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Hello, and welcome to The Hitchhiker's Guide to CFD! As the name implies, this series of articles is aimed at people who are new to (and completely baffled by) CFD and want to find out more about this exciting subject but do not have a background in fluid dynamics/heat transfer, numerics and/or computer modeling. The series is aimed at introducing many of the fundamental concepts and ideas that form the basis of CFD without having to resort to complex numerical formulae and terminology laden descriptions that so often pepper articles of this type. Instead, the use of images and practical examples will be used to emphasize topics of importance. The use of plain English (as the author knows no other!) will also be enforced. If formulae will be used (which unfortunately at some point we will have to discuss), then the author promises to make the experience as painless and worry-free as possible! Don't reach for that math book quite yet!
As with any text on a complex subject of this type, not everything will be covered in detail. In general, anything related to the physics of fluids flow and heat transfer will not be discussed, as there are many good books available on these topics already. However, pertinent examples will be included and reference will be made to certain types of flows that the user is expected to know about. Additional notes and advice will be given later on this, so don't go spending any money yet!
The author's own experiences will also be included in various areas (for better or for worse!). Although this was primarily gained using the STAR-CD commercial CFD code, the information will be general and applicable to all popular CFD codes that employ the same methodology.
In short, the aim is to provide engineers, scientists, designers and anyone with a interest in this subject with a complete overview of what CFD is about and how to get the most out of it. If you already have a Masters or PhD in this subject and are thinking of writing your own CFD code, then this is not the guide for you! If, however, you think that CFD stands for Complicated Formulae Descriptions then read on!
The other chapters that will be discussed in this series will include the following subjects:
This is a just a subset of the most important topics within this subject, but will be enough to provide the reader with a basic understanding of what they need to know in order to further explore their own areas of interest. It should also allow the reader to confidantly approach a commercial CFD code vendor and ask the right questions for the problems he or she wishes to tackle. Particular emphasis will be given to grid meshing techniques, range of modeling options and versatility of post processing. If any or all of these terms are new to you then don't worry as they will be explained in due course.
CFD stands for Computational Fluid Dynamics. It means predicting physical (and non-physical!) fluid flows and heat transfer using computational methods. Over 40 years ago, this was an emerging field that owed everything to the advent of the modern computer. Although the equations describing fluid flow and heat transfer (i.e. conservation of mass, momentum and energy) had been developed many years earlier, it required the fast automated processing of mathematical instructions before it was considered to be a practical tool. Forty years later on and computers that we carry around in briefcases and handbags are more than powerful enough to perform the required calculations to get a CFD solution in a matter of minutes! As computers get even more powerful in the future then the accessibility and efficiency of CFD will become more and more commonplace.
Fluid flows and heat transfer are important as they affect everything in our everyday lives. For example, breathing, eating, walking and exercising all involve fluid flow and heat exchange. Our bodies are in a constant battle with the environment in order to try and equalize the amount of heat and moisture exchanged with it, depending on what we are doing and where we are. Fluid flows are also encountered in virtually all parts of industry, especially during the manufacturing and operation of various machinery and components that we encounter. For example, the automobile includes a whole world of different fluid and heat transfer mechanisms, such as cooling, combustion, ventilation and aerodynamics. Understanding how all these fluid and heat transfer mechanisms work is important for engineers and scientists to improve the operation of the mechanism and reduce its impact on the environment.
As well as solving plain vanilla fluid flows (such as flows in pipes and
around obstacles such as cylinders), CFD can also include additional phenomena
such as chemical reactions, phase changes, acoustic noise and thermal radiation,
etc. In fact, the speed at which commercial and research codes are progressing
means that virtually any kind of real life industrial application, from high
speed re-entry vehicles to macroscopic electromagnetic effects can be studied
using CFD. Heat transfer within and across solids can also be included. This
means that the analysis can include any combination of gas, liquid and solid
materials and all appropriate physical effects that would normally occur in
reality being included.
There are many different ways by which equations describing fluid flow and
heat transfer can be solved using computational methods (but note: the solutions
obtained are only approximate, whichever method is used!). Most commercial
and research codes rely on one of the following:
Each of these methods requires the definition of discrete points in space at which variables like velocity, pressure, temperature etc. will be computed. While the governing equations are always the same, the particular geometry with initial and boundary conditions determines a unique solution for each particular problem.
The Hitchhiker's Guide will concentrate on the Finite Volume Method, as it is the method used by most of the popular commercial CFD codes currently available. The other methodologies are still commonly used in industry and yield good results for certain types of applications. Additional information on each will be provided later.
