bernulli

24 ms·

211.
by bernulli 6y ago
Ok, omega * R is what I referred to as tangential velocity, you’re right. I don’t know if that’s a useful way to look at it, though. It’s like saying “if I just made my car tires a mile in diameter, geez, I could be so much faster!”
212.
by bernulli 6y ago
Oh interesting, I’ll need to look into where that discrepancy comes from!
213.
by bernulli 6y ago
It probably (?) wouldn’t increase the drag of the aircraft significantly, but you’re still spinning a cylinder in a viscous fluid and that takes energy. So over all, you increase the power you need to put into the system to create a certain
214.
by bernulli 6y ago
But the whole game of airfoil and wing design is to optimize the lift to drag ratio, i.e. how much lift is produced per propulsive power needed. Sure, given enough power, you can just remove wings altogether and simply fly an engine.
215.
by bernulli 6y ago
Potential flow theory tells us that the lift L is L = rho U Gamma, where rho is the air density, U the flight velocity, and Gamma the circulation, i.e. the tangential velocity of the rotating surface times the circumference. L is then only
216.
by bernulli 6y ago
It is to realize whether learning is possible at all. What can you learn from a successful game of rolling dice, from a successful game of flipping a coin? Was it the way you moved your wrist or did you you just luck out?
217.
by bernulli 6y ago
One of the last things my mother told me before dying in her fifties was “but there’s still so much I wanted to do”. I drew a “life calendar”, crossed out all the years I had already spent, marked her age when she died and more statistical
218.
by bernulli 6y ago
Also, in terms of chemistry, measuring the volume of components (e.g. of powdery substances like flour) rather than their mass is really not a good way to determine the exact amount.
219.
by bernulli 6y ago
Wow, that view is even grimmer than just jobs being automated away...
220.
by bernulli 6y ago
Is it, though? How many jobs will there be 10-20 years down the road where the main requirement is to be able to sit still for 8 hrs, ask before you go pee, and do as you're told? How many of these jobs will be automated instead? Or, d
221.
by bernulli 6y ago
Reusability is not binary, saying 'all liquid rocket engines are reusable' is pointless. It's like saying surely a Formula 1 car is equivalent to a Tacoma in terms of 'reusability', because both can be restarted. Sp
222.
by bernulli 6y ago
Most boats/ships already get wet with (sea)water, alternating with air at the water level. Hardly a new problem.
223.
by bernulli 6y ago
Those are two very different things. The OP article talks about introducing air bubbles into the stream along the hull, the articles you linked talk about cavitation bubbles, i.e. water that vaporizes when the pressure locally drops. The la
224.
by bernulli 6y ago
I haven't seen any of pfdietz' arguments in a book. But ThenAsNow already recommended the excellent "Modern Compressible flow" by J.D. Anderson, and, in extension, I can recommend "Hypersonic and High-Temperature Ga
225.
by bernulli 6y ago
Well, but the gas does not reversibly expand again between the shock and the stagnation point. So clearly it is heating up, even if it were compressed isentropically. The position is far from incoherent. Temperature rise during isentropic c
226.
by bernulli 6y ago
Yes you do, you keep repeating that 'proper' compression is isentropic. That's simply not the case, certainly not across a shock. The second part of your argument is well defined and makes sense, and no-one argued that. But e
227.
by bernulli 6y ago
Exactly. There's no mention at all about the foundation of all your arguments, namely that compression be inherently isentropic. It simply is not. You can add it as an additional constraint, if you want to. What I say is that even when
228.
by bernulli 6y ago
I have never seen compression to be considered inherently reversible, certainly not in the aerospace or hypersonics/reentry communities. I assume you come from a different background? Care to explain your second argument? Even when the
229.
by bernulli 6y ago
I am having trouble seeing how compression in a shock would be isentropic? Care to elaborate? And why passing through a shock (compression) is akin to friction, but not to compression?
230.
by bernulli 6y ago
The specific impulse does not take the engine (or reactor) mass into account, but it's (approximately, if you neglect pressure effects) proportional to the exit velocity of the gas flying out of the nozzle. The exit velocity in turn is
231.
by bernulli 6y ago
It's really not the most important parameter, though. Unless you get the necessary thrust levels, the efficiency (Isp) of electric propulsion will get you very efficiently nowhere. And don't get me wrong - electric is amazing and
232.
by bernulli 6y ago
It's literally the title: "Ask HN: What is the best way to learn how to build your own hardware" ...
233.
by bernulli 6y ago
There's a lot more to learning a different language than learning substitutions. For starters, there is hardly any bijective mapping between words. Translating 'run' to German could mean any of 'rennen, renne, rennst, re
234.
by bernulli 6y ago
I know. And by local median income, and by national interest, and by .... My point is that the ‘simply auction it off’ approach is bad. [edit: typo]
235.
by bernulli 6y ago
Ok, I’ll go for the bait. How does this help OP in his goal to learn how to build devices like this?
236.
by bernulli 6y ago
So only visa for FAANG and finance? Who is going to work outside of New York and Silicon Valley? Who is going to work at the universities?
237.
by bernulli 6y ago
In my experience 'Kohlefaser' is a pretty common technical term, I've only ever encountered 'Karbon' w.r.t. bicycles (but that just be my limited view).
238.
by bernulli 6y ago
Unfortunately, it turns out that (for ideal gases, such as our atmosphere) a 'negative shock', i.e. a rarefaction shock, is impossible. It would violate the 2nd law of thermodynamics! And if you have multiple, subsequent shocks (o
239.
by bernulli 6y ago
The key thing is that the absolute pressure rise through the shock wave isn't very high - it's just perceived as a loud boom when it is reached in a very short time. The pressure wave is ultimately caused by displacement because o
240.
by bernulli 6y ago
This is then made even more frustrating due to the asymmetry when the opportunity cost for you for not having your problem fixed is much higher than for the person you are trying to reach. Or, more succinctly, asking `Who has the problem?&#

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