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But if I had to guess, I'd say that the counterargument remains that the exhaust is accelerating the dust near its exit and propels it outward before being picked up by the freestream, while the exhaust itself cools, loses velocity and turns in toward the car. Not terribly convinced by this either, but I don't think they're terrible people for bringing it up.

Those 3 things are counter thesis to any blown diffuser. In fact any blown device.

Let us break the argument.
Do dust go under the diffuser? No
Do exhausts and dust follow the same trajectory? No (maybe only ringo has not got it yet)
Where do exhausts go? We can discuss it, but dust gives very little answer to this question
I have understood the push-pull issue, which is blown away (pun!) if we consider negative relative pressure (so the vaccum cleaner example fits)

I fully concur with that statement of what is certain & what is still in debate. I too checked up on the relative mass of the particles & now appreciate how much more inertia the cement has than the air. An analysis of the forces on an air particle would be helpful.

[...]

Guys,

do you REALLY belive that bullet's and gases will follow THE SAME PATH in 50m/s crosswind for the next 1m ?



BTW: ringo, do you see this small part of exhaust bend more then 90 degree just after the barrel ? Without any crosswind. At > 1000m/s exhaust speed.


n_smikle: Just to prove how much of a nitpicking bich i am :
gas molecules, do excersise some pulling forces - due to gravity (REAL nitpicking, i know) and van der waals force (not so much nitpicking, it's where most of our liquid gases supply comes from, just google for joule-thomson effect). Of course those forces are negligible in our analysis.

tok-tokkie: long time ago (i'm wondering if renault spent so much time as we did on this topic , back on page 33 of this thread we've already discussed those trajectories.

strad: im not angry at all, but can you please elaborate a little bit more on why you think the spray paint ia a good example ? maybe using your physics and real world numbers knowledge ? Would be a welcomed contribution to our research.

BTW: ringo, do you see this small part of exhaust bend more then 90 degree just after the barrel ? Without any crosswind. At > 1000m/s exhaust speed

It's caused by backpressure....totally different

strad wrote:I think the spray paint is a good example Ringo. [...]

The spray example is wrong, because it does not take intpo account the air flowfield and its effects. [...]

Frustrated would be a better word..
In the bullet example..gasses are pushing the bullet...and the back flash often carries burning gunpowder with it and can actually burn you and is a good reason to wear eye protection when shooting.
I am open...Where is your video? Proof other than your insistence you are right?
What I'd really like is for the team to come out and say..."ok, this is how it works" ... little levity...bout as much chance of that as either of us changing our minds and going...I see,,,you're right.

I think the bullet example is good even if we do not take into account reversed flow. It shows that the gases do not reach as far as the bulet. It is the same as the feather- ball example I posted.

I think you can not disagree that heavy and light particles have different trajectories in a flow field.

It shows that the gases do not reach as far as the bulet. It is the same as the feather- ball example I posted.

Nobody is disputing that...Only whether the gas and the bullet initally follow the same trajectory.

From my understanding the discussion is about dust going outside the rear wheel being the proof that exhaust go wide also.
If you also agree that the trajectories are different, then the discussion is over: it is not a proof.

My first thought on seeing the video is that the car is turning, the exhaust is kicking up the dust, and we are looking at a trace of the car's path around that bend.

We can't discount that the car is turning. The exhaust kicks up some dust, the car travels forward and turns right, so the point at which the dust is kicked up is then offset to the side of the car.

There is also some merit in the water splash analogy. The initial impact of the exhaust would be outward. However, would the dust continue on that path if the exhaust formed a vortex down the edge of the car? Is it reasonable to expect the dust to form a nice vortex like smoke in a wind tunnel, or would the dust kick up and get flung out as the vortex hit the ground?

Finally, how long had the dust lain there? Had it previously been stored in a bucket, open to the damp sea air?

Richard you said the exhaust kicks up the dust(and as seen in the video), So can we all agree that the initial velocity of the gas is outwards and away from the car?

richard_leeds wrote:My first thought on seeing the video is that the car is turning, the exhaust is kicking up the dust, and we are looking at a trace of the car's path around that bend.

We can't discount that the car is turning. The exhaust kicks up some dust, the car travels forward and turns right, so the point at which the dust is kicked up is then offset to the side of the car.

There is also some merit in the water splash analogy. The initial impact of the exhaust would be outward. However, would the dust continue on that path if the exhaust formed a vortex down the edge of the car? Is it reasonable to expect the dust to form a nice vortex like smoke in a wind tunnel, or would the dust kick up and get flung out as the vortex hit the ground?

Finally, how long had the dust lain there? Had it previously been stored in a bucket, open to the damp sea air?

Actual drive line relative to the track plays obviously significant role to our observations, and i agree it seems to go away from the dust.

Don't think it's reasonable to expect dust/water to form a vortex.
Regardless the speed of the car, speed of external flow in reference to dust/water on the track is more or less zero. Dust/water particles get kicked off the track by colliding with high momentum (both speed of the exhaust flow and thermal energy of molecules add to that) gas particles - they'll gain high enough momentum to overcome adhesive and gravitational forces acting on them.

As n_smikle mentioned already, there are no significant pulling/sucking forces from the exhaust gases itself involved, so you rely on external atmosphere's molecules filling the gap (low pressure under the car and diff) - and obviously those molecules (cold, just started to accelerate towards low pressure area) are by far not energetic enough to carry dust/water particles with, not to mention the fact that we are pushing from 8cm diameter pipe (jet, sort of), but pulling from all ovr the wide open space.
For the same reason 747's turbofans can easily kick off the car behind them like a paper toy, but you are more or less safe standing few meters in front of them at the same time.

n smikle wrote:Richard you said the exhaust kicks up the dust(and as seen in the video), So can we all agree that the initial velocity of the gas is outwards and away from the car?

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