How much downforce does the 197 diffuser give?

[Marv-C]

You funking idiots lol!!!

Just to clear this up, my mate drove his 197 round a corner and it felt very much on the limits, so we took the diffuser off, and he wrote it off at a slightly lower speed.

FACT. Discuss.

Fook :S

 

[Jasey]

You funking idiots lol!!!

Just to clear this up, my mate drove his 197 round a corner and it felt very much on the limits, so we took the diffuser off, and he wrote it off at a slightly lower speed.

FACT. Discuss.

Was there a tractor round the corner the second time ?

 

[-Leigh-]

No, it was the lack of downforce that caused it!!!!!

 

[_WILL_]

_WILL.: please run a CFD model with/without a diffuser! LOL

If i had the time i would, just to end this thread!!!!!!!!

 

[realnumber 1]

You funking idiots lol!!!

Just to clear this up, my mate drove his 197 round a corner and it felt very much on the limits, so we took the diffuser off, and he wrote it off at a slightly lower speed.

FACT. Discuss.

Could be a 1000 reasons for that but I doubt it was the diffuser imo. I'm not even convinced the Clio rides low enough for the diffuser to have any effect tbh.

 

[Eddyg]

Please can you explain some of these statements in more detail, I am an engineering student and have studied a reasonable amount of fluid dynamics and have used wind tunnels (only on a small scale). Some of the stuff you are saying doesn't seem to quite make sense to me? but i may be wrong

What you studying? i am doing aerospace at the moment. I am having a lecture next week on race car aerodynamics so i will be able to explain all after then.
Using what i know at the moment a diffuser is just a way of increasing the pressure of air (hence slowing it Bernoulli's) without massive losses.
On a car its purpose is mainly to expand the faster moving air under the car back to ambient pressure as it exits. This stops a big pressure differential between the low pressure air underneath and the relatively high pressure air at the back which would result in higher levels of drag.
The diffuser itself will cause a low pressure suction peak just ahead of it like the middle of a venturi.
The underside of a clio isn't exactly great for smooth air but there will some flow coming under and as the Reynolds number increases it will have the effect of dampening out some of the turbulent eddies caused by all the gubbens underneath. I think 40kgs it plausible. TBH the only way you will know is windtunnel testing.

Give me a week and i will talk to my lecturer!

 

[Mike_182]

The underside of the Exige with the diffuser removed - you can see the flat surface that is being referred to in one of the posts above. THIS is required for the diffuser to function effectively.

 

[Pinkorstink]

THIS is required for the diffuser to function effectively.

Effectively or optimally?

 

[Mike_182]

Effectively or optimally?

Effectively.

The pisch underside of all hatchbacks causes a massive amount of turbulence and a low pressure rolling area at the boot when moving. The only fix is a slab of aluminium underneath it and a diffuser which causes the air to 'flow' and the air to separate at the front and then almost join back together at the back with the two areas travelling at similar speeds just at different pressures. As has been mentioned, your firewall and the exhaust tunnel creates a massive amount of buffeting under the car - one of the reasons hatchbacks are so noisy on the motorway when an Exige with no sound insulation is much quieter.

Take a hammer to a slab of butter, and then take a knife to it - that's sort of what I mean...

 

[bradharding24]

Please can you explain some of these statements in more detail, I am an engineering student and have studied a reasonable amount of fluid dynamics and have used wind tunnels (only on a small scale). Some of the stuff you are saying doesn't seem to quite make sense to me? but i may be wrong

Sure be precise as to which parts dont make sense and i will elaborate. If you have a basic grasp of aerodynamics i can include equations and examples if it would help.

 

[Pinkorstink]

Sure be precise as to which parts dont make sense and i will elaborate. If you have a basic grasp of aerodynamics i can include equations and examples if it would help.

What, no pictures needed?

 

[pritchard182]

good to see a bit of french kit that dus wat it says on the tin

 

[daydotz]

haha

 

[_WILL_]

What you studying? i am doing aerospace at the moment. I am having a lecture next week on race car aerodynamics so i will be able to explain all after then.
Using what i know at the moment a diffuser is just a way of increasing the pressure of air (hence slowing it Bernoulli's) without massive losses.
On a car its purpose is mainly to expand the faster moving air under the car back to ambient pressure as it exits. This stops a big pressure differential between the low pressure air underneath and the relatively high pressure air at the back which would result in higher levels of drag.
The diffuser itself will cause a low pressure suction peak just ahead of it like the middle of a venturi.
The underside of a clio isn't exactly great for smooth air but there will some flow coming under and as the Reynolds number increases it will have the effect of dampening out some of the turbulent eddies caused by all the gubbens underneath. I think 40kgs it plausible. TBH the only way you will know is windtunnel testing.

Give me a week and i will talk to my lecturer!

I am studying mechanical engineering at leicester uni. We have done a fair amount of fluid stuff, but im not confident enough with it to apply it to this situation and i do think that the only way to really know is to windtunnel test it, which im guessing renault has done already.

Sure be precise as to which parts dont make sense and i will elaborate. If you have a basic grasp of aerodynamics i can include equations and examples if it would help.

