Influence of vortices on underfloor aerodynamics

[chuckdanny]

Anyway, you can't completely seal the floor

Of course, i never said otherwise.

if for no other reason than it would stall the diffuser.

Didn't disagree trying to explain why, the divergent part being such in need of air to comply with Rho=cste, that without flow from outside (which the horn vortex still provide) it would take it from behind hence flow separation recirculation etc.. well very simplified view on it.

[bhall II]

This is where the language barrier becomes an issue.

When I said, "you can't completely seal the floor," it was in response to this...

And i wil say that my exotic research into a parallel world may have given me the answer to why maybe you shouldn't completly seal the divergent part because you can massively benefit from vortex enhanced suction.

I guess a better way to put it would have been for me to say, "you can't completely seal the floor even if you wanted to."

Otherwise, yeah, I agree. If you can somehow get this to happen...



...it would be fantastic. It seals most of the floor, and it puts the center of pressure very close to the center of the car.

In fact, I think that's probably the goal of secondary vents (flip-ups, little diffusers, whatever) like this...



In contrast to the forward vent (flip-up, little diffuser, whatever), the FloVis has been completely removed by constant and/or strong air flow.

This whole thing reminded me of the floor duct McLaren ran last year...



Now I really want to know the purpose of that feature. Anyone know if there's any sort of consensus view on the matter?

[Thunder]

I may be wrong but McLaren only used that Duct at the hotter races for additional ERS cooling. Also i think they only had it on one Side of the Car.

Also: Great to follow your Aero conversation. =D> Keep it up.
But beeing an Aero Noob AND english not being my first Language makes for a tough read.

[bhall II]

But beeing an Aero Noob AND english not being my first Language makes for a tough read.

Super-simplified version: if you can get the vortices under the car to exit somewhere along the sides of the floor rather than the diffuser, the resulting v-shaped flow structure will combine with the splitter (yellow) to create a sort of "air plow" that enhances suction under the car.

[Jef Patat]

I may be wrong but McLaren only used that Duct at the hotter races for additional ERS cooling. Also i think they only had it on one Side of the Car.

Also: Great to follow your Aero conversation. =D> Keep it up.
But beeing an Aero Noob AND english not being my first Language makes for a tough read.

I was going to post exactly the same. I find these aero discussion mighty interesting but I have no knowledge about the topic at all.

The duct has been there for cooling indeed. See for example:
http://thejudge13.com/2014/04/02/f1-for ... mpionship/

[variante]

Super-simplified version: if you can get the vortices under the car to exit somewhere along the sides of the floor rather than the diffuser, the resulting v-shaped flow structure will combine with the splitter (yellow) to create a sort of "air plow" that enhances suction under the car.

The question is: is it worth triyng to place the centre of pressure as forward as possible, thus sacrificing the diffuser? Isn't many car's bottleneck the rear end of the car rather than the front?

Don't get me wrong: there's no doubt that those airflows function as sealing device, and that the sealing is more effective at the leading edge of the floor. But it would be worth considering other functions of those vortices (especially those coming from the lower corner of the bardgeboards).

Maybe F1 aero guys are trying to build an organic system where those vortices do the following: (1) generate downforce by themselves across the floor, (2) seal the floor, (3) increase airflow underneath the floor without increasing static pressure, (4) merge with diffuser's vortices (as they rotate the same way).

[Just_a_fan]

My understanding is that they run vortices along the underside of the floor to generate downforce - a vortex is a low pressure zone after all. The problem is that the vortex gets wider as it goes downstream. That's why, I think, rake is so important - it gives the vortex space to grow rather than squeezing it till it breaks down. The vortex also helps to limit lateral flow under the floor so "sealing" it.

In effect the vortex and the rake work together to create downforce.

As always, one shouldn't try to look at the vehicle as a series of single features. It's one big aero device and it all works as a single entity. Or at least the intention.

[hollus]

If they could do this...

Since we are all doing eyeball CFD already...
Vortexes are much wider than the visible condensation core. If you were to put vortexes like that under the floor, they'd occupy all the space from tarmac to floor and hence block all flow to the back of the car. If vortexes go under the floor, I think they either enter already quite near the outside, allowing an unimpeded stream of air to pass near the center of the car, or they die under the floor and are used for direct downforce generation. The vortex plow sounds to me like the easiest way to stall the diffuser, which surely is not the intention.
Again, eyeball MK1 CFD at work here, zero evidence base. IMO, IMO, IMO.

[bhall II]

Didn't mean to criticise...

I didn't take it as criticism.

The question is: is it worth triyng to place the centre of pressure as forward as possible, thus sacrificing the diffuser? Isn't many car's bottleneck the rear end of the car rather than the front?

Have you tried the unsimplified version?

I think I probably omitted something important in the simple one...

Super-simplified version: if you can get the vortices under the car to exit somewhere along the sides of the floor rather than the diffuser, or if you can pull the middle sections of the vortices outboard and forward, toward the edges of the floor, the resulting v-shaped flow structure will combine with the splitter (yellow) to create a sort of "air plow" that enhances suction under the car.

Basically, it's taking advantage of what's already happening with or without manipulation...



The bottleneck is in the middle of the floor where air flow slows down significantly. So, you give that air flow an opportunity to leave, and any flow that doesn't leave will continue to the diffuser as normal. The important part is to keep the system moving quickly, because it can only accept air flow as fast as it can vent it.

