HN user
Maybe ;-)
I do know that this course is still offered widely across the US, but maybe not within CS, and maybe not as a mandatory course.
But, bottom line: if you're interested, it's very likely you can take it - you may have to check your school of engineering or applied math department.
Sounds like a vanilla numerical math course?
It really is a tragedy of the commons with a changing demographic, where DINK are more prevalent that, say, 50 years ago, where most tried to contribute to raising the next generation. But the amount of entitlement still baffles me - where do these people think everyone younger than them comes from? How much time and money do they think it costs to train a human being to be a member of society, of benefit to them!, and why do they just assume it is someone else's duty to provide this to them, free of charge to them?
But you still benefit even from the mere _existence_ of human beings younger than you are (e.g. service, manufacturing, research, medical, etc.). If you think you don't, think about the fraction of people you will interact with in 20 or 40 years that are younger than you will be, or about the innovations you will be using in 20 or 40 years that were developed by people younger than you. Those human beings have to come from somewhere - why do you insist it my obligation to make you that gift, at exorbitant expense of my time and money?
There's a simple word for people with that attitude: freeloader. It's pretty easy to understand:
I can not image the extra amount of work that get places on people that do not have children...
What you're forgetting is the amount of work required to raise children. That's an externality that is only covered through a social contract, where everyone contributes to a next generation. Why should parents put in the extra work to raise children that, god forbid, would ever do something you benefit from?
What you want is someone else doing the work so that you can just reap the benefits. You're a freeloader, plain and simple (other, less favorable words come to mind).
Unless, of course, you will never in your life use the services or inventions of anyone younger than you. But, somehow, I doubt that - am I right? You just expect those cohorts to just appear out od thin air to your benefit.
Just a few words from the other side of the table: I have the highest respect for students sitting there who juggle college, family, sometimes job, after having worked for a while already.
It’s wonderful to have someone listening because of a conscious decision rather than just following the obvious next step. Interact with your professor, ask questions, I’d hope most will try to be supportive, especially when you’re their age or even older. Best of luck to everyone doing this, it’s amazing.
Nitnitpick: and we know it isn’t and the corrosive propellants will contaminate surfaces:
Absolutely! I’m sure they analyzed for that, but backflow to the opposite direction of the nozzle is quite unintuitive but normal in rarefied flow, with propellant deposition next to the nozzle possible and documented [1]. Just because you point it away from the sensitive stuff does not mean it won’t get there and leave nasty traces.
Thank you!
I don’t think I understand the resolution then, could you explain a bit more? Say, I request data along a 100km line, every 10m. Do I get the same numbers if it’s in the same mesh cell with a sudden change when it’s crossed, or do I get some (bilinear?) interpolation?
Hi meteo-jeff, this looks really cool!
I have two questions:
1) How does the spatial resolution come into this? Is it constant data all across the 2kmx2km (?) parcel with an abrupt change, or is it interpolated in some way? Can I query the coordinates of the mesh?
2) How 'historical' does it get? How far back can I go with this?
Thank you!
Hi Otras,
a few things to wrap this up for me, and thank you for your civil answer to a not so civil comment.
1st: I should have made this a 'yes and' rather than a 'no but' comment. It's a well written article with nice illustrations, and you put in the time to do so and share it with the world. Thank you for that!
2nd: Obviously you're right that it's possible to calculate the distance to plane by observing when it passes a certain point and then measuring the time until the sound hits you from that very point.
3rd: My smartassery still stands that "How far behind a plane is its noise?" is a misleading way to frame it - the noise is not behind the plane at all, it's actually significantly ahead of it. But (!), again, I should have made that a 'yes, and' instead, to add a facet to your nice article.
Cheers, and hope to read from you again!
Not in the countries you refer to, and not in gliders in many European countries. Not 'everywhere' then.
That we can calculate the distance to a lightning to great accuracy (my example in the very first sentence) by accounting for a finite speed of sound only and assuming an infinite speed of light? Yeah, I'm pretty serious that we can neglect that error of some microseconds when concerned with a process that takes many orders of magnitude more.
