71–80 of 165 posts
It's pretty rare to read an article in which the degree of previous error is sixteen orders of magnitude.
Earlier quoted context omitted.
No, actually friction is closer to the truth. It's air running into air at a shock, a process that generates entropy, just as friction does. Compression is a process that is adiabatic, generating no entropy. And in an ideal gas the density increase across a shock reaches a finite limit even as the mach number goes to infinity.
I can't make sense of your claim. Compression could be done adiabatically, or not, but that doesn't mean it's isothermal or somehow inherently adiabatic. In fact, for it to be isothermal would require the transport away of compression heat, which would be decidedly non-adiabatic. The parent is correct.
The parent is not correct. Compression occurs at a shock, but this is not heating due (solely) to adiabatic compression. This is clearly seen by considering the case where mach number goes to infinity: in that limit, the fraction of heating due to compression goes to zero (as the gas density remains below some finite limiting value.) What is causing heating is a dissipative process in the shock itself. This dissipative process is more akin to friction than it is to compression.
Earlier quoted context omitted.
I mean, the HS teacher that told me this had a masters in Mech E. Or so he claimed, hmm...
It’s amazing a man would work for a masters degree and end up teaching mainly below average people at a public institution. What’s a masters degree worth?
Some people are teachers because they want to be teachers, period. They find fulfillment in it.
That’s not even touching the unnecessary dig at public institutions.
Earlier quoted context omitted.
Is it regarded as a failure? Teachers in public schools work part time for 25 years and get a scaling pension they “negotiated” with the politicians their union dues elected. I’d call them fantastic winners. Grifters? Sure. But certainly not losers.
> Teachers in public schools work part time Typically full-time and usually year round, though some work seasonally for about 3/4 year (but still full-time), with commensurately reduced annual pay.
Earlier quoted context omitted.
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?
Compression due to a shock is not isentropic. Parent is incorrect. This is basic gasdynamics. An accessible reference: Modern Compressible flow, by Anderson.
Earlier quoted context omitted.
Because compression, as normally considered, does not generate entropy. It causes gas to become hotter because the temperature has to be higher for entropy to remain constant, as the volume is reduced. This is a reversible process. But shocks are inherently irreversible. But as my last sentence said, as the mach number goes to infinity the density increase across the shock is finite. Therefore, compression cannot be…
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 density reaches a limit, the change in pressure over the shock does not, so you can pump in energy arbitrarily (of course your gas would dissociate at some point). The total pressure l…
"Compression: decrease in volume of any object or substance resulting from applied stress."
Compression refers to a reduction in volume, not an increase in pressure. This is finite for gas crossing a shock, even as the mach number goes to infinity.
Earlier quoted context omitted.
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?
Compression due to a shock is not isentropic. Parent is incorrect. This is basic gasdynamics. An accessible reference: Modern Compressible flow, by Anderson.
Earlier quoted context omitted.
It’s amazing a man would work for a masters degree and end up teaching mainly below average people at a public institution. What’s a masters degree worth?
Good lord this comment is condescending. Some people are teachers because they want to be teachers, period. They find fulfillment in it. That’s not even touching the unnecessary dig at public institutions.
Why go to the trouble of getting a masters to waste it like this?
FWIW I don’t believe teachers should need a bachelors. A 2-year trade school should suffice.
Earlier quoted context omitted.
> Teachers in public schools work part time Typically full-time and usually year round, though some work seasonally for about 3/4 year (but still full-time), with commensurately reduced annual pay.
You are simply full of it. Public school teachers are booked for around 180 days of teaching + admin work. I read my tax bill. This is like 36-38 (38 being generous) weeks of work per year. It’s 3 months off.
Except your calculations support exactly what I said.
> It’s 3 months off.
Yes, it's typically 3 months off if they don't work summer session in a traditional schedule.
Which most who are able to (there are reduced available positions) do.
Hence, the full-time but, for some, 3/4-year seasonal full-time that I stated.
Earlier quoted context omitted.
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?
Because compression, as normally considered, does not generate entropy. It causes gas to become hotter because the temperature has to be higher for entropy to remain constant, as the volume is reduced. This is a reversible process. But shocks are inherently irreversible. But as my last sentence said, as the mach number goes to infinity the density increase across the shock is finite. Therefore, compression cannot be…