Renault are not only unique in their exhaust location, but also their sidepod cooling set up. Its possible that the two solutions are related.
Teams have to find the most efficient way to vent hot air from within the sidepods. This air has to pass through the radiators and coolers, as it passes from the sidepod inlet to the tail of the coke-bottle shape. In order to keep the car as slim as possible for reduced and drag and better airflow to the diffuser and beam wing, teams adopt different cooling outlet solutions to work with the other aero flow structures around the car. In Renaults case the majority of the outlet area is in three places: a tall narrow outlet above the gearbox and two low and wide vents either side of the gearbox. We can see the gearbox oil cooler within the upper outlet, so some of its volume is used for venting this heat. Reducing its ability to vent air from the sidepods.
Renault appear to have chosen the two low\wide outlets in order to boost airflow to the beam wing. The compromise in doing this, is the airflow over the diffuser. Airflow over diffusers might be considered as unimportant, as its the flow underneath creates the low pressure and hence downforce. But with a restricted diffuser height , the use of directing high energy airflow over the top of the diffuser and its trailing edge gurneys, helps the airflow beneath. Effectively making the diffuser act like its larger than it is. With other teams, they use the exhaust gasses or very narrow sidepods to direct as much high energy flow as possible over the diffuser. In Renaults case, the airflow running along the floor does not flow directly over the top surface of the diffuser, as these wide vents are in the way. Some people have suggested the teams are directing the heated airflow from within the sidepods out of these vents and over the diffuser for greater aerodynamic effect. However the reality is different, the air coming out of these vents will be of low energy, having passed through the various cooler matrices. Thus its effect in aiding the diffuser is much reduced.
So why have Renault thrown away some aero gains in this area? Probably because of the exhaust solution, As the flow under the diffuser is accelerated by the exhaust gasses passing under the floor, the diffuser does not need the effect of airflow blowing over the top. Thus they moved to the sidepod outlets to this area and played better airflow over the beam wing.
So far no other team have gone for low wide cooling outlets, but equally contrary to the rumour mils no teams has adopted front exit exhausts either. It will be interesting if any team follows Renaults solution in either of these areas.
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Somebody on F1Technical forums was suggesting that due to the rather large sidepod inlets and small outlets that the air is being accelerated out of the back. Is it possible they went for higher drag inlets to get better exit speed from the outlets for the diffuser (Even if it isn’t faster that surrounding flow, it should be faster than Red-Bull cooling outlets for example)
If the air coming from those outlets had low energy why did they put gurney flaps on the diffuser ? (and they modified the gurneys lately by adding a slit in the center)
All cars have them.
Thanks, as usual, very interesting.
Is there any picture on the internet were we can see the gearbox oil cooler through the outlet, as you said in the article ?
Surely the air flow exiting the sidepods and central vent has been accelerated by energy input from the radiators/oil/hydrafluid coolers and you have in effect a weak ramjet effect, this also speeded by the reducing cross section of the exit ducts.
To back up my article, I had an ex F1 aero man give me some insight, his comments were “The coke exit flow here is very very tired, it will be hurting diffuser performance”
I’m not very convinced 😛
If their only aim was just ”to boost airflow to the beam wing” they could place the outlets where they placed them in the R30 and late R29, but low, inorder to make a tighter cocke bottle, a la RedBull, and free up a lot of space above the floor and in front of the diffuser.
Or they could adopt RedBull’s central cooling outlet solution.
And why is there big gurneys if the airflow above the diffuser is weak ?
Can they be doing this to intentionally slow the airflow over the diffuser, to create a larger flow difference between the areas above and below the diffuser? And the gurney flaps add to the low pressure zone behind the diffuser? All of this creating a small suction effect, as the low pressure area above and behind the diffuser tries to fill itself by pulling air from under the car?
Regarding the flow energy etc. Renault has (or had) very big sidepod inlets. I haven’t seen any clear picture, but for me it looked like the radiators were smaller than the inlets.
This made me thinking – if the flow quality there is good, why not duct some of the unrestricted flow directly to blow over the diffuser (or wherever you want it)? At first glance, this idea looks really good. A much more direct method, compared to what STR or McLaren did.
But I imagine that there could be many problems – compromised to some extent cooling, additional resistance. Maybe the gain is not good enough? On other hand, it could decrease the negative effect you’ve written about.
The air could also be ducted to cool something else, like electronics.
Perhaps you know something more about this? Maybe you’ve seen their sidepods in detail?
“Airflow over diffusers might be considered as unimportant, as its the flow underneath creates the low pressure and hence downforce.”
As a professional aerodynamicist I feel compelled to comment this. For a sharp trailing edged diffusor (as oppposed to blunt end diffusor) the flow over top is crucial. It is the shear flow (sometimes referred libearally as Kutta-condition) aft trailing edge what is the “motor” of the diffusor. It is is very important to have hi-energy flow over top. This high energy flow then “grabs” or more precisely shears the under flow aft the trailing edge and pulls the flow from under. The whole idea of the waisting is here!
The benefits of the Renault solution could be that you can pinpoint draught flow to the lower sides of the gearbox. Also remember that shocks are thermal devices in the end too, they also need cooling.