Exhaust Blown Floor - Forward Exhaust Exit

[ringo]

Ringo

1) "When air is fed under the car, it is really just to prevent stagnation and circulation from occurring."

What is meant by "circulation from occurring." What type of circulation and where exactly?

Yeah, less air as possible is good as long as the velocity is very high, so there is a balance. Chances are there is a limit to how low a mass flow you can push without creating eddys.
I shouldn't have said circulation, as that will create confusion. It's more eddy formation. I have't looked in a fluids book in a while so i am rusty on the subject.

2) May I assume that there would never be a reason to inject flow under the side-pods because of their distance from the ground? There is no chance of accumulation or stagnation?

You don't want to inject air under the side pods. If the pressure is not lower than under the flow then forget about it. If the floor pressure is say 90 kpA, injecting 101 kPa air under the car will just raise the average presure under there.
Any injection should be behind the diffuser throat, or ahead of the splitter where the pressures are higher than the under floor.
Sucking away from the floor is better than pumping in towards the floor.

Even back in the day, there was never a blown diffuser with the exhuasts under the floor. All were ahead of the diffuser.
The fan cars sucked away, they never pumped towards.

[hardingfv32]

shelly

"barge boards do not push flow away from the floor, they increase flow under the floor"

I wish to learn. Exactly how do the barge boards increase flow under the floor? Accurately describe the path of the air flow follows.

Thanks
Brian

[shelly]

shelly

"barge boards do not push flow away from the floor, they increase flow under the floor"

I wish to learn. Exactly how do the barge boards increase flow under the floor? Accurately describe the path of the air flow follows.

Thanks
Brian

Barge boards:

-prevent wake from the front tires going under the floor
-in top view are angled to the local flow so that they act like low aspect ratio wing. Air is accelerated in the channel between the bargeboard and the tub (lower pressure)
-they release two vortices from their top and bottom edges: the bottom vortex is sucked under the floor where the low pressure of its core generates downforce.(vortex generators on the tea tray serve the same function: they generate vortices that go underneath the floor and meke their low pressure in the core be felt as downforce until they eventually lose energy)

[shelly]

[quote="ringo"]
I have't looked in a fluids book in a while so i am rusty on the subject.

[quote]

I suggest you have a look at the books before discussing, so that you could get rid off of some inconsistencies in what you write. You seem to forget that for given geometry and density, mass flow rate and velocity are synonimous; and also you say that if you push air faster than it can escape, it will accumulate, forgetting that you can not push more in than it is able to get out. Also it is impossible for air to leave "quicker than it enters": mas in must equal mass out if no sources are present.
in the fw 33 picture, air comes in from the sides in the floor zone which on top is exposed by the coke bottle shape of the sidepods.

vacuum example does not hold because under the car you will have a pressure mostly influenced by the base pressure of the rear of the car, much higher than 0.

I agree with you o this: the air diverges after the first suction peak, but then the second sction peak calls it back again: under the floowr you will see a typical bow trajectory

[ringo]

Shelly the density changes.

you can't use simply formulas either as the system bounday is not well defined.

So saying given density, just wont apply becuase it is not give, as it changes.

Shelly mass can accumulate in a system. Things like capacitors, compressor air tanks demonstrate this.

mas in must equal mass out if no sources are present.

But you are forgetting one thing, you only have 2 parts to that rule. There are 3; in, out, and then part in between those 2. Mass is still conserved, you just aren't considering the part existing between the in an out.

I will try and do a crappy drawing of what happens under the floor. This is what i observe. It doesn't have to be related to any theory, and so it could be wrong, but this is what i see and it makes sense. Gimme a sec.

[ringo]

The lower half is not what happens with a T floor; IMO. We can't stamp rules of thumb onto it because the situation is just specific.

The diffuser is not a vacuum suction, it does no work so it cannot do what the lower half is demonstrating.

Using the top half, it's easy to see why the exhuast does not get pulled back under the car. Flow lines cannot cross, so it wont happen, neither would it create down-force if it did somehow.

[shelly]

ringo, what you call "accumulation" is not possible:
-even if density would change, mass flow in mass equal mass flow out for any given control volume
-if there is no thermal energy exchange, density of air around a car at subsonic speed does not change. It is a completely diffiernt situation from the air reservoir that you have cited as example, where air is forced into a closed volume.

About your sketch: you are correct in saying that pathlines do not cross, but the bottom-green part is a more correct sketch fo what happens under the floor than the top-blue one.
The more correct representation is a mix of the two: pathlines behave more like the blue in the barge board, front leading edge zone, and like the green in the rear part. Flow on the forward pat of the bargeboard does not get sucked bak in under the floor, but pathline on the external part of the leading edge go outside laterally and then come back in.
If you think that pressure is lower on the leading edge and on the kink line, it comes straight forwardly that the stream tube must be larger where pressure is higher compared to thes two low pressure regions.

Pathlines do not need to cross to behave like you have quickly sketched in the green part; it is only it being a quick sketch that gives you the wrong impression that they would have to.

Do you have a cfd floor streamline image of a f1 model (without exahust)? This is a basic flow feature, so if the diffuser works we should see the pattern

[ringo]

Accumulation is actually a part of the conservation of mass flow basic principle.

you are misunderstanding it.

rate of change of mass of a system = rate of change of mass in - rate of change of mass out.

Nowhere does it says in has to be equal to out.

only when the system is steady state does in equal out.

as 0 = m dot in - m dot out; which is m dot in = m dot out.

If it's not zero, then you have accumulation, which doesn't break any laws of physics because the mass is accounted for.


