Tech talk: The new blown diffuser

Will Gray

The ban on blown diffusers has resulted in several innovative design approaches to exhaust flow this year — so with four races gone what are the trends and is there a consensus on the right way to go?

In 2011, the approach to the rear end design was obvious, although not exactly simple. Red Bull led the way in steering their exhaust exits through the floor and into the flow in the rear diffuser, with the aim of increasing the downforce in this area. The exhausts provided an injection of high-energy air and clever engine mapping ensured the exhaust flow was constant whatever the throttle position.

The ban on off-throttle blowing engine maps, brought in mid-2011, tried to put an end to the blown diffuser and this year an added stipulation for the exhausts to exit from a specifically defined area in the upper part of the sidepods at an upwards angle of 10 degrees has forced teams into a re-think.

This new ruling has cut out the dramatic advantages of the blown diffusers - but there are still gains to be made.

The teams quickly worked out that upper-exiting exhaust air could still be directed once it left the pipe using a mix of angled scoop channels, tunnels and turning vanes to manipulate the airflow. These achieve flow steer by creating low-pressure areas further downstream of the exhaust that help to pull the air in the required direction. It was then just a matter of working out which way to direct it.

In pre-season testing, there were a variety of approaches — pointing the flow up to the upper rear wing, rearwards to the beam wing, down towards the central diffuser, wide over the brake ducts or wide and down into the gap between the rear tyres and the diffuser.

The final one in that list - blowing into the gap — tries to replicate one key benefit of the blown diffuser, which was the creation of an air 'skirt' on the outer edge of the diffuser. The fast airflow effectively created vertical barrier from the upper edge of the diffuser to the ground, which prevented the low-energy turbulent air from the rear tyres flowing sideways and mixing with the diffuser flow.

The new rules make that harder to achieve — but, as teams have discovered, not impossible.

The FIA's ideals of a simple exhaust outlet hole in the sidepods did appear on some cars — notably the Red Bull — at launch events but now all top teams except Lotus have some kind of flow-steering solution.

All around the rear of the cars, there are lots of very small vanes, all of which are designed to coax the air through this complex region and down into the area between the tyre and diffuser. This technique is only possible due to the clear understanding of flow regimes made available by CFD, and some of these guiding surfaces are extremely small but have a dramatic effect. To encourage the flow downwards from the outset, some teams have also created steep drop-offs at the rear of the sidepods, which lead the flow from the top of the sidepods to rush over the exhaust exits and force it down.

However, there is another flow regime in this area that can also provide benefit.

The undercuts on the sidepods allow teams to steer clean airflow around the sides towards the rear to help add downforce, in this case by directing it into the central area of the diffuser. The exhaust flow is stronger, so that is the one that needs pointing at the diffuser/wheel gap — and so the two flows have to cross.

McLaren and Ferrari have both gone for wide outboard bodywork that aims to allow the undercut flow to travel underneath, while the exhaust and upper sidepod flow goes over the top of that bodywork and is then steered down towards the tyre gap.

Doing that, however, appears to compromise both flows — which is why Red Bull have tried another solution.

The champion team started off with a basic exhaust, but that was never going to run in races — it was just to get the basic car settled in testing before trying a more radical approach.

They replaced the early design with outlets much further towards the front of the sidepods and a channel designed to direct the exhaust air down while the air flowing around the sidepod undercut was fed through a tunnel into the central rear area, to avoid mixing with the exhaust air.

This complicated system was not to Vettel's liking, and subsequently a further modification was made for Bahrain to close off the tunnel — effectively limiting the effect of the undercut flow but improving the exhaust flow effect. New slots in the floor have also been created to feed energised flow into the underside of the diffuser.

So it looks like Red Bull, for now, have also had to compromise. But it is clear that the ideal solution is to make both individual flow routes work together — so it will be interesting to see the next iterations...

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