The noise is always pointing behind the aircraft or any moving object due to lag.
I don't know what a pointing noise is.
Light also encodes the direction to an objects past location, even though light is always moving faster than the object.
Yet the light is never outrun by the object, much like the subsonic plane never outruns its noise. However, here, the (fast) noise arrives after the (slow) plane. In your analogy, that would correspond to the (slow) object arriving before the (fast) light.
You're the one bringing up the finite speed of light that has zero relevance to the problem at hand.
Seems to cut either way.
The title is "How far behind a plane is its noise?" It's not. It's ahead of the aircraft.
" These statements exhibit a fundamental misunderstanding of the phenomenon. It's not that the sound outpaces the aircraft. It's that light from the aircraft (reflected or transmitted, e.g., by landing / navigational lights) travels faster than sound."
Explain to me again how any of that explains why the airplane passes me (and I see it 1e-6s later which seems to be somehow super important to you) before I hear it, even though its sound is traveling towards me much faster than the plane.
It's just completely unrelated to why the aircraft passes you before you hear it.
Thank you.
For all practical purposes it's really completely unrelated to the speed of light. Nothing would change in the argument if light travelled instantaneously. Sure, the numbers would change by O(1e-6s), but I'll admit that I wouldn't be able to tell the difference when watching an aircraft.
Because an aircraft does not make a sudden noise? At least where I'm from aircraft don't sound like discrete booms. I'm not sure I understand your question.
But when the aircraft is closer to you, the attenuation is lower so you can hear the sound.
So we agree after all.
"Of course you hear the siren or crazy man or anything, before it passes you if the component of the velocity vector pointing to you is slower than the speed of sound."
So, only in aircraft it is different? Magical aircraft physics after all?
" If some one was standing 1000 meters away from you, and had a sign that flashed a sequence of numbers, 1,2,3,4,… once per second, and at the same time as the number flashed, they shouted the number loud enough that you could hear it, do you think what you heard and what you saw would be in sync?
Of course not.
But to humor you: which is the distinct event in a normally flying aircraft in which you can tie the exact point at which the light and sound signal leave the aircraft towards you so you can use that to calculate the distance? Spoiler: there isn't, you cannot, and that is precisely the point.
There are some examples currently in Ukraine, in which you could use your argument.
Thanks for chiming in!
My point is that your one instant in time is completely arbitrary. You do not know where the position is, and you do not know when the sound was emanated. I.e., you cannot calculate anything.
Your math would work iff you observe a discrete event where you can tie sound and light - engine blow-up, for example. In all other cases, it means nothing.
Again - why don't you hear the sound of the aircraft taking off if speed of sound is the only effect?
Well yeah, if the plane is faster than its sound (and flying towards you), the plane will reach you earlier than the sound does. The plane does not get attenuated by flying farther, and your seeing threshold is helped by the sun or the lights the aircraft turns on at night.
Air properties are actually super relevant, the speed of sound distribution (slower with increasing altitude) can have lensing effects for sonic booms.
Sorry, I thought the first answer was so short it seemed rude, which is not what I intended.
Haha, yes, you found the one case (which is unrelated to the article) in which sound weakening is not a function of time, as the distance to you stays constant.
"Even if the sound travelled faster than sound [sic] you would still see the airplane passing over you before the sound emitted from the airplane when it passed over you reaches you."
Absolutely not. It depends on the Mach number, distance, sound weakening, and your hearing threshold.
You cannot hear some crazyman running at you, screaming, until he has passed you? You cannot hear the stereo in some guy's car until after he passed you? You cannot hear a siren of police until the car has passed you? Or are what you describe special magical airplane-only physics?
"Why does it have to be a sudden sound? The effect the article describes is the same as eg thunder"
Well yeah, that's a sudden sound. My point precisely.
So why don't you hear from your observation point the airplane (or all airplanes for that matter) as it takes off, which is when it makes its first noise? And by all means, account for a few ms of light movement if that makes you happy.