That said, I think it might be telling that only one team continues to run a secondary vent (flip-up, little diffuser, whatever). I imagine the lack of EBD super-seals means teams have to be careful about errant flow in that area.



Otherwise, I don't think sealing the floor and increasing diffuser efficiency are mutually exclusive. Stronger seals allow for higher rake angles, which increases downforce generated by the diffuser (and the front wing). The worry, I suppose, would be stalling the diffuser. But, there are ways around that...

[variante]

I think everyone who's written here is pointing out pretty much the same thing, then.

...apart from a couple of points:

The bottleneck is in the middle of the floor where air flow slows down significantly. So, you give that air flow an opportunity to leave, and any flow that doesn't leave will continue to the diffuser as normal.

If that "bottleneck airflow" leaves, then the airflow heading to the diffuser will be slowed down... So, again, is it worth sacrificing the diffuser in favour of the leading edge of the floor? The answer can only be found in a full and accurate CFD simulation; also, the answer may vary depending on the particular CoP location of a car's aero layout.

Stronger seals allow for higher rake angles, which increases downforce generated by the diffuser

I've heard this a lot of times, but it is yet to be proved. Actually, i think it happens the opposite: downforce coming from the floor in general is much increased, while downforce generated by the diffuser decreases.

BTW these are just details that have nothing to do with the general topic.

[bhall II]

I think everyone who's written here is pointing out pretty much the same thing, then.

...apart from a couple of points:

The bottleneck is in the middle of the floor where air flow slows down significantly. So, you give that air flow an opportunity to leave, and any flow that doesn't leave will continue to the diffuser as normal.

If that "bottleneck airflow" leaves, then the airflow heading to the diffuser will be slowed down... So, again, is it worth sacrificing the diffuser in favour of the leading edge of the floor? The answer can only be found in a full and accurate CFD simulation; also, the answer may vary depending on the particular CoP location of a car's aero layout.

Stronger seals allow for higher rake angles, which increases downforce generated by the diffuser

I've heard this a lot of times, but it is yet to be proved. Actually, i think it happens the opposite: downforce coming from the floor in general is much increased, while downforce generated by the diffuser decreases.

BTW these are just details that have nothing to do with the general topic.

You might find this interesting.

Ground Effect Aerodynamics of Race Cars by Xin Zhang, Willem Toet (former head of aero at Sauber), and Jonathan Zerihan (former aero team leader at Mercedes AMG Petronas).

http://eprints.soton.ac.uk/42969/1/GetPDFServlet.pdf

Bon appétit!

[variante]

Bon appétit!

Merci for making me read several pages of obvious conclusions!
Have i lost the steps directly concerning our conversation, possibly? What do you find particularly interesting about that article?

[bhall II]

Is that not the one with a section on diffuser ride height/angles?

I dunno. Stripped of its original context and then flavored for mass consumption, it seems this conversation has become something it was never really supposed to be. It looks like I'm making outlandish claims, but I'm not.

As far as I'm concerned, the main idea is that the use features like those seen below is a way for designers to maximize sealing/suction in a way that takes advantage of the naturally occurring parabolic flow structures under the floor. That's it. Everything else belabors that point.

[variante]

Is that not the one with a section on diffuser ride height/angles?

It is, but it doesn't talk about rake condition or CoP moving about... Unless i missed those parts. Even if tose parts are present, i suspect they couldn't help us more than our intuition, as the shown experiment are quite basic, generic and...ancient (as 99% of the essays)

As far as I'm concerned, the main idea is that the use features like those seen below is a way for designers to maximize sealing/suction in a way that takes advantage of the naturally occurring parabolic flow structures under the floor.

And i agree with your interpretation. Getting deeper into the topic would be interesting, as the actual behaviour of underfloor airflows is an unknown even to most aero engineers.

Trinidefender mentioned reading about changing of vorticity under certain condition, which is a pretty curious aspect. It would be interesting...

[godlameroso]

Ultimately the vortex is an end result, it's the simplest way for nature to organize air with a continuous difference in pressure. Imagine this shape ___/ travelling -> direction, how fast would this shape have to travel before the inside of the leading section no longer receives any air? In other words this shape, while not moving, has air surrounding it. You see under normal circumstances air is really full of itself, it's not an attack on it's character it really is, our atmosphere is nice and dense and pressurized and such tries to maintain everything in it's natural pressurized state.

Now back to our sled like shape, if it moves fast enough in the aforementioned direction, part of that shape will be going so fast that no air can fill that space, and just like when a piston causes vacuum in the intake manifold, the space that is devoid of air causes that same air to do everything in it's power to fill the low pressure zone. Because our air is pressurized at a nice 1 bar at sea level. However, as long as the car is moving the low pressure zone is never filled with air. The rooster tail you see when the cars are in the rain is the result of air trying and failing to fill the pockets created by the bodywork with itself. Air can't stand being not full of itself and will lift boats and planes and smash cars into the ground in order to continue being a narcissistic piece of $h!t all full of itself.

In a diffuser the faster the directional vector of the vortex approaching the diffuser keeps it from filling the space of the diffuser, that's where the vacuum is generated, so for a diffuser to really work the air flow has to have a lot of directional integrity, no atmospheric phenomena fits the criteria better than a vortex.

Another curious thing about air is that you have to give it an out if you want it to create lift, if air is trapped and has no where to flow the pressure in that crevice will create it's own boundary layer and prevent any more air from entering it. That's why pickups actually have less air resistance with the tailgate up than down. As long as air has an out you can do work with it by placing obstructions to create vacuum pockets in that air stream.