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Glass viscosity calculations debunk the myth of flow in medieval windows (2017)

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Re: Glass viscosity calculations debunk the myth of flow in medieval windows (2017)

#151

Post by bernulli »

pfdietz wrote: post #148
ThenAsNow wrote: post #144

Earlier quoted context omitted.

The semantic error you're making is redefining the general term compression as the more specific "isentropic compression". The process by which the compression takes place is not prescribed just by the term "compression" as you did originally. Just saying "compression" specifies nothing about the process by which it happens. Compression can take place (theoretically) through an isentropic process, or a non-isentropic…

Consider the following extension of this thermodynamic process. The gas is shocked, and then we adiabatically expand it back to its original density. Because entropy was added, the gas is now hotter than initially. But it is not compressed. The heating wasn't due to compression, it was due to dissipation at the shock. That dissipative process is separate from compression, and you are confusing the two. The incoherenc…

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 compression is well understood, as is the temperature rise and total pressure loss through a shock.

Re: Glass viscosity calculations debunk the myth of flow in medieval windows (2017)

#152

Post by bernulli »

bollu wrote: post #114
pfdietz wrote: post #72

Earlier quoted context omitted.

I didn't say the compression was isothermal. 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 dissipativ…

This is very interesting, is there a textbook for this?

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 Gas Dynamics" by the same author.

Re: Glass viscosity calculations debunk the myth of flow in medieval windows (2017)

#153

Post by dang »

nemo44x wrote: post #57
bosswipe wrote: post #54

Earlier quoted context omitted.

This idea of a shadowy unified people that control everything is a poor way to try to make sense of the world.

“They” refers to the teachers in this context. Sorry to assume you spoke English as a first language.

If you continue to post personal attacks and name-calling like "You are full of it" to HN, we will ban you. We've already had to warn you about this.

https://news.ycombinator.com/newsguidelines.html

Re: Glass viscosity calculations debunk the myth of flow in medieval windows (2017)

#154

Post by dang »

chowned wrote: post #91
nemo44x wrote: post #57

Earlier quoted context omitted.

“They” refers to the teachers in this context. Sorry to assume you spoke English as a first language.

Who hurt you? Seriously, what's your problem?

We've banned this account for repeatedly breaking the site guidelines. Please don't create accounts to do that with.

If you don't want to be banned, you're welcome to email hn@ycombinator.com and give us reason to believe that you'll follow the rules in the future.

https://news.ycombinator.com/newsguidelines.html

Re: Glass viscosity calculations debunk the myth of flow in medieval windows (2017)

#155

Post by gattr »

Traster wrote: post #135
heikkilevanto wrote: post #113

Yeah, I learned this "fact" in school as well. Later I found a simple refutation: If a church window pane would deform by a measurable amount in 1000 years (say 0.5mm), then a 100-year old telescope or camera lens would be totally useless, and yet they seem to function all right.

I mean... the obvious answer to that is that we don't use the same techniques to manufacture a telescope or camera lens that we used to manufacture church windows. In fact we don't use glass for large telescopes specifically because they'll deform under their own weight. I'm not saying that original fact is correct, I'm just saying I don't think what you're saying disproves the idea that that glass could deform over…

That's incorrect. We do use glass for large mirrors and it works just fine ([1], [2.1], [2.2]). Do not confuse these two effects:

1. Viscous liquid-like creep (does not occur).

2. Elastic deformation (sagging) under gravity - this of course takes place, and is compensated for by multipoint support and, in larger professional telescopes, "active optics" (actuactors that counter the deformation depending on mirror's orientation; not to be confused with "adaptive optics", which deform a mirror with high frequency to counteract atmospheric image distortion).

[1] https://en.wikipedia.org/wiki/Hale_Telescope

[2.1] https://en.wikipedia.org/wiki/Large_Binocular_Telescope

[2.2] https://www.newswise.com/articles/steward-observatory-mirror...

Re: Glass viscosity calculations debunk the myth of flow in medieval windows (2017)

#156

Post by nemo44x »

dang wrote: post #153
nemo44x wrote: post #57

Earlier quoted context omitted.

“They” refers to the teachers in this context. Sorry to assume you spoke English as a first language.

If you continue to post personal attacks and name-calling like "You are full of it" to HN, we will ban you. We've already had to warn you about this. https://news.ycombinator.com/newsguidelines.html

Understood and I didn’t mean to go about it that way. Was meant in jest - but I understand that it was not good for discussion to insult a community member this way. Thank you for the warning.

Re: Glass viscosity calculations debunk the myth of flow in medieval windows (2017)

#157

Post by pfdietz »

bernulli wrote: post #151
pfdietz wrote: post #148

Earlier quoted context omitted.

Consider the following extension of this thermodynamic process. The gas is shocked, and then we adiabatically expand it back to its original density. Because entropy was added, the gas is now hotter than initially. But it is not compressed. The heating wasn't due to compression, it was due to dissipation at the shock. That dissipative process is separate from compression, and you are confusing the two. The incoherenc…

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 compression is well understood, as is the temperature rise and total pressure loss through a shock.

No, it is quite incoherent. Specifically, it fails to even offer what any physical theory must: a quantitative prediction. Compression, in your view, can be allowed to produce variable amounts of heating, for the same change in density. Since you aren't making a quantitative prediction, how is what you are saying even testable?

(The answer is that compression produces a specific amount of heating when it is adiabatic, and additional heating comes from dissipative processes that are occurring alongside the compression, but that are not themselves compression.)

Re: Glass viscosity calculations debunk the myth of flow in medieval windows (2017)

#158

Post by pfdietz »

bernulli wrote: post #150
pfdietz wrote: post #75

Earlier quoted context omitted.

I didn't say compression at a shock was isentropic. I was saying the claim that compression at the shock could not be the cause of (all) the heating, unless you simply define heating at a shock as being due to compression. Instead, I think you should separate out the heating there into two parts: one due to isentropic compression, and one due to injection of additional entropy by dissipation at the shock.

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 explain to me again how this is now 'like friction'?

Requiring compression to be isentropic is just requiring "compression" to be a well-defined term with a specific quantitative result. Otherwise, it's a vague term that offers no quantitative prediction for how much heating it produces for a given change in density. That's not science.

Re: Glass viscosity calculations debunk the myth of flow in medieval windows (2017)

#159

Post by pfdietz »

bernulli wrote: post #149
pfdietz wrote: post #76

Earlier quoted context omitted.

Let's look up a definition. "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.

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 dV approaches a constant, the pressure still rises, so the product also rises. [edit: removed snark]

Requiring compression to be isentropic is just requiring "compression" to be a well-defined term with a specific quantitative result. Otherwise, it's a vague term that offers no quantitative prediction for how much heating it produces for a given change in density. That's not science.

Re: Glass viscosity calculations debunk the myth of flow in medieval windows (2017)

#160

Post by pfdietz »

ThenAsNow wrote: post #80
pfdietz wrote: post #53

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…

Look, where does this idea come in that compression "as normally considered, does not generate entropy"? The inlet of a supersonic aircraft or a ramjets/scramjet performs non-isentropic (shock-based) compression. You're coming up with your own definition of compression that is inconsistent with gasdynamics practice.

Considering compression to be isentropic is just nailing down what compression means enough for its heating to be actually definable. Otherwise, the argument is just "if there's compression occurring, all the heating must be considered as being due to compression" which is just bullshit.
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