point on the rear wing wasn’t properly reproduced. Therefore, we introduced artificial intensity into the tunnel and suddenly they had to be re-optimised and then they began working on track due to a better correlation.’
This is the key to achieving repeatable and accurate results – optimising the correlation of the Wind Tunnel with reality and this is achieved by using techniques such as Boundary layer control and moving belts underneath the car.
Why use Wind Tunnels?
In terms of accuracy and repeatability, Wind Tunnel testing was described as the ‘least worse’ when compared to CFD. Although a truthful yet pessimistic view, Wind Tunnels still offer engineers a wide variety of capabilities, some of which are not purely engineering.
For example, Wind Tunnels can help to improve the relationship between the aerodynamicists and stylists of automotive companies, which continues to be an issue. Essentially, stylists have minimal concern for the performance of the car, they are simply interested in developing the most aesthetic design for their target market. Aerodynamics on the other hand, do have performance targets to meet, and so every curve, groove or inlet needs to be aerodynamically efficient.
Therefore, it is a constant battle between the two. However, some manufacturers such as Porsche include the stylists in the very early stages of Wind Tunnel test programmes to improve their understanding on aerodynamics and thus, how their design ideas affect the overall performance of the car, in the hope that they will consider aerodynamics as part of the concept development process.
This method of educating stylists is becoming more important and is one of the reasons behind the increased use of visualisation techniques such as using smoke or Particle Image Velocimetry (PIV)
during a Wind Tunnel run to demonstrate the turbulence and wakes produced from certain shapes. In fact, this is even becoming important within the CFD world as Exa Corporation illustrated by exhibiting their new software aimed for stylists where CFD visualisation simulations can be watched through 3D glasses.
This need to interact with a physical model in the Wind Tunnel is also down to our current engineering culture. ‘We have to put a clay model in a Wind Tunnel because we want to see and feel it whether you are a stylist or an aerodynamicist,’ explains Andreas Kremheller, Aerodynamicist at Nissan Technical centre Europe (NTCE).
‘Aerodynamics is still seen as a black art. We could develop cars using CAD alone just as a customer can buy a car from a website but they still want to go and see it to get an appreciation. Also, if you analyse a physical model, you may see any discontinuities on the surface so you know immediately which areas you need to focus on.
’Although physical testing is currently part of our engineering behaviour, with the next generation playing on iPads at the age of three, it may be a different story, particularly if CFD software capabilities develop to allow the user to interact with the virtual model.
Using clay models may seem somewhat outdated, but it really is the most flexible and practical method to developing cars in Wind Tunnels. ‘In my opinion there is no quicker method then removing or adding some clay in a couple of minutes and being in touch with that model, both for styling and aerodynamic purposes,’ highlights Marcel Straub, Complete Vehicle Aerodynamicist at Porsche Engineering services GmbH.
‘I think this is the only way you can combine having direct interaction with styling together with the model and obtain reliable measurements all in one package.’
In the past, making a small change in CFD and consequently re-running the case used to take hours, which is why clay models remained attractive as changes could be added or removed within minutes. Of course, by physically manipulating the shape of the model it is a challenge to perfectly model it back to baseline condition, unlike a CFD model and thus there lies some inaccuracies. However, arguably now CFD can be quicker
because the simulations can not only run faster, but if clay models require the production of Rapid Prototyping parts then clearly CFD is going to be the fastest method.
Furthermore, with the introduction of the WLTP to generate new emission targets for 2020, Wind Tunnels will be required even more – which surely still proves their worth? ‘The new WLTP has a ‘combined approach’ where you can declare your minimum and maximum drag coefficient values in the Wind Tunnel. Therefore, this is your ultimate tool,’ highlights Andreas Kremheller, Aerodynamicist at NTCE. ‘The sooner you can utilise that tool to shape your vehicle, then surely that minimises the risk as you are already using that tool to develop your vehicle, rather than a coast down test.’
Wind Tunnels have a bright future, as both motorsport and automotive companies will continue to utilise them, even if it is not a 100% accurate method, it remains the closest thing to reality. Especially when both the regulations and companies keep pushing to reduce costs and consequently the amount of physical testing will continue to diminish to a minimum.
Wind Tunnels are used for a whole variety of tests, such as cabin comfort. This is where different configurations are tested and the flow is analysed to improve the airflow surrounding the passengers which is particularly important with convertible vehicles. This also illustrates the use of smoke which is a common visualisation technique.