Aerodynamic study of the CDG concept

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The FIA (Federation Internationale de l’Automobile) has proposed a set of new rules for the 2008 FIA Formula 1 season. The purposes of these rules are to reduce costs, and most importantly make racing more exciting by encouraging more overtaking during the races. Part of the changes proposed involves major dimensional changes and the introduction of split rear wings also known as the CDG (Centreline Downwash Generating) wing. The aim of this rule was to produce a wake behind the lead car that would allow a following car to produce less drag and more down-force, hopefully resulting in more overtaking during the races. The question arises, will this really work?

Previous draft studies provide significant information about the current situation where by the following car produce less drag and down-force compared to the leading car. Up to 45 percent reduction in down-force and 28 percent reduction in drag are experienced by the following car when travelling at a half car separation behind the leading car and the figures drop as the separation is increased. One of the advantages of this situation is that it helps the following car to slip stream the leading car on long straights; however, the loss of down-force encountered by the following car makes it difficult and in most cases impossible to overtake in fast corners.

Presented in this article are the results of the CFD analysis conducted on an F1 CAD prototype designed to meet the 2008 Formula 1 regulations. The purpose of this study was to analyse and investigate several aerodynamic aspects of the 2008 car evaluated at a velocity of 250 km/h and most importantly the outcome when run in a draft dodging situation at a separation of one car length.

The following image shows two views of the model being used for the purpose of this analysis.


From the figures shown it can be seen that the change from 2006 to 2008 would be quite significant and several changes are quite important to analyse in terms of aerodynamics. As an example, a lot of down-force is produced by the rear wing assembly of a current Formula 1 car; however in 2008 the effect of wheel spin on wing performance can play an important role in rear wing design.

Optimising the CDG wing design


Initial two-dimensional studies show an increased trend in down-force when a twin element wing is analysed close to the spinning tyres compared to the analysis in isolation at a height specified by the rules. The following figures show a comparison between the airflow profile over the same wing in free stream (left) and with close proximity to a spinning wheel (right). It can be clearly seen that the spinning wheel has helped the flow remain attached at the trailing edge of the main element where as the flow separates when the wing analysed on its own.

Flow structure

Indeed, two-dimensional studies provide a good indication of the big picture, however it should be noted that the effects can greatly change when similar analysis is performed in 3D. A good example would be the effect of trailing vortices produced by the wings and the spillage of flow over a spinning tyre. In a real case scenario the effect of extra turbulence produced upstream of the wing by the front tyres and wing can change the situation further.

The following figure is an illustration of the flow being disturbed by the front tyres and flowing over the rear wing.

Streamlines over front wheel and rear wing assembly

The purpose of the split wing methodology was to produce a laminar region behind the car where by the loss of total pressure would be minimum resulting in the following car to maintain down-force. Since CFD offers a range of visualisation techniques, the following figure shows the wake behind the CDG car on a plane at a distance of one car length and is coloured by velocity magnitude. Several things are apparent; the downwash produced by the car can be clearly seen in the middle of the wake, however at the same time a lot of turbulence is created by tyres and wing tip vortices.

Wake behind the CDG car

For comparison purposes, the following figure shows the wake produced by the current regulations and the change is quite visible. Currently the diffuser contributes enormously in producing an up wash which later combines with the wing vortices resulting in a huge wake structure; however in 2008 the wing tyre interaction can produce a similar scenario.

Wake structure behind a 2006 F1 car

Furthermore, the following figure represents the airflow velocity over the car’s centre lines when run in a draft dodging scenario.

Airflow velocity contours on the car's centre line

The velocity profile shows several regions that should be analysed carefully. The flow over the front wing of the following car is slower compared to the leading car which means the front wing performance can suffer and at the same time the under body down-force. All these changes can promote a change in the balance of the car affecting its handling in corners and reducing its possibility to overtake.

Moreover, the effects can be clearly seen by analysing the change in surface pressure over both the car’s surfaces. The following figure shows the difference between the surface total pressures over the car’s under body surfaces. In simplistic ways, the red regions would be more efficient in producing down-force and drag compared to the yellow and green regions since the air would be carrying less energy there.

Comparison of total pressures on the lower surfaces of both cars

Similarly, the following figure shows the surface total pressures on the upper surfaces of the leading and following cars.

Comparison of total pressures on the upper surfaces of both cars

Indeed the wake shown in figure 4 show a region that would help maintain down-force on the following car, however, by analysing the change in surface pressures over both cars it can be clearly seen that the change is significant. The analysis showed the following car experiencing a down-force reduction of approximately 28 percent and at the same time a drag reduction of about 23 percent. The reduction in drag would promote slip streaming in long straights and the reduction in down-force can affect overtaking possibilities, keeping the situation similar to what it is now!

Furthermore, the affect on the balance of the following car can be analysed by the following figure. It shows surface static pressures on front and rear wings of both cars.

Surface static pressure over front and rear wings of both cars – Lead (White) Following car (Red)

Based on the surface pressure legend shown above, dark blue corresponds to more down-force and a linear decrease in down-force as we move towards green and pink regions. The following image further illustrates the complex flow in three dimensions during a draft dodging scenario.

Three dimensional representation of streamlines

While there is an obvious loss of down-force, the numbers show a down-force reduction of 27.5 percent experienced by the following car’s front wing and a reduction of 25 percent experienced by its rear wings. The down-force reduction is consistent, however it can change when analysed in a different position such as in half of the wake of the leading car.

The cause of down-force reduction can further be investigated in the following image where by the streamlines are released in front of the following car. Since the flow is over the front wing of the following car, the sources can be identified using CFD. As it can be seen some of the air is flowing over the leading car’s downwash region and some of the flow is coming from the diffuser carrying less energy and affecting the front wing performance. Moreover, the same flow then travels over the rear wing of the following car reducing its efficiency. Since it explains the cause of down-force reduction in this scenario, special rules limiting the size of the diffuser can some how cure the problem.

Streamlines over the front wing of the following car and passing over its rear wing

Conclusion


Although the analysis proved useful and highlighted several issues that may arise during the 2008 season at a separation of one car length, it would be worthwhile studying the wake of the leading car at more lengths behind the car and investigate its development while it moves away. Such analysis can indicate on what may happen when the car is run at different separation lengths. Moreover, lots of effort can be put in the development of front wings that would minimise the effect of the on coming flow on the under body down-force.

Another solution to promoting more overtaking, on top of the CDG wing, could be the use of moveable aero devices. Such devices can help further reduce drag on long straights and can minimise the effect of oncoming flow by the lead car. Indeed the new rules have opened doors for new aerodynamic research and development; however teams paying the closest attention towards this would be able to stay in front.

Words and Images by Miqdad Ali - F1 Technical
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