MUR Blog - CFD: Using CAD Effectively
Computational Fluid Dynamics, or CFD, is the indispensable tool of any aerodynamicist. It provides velocity profiles, pressure contours, vortices, and streamlines in pretty colours, alongside valuable data such as wall shear, coefficients of lift and drag.
Through the Power of CFD, We Ascertained a Cow Travelling at 1,300km/h Could Fly
Its usefulness stems from providing quantitative insight into the effects and nature of fluid flow around any object, from race cars to cows. When implemented successfully it releases colossal volumes of dopamine into the brain and triggers spiritual enlightenment; my fellow sub-team member Andrew Chak once did CFD for 68 consecutive hours in a full-lotus, attaining nirvana. However, CFD is as petulant as it is powerful, and has left me weeping in the foetal position on numerous occasions. This brief chronicle of learnt lessons is written to provide any fellow CFD novice a leg-up for its steep learning curve. As a starting point, I’ll be discussing how to utilise Computer Aided Design (CAD) for simulations.
CAD of a body is the first step in running CFD; it is where the shape that fluid flows around/through is created. When creating bodies, it is essential to keep bodies as clean and simple as possible. I cannot stress this point enough. As Andrew detailed in his blog, CFD basically works by applying Newton’s equations to millions of ‘mesh’ elements, and it’s very difficult to create this mesh with small and irregular shapes. Further, simulations take several hours to complete, and eliminating some elements could be the difference between being able to run one or two simulations in a day. Thus, where possible, complicated geometries should be approximated with simpler shapes.
The Difference Between Accurate CAD and CFD CAD
Above are two CADs: one a depiction of what the design should physically look like, the other an approximation for CFD purposes. You may notice several differences:
- “Chiselled Red Diamond Man” is replaced by “Pawn Man”, whose design was inspired by the East-Indian board game, Chess. Pawn Man’s dearth of arms will make it difficult for him to steer, but easy for us to simulate
- Tyres approximated with simple cylinders
- Nuts, holes and bolts removed
- Chassis interior “filled in”
- Engine represented by a comparatively simple block
The thought of attempting CFD on the former makes me physically ill and I would only recommend it to those who enjoy pain. Not only does the high level of detail require a bevy of mesh elements and computational time, simulation attempts would be dogged by hordes of errors due to poor mesh quality. Conversely, the approximated CAD has fewer and higher quality elements, reducing error sources and computational time.
While simplicity is our friend, it’s important to understand what aerodynamic ramifications your approximations have. The aforementioned changes won’t appreciably affect flow; there’s still a helmet impeding rear wing access, spinning cylinders are a reasonable representation of tyres, and the effect of bolts are small enough to ignore. Contrarily, aerodynamically significant features should be modelled as close to reality as possible. Our wing elements are smoother than Andrew Chak on a Saturday night, and diffuser angles are carefully designed to maximise the production of sweet, delicious downforce. The extent of your simplifications will be dictated by project scope; a PhD student investigating flow across a turbine blade will require much higher fidelity CFD than we do. Nevertheless, in any case, appreciating the disparity between your approximation and reality is crucial to getting the most value from your simulations.
Despite your best efforts, any aerodynamicist inevitably encounters some difficulties with successfully running simulations. Upcoming blogs will detail how to remedy this using other steps in the process, such as mesh sizing, boundary conditions and use of geometry repair tools. Until then, heed these CAD tips and you’ll find taming the CFD beast a little easier.
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About the Author:
Senior Aerodynamics Engineer, 2017
Junior Aerodynamics Engineer, 2016