Its just that my understanding the part about the diffuser sending the air upwards creating a force, the equal and opposite force bit(sorry i can't remember the exact quote). I was under the impression that the purpose of the diffuser is to reduce the flow rate of the air and so increasing its air pressure back to/close to the ambient air pressure. Basically its helping to get the car to work like a plane wing, but the other way up? I don't understand how the air causes a force?

 

[bradharding24]

I was under the impression that the purpose of the diffuser is to reduce the flow rate of the air and so increasing its air pressure back to/close to the ambient air pressure. Basically its helping to get the car to work like a plane wing, but the other way up? I don't understand how the air causes a force?

Ok, my background is in geology and hydrogeology, where i learnt about fluid flow, but the principle is the same in engineering and aerodynamics.

The fluid flow under an object (lets say a car) is accelerated relative the ambient air because the gap between the two interfaces is narrowed. For this to happen the floor must be flat, as otherwise the air becomes turbulent (ie the reynolds number increases), and also close to the ground. This increase in speed increases the kinetic energy of the fluid which causes an opposing decrease in static pressure which effectively adds downforce to the car, because the static pressure under the car is lower than the relatively higher static pressure above the car.

The speed and thus the static pressure is lowest just before the diffuser. Now if there was no diffuser, the low static pressure fluid would cause a large amount of drag as it exited the rear of the car.The diffusers causes a static pressure rise by recovering the kinetic energy of the fluid flow (increased by the initial increase in fluid flow). It does this because the fluid velocity decreases as the diffuser area increases.

Because a diffuser sticks out behind the rear of the car (like a proper diffuser on a racing car) it also extends the area of dowforce producing underfloor. As the static air pressure is increased and the fluid flow decreased the amount of drag is reduced and a certain amount of 'wake fill' occurs.

The ground effects cars in F1 in the 1970s where in effect upside down aeroplane wings that produced a force perpenductular to the direction of travel (downforce). I was using this to demonstrate the concept behind the diffuser, which is a development step from the banning of ground effects in some forms of racing, showing how a movement in one direction results in the generation of a force in a different plane.

 

[_WILL_]

Ok, my background is in geology and hydrogeology, where i learnt about fluid flow, but the principle is the same in engineering and aerodynamics.

The fluid flow under an object (lets say a car) is accelerated relative the ambient air because the gap between the two interfaces is narrowed. For this to happen the floor must be flat, as otherwise the air becomes turbulent (ie the reynolds number increases), and also close to the ground. This increase in speed increases the kinetic energy of the fluid which causes an opposing decrease in static pressure which effectively adds downforce to the car, because the static pressure under the car is lower than the relatively higher static pressure above the car.

The speed and thus the static pressure is lowest just before the diffuser. Now if there was no diffuser, the low static pressure fluid would cause a large amount of drag as it exited the rear of the car.The diffusers causes a static pressure rise by recovering the kinetic energy of the fluid flow (increased by the initial increase in fluid flow). It does this because the fluid velocity decreases as the diffuser area increases.

Because a diffuser sticks out behind the rear of the car (like a proper diffuser on a racing car) it also extends the area of dowforce producing underfloor. As the static air pressure is increased and the fluid flow decreased the amount of drag is reduced and a certain amount of 'wake fill' occurs.

The ground effects cars in F1 in the 1970s where in effect upside down aeroplane wings that produced a force perpenductular to the direction of travel (downforce). I was using this to demonstrate the concept behind the diffuser, which is a development step from the banning of ground effects in some forms of racing, showing how a movement in one direction results in the generation of a force in a different plane.

This is pretty much the same as my understanding.
I think it would be safe to say that you would agree that the diffuser does work (it looks pretty well designed to me) in isolation (in a test situation not on a car) or if it was fitted to a clio with a modified underside (flat).

I also understand the argument that the flow has to linear (smooth) and not turbulent for the diffuser to work effectively. But i would personally say that the underside of a clio 197 is flat enough for the diffuser to have an effect, yes there will be turbulence, and a flat underside would make it much better, but having a flat underside would only further reduce turbulence, not get rid of it completely?

 

[theduck]

So the out come of this is, I need a sheet of alumminum? lol

 

[c__w]

Effectively.

The pisch underside of all hatchbacks causes a massive amount of turbulence and a low pressure rolling area at the boot when moving. The only fix is a slab of aluminium underneath it and a diffuser which causes the air to 'flow' and the air to separate at the front and then almost join back together at the back with the two areas travelling at similar speeds just at different pressures. As has been mentioned, your firewall and the exhaust tunnel creates a massive amount of buffeting under the car - one of the reasons hatchbacks are so noisy on the motorway when an Exige with no sound insulation is much quieter.

There is probably truth in that but what hatchback have you been in that is noisier than an Exige! they're known for high road and wind noise.

Most of the noise from hatchbacks is the road noise made worse by the resonance in the open rear IMO. They're not the quietest cars available but saying an Exige is "much quieter" is going a bit too far IMO.

 

[Counterdoc]

This thread is super super super duper old, but the 40 kg mentioned are just marketing and not possible with the 197 diffusor. I owned a Lotus Evora 400 with a complete flat underbody and a properly designed diffusor + rear wing and it only made 20 kg of downforce. The Evora GT430 Sport made "only" 100 kg downforce (front and rear combined) at 315 km/h...

So 40 kg in the rear at 70 mph with a Clio - nope!