more aero...Trapped Vortex

[Locost47]

Thanks Pheonix. I agree that my new boss may not be as accommodating as my previous one and is not the kind of guy you want to annoy! Then again, maybe as long as it doesn't involve Ferrari or any of those '2nd hand cars' then he might let me off...

meb - of course you can PM me.

[phoenix]

Originally Posted by Locost47

Thanks Pheonix. I agree that my new boss may not be as accommodating as my previous one and is not the kind of guy you want to annoy! Then again, maybe as long as it doesn't involve Ferrari or any of those '2nd hand cars' then he might let me off...

Ok, my question is then:

If you have a rear wing on the car and therefore the pressure underneath it is low relatively speaking, how does this make it possible to have a diffuser with a steeper angle?

I understood that, in simple terms, separation generally occurs on steeper angles than about 7 degrees. So what is the interaction wit the rear wing that allows the steeper diffuser angle?

Thanks

Phoenix
c/o Torro Rosso

[Locost47]

If it's for Torro Rosso then the diffuser needs to be as flat as possible and with the rear wing miiiiiles away.

Nah, seriously, from the air's point of view the diffuser's job is to gradually recover the static pressure from the low value in the narrow region between the car and the ground and the almost-freestream value in the near-wake. If you do this by a gradual vertical expansion then you get additional downforce from the upturning of the air as well but a horizontal diffuser would actually work too. (This also means that if you can't go any steeper vertically then you can expand laterally and still see a benefit...)

The problem with this pressure recovery is that the flow is going 'uphill', in that it is flowing from a region of low pressure to a region of higher pressure. Note that it is compelled to do so because of its existing momentum; the pressure gradient isn't 'driving' the flow, it was already flowing and this is just what it encounters along the way.

Oversimplifying a little, if the diffuser angle, and therefore the pressure gradient, is too steep then you'll get separation.

The main thing the wing interaction does is reduce the pressure at the outlet of the diffuser and therefore lowers the 'hill' that the air under the car has to climb on its way out. This is why any device at the rear which reduces the base pressure of the car will also help the diffuser. There will be a drag increase due to the extra suction on the back face of the car, but the downforce gain is usually easily worth it.

Normally people design diffusers which look the part but are way too steep, especially if they haven't carefully protected the air on its way under the car and it is a mess before it even gets there. 7degrees is pretty safe, but you can normally push to 12-15degrees with a smooth floor & decent endplates.

Other than going too steep, the most common problem I've seen is the construction of the diffuser itself. Often people make the flat part of the floor and upsweep of the diffuser from separate panels, for simplicity's sake, but then any mismatch or discontinuity at the diffuser apex will render it almost useless. It is much better to make that area from one single panel, bent into a flat V-shape. With deep, long endplates and decent management of the upstream flow plenty of cheap downforce can be yours! Ok, I'm getting carried away now. Sorry.

[AU N EGL]

Originally Posted by meb

No, but I also don't know this course as well. My only opportunity is thru the esses and into the bus stop - wicked fun that area!!!

YES IT IS

[rcarr]

Originally Posted by Locost47

If it's for Torro Rosso then the diffuser needs to be as flat as possible and with the rear wing miiiiiles away.

Nah, seriously, from the air's point of view the diffuser's job is to gradually recover the static pressure from the low value in the narrow region between the car and the ground and the almost-freestream value in the near-wake. If you do this by a gradual vertical expansion then you get additional downforce from the upturning of the air as well but a horizontal diffuser would actually work too. (This also means that if you can't go any steeper vertically then you can expand laterally and still see a benefit...)

The problem with this pressure recovery is that the flow is going 'uphill', in that it is flowing from a region of low pressure to a region of higher pressure. Note that it is compelled to do so because of its existing momentum; the pressure gradient isn't 'driving' the flow, it was already flowing and this is just what it encounters along the way.

Oversimplifying a little, if the diffuser angle, and therefore the pressure gradient, is too steep then you'll get separation.

The main thing the wing interaction does is reduce the pressure at the outlet of the diffuser and therefore lowers the 'hill' that the air under the car has to climb on its way out. This is why any device at the rear which reduces the base pressure of the car will also help the diffuser. There will be a drag increase due to the extra suction on the back face of the car, but the downforce gain is usually easily worth it.

Normally people design diffusers which look the part but are way too steep, especially if they haven't carefully protected the air on its way under the car and it is a mess before it even gets there. 7degrees is pretty safe, but you can normally push to 12-15degrees with a smooth floor & decent endplates.

Other than going too steep, the most common problem I've seen is the construction of the diffuser itself. Often people make the flat part of the floor and upsweep of the diffuser from separate panels, for simplicity's sake, but then any mismatch or discontinuity at the diffuser apex will render it almost useless. It is much better to make that area from one single panel, bent into a flat V-shape. With deep, long endplates and decent management of the upstream flow plenty of cheap downforce can be yours! Ok, I'm getting carried away now. Sorry.

Wow! Couldn't put it better myself!

