bernulli

Ok, you can adjust my illustrative example of O(100s-1000s) by the 30ms to account for a finite speed of light if you think that makes any difference to the argument. Let me know.

For another thought experiment: if you cannot hear that original first sound on take-off, which one can you hear? 10 miles from you? 1 mile from you? That will be the virtual first sound to you, determined by how much weaker the sound has become on its trip through the atmosphere, and how that relates to your hearing threshold. But it will not always and exactly be at the point where the plane has reached its closest point to you (as in the article).

Precisely, "sound attenuation in the atmosphere and your hearing threshold".

So why don't you hear the sound when it all begins, right at take-off? That should be the first sound to reach you, no?

That's funny for someone with aero-glide2 as user name. I'm used to metric variometers in gliders.

Use the construction the author is using, i.e. the emanating sound waves, but you'll have to start them where it all starts, i.e. at take-off, and not simply appearing right next to you. Then, correct the drawing by having the plane move slower than the sound waves. The first sound you should be hearing as observer will be the take-off (if you could hear it) at the airport, and the aircraft will be wherever it is afterwards.

Nice, but very wrong. This describes the case of a plane suddenly appearing in mid-air and starting to make noise, something, planes rarely do (maybe in the Bermuda triangle). It's like thunder after lightning, or seeing a ball fly before hearing it being kicked when you're far away.

The aircraft, however, is flying for a long time, certainly it was flying and making noise much earlier than when it is passing the observer. As long as it flies subsonically, i.e. sound outpaces the aircraft - which is the case for every single commercial plane - the sound may be able to reach you much much earlier than the plane: As an example, take an aircraft flying with 100 m/s directly towards you. With every second flying, the sound will gain another 200 m distance relative to the aircraft (speed of sound ~300 m/s).

If you're 100km away, the aircraft will reach you after 1000s, the sound has reached you after 333s, i.e. ahead of the aircraft. If you're 200km away, the aircraft will reach you after 2000s and the sound has reached you after 667s.

So, how come it sounds like the sound of the plane is behind the plane? It's got to do with sound attenuation in the atmosphere and your hearing threshold.

So, it's not at all like in the article.

Somewhat minor nitpicks:

- The aircraft is drawn to essentially fly with Mach 1, i.e. at the speed of sound, as the position of the plane relative to the wave does not linearly increase with time. Essentially all airplanes you see are flying subsonically (unless you're in the military).

- "If the plane was moving very slowly, it wouldn’t outpace its sound by much." That's completely wrong. "very slow" aircraft are much slower than their sound, and all commercial aircraft still are slower than their sound, all of them are outpaced by their sound rather than the other way around.

[Edit: typos & math]

Haha, thanks! Let me think about this! ;-)

You can create audible shockwaves, like the crack of a whip or a sonic boom, easily with your hands. It's called 'clapping'.

The conditions to reach sonic flow are surprisingly modest!

It's fully characterized through the pressure ratio between high-pressure reservoir (here: inside of bottle) and low-pressure surroundings, and a parameter characterizing the molecular structure, the isentropic exponent.

For diatomic molecules (our air), the isentropic exponent is 1.4, and the critical pressure ratio at which Mach 1 will be reached is ~0.5, i.e. as long as the high pressure is twice as high as the surrounding pressure, the flow will reach the speed of sound. For more complex molecules the isentropic exponent approaches 1.1, and for steam 1.14, with a critical pressure ratio of ~0.58.

I.e. when you release air from a >2 bar (30psi) car/bike tire, you have sonic flow right there!

Well, yeah.

I always like to see it the other way around, it eliminates entities that are not a good fit, and the remainder is what drives adaptation. Of course, people could argue that's just like Michelangelo [1], and we're back at 'designed'.

[1] “The sculpture is already complete within the marble block, before I start my work. It is already there, I just have to chisel away the superfluous material.”

Haha, I wonder how this maps to the responses to the linked article in this very discussion forum ;-)

> elsevier is working with materials they dont own. however when substantial changes are made and accepted it could be argued that is a publishing partnership.

I don't know what you are trying to say and how that addresses the question.

> you dont need elsevier anyway, all you need is to create awareness of your work among your peers.

Unfortunately, that's not how it works in most scientific career paths.

I can tell you exactly where it came from, it's being spun up in that launcher thing, obviously, that's the whole point of spin launch. You can see it for yourself in the video. Is the pointy end of the rocket always facing in the same direction, e.g. upwards? No, it's changing its orientation continuously, it's always facing 90 degrees from the arm of the launcher, which, you guessed it, spins.

It is indeed exactly like a bucket. And the bucket changes its orientation throughout you spinning it, that's the whole thing, that's why the water stays inside. The top side of the bucket is always facing you. For that to happen, it has to rotate around its COG with the same rate it's rotating around.

A different example: the moon is tidally locked to the earth (like the bucket to you, or the rocket to the spin launcher) - very obviously, it needs to spin around its axis to achieve that, and it does so at exactly the same rate as it is rotating around the earth.

That momentum doesn't just disappear.

How could it not? The angular momentum is just disappearing?

Edit: Here's a link to an illustration, as your link provides zero information in that regard.

https://www.youtube.com/watch?v=TGO4LtCctTk&t=141s&ab_channe...