The Finite Volume Method (or FVM as it will hereafter be known) starts with the integral form of the governing equations, involving surface integrals (e.g. convective and diffusive fluxes) and volume integrals (e.g. those describing sources and sinks). In case of a transient flow (ie unsteady flow that changes over time), there is also a rate-of-change term. The FVM represents the integration of the governing equations over (a finite number of) contiguous control volumes (CVs) representing the solution domain. Since variable values are computed only at discrete points (usually centroids of control volumes), approximations must be used to express the integrals in terms of unknowns at discrete locations. Three kinds of approximations are involved:
In this way, one algebraic equation per CV is obtained, linking variable value at the centroid of that CV with those at neighbor CVs. For the solution domain as a whole, a large system of algebraic equations is obtained. To make things more complicated, one such system is obtained for each governing equation (e.g. for three velocity components, pressure, temperature etc.). Since these equations are in general non-linear and coupled, the solution must be sought using iterative solution methods. Iteration means repeating a sequence of operations over and over, until changes in computed variables become negligible and we declare the process to have “converged”.
Most of the main commercial CFD codes, such as STAR-CD, Fluent, CFX and Phoenics are based on the FVM scheme. One of the reasons why FVM has succeeded over the other methods is that it is inherently conservative: irrespective of errors in various approximations, the discretized equations still fulfill the conservations laws exactly (e.g. mass entering solution domain equals mass leaving it). In other words, the errors introduced through various approximations affect only the distribution of variables within the solution domain without violating conservation principles. The FVM is also easier to understand by engineers than some of the other, more mathematically-involved methods, since the terms that need to be computed have a clear physical meaning (e.g. mass or heat flux through a CV face, force at a CV surface etc.). More will be discussed about this topic later on in the series.
What all the above means is that we first have to subdivide the solution domain into a finite number of CVs (also called cells) using a suitable grid (also called a mesh). The grid can be composed of hexahedral, tetrahedral, prismatic, pyramid or polyhedral cells. Any of these cell shapes can be used to construct grids suitable for CFD domains but for historical and other reasons, hexahedral and tetrahedral are the pre-dominant shapes used. Polyhedral grids are rapidly catching up and are likely to be the standard in the future. Examples of each of the above cell types are shown below:
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Commonly used CFD Cell Shapes Click on a picture above to see a bigger image |
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Sounds easy? Well, for simple geometries such as cylinders and squares, creating the grid is usually easy. However, for more complex shapes such as aerofils, car bodies, ships and just about everything else the challenge is to create the best quality grid possible in a reasonable time. We will define what we mean by grid quality and reasonable time later on in the next article, plus also discuss the different cell shapes and grids they result in.
CFD has been used in industry since the 1960's onwards. Its use has now grown such that it is used in a increasingly varied range of industries, from Meteorology to Biomedical (see the list below). The reasons for this are varied, but a number of key elements emerge for its rapid rise and usage over the past 10 years.
It has proven to reduce lead times and cost of new designs:
CFD is now routinely used in the aerospace, automotive and other industrial engineering fields as an intrinsic part of the design process. For example, all new aircraft and missile shapes are tested and optimized using CFD, as well as virtually every single component used in the modern automobile, from the radiator to HVAC's to the windshield wiper spray pattern. Designers are now able to run the initial fluid dynamics and heat transfer analyses themselves without having to wait for experimental prototypes to be setup and run. Once the initial design criteria have been met, more advanced CFD calculations can be performed by specialists within the group while the designers continue to refine the design based on the results.
It can help verify existing experimental data or provide data when none exists:
Historically, experimental results have always been used as the guide for designing fluid and heat transfer based systems. However, as these involve a large amount of human interaction then they were always prone to human (and other) error. CFD can now provide an alternative source of data which can be compared directly with the experimental results and verify their validity (or not!). For this reason, it is often called "The Virtual Wind Tunnel."
One of first main applications of CFD was in the design of high speed re-entry vehicles for space exploration. Engineers involved in these projects had very little in terms of hard data and universal theories to help them construct and test their vehicles. By developing appropriate numerical techniques, early CFD engineers were able to provide the first practical and complete engineering solution to a problem that had taxed the minds of many during the 1950's and early 1960's.
Similarly, in the nuclear power industry, it is too dangerous to operate plants under hazardous conditions to test equipment. CFD can be used to predict what will happen under failure conditions, such as coolant pipe breaks and contamination.
It can easily be configured to provide parametric results of almost unlimited detail:
CFD is typically used to verify one set of experimental values in order to ensure that the correct solution is being produced. Thereafter, it is often configured to produce a range of results based on any number of input variables so that a complete insight of the operating envelope is developed.
For example, an aircraft manufacturer might need to produce a safety zone contour for airlines who purchase their aircraft so that they can judge how close personnel can be when the aircraft is taxiing into or out of the terminal. In this case, the variables might be the aircraft type, the engine type and the throttle/power rating at which the engine is being run at. The CFD analysis can be set up so that any of the above variable combinations can be studied at the push of a button, or alternatively, all such combinations can be run automatically while the engineer has gone home for the night or is even away on vacation!
