As its my day off I thought I’d explain about the tools I use to do what I do rather than go into details on what I’m doing. (OK so I ended up not having time on my day off to finish writing this and have ended up writing it whilst waiting for a project to solve)..
There are many tools (software) that I need to employ in order to carry out these complex equations in the short space of time required; one of these is Computational Fluid Dynamics or more commonly CFD - which you may hear mentioned throughout the coverage of an F1 race.
So what is CFD? In simple terms it’s a fairly fancy scientific calculator that carries out millions of equations very rapidly by way of a computer to solve the fundamental conservation laws of mass, momentum and energy using the Navier-Stokes equations.
Now these equations are around 200 years old and were not essentially solved due to their complexity… ????? you’re now saying to yourself. It was only when the computer arrived in the 1970′s that these very complicated and time-consuming equations were solvable which was when CFD was considered to be born.
OK, that is the tool I use, but how do I use it….. Lets use an F1 car as an example. Imagine the car as one solid block rather than 1000′s of parts and components sat in a large sealed box. You then discretise the volume within the box with millions of tiny volumes (cubes, pyramids, wedges, prisms and polygons) so you have a virtual ‘mesh’ all around the car. [Please not that the image below has the mesh within the volume cut-away so you can visualise the mesh topology around the model (car) - ordinarily when solving you will have a domain that is filled with a mesh. It is also utilising a polyhedral mesh]
Now we all know that in life, equilibrium must exist, and the same applies in CFD – you must also have a balanced system, so what (energy, mass balance etc) goes in, must come out. For example, energy, either gets converted as stated in Newton’s Laws or Laws of Thermodynamics at the ‘inlet’ of a closed system or simply passes through to the ‘outlet’ unaffected.
In the case of CFD you have many equations being solved within each tiny volume (such as the 3 components x,y & z of velocity, pressure, turbulence etc) and the resultant output of each equation is then used as an input variable for its neighbouring volume and so on. During the solving process you will see a residual curve indicating the ‘state of equilibrium’ within the domain essentially satisfying the criteria that what goes in, comes out. In other words you have a converged solution (usually when residuals are within 1x10e5 but 1x10e3 is also accepted in some cases or if a paticular variable such as mass-flow or pressure outlet flat-lines) from which you then can post-process and start to make design changes.
In the image above you can see these pressure contours graduated by colour – notice the intensity of the pressure waves at the nose-cone, tail uprights or wing tips – this is how shock-waves develop (These sudden increases in pressure, temperature and density play a huge part of the design process for supersonic flow problems and are also part of the ‘sonic boom’ you hear about – this is where the expansion wave and the shock wave momentarily merge and degrade, cancelling one another out. They also can play a big part in designing out drag characteristics associated with this phenomenon.
You can extract a huge amount of data from this kind of simulation using CFD – essentially this is virtual prototyping so you are creating your designed objects in within a computer program which reduces the need for any physical testing or expensive wind-tunnel time.