Diffuser: Effect of Upper Surface Quality & Rear Lip

[Facts Only]

On the Force PT in the picture below the diffuser is essentially a single part with airflow over both sides.

Diffusers are generally tooled from CF on the underside with the 'B' Surface on top, usually this is covered by upper bodywork but on cars like the Force its not. What negative effect (if any) will an un-tooled surface have on the airflow at the rear of the car. It appears that the Force has some sort of tooling or cleaning on the upper surface.



Also for tooling and support many diffusers like this have a large return lip pointing upwards, I assume this will act as a gurney flap but would using a larger lip like a blocker have positive or negative effects?



You can see the lip in the lower centre section of the diffuser on the same car.

Just wondering what peoples thoughts are.

[godlameroso]

That seems like a very draggy diffuser, and doesn't seem like it gets much airflow to the upper surface, but that's just my worthless opinion.

[Facts Only]

Hillclimb (British) seems to push for low speed down force most of all with drag playing second fiddle.

I'm wondering more about the effect on down force of the upper surface and rear lip.

A car I'm working on could be easier to make, simpler and lighter with a larger rear lip and cheaper with an un-tooled upper surface so I'm trying to gauge the the negatives on performance.

[Pierce89]

Hillclimb (British) seems to push for low speed down force most of all with drag playing second fiddle.

I'm wondering more about the effect on down force of the upper surface and rear lip.

A car I'm working on could be easier to make, simpler and lighter with a larger rear lip and cheaper with an un-tooled upper surface so I'm trying to gauge the the negatives on performance.

You'll be quite okay with an un-tooled top side. With the larger lip, to find out what height stops being beneficial and starts hurting performance would change car to car, but I would limit the lip to 25-30 mm .

[henra]

What negative effect (if any) will an un-tooled surface have on the airflow at the rear of the car. It appears that the Force has some sort of tooling or cleaning on the upper surface.

Diffusers work by having fast Airflow above the rear exit of the diffuser creating a pressure differential that accelerates the air underneath. Thus you want as clean airflow over the Exit of the diffuser as possible. That said in the given example I wouldn't worry about the inlet lip. Max 10 - 20 inches behind the lip you will have more or less clean attached flow again. In the given example it is the rear suspension that will significantly cost DF by massively disturbing the flow over the rear exit.

[FW17]

That is a venturi tunnel not flat bottom diffuser

[andylaurence]

Rob sold that car at the weekend. How did you get the second photo? That's the car outside the place it's kept.

From memory, the radiator sits over the tunnel on one side. The floor is actually from the Empire Evo/001. Bill fitted it for the previous owners, I think. I suspect the floor was developed when Bill had a student working for him on aerodynamics at the same sort of time they were developing the Wraith with Willem Toet.

As for downforce and drag, most hills have a top speed of under 100mph and despite this being an 1100cc class car, the emphasis is on downforce. This might be because of the light weight of the car, which I seem to remember is under the magic 300kg. Shelsley is one obvious exception as it's absolutely flat out much of the way up - Rob takes all the wing off there.

[Facts Only]

The pics are from the Racecars direct advert, I'm not the new owner or Rob.

Its a lovely car but I only really used the photos as they were good close-ups of the rear end to illustrate what I was asking about.

Basically it seems that an untooled upper surface wont have much effect and the suspension/radiator do a good job of blocking the airflow anyway.

[Vyssion]

What negative effect (if any) will an un-tooled surface have on the airflow at the rear of the car. It appears that the Force has some sort of tooling or cleaning on the upper surface.

Not much to be honest... Comparably, the decrease in dynamic pressure from the acceleration of the air below the car (and inside the venturi tunnel) will cause a much larger pressure differential than any sort of "top surface" panel design would - so you can get away with not having a properly tooled or formed outer surface. Obviously there will be some amount of turbulence or dirty air generated simply because it isn't a nice flush surface, but it will be minor compared to the wheels around it.

Also for tooling and support many diffusers like this have a large return lip pointing upwards, I assume this will act as a gurney flap but would using a larger lip like a blocker have positive or negative effects?

Gurney flaps only work up to a certain height which is related to the thickness of the boundary layer which has formed on that surface to that point. Beyond that, any extra height just begins to become more draggy. You want a gurney flap to induce a vortex that is rolling in the 90° angle of it followed by two counter rotating vortices on the other side of the flap (off the vehicle) which then serves to virtually increase the effective angle of attack of the surface. But again, most of the performance gain that would come from those tunnels would be due to the internal and underneath geometry, rather than any sort of top surface finish or design.

[Facts Only]

Interesting reply, cheers Vyssion

[godlameroso]

Food for thought regarding rough untooled surfaces.

http://ocean.si.edu/ocean-photos/biomim ... -denticles

[Facts Only]

Just to drag this thread back up again, on a related note; what sort of effect on performance would any intrusions have on the diffuser.

IE if a bolt head was sticking out into the underside? Or the tail of a rivet? Especially in the corners where the end fences meet the upper surface?

Will they ruin the air flow characteristics or add more energy to the flow?

[Vyssion]

Just to drag this thread back up again, on a related note; what sort of effect on performance would any intrusions have on the diffuser.

IE if a bolt head was sticking out into the underside? Or the tail of a rivet? Especially in the corners where the end fences meet the upper surface?

Will they ruin the air flow characteristics or add more energy to the flow?

They would induce separation which, depending on how strong the adverse pressure gradient was around that area, may induce a larger area to separate permanently due to the additional vorticity production. If the bolt was in a region where the adverse pressure gradient wasn't as strong, you would get a separation "bubble" like thing behind the bolt head, but then the flow should re-attach. Something as small as a rivet tail you could probably get away with... but a large hex headed bolt (i.e. M12 or something) probably not.

[Facts Only]

Cheers Vyssion,

It will be in a low adverse pressure gradient area (the shallow angle under the rear axle), nothing like an M12, maybe some M5 button head bolts or rivets.

The whole thing is an exercise in Cost & Convenience vs Performance, how much can costs and time be reduced without notably affecting performance..

[godlameroso]

Countersunk allen keys as an alternative?