Blown Diffuser: How does it work actually and why it's there

[n smikle]

Vortex generated in free flow air has little or no compressibility and is not as tightly wound

Vortex generated from exhaust is essentially a gas that the vortex generator is acting on and has high compressibility, forms a more tightly wound vortex with the "tornado" effect able to compress the gasses within the vortex...

IMHO is the energy he's talking about...and resembles more closely how a skirt would act on the sides of the floor or the diffuser creating better sealing effect.

The "real" question is would a skirt to the ground be better than blowing the diffuser with the exhaust? I say it would....

They are replicating the effect with high energy gas...

You could be right, but in the videa James Allison says the RB style diffusers use the extreme low pressure in the vortex core to reduce pressure under the floor. These are two different principles obviously. I just know you can get EXTREMELY low pressures in a vortex core, and I believe this is a greater pressure reduction than you could get by using the vortex to "seal" the edges of the diffuser. i just don't like the "sealing the edges of the diffuser" theory because the extra turbulence might cancel out the slightly increased vloume.

It is one in the same to me, The vortex fringes are the seal, and the vortex core is the low pressure zone. So even if you seal the sides with the vortex fringes, your vortex core will be under the diffuser. I think that the trick is get it so that the vortex fringes from both the left and right, do not curl so tightly that they get enough space to come into the center of the diffuser thereby "splashing" together and compromising the low pressure zone created by the two vortex cores (left and right).

[hardingfv32]

How is this possible and still be called a vortex?

Or is the low pressure harvested just as the vortex is formed soon after the exhaust exit opening?

Brian

[speedsense]

Vortex generated in free flow air has little or no compressibility and is not as tightly wound

Vortex generated from exhaust is essentially a gas that the vortex generator is acting on and has high compressibility, forms a more tightly wound vortex with the "tornado" effect able to compress the gasses within the vortex...

IMHO is the energy he's talking about...and resembles more closely how a skirt would act on the sides of the floor or the diffuser creating better sealing effect.

The "real" question is would a skirt to the ground be better than blowing the diffuser with the exhaust? I say it would....

They are replicating the effect with high energy gas...

You could be right, but in the videa James Allison says the RB style diffusers use the extreme low pressure in the vortex core to reduce pressure under the floor. These are two different principles obviously. I just know you can get EXTREMELY low pressures in a vortex core, and I believe this is a greater pressure reduction than you could get by using the vortex to "seal" the edges of the diffuser. i just don't like the "sealing the edges of the diffuser" theory because the extra turbulence might cancel out the slightly increased vloume.

It is one in the same to me, The vortex fringes are the seal, and the vortex core is the low pressure zone. So even if you seal the sides with the vortex fringes, your vortex core will be under the diffuser. I think that the trick is get it so that the vortex fringes from both the left and right, do not curl so tightly that they get enough space to come into the center of the diffuser thereby "splashing" together and compromising the low pressure zone created by the two vortex cores (left and right).

I assume your talking left and right vortex from the same exhaust pipe? As the design of the exit pipe has a strake in the center of it creating counter rotating vortices on each intersection of the strake and the pipe. A single strake (intersecting top and bottom) would create four vortices from the same pipe. two on the left, two on the right.. is this what you are referring to?
Wouldn't the "problem" be the high pressure wrapping of the low pressure center also getting into the diffuser?

[marekk]

Wouldn't the "problem" be the high pressure wrapping of the low pressure center also getting into the diffuser?

Don't think there is any high pressure region "wrapping" low pressure center of free flow vortex. It's all below ambient. There is no compression of gases within the vortex either IMO.

If you manage to place your vortex along the side of the floor, it will try to suck everything around, including air beneath and to the side of the car and the floor itself.

It's true that confined space under the car prevents further buildup of those vortices, but on the other hand negative pressure vector (low pressure at the rear of the car) helps with stretching and tightening of vortex tubes, making them stronger, so with carefully planing and some luck one can keep them alive all the way to the diffuser.

In fact all current teams manage to do this IMO, if you watch closely it's easy to spot something like this (strong vortexes clearly visible under both sidepods):


(credit for this enhanced picture to formula_none).

[hardingfv32]

For a surface to get the most benefit from the lower pressure center of the vortex, must it not be in contact with this center area? It would then seem that it would be difficult to maintain the helix shape of the vortex in this case.

Here is a paper with some pressure distributions for vortices on a delta shaped wing.

http://www.cfd4aircraft.com/int_conf/IC ... hummel.pdf

Brian

[marekk]

For a surface to get the most benefit from the lower pressure center of the vortex, must it not be in contact with this center area? It would then seem that it would be difficult to maintain the helix shape of the vortex in this case.

Here is a paper with some pressure distributions for vortices on a delta shaped wing.

http://www.cfd4aircraft.com/int_conf/IC ... hummel.pdf

Brian

What really counts is pressure difference x area. You can go for lower pressure difference, but acting on longer path and bigger area, doing some additional usable work along (sealing the sides, preventing flow separation in diff), or you can quickly kill your vortex directing it at steep angle into the floor.

There is one big difference between vorticies known to avionics and those under the floor - the later are positioned in accelearting flow, and you can use this to drive vorticies stronger - but you need keep them alive long enough.

As there are many sources of vortexes on current F1 cars, and vortexes rotating in the same direction tend to curl together, making one big, strong vortex, you want to give them enough lifetime to do so - one more reason to not use all of vorticies energy to soon.

