimensions that need to be considered. CFD typically uses the most complex fluid equations known as Navier-Stokes equations make the least assumptions. So if you only need 1D or 2D equations for CFD you will additionally make life a lot easier. In some future content I will go through the fluid flow equations in more detail however, more information can be found Ref [1].
Numerical methods use huge numbers of simple arithmetic operations to solve the Navier-Stokes equations. This is usually by drawing a grid around or inside the object that fluid is going to flow around or inside depending on whether you are looking at external flow such as an aircraft or internal flow such as a pipe.
The more dimensions you need and the more complex your object then you will probably need more grid points which require more computing power and time. More grid points means more accuracy but at the cost of more effort. In some future content I will look at the numerical methods used in CFD in more detail but also see Ref [1].
CFD has lots of applications primarily in the aircraft industry to predict drag and lift. A wide range of industries use CFD from chemical manufacturing industries to civil engineering. CFD can be used to solve any problem involving fluids whether that is air, water, sulphuric acid, molten lava, etc. Obviously all of my content of CFD be for helicopters or aircraft with some on sailboats.
I mentioned FreeCAD [2] previously and how I hope to produce future content on it. CAD models created in FreeCAD can be exported as a '.stl' file for use OpenFOAM.
OpenFOAM is free opensource CFD software package. OpenFOAM is a linux based system but can be also used in windows through a virtual machine application. It's not a particular user friendly package not that any CFD software package is. Any future content where I am demonstrating the use of CFD will use OpenFOAM or one of my own software codes. I hope to do future content looking at OpenFOAM in more detail. More information on OpenFOAM can be found on their websites [3,4]
Having discussed CFD and what it is, you should now be able to see the advantages of it. One of the main advantages of CFD is it gives a lot information about the flow for a particular problem you are interested in the case of my content that would be the flow around an aircraft. The other great advantage the information can be used to produce lots nice colourful pictures such as contour plots, flow paths, etc.
Disadvantages of CFD are that it's takes a lot time and effort to get CFD to work and to give results. In addition the results are still theoretical and may not give a better answer that a simple calculation. CFD results must be validated before they can be trusted as fact.
Unfortunately I have spent most of my professional career sitting at a computer and still not a fan of sitting at a computer all day. CFD as computers in general are is frustrating and tedious. My primary enthusiasm is for aircraft and more particularly helicopters , CFD is just means to end for me however CFD can be end in itself for some people in the aircraft industry and is their primary area of enthusiasm. But I prefer to spend as little time at a computer as possible just find out what I need to know.
How I intend to use CFD in my future content is give examples when a simple calculation would give as good a result as CFD. Use CFD to generate simple equations that can be used without repeating a CFD analysis. Use CFD to give graphical illustration of complex aerodynamic phenomenon such as vortex ring, autorotation, etc. Look at ways CFD principles can be applied to flight simulators, while I would not say I'm an expert on CFD or flight simulators I do believe modern professional flight simulators use CFD type calculations in their software code to make the flight simulator behave more realistic.
Please leave a comment on my facebook page or via email and let me know if you found this blog article useful and if you would like to see more on this topic. Most of my blog articles are on:
Mathematics
Helicopters
VTOL UAVs (RC Helicopters)
Sailboat Design and Boatbuilding
If there is one or more of these topics that you are specifically interested in please also let me know in your comments this will help me to write blog articles that are more helpful.
References:
[1] Computational Fluid Dynamics: The Basics with Applications, John D. Anderson Jr., 1995, McGraw Hill
[2] https://www.freecadweb.org
Disclaimer: Eiteog makes every effort to provide information which is as accurate as possible. Eiteog will not be responsible for any liability, loss or risk incurred as a result of the use and application of information on its website or in its products. None of the information on Eiteog's website or in its products supersedes any information contained in documents or procedures issued by relevant aviation authorities, manufacturers, flight schools or the operators of aircraft, UAVs.
For any inquiries contact: [email protected] copyright © Eiteog 2023