Wikipedia:Reference desk/Archives/Science/2017 December 13

= December 13 =

Are these oranges there in the background?
Are these oranges there in the background?

Thanks, ClinicalCosmologist (talk) 10:35, 13 December 2017 (UTC)
 * It's hard to be sure without knowing the scale, but the bush looks more like a kind of Cotoneaster to me. AndrewWTaylor (talk) 11:40, 13 December 2017 (UTC)
 * A little searching found that image on the seller's website, where it it labelled as being a 17 inch / 43 cm statue. A bit of measuring suggests that those fruit are therefore about 1 cm in diameter: too small to be oranges. The colour doesn't look right either: I am inclined to agree with Cotoneaster. Wymspen (talk) 22:19, 13 December 2017 (UTC)
 * Oranges grow on trees and are typically a few inches in diameter. Assuming the step there is about 4-6 inches in height, the fruit is obviously too small for orange - maybe a quarter of an inch at most. I think AndrewWTaylor has it right. Matt Deres (talk) 13:45, 13 December 2017 (UTC)


 * Yes, when I looked much more closely these didn't seem like mini-oranges even ("dwarfy oranges" as used for jams ETC), but rather like a very special kind of Cotoneaster as noted by Andrew Taylor. Thanks guys!


 * To me it looks like Cotoneaster horizontalis, a fairly common garden shrub in the UK. Richard Avery (talk) 08:39, 14 December 2017 (UTC)

Thermodynamics of flame front propagation in a closed chamber (two-phase model)
I have the feeling that should exist somewhere in a textbook, so I am asking if someone has seen it before.

Assume a closed chamber at volume V containing a flammable homogeneous mixture of gases, which we ignite (e.g. by spark ignition). As the flame front propagates, the fraction of burnt gases increases, heat is generated by the combustion and temperature/pressure rises. Under reasonable combustion conditions, the pressure is at equilibrium between fresh and burnt gases, but the temperature is not (burnt gases are much hotter).

Experimentally speaking, accessing to instantaneous pressure inside the chamber is easy, but transient temperature is trickier to measure. I would think that under reasonable hypotheses (described below) the pressure signal is enough to solve a simple two-phase-at-pressure-equilibrium model and recover the full history, but I have not found a ref that does so yet. The trick is that during combustion, temperature increases, hence burnt gases expand and compress the fresh gases (which heats them) until the pressure is at equilibrium between the two phases. Hypotheses: You have of course access to all relevant constants (heat capacity, molar mass of the gases, lower calorific value for the fuel, etc.). I tried doing it by hand, but after an hour of paper-inking I had no success. Bonus point if the answer allows for a variable (but given) total volume V, but I think I can fiddle with a constant-V solution easily enough. Tigraan Click here to contact me 10:48, 13 December 2017 (UTC)
 * 1) No mass or heat transfer through the chamber walls.
 * 2) Two-phase model: all burnt gases are at the same temperature, all fresh gases are at the same temperature, flame front infinitely thin, combustion is locally instantaneous and complete (i.e. the flame front propagates with a finite speed, but as it passes a chunk of gas it immediately burns it).
 * 3) Each phase is an ideal gas (but the γ and average molar mass are not the same for the two phases)
 * 4) (optional) Assume the flame front is adiabatic, i.e., when a chunk of gas burns the heat this generates is averaged over all burnt gases (including itself) but not on fresh gases (those will still heat by compression though). (This is probably not strictly true, but while keeping with a two-phase model, it is more physical than assuming the heat is averaged over all gases (because then the burnt gases would not be at a temperature much different from the fresh ones', which is known to be false))
 * There is a massive amount of research on just this problem - it's the combustion chamber of an Otto cycle petrol engine. The transient pressure and temperature are difficult to measure - even more so to measure them across a spatially-distributed set of measurements - but it's such an important problem that serious effort has gone into doing just that. It resisted modelling for a long time, even when the computing power to do so became available, because the underlying processes weren't well enough understood - and of course, it's fluid dynamics and that's just hard.
 * Still the best primers I know on this are Harry Ricardo's series of books (they're not just editions, each edition is pretty much a re-write with the new understanding of that decade) The High-Speed Internal Combustion Engine. These go through the best understanding of the day, from pre-WWI to the 1960s, and cover the combustion chemistry, the instrumentation engineering, and of course the engine design as it developed through the 20th century. Sadly they're expensive for most editions (cost me a fortune to complete the set), although the last edition is a reasonably priced university textbook. The material in here is essential for anyone who really wants to understand 20th century engineering. Like some other books (Richard Rhodes' The Making of the Atomic Bomb is another) the strictly chronological treatment (across the editions) means that the development of knowledge is more clearly visible than in most textbooks presenting only the current best knowledge, and this can make it easier to comprehend, albeit lengthier to read.
 * As to the underlying dynamics of the situation, then that's too much to describe here. But there aren't shock waves (there can be, but they're avoided), multiple accidental ignition points ("hot spots") are avoided as they're uncontrollable for timing and most importantly, there's a significant energy transfer around by optical means. If this causes a pre-ignition ahead of the designed flame front, that's a bad thing and the cause of "knock" (WP has no article on knock, as it confuses several unrelated effects). Andy Dingley (talk) 12:45, 13 December 2017 (UTC)
 * I am aware that it is a difficult topic to handle seriously. I still wish to see the result of the simple, grad-student-level computation that I described above, in spite of its unrealistic assumptions (adiabatic walls, 0D-model with two homogeneous areas, no radiative transfer...) that make for a limited applicability to real-world scenarii. This seems simple enough that it has already been done, but hard enough that I will screw up performing the thermodynamics myself.  Tigraan Click here to contact me 17:08, 13 December 2017 (UTC)