Congratulations on the job Locost.
I've heard on the grapevine, F1 is not a nice place to work. I have applied for a couple of jobs that I have seen in Autosport but out of choice I wouldn't want to go into the F1 game.
Saying that, Williams has some excellent facilities, we went on a factory tour at the very end of 05' when the cars still had BMW stickers on them, when they hadn't sorted out a contract with Cosworth, they were testing them on the sly.

[meb]

The front straight is another opportunity, but haven't quite put turns 9,10 and 11 together...high 1:04s at LRP ain't too bad in my daily driver...but I have lots to learn at the Glen. My best times at LRP are always in open passing instructor events...the testosterone level seems higher on those days...just something odd about a daily driven Mini and a few Radicals racing around a track - got keep a close eye on those boys...wicked fast closing rates! And no, I'm not an instructor...took a fellow out once who wouldn't listen and that did it for me. I'll die in my own hands thank you very much.

Originally Posted by AU N EGL

YES IT IS

[zac510]

For my own interest I looked up the R56 mini and found this photo so decided to share it:



So far searches for 'Fiesta spoiler' are not really producing the results I was expecting

[rcarr]

What is the difference?

[Locost47]

Flipping heck! No-one's supposed to have a photo like that! That's a pilot build R56 kitted up for a hot or cold environment test. Someone must have been pretty opportunistic with their camera.

The difference between the C-pillars on the R50 and R56 Minis was that the newer one replaced the longer glass side window with a short one plus plastic body panel. This panel had a sharp edge moulded into it to fix the separation point on the corner and help improve drag (and cross-wind stability, to a lesser degree).

[zac510]

Is that the final production C-pillar panel though?

[meb]

What's interesting, is I have given a lot of thought to that area and wondered how it might work with a diffuser and wing designed for down force or for vortex generation...but in addition, I also gave some thought to creating an opening from the inside of the car along the vertical length of the C pillar where it mates to the rear hatch...trying to eliminate the parachute effect created with side windows open. Another interesting complication.

In essence, the ridge in the pillar help to create a vortex??? How the different are masses merge from the roof, C pillars and from under the car is a curiousity of mine...can you spare a wind tunnel for a few days???

[rcarr]

Originally Posted by Locost47

The difference between the C-pillars on the R50 and R56 Minis was that the newer one replaced the longer glass side window with a short one plus plastic body panel. This panel had a sharp edge moulded into it to fix the separation point on the corner and help improve drag (and cross-wind stability, to a lesser degree).

Doesn't the sharp edge also help with soiling on the rear window?

[Locost47]

I don't think that's the final production c-pillar panel. It looks too much like what it should be like and not what a stylist has 'prettied-up'.

I don't know much about soiling at all, although that didn't stop them from putting me as chairman of a soiling session at a big international aero conference and making a muppet out of myself by trying and failing to ask intelligent questions about stuff. D'oh!

My guess would be that the c-pillar trip line would worsen soiling on the rear screen by reducing the local flow velocity there. Most estate cars and steep hatchbacks suffer from soiling issues so I would figure that these devices making the car more like an estate, from the air's point of view, would have the same effect. It may well be more complicated than that though, i don't know.

[rcarr]

Originally Posted by Locost47

My guess would be that the c-pillar trip line would worsen soiling on the rear screen by reducing the local flow velocity there. Most estate cars and steep hatchbacks suffer from soiling issues so I would figure that these devices making the car more like an estate, from the air's point of view, would have the same effect. It may well be more complicated than that though, i don't know.

Most well aerodynamically designed cars have a device such as a "spoiler" to encourage the dirt away from the opening button/handle and the rear window. So the users hand doesn't get dirty and the vision through the rear screen isn't impaired. Such devices can be seen on Mercedes A-Class, Honda Civic, Rover 25 etc.

[meb]

But if it works from an aero perspective it's a great thing...especially if the car in question comes with a rear wiper ...which might also create lots of drag???

[GordonG]

So, a slightly off topic question, but closely related to the discussion on diffuser angles: Suppose you're not allowed by the regs to run an effective side fence on your diffuser (but you are allowed an inclined rear floor so long as it's flat across the car). Does that increase or decrease the optimal/maximum effective angle of the floor/diffuser?

I'm not sure if this floor will operate more as a diffuser or a delta wing? Can it be a delta wing if the sides are parallel?

G

[Locost47]

"I'm not sure if this floor will operate more as a diffuser or a delta wing? Can it be a delta wing if the sides are parallel?"

While it's true that increasing the 3-dimensionality of the flow can help wings and diffusers be run at steeper angles, the complication in this case is that the wakes of the rear wheels will be drawn in towards the centre of the car and mess up the diffuser. What you are trying to do is provide a source of 'fresh' air to re-energise the tired air which is about to separate. Unfortunately the wakes of the wheels are seriously low-energy and won't be helfpul for this purpose.

That said, if you can start the diffuser further forward and then shape the sills so that the air drawn in towards the low pressure region under the car forms clear and defined vortices then you get a little of the delta wing effect and also some of the upwash/re-energising effects too. This was used in Group C sportscars, amongst other formulae, and quite heavily in F1. There's a much better explanation of this (with helpful diagrams!) in Joe Katz's book on race car aerodynamics.