Exhaust Blown Floor - Forward Exhaust Exit

[strad]

I don't see how it can follow any other path,,the gasses propel it, so it has no choice but to follow the same path.

[shelly]

Gas and dust do not follow the same path, because they have different inertia so dust will react less promptly to forces imparted by airflow.

It is somethingt similar to this example: imagaine that you are holding in your hand a tennis ball and a feather, and that you launch them forward together. The ball and the feather will have differnet trajectories, with tha ball goging longer.
So the dust once accelerated to the side of the car will keep momentume more than exaust gases and will follow a more external trajectory

[marekk]

I don't see how it can follow any other path,,the gasses propel it, so it has no choice but to follow the same path.

1. Fluids, unlike water droplets or concrete dust are not just bunch of molecules - they interact, forming pressure/accoustic waves, so if you see water/dust particles accelearted, this doesn't mean they are hit directly by one of exhaust gas molecules.

2. Inertia. As shelly said, we are talking about particles several orders of magnitude heavier then gas molecules. Very simple experiment - put something small and light (grain of rice will be perfect) on the table and try to blow it away - it will go much further then air used to move it. If you add crosswind, it's easy to understand why air molecules and rice take different pathlines.

Lack of visible dust sucking under the floor - not expecting it at all, even Jumbo Jets engines have hard time trying to suck some dust/water from the runway, and they are slightly more powerfull then F1 diffuser. But this doesn't mean they can't suck any air.

[ringo]

Shelly you are not making sense.
Your opinion would be logical if the dust was being pulled. However in this case the dust is being pushed.
We know the dust is heavier than the gas. So if it is being pushed, then the lighter gas has to be be pushing out against it.

No other particles in the atmosphere are responsible for the push.

If anything the gases are flying wider than the heavier dust particles, so heavier dust only helps the argument.

So again the fact that there is a push or "squeeze out" as suggested by the CFD and not a "suck in" is very telling.

The gas wont push out the particles, then turn back under the floor. That makes no sense. The particles are proppeled by exhaust gases and nothing else.
For if it was something else we would see the same from other cars, and it is not the case.

COncrete dust is probably as good as its going to get, better than water droplets, almost as good as this:

or this:

wouldn't you say that the gases are propelling this dust or vapour or whatever it is?

[ringo]

Gas and dust do not follow the same path, because they have different inertia so dust will react less promptly to forces imparted by airflow.

It is somethingt similar to this example: imagaine that you are holding in your hand a tennis ball and a feather, and that you launch them forward together. The ball and the feather will have differnet trajectories, with tha ball goging longer.
So the dust once accelerated to the side of the car will keep momentume more than exaust gases and will follow a more external trajectory

Ok answer this:

Is this a push or pull happening in the video?

Following your reasoning about inertia, wouldn't you agree that the gas would push out more than the heavier concrete dust?
You must agree with this if you are to apply your principle.

[malcolm]

The exhaust could accelerate the dust, using the drag-forces of the dust to accelerate it with the stream of exhaust (the higher mass of the dust being accelerated in the Y direction by the decelerating stream of exhaust); then the exhaust could be affected by other factors (diffuser, cross-flow, etc), and the higher momentum of the dust would preclude from accelerating in the X direction, despite the lower-momentum-yet-higher-speed exhaust from accelerating in the X direction. (X is longitudinal with respect to the car, and Y obviously lateral)

Therefore, closer to the exhaust exits, the exhaust is going faster in the Y direction than the dust particles, but as the exhaust slows down in the Y direction and accelerates in the X direction, the dust particles end up with a Y component of velocity that is greater than the exhaust and air mixture, but an X component of velocity that is lower than the exhaust and air mixture.

So Shelly was making sense.

[PlatinumZealot]

Not complete. You neglected the air in the atmosphere.

We all know that gasses cannot Pull objects.

This is Newtons 2nd Law.

Only Push. There is not particle that exits that will be pulled by gas.

So... We have to consider the external atmosphere around the car. This will push the concrete dust under the car if that is where the gas is flowing.

So yes, the concrete dust can be pushed under the floor, or at least attempt to curl into the diffuser IF that is where the gasses are going; But alas, the video clearly shows that the dust is push aggressively wide of the car.

[strad]

I side with Ringo on this..it only makes sense to me.

[shelly]

malcom and marekk have given good explainations, which are more detailed than mine: that maybe will make it easier for ringo, strad and n_smikle to understand my point.
@ringo: the picture you posted of flow viz with msmoke prove my piint, not yours: smoke is used because of its low density.
n_smikle is partially right: it is the external flow that plays a big part in determine the trajectory, and lighter parts are more heavily affected by it. My example was focused on this.
As for the push-pull argument, I do not fully understand the issue. Is it push=blowing and pull=sucking? Like malcom said the gas push the dust at the beginning, but that does not put any constraint of their subsequent trajectories in a field of moving air

[strad]

We have said air is a fluid yes?
So if I substitute a water jet the concrete will go where the water is aimed.
It will follow the ballistic force propeling it. It will not take on a life of it's own and just go trotting off where it pleases.

[tok-tokkie]

Dyson vacuum cleaners & industrial cyclones use the inertia of the dust particles to separate them from the airsteam. Having said that I must say the cement dust really makes me believe the exhaust is also going there.


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

[shelly]

strad and tok tokkie: you are missing basic point, whih is that both gas and dust are immersed in the whole air flow field and due to their different inertia they will rect differently.
Your example tok tokkie is right on the spot, but you have to consider that there are three actors: air, exhaust and dust. So both exausts and dust will be drivn by air, along different trajectories due to theor different inertia.

[PlatinumZealot]

Shelly, you are missing the other half of the system... there are three "bodies" of air:

1. the exhaust gasses
2. the ambient air
3. the air under the car

Gasses cannot pull objects. Smoke or dust it doesn't matter a gas cannot pull it, right.

So the point is, that even if the air curls under the car the cement dust, you cannot expect it to pull the dust in... BUT.. you can expect the ambient air around the car to PUSH the dust in. - And that did not happen.

And Cement dust, have anybody here ever opened a bag of cement? It is lighter than baby powder - you can reasonably expect it to give a good trace of the exhaust. It is not like the exhaust is in a solid walled tube - you have bodies of air around it that push on it - and guess what.... the push wasn't hard enough to curl the exhaust nor the cement dust under the car. So there is only one reasonable conclusion...

[strad]

Christ...That's because they are purposely separated...The dust will ride the air as long as it can...it just won't go AS FAR.

[strad]

.... the push wasn't hard enough to curl the exhaust nor the cement dust under the car. So there is only one reasonable conclusion...

TA-DA...Thank you.