I clearly see their rocket rotate around its center of gravity, with the same angular velocity as the arm it's sitting on, while the center of gravity is moving on a circular path.

So, again, how is it not spinning? Where does the angular momentum go?

That's barely enough for attitude control, do you have numbers how that could be used to despin a large rotating mass, as it is exiting the launcher?

The only thing that’s valid (and I didn’t really understand if the video maker understood it bc he wastes so much time on the wobble) is that if you spin sth up to, say, 200 /s, then your rocket will spin, with its full length, at 200 /s. I don’t know how they want to address that.

Also - could you explain why you don't count van der Waals' equation among the cubic EOS?

That's what I work on, I'll send you an email (if I find your address somewhere)!

Regardless of why you get to supercritical conditions, once you have them, that's plenty of reason to care about what supercriticality does in industry.

You want high pressures to achieve a high efficiency of a process (gas turbine, rocket, Diesel), then it's more of a side effect that the pressure is supercritical - I think that's what you allude to.

However, you may also want your fuel jet to mix more efficiently, so absence of phase equilibrium with surface tension may be advantageous.

You may also want a working fluid in a power cycle where expansion through a turbine does not end in subcritical spray that destroys your turbine blades.

You may also want to have a heat exchanger that does not have a 'boiling crisis', i.e. a subcritical vapor film that drastically reduces your heat flux when you exceed a certain limit temperature: think about it, the situation when your system gets unnaturally hot, and you really want to get rid of the heat, is when the heat transfer collapses. Now, there are subtleties about whether or where this can still happen at supercritical conditions, but let's just say for high enough pressures, a distinct phase transition no longer occurs (somewhere beyond 3 to 10 times the fluid critical pressure).

I mean something different: no one has ever measured a critical volume, your dp/dv flatlines at the critical point, i.e. there is no driving force to make the fluid balance out the naturally occurring fluctuations, indeed relaxation times go to infinity at the critical point. An EOS only gives you the thermodynamic equilibrium value, attained when you let it sit for times much larger than the relaxation times.

The Widom line(s) are somewhat arbitrarily and ambiguously defined, so I'm not a big fan (pseudo boiling all the way!), but the whole point there is that there is no phase equilibrium at supercritical conditions. However, just because we never observe supercritical liquids and gases simultaneously in equilibrium, does not mean they don't exist. They absolutely do and do lead to a phase transition between liquids and gases - just not coexisting.

This also means that your other comment needs a revision: we absolutely do have something akin to boiling at supercritical conditions, even with macroscopic effects, such as the (subcritical) boiling crisis <> (supercritical) heat transfer deterioration.

Super interesting, and I watched the video on GitHub!

Of course, for an EOS to be useful at the critical point you have to believe that there is such a thing as continuum or even thermodynamic equilibrium at the critical point ;-)

There’s plenty of applications beyond using them as solvents, e.g. fuel injection in jet engines, Diesel, or rockets; cooling in power plants or rockets; drilling; power cycles, etc.

There is nothing magical about a supercritical fluid (SCF), i.e. a fluid at a state that has a temperature and pressure higher than the respective critical values. The article repeats the same crap that you read other places: it does not have liquid and gaseous properties simultaneously. At low temperatures, the SCF is a liquid; at high temperatures (and not too high supercritical pressures), it's a gas or even an ideal gas. The 4-quadrant way of showing the 'supercritical domain' is super misleading in this way. Nothing happens when you cross those imaginary boundaries.

What is almost magical is a fluid at near-critical conditions. There's a whole lot going on we don't properly understand yet.

Same thing, you will want to reuse existing code, and you do not want to split knowledge in your student group. There is no such thing as 'gradual' migration: I either have to support Python in my group, or Python and Julia -- until everything is migrated.

If I have a working ecosystem using Python, with students trained in Python, and all previous work in Python, there's a whole lot of opportunity cost associated with me deciding to have the next student use Julia. I'd rather have that student build on existing tools and knowledge and do something new with their time.

Its third, and major, drawback is that Python has a legacy in many groups.

For those unfamiliar, CERFACS (Centre Européen de Recherche et de Formation Avancée en Calcul Scientifique, i.e. European center for research and advanced training in scientific computing) is a leading research institution, with two main branches: meteorology, and engineering computational fluid dynamics. I am not affiliated and can only evaluate the engineering part, their combustion modeling group is one of the best in the world.

That’s also Newton’s 3rd.

How I hate that article. Poor John D. Anderson will forever be associated with this ridiculous claim that no one knows why aircraft fly. Also note, that he does not say that at all:

“What Anderson said, however, is that there is actually no agreement on what generates the aerodynamic force known as lift. “There is no simple one-liner answer to this,” he told the Times.

Not only does he not say that no one understands lift, he also doesn’t say that there is no agreement. What he says is that there is no simple answer. That means something completely differently.

Edit: there are two fundamental ways to describe lift: as the resulting force from integrating pressure over the surface of the wing (here discussed in a distorted way as ‘Bernoulli’), and a control volume way of Newton’s 3rd axiom, where air is pushed downwards by the wing and the wing in turn experienced a force in the opposite direction.

There is no mystery. Both views are equivalent. What is hard is to predict the exact force, as one would have to predict the flow field around the wing.