[strad]

Look where the burns are from the exhaust heat...on the outsides,,,not under...

To me it's using exhaust gas to seal the diffuser much like the FEE seals the whole sidepod.

[hardingfv32]

Maybe so, but why don't we focus on what Allison said about using a vortex between the tire and diffuser to make use of the vortex's low pressure core.

The physics of this technique are not apparent to me.

As an example, in the photo above which way is the vortex rotating on say the left side of the diffuser next to the tire?

Brian

[shelly]

Brian, the virtex is already there and the exhausts enhance it, as I wrote in this post

viewtopic.php?f=12&t=9178&start=2190

[MIKEY_!]

All this talk of the diffuser creating a vortex and the teams using the exhaust to enhance it is bothering me... Won't the vortex suck the car backwards creating considerable drag like it would do on any other part of the car.

[Pierce89]

Look where the burns are from the exhaust heat...on the outsides,,,not under...

To me it's using exhaust gas to seal the diffuser much like the FEE seals the whole sidepod.

Low pressure works on any surface. The large flat surface outside the edges of diffuser, could be large and flat to give a good area for the reduced pressure to act on. The vortex's DON'T NEED TO BE INSIDE THE DIFFUSER TO LOWER THE PRESSURE UNDER THE FLOOR. Also, of course the exhaust burning is where the exits are, the exhaust loses temp quickly in freestream. It will only burn right next to exits.

[horse]

The large flat surface outside the edges of diffuser, could be large and flat to give a good area for the reduced pressure to act on

That's a really interesting point, Pierce, I hadn't thought that there could be such a localised effect happening here. Obviously, this is just the Ferrari design here and that is not the best example of this technology, but, still, this is an informative insight.

One wonders, then, about the balance between this local effect and diffuser efficiency enhancement.

[marekk]

Maybe so, but why don't we focus on what Allison said about using a vortex between the tire and diffuser to make use of the vortex's low pressure core.

The physics of this technique are not apparent to me.

As an example, in the photo above which way is the vortex rotating on say the left side of the diffuser next to the tire?

Brian

Left one should rotate clockwise when seen from behind the car IMO.
But the direction of rotation is not that important anyway.

Physics behind free flow vortex as i understand it:

Air (or any other fluid) molecules start to rotate due to pressure difference between both sides of generating device (wing, bargboard, vortex generator ...). Sharp edge between both surfaces help keep radius of this rotation small.
They gain angular speed and momentum, and at the same time (as described by bernoulli's principle) loose some of static pressure. Higher pressure ambient air rush to fill this gap, due to viscosity forming well known helical pattern. As they gain angular speed, static pressure in this region drops (with lowest value at vortex center). You can use this low pressure at will.

In free space they grow bigger and bigger in diameter, dissipating their energy due to mediums viscosity and eventually die (but it takes some time - vorticies behind starting 747 can still rip off the wings of small GA airplane after 15 min.)

In constrained space under the floor they are stretched in interaction with accelerating external flow, and due to (angular) momentum conservation have to go quicker, which means even lower static pressure.

@MIKEY_!: Of course, creating vortexes means drag (no free energy in our universe), but they are there anyway (no vortex-free airfoils in our universe either).
You can just throw them away (as seen clearly today on ML's rear wings), or you can try to gain some of this energy back.

[speedsense]

Wouldn't the "problem" be the high pressure wrapping of the low pressure center also getting into the diffuser?

Don't think there is any high pressure region "wrapping" low pressure center of free flow vortex. It's all below ambient. There is no compression of gases within the vortex either IMO.

If you manage to place your vortex along the side of the floor, it will try to suck everything around, including air beneath and to the side of the car and the floor itself.

It's true that confined space under the car prevents further buildup of those vortices, but on the other hand negative pressure vector (low pressure at the rear of the car) helps with stretching and tightening of vortex tubes, making them stronger, so with carefully planing and some luck one can keep them alive all the way to the diffuser.

In fact all current teams manage to do this IMO, if you watch closely it's easy to spot something like this (strong vortexes clearly visible under both sidepods):


(credit for this enhanced picture to formula_none).

My mistake, should read "a higher pressure wrapping a lower pressure core"

With exhaust gases generated w/o water or O2 (having burned off) and the heated gas is lighter than ambient air, the "potential" compressablity exists as the generated heated gas vortex has different properties than an ambient free flow generated vortex.
It only stands to reason that the duration of the vortex would be longer lasting and less vunerable to the friction of the air around it acting as quickly on the vortex, and being lighter than the air around it, with higher accelerations of the air flow (over a free flow vortex) causing the tornado.
Going back to a point made earlier, if I put in a skirt (to the ground) on the diffuser to contain the flow inside the diffuser and prevent outside intervention of the wheel/tire "air pump action" on the air gap, the diffuser would be the most efficent it could be, wouldn't it? I believe yes.
So replicating this non-legal skirt with an ambient "free flow" vortex would improve the same diffuser with an air gap. With an even further improvement coming from a heated gas generated vortex that has more energy. IMHO.

[hardingfv32]

Is the exhaust is going to reduce the angle (stretch) the helix shape of the vortex?

In the case of most of the cars, the exhaust driven diffuser vortex only has the about length of the tire to function. How may helix revolutions might be formed in that distance? Could there only be enough distance to form the initial rev/twist?

What might I research under to gain more info on the this type of vortex formation? Almost everything I have found is aviation specific.

Brian