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WEC Technical Analysis: Rear Diffuser

Audi R15 spec. 2009
09/05/2017 -

WEC 2017 has recently started and once again the discussions related to rear diffuser confirms that this topic is still actual.

Let’s  review very briefly why rear diffuser concept is a key point on race car design: firstly underbody flow is responsible of at least 30% of total downforce generation but most importantly this share of downforce is generally created with high efficiency value (L/D).

All prototypes cars are creating great level of downforce using front splitter which is practically a front wing in ground effect. Splitter act as main elements and a second element as blow flap complete the front aero device.

Audi with his extreme R15 -2009 was the first using this concept, and opening massive discussion about the legality according to the tech rules.

Drawing below shows very clearly the front wing operating in ground effect:



Rear diffuser is responsible to create the correct pressure distribution on flat bottom part of the car generating downforce and increasing underbody flowrate that in turn, feeding with more airflow the diffuser, is creating more intense pressure peak and ultimately downforce.

Over the years several tricks have been used to feed the diffuser with airflow that does not comes from the underbody. Typical example is Exhaust gases from the engine, or clean air taken form upper/side parts of the car.

Exhaust gases are particularly efficient in boosting the airflow  thanks to high momentum, but they introduces also an undesired “throttle sensitivity” on downforce generated, as their blowing are depending on engine operating point. When driver is in high speed corner, a short lift up on throttle is responsible of abrupt drop on exhaust flow and rear downforce. The use of exhaust gas to enhance the diffuser performance is clearly banned by technical rules, see LMP1 Tech rules art. 3.4 that clearly defines not permitted location of exhaust pipe.

Being the topic very important part of aero design several ways, less evident and therefore more easily compliant to the tech rules, of enhancing rear diffuser have been developed over time.

Following drawing of Audi R15-2009 shows how part of the flow running along the sidepods could be taken inside the car and ejected over the diffuser upper surface:



This flow is not discharged inside the diffuser volume, and therefore could not be strictly considered as blowing into it, nevertheless with proper design of micro aero devices like nolders or turning vanes, the flow is directed upward and creates upwash on the air exiting from the diffuser, pulling it upside like if the diffuser has stepper angle.

Several variation of the same concept could be identified on following drawings where the intake location and internal flow arrangement are different but still designed to improve diffuser performance with smart conditioning of airflow at his exit section.

Intake for heat exchanger could be split and part of it directly blown over the diffuser, this concept has been adopted on AMR-One:



Rear fender pressure area over the floor is used to direct part of sidepod flow inside the car and discharged over the diffuser, like on Audi R18:



“Flow through” concept of Nissan GT-P is clearly visible here:



Unique combination of “flow through” and exhaust gases blowing into a side tunnel closed to the rear diffuser was used on Lister LMP1:



Very refined design of rear fender exit and top diffuser of Toyota TS050:



Very interesting to note the rear crash cone lifted by fender upper surface, creating more cross section to evacuate internal flow.

Aerodynamics concept of race cars tend to stretch at the limit the spirit of technical regulations, regardless the class and sophistication of the rules. Given a box rules there is always a corner that make sense to be pushed a step further.


article by Ing. Riccardo Romanelli
Drawings Antonio Pannullo

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