Anyway, the green flow doesn't happen.

I don't even have to use one of my CFDs i can google an example done by a big team.



these lines cannot be crossed, no room for any intrusion.

If any flow come in from the sides, it's not enough to completely pull in exhuast or barge board flow. The pull is not that strong under the car.

These lines here in gray are why anytring trying to come in simply wont have room.

So you see how the drawing relates to this.

[shelly]

As for mass conservation: we ARE in steady state, so mass in mass equal mass out!
We can not have accumulation. You stated the principle correctly, but you do not aplly it correctly.
As for the picture: the perspective is not good, but it is definitely more similar to your green example than to your blue. Do not forget that in the picture we are just seeing the innermost streamlines.
If you had read my post, you would have found that flow on the forward pèart of the bargeboard is not deflected inside.
As far of stregth of the pull: there is the external flow thet alignes the exhuasts, and then thy are deflected inwards.
From which F1team advertisement is the picture from?

[Lurk]

I know barely nothing in terms of fluid dynamic so I have a question about the "air accumulation" stuff:
Is ground effect on airplane (I don't know if it is the right word in english: it is when the plane is close to the ground and tends to lift by itself) a kind of accumulation of trapped air which lift up the plane? How does it work exactly? It sounds kind of related for me... (even if the plane is designed to lift and race car to be stuck)

[ringo]

As for mass conservation: we ARE in steady state, so mass in mass equal mass out!
We can not have accumulation. You stated the principle correctly, but you do not aplly it correctly.
As for the picture: the perspective is not good, but it is definitely more similar to your green example than to your blue. Do not forget that in the picture we are just seeing the innermost streamlines.
If you had read my post, you would have found that flow on the forward pèart of the bargeboard is not deflected inside.
As far of stregth of the pull: there is the external flow thet alignes the exhuasts, and then thy are deflected inwards.
From which F1team advertisement is the picture from?

An F1 car is hardly at steady state.
The floor works the suspension, and then there is heat exchange accross the system and within the fluid.
Hardly anything in reality is steady state. Especially that renault r31 sytem.

I don't agree with this floor pulling anything 2 feet outside of it into the diffuser.
I have never observed that, neither on a dusty track, in the rain, or in any representation of a modern F1 car.

The exhuast wasn't designed to go into the diffuser, and that's the crux of the arguement.
The diffuser does not perform work, and thus it would be up to the fluid itself to expend energy to pull in the exhaust, that is counter intuitive and it doesn't happen.

Th r31 exhaust is behaving like an ejector for the floor. ejectors pull vacuum by entraining fluid. That is actually the first benefit of their floor.

It's not a blown diffuser, that's just nonsense with no thought behind it at all.

[hardingfv32]

Shelly

Non confrontational question:

"As far of strength of the pull: there is the external flow that aligns the exhausts, and then thy are deflected inwards."

Why is the exhaust exit not aligned properly with the flow? Assume there is no physical reason to prevent this.

Brian

[hardingfv32]

Ringo

"The r31 exhaust is behaving like an ejector for the floor. ejectors pull vacuum by entraining fluid."

Can you provide details of what type of flows and what body parts create this effect?

Thanks
Brian

[malcolm]

1) The screen-shot that Ringo posted is from Speed Channel (a US channel). The front wing looks like the HRT F110, but the air intake doesn't... I'm at a loss.

2) Ringo: Note the suction peaks at the leading edge and trailing edge of the floor. There is a really big blue area near the diffuser kink line, and if more streamlines were included in this plot, I'd bet that low pressure zone would be sucking more of them into the diffuser. Also, given how large the blue area is at the leading edge, I would assume that would definitely help straighten the exhaust flow, relative to the centre-line of the car. Since you have a differing opinion, why do you think those low pressure zones wouldn't be sufficient to pull in some exhaust under the floor toward the diffuser?

3) Brian: Look at the streamlines that go under the nose, then over the splitter, and finally under the floor. Those were what I was referring to in an earlier post, where teams are essentially using the raised nose to act like a nozzle of sorts for the floor.

[ringo]

http://en.wikipedia.org/wiki/Injector

Info on the ejector here. The r31 is more like half of the sytstem in that article.

2) Ringo: Note the suction peaks at the leading edge and trailing edge of the floor. There is a really big blue area near the diffuser kink line, and if more streamlines were included in this plot, I'd bet that low pressure zone would be sucking more of them into the diffuser. Also, given how large the blue area is at the leading edge, I would assume that would definitely help straighten the exhaust flow, relative to the centre-line of the car. Since you have a differing opinion, why do you think those low pressure zones wouldn't be sufficient to pull in some exhaust under the floor toward the diffuser?

You want to see a plot with more stream lines?

Here is a shot from a honda CFD, not mine:


I've been over this before in this same thread. Follow the blue arrows.
Divergence, agree?
Then notice that only the lines that crash into the tyres first have any hope of squeezing into the gap between wheel and floor. Nothing crosses the side edge of the floor.

There is no outside flow coming into the side of the floor, none. Only that which originate from the splitter dominates the space under the car.

Now following this, how would hot gas shooting out at 220m/s at 90 degrees, take a detour and get sucked under the car?

This was my first attempt at the front exhuasts. More generous pipe angles and a lack of information as we have now about the geometry.

Look on the similarity between this and the Honda, or the USF1 car from speed.

It can be seen that only that which hit's the rotating tyres, lose enough energy then slips by and goes under the floor.
The majority of it, deflects to the outside and goes past the tyre.