CFD is now used in an extremely wide range of industries. Any industrial process that involves fluid flow and/or heat transfer can benefit from CFD analysis. New applications are continuously being found for it, especially as the relative cost of introducing CFD is dropping and more and more small companies and institutions are beginning to apply it. The body of experienced users is also growing in line with ease of use, making CFD easier to introduce.
Listed below are a typical list of industrial and academic areas where CFD is commonly used. Examples of these applications are all over this website (Explore the articles in the navigational panel on the left!)
| Aerospace | Aerodynamics, wing design, missiles, passenger cabin |
| Automotive | Internal combustion, underbody, passenger comfort |
| Biology | Study of insect and bird flight |
| Biomedical | Heart valves, blood flow, filters, inhalers |
| Building | Clean rooms, ventilation, heating and cooling |
| Civil Engineering | Design of bridges, building exteriors, large structures |
| Chemical Process | Static mixing, seperation, reactions |
| Electrical | Equipment cooling |
| Environmental | Pollutant and effluent control, fire management, shore protection |
| Marine | Wind and wave loading, sloshing, propulsion |
| Mechanical | Pumps, fans, heat exchangers |
| Meteorology | Weather prediction |
| Oceanography | Flows in rivers, estuaries, oceans |
| Power Generation | Boilers, combustors, furnaces, pressure vessels, nuclear |
| Sports Equipment | Cycling helmets, swimming goggles, golf balls |
| Turbomachinery | Turbines, blade cooling, compressors, torque convertors |
Within the list of industries and applications listed above, CFD can include any of the following phenomena and flow regimes:
As CFD is still in its relative infancy as a subject, there are a number of things that CFD cannot currently do accurately at an affordable cost. The reason is that some physical phenomena are not well enough understood to be able to postulate a mathematical model, or the required models are so complex that the required computing effort is excessive. Therefore, the governing equations are simplified using models of physical phenomena that are affordable but not exact, leading to errors even if one was able to solve the equations analytically. The phenomena affected include:
One of the primary reasons for the rise of CFD over other traditional methods of producing flow results is due to its effectiveness at displaying a complete description of the flow field. By this we mean that the result of any flow quantity such as velocity, pressure, temperature, turbulence etc, can be shown for every part of the domain. Although the initial cost of producing a CFD result might be considered relatively high at the beginning, the wealth of available information at the end of the analysis can make it a very cost effective option for designers on the whole. The repeatability of the results is also important.
 Blunt Body Example: CFD Domain Geometry |
Listed below are some examples of what CFD can provide the end user. A simple blunt body example (above) is used to illustrate each of the items.
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Examples of CFD results Click on a thumbnail to see the full image with color scales |
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) Stagnation points |
) Recirculation zones and re-attachment points |
) Pressure drop and rise across a system |
) Hot spots and cool zones |
) Flow rates and flow split |
) Density variation |
) Shock Waves |
) Lift and drag forces acting on surfaces |
) Concentrations and mixing |
) Particle tracks and streamlines |
) Turbulence levels and vorticity |
) Heat Flux and Exchange |
A CFD analysis can be carried out without the requirement of extensive equipment
and supporting instrumentation. Starting only with a sketch of the application,
a CFD-code and a PC, it is possible to produce a CFD solution that can yield
a useful understanding of the flow and heat transfer.
However, in general, CFD is slotted into what's known as the CAE (Computer Aided
Engineering) environment in which other software is extensively used, such as:
Most CFD software is able to accept input from all of the above software packages
and manipulate it (if required) so that it produces the required grid types.
In addition, once the CFD analysis has been completed, the results (including
the grid) can be exported for further analysis in another package. For example,
heat transfer coefficients generated from the flow can be mapped to structural
analysis software in order to generate thermal stresses. In addition, certain
CFD software are able to communicate directly with structural analysis software
in order for fully coupled fluid-structure interaction calculations to be performed.
Listed below is basic process followed by engineers in industry who commonly
use CFD for analysis:
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CFD Process Click on figure below to see a slide show of the process |
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On the face of it, the process looks straight forward and would appear to involve roughly the same amount of time for each step. However, engineers and scientists familiar with the process will know that upto 80% of the their time will be spent on steps 2 to 3 alone! For this reason, a lot of recent commercial and research expenditure has been on the development of more efficient tools for purely surface preparation and volume mesh generation, with the ultimate Holy Grail being the "Push Button Mesher". While many companies would claim they now provide this, no single piece of software is currently able to deliver across the wide spectrum of CFD applications. However, it will only be a few years before this aim will be reached and the engineer will truly have a powerful "one size fits all" tool within their hands and the time allocated to process will drop significantly. Until then, the CFD analyst will still have to toil away at generating the grid and learn which tools are best for which application.
This completes the first article in this series! The next article on meshing and grids will begin looking at the practical aspects of creating a solution domain for a CFD analysis.
By Kevin Jones
The opinions stated in this article are purely those of the author and are not neccessarily those of the CD-adapco Group.