 * We can directly lwrite down a system of ODEs here. The total energy $$E_1$$ of the unburned phase changes for two reasons: mass defects to the burned phase, taking energy with it; and the burned phase does work on the unburned phase.  (If we don't take your "optional" hypothesis, there is also some fraction of the burn energy that is applied to the unburned phase.)  So $$dE_1=-e_1dm_2+p\,dV_2=\frac{E_1}{f-1}df+p\,dV_2$$, where $$e_i:=E_i/m_i$$ and f is the (mass) burn fraction $$m_2/(m_1+m_2)$$.  Similarly, $$dE_2=(e_1+\varepsilon)dm_2-p\,dV_2=\left(\frac{E_1}{1-f}+m\varepsilon\right)df-p\,dV_2$$, where ε is the specific energy of combustion.  Finally, equating the EOS $$p=(\gamma-1)\rho e$$ for the two phases, we have $$V_1=\frac{(\gamma_1-1)E_1}{(\gamma_2-1)E_2}V_2$$; combining that with $$V=V_1+V_2$$ yields $$V_2=V^{-1}\left(1+\frac{(\gamma_1-1)E_1}{(\gamma_2-1)E_2}\right)$$.  Even assuming $$dV=0$$, however, $$dV_2$$ is a complicated nonlinear function of $$E_i$$ (albeit linear in their differentials).  Solving the combined system directly to obtain energies as a function of burn fraction (which could be inverted to obtain burn fraction and other properties from the observed pressure) seems unlikely, although the linear dependence on the derivatives suggests that a numerical solution should be reasonably straightforward.  --Tardis (talk) 07:33, 15 December 2017 (UTC)

Looking for youtube video that showed a cartoon train in scenarios near the speed of light?
I remember seeing a series of 5-10 videos that helped explain relativistic speeds of a train that showed how 2 observers would witness seemingly paradoxical behavior of the train (such as passing through a tunnel that was shorter than the train's length) but that to one observer, the train would appear longer than the tunnel. It then took this paradox to further extremes such as having 2 gates that would "close" for a millisecond while the train was inside the tunnel, but to one observer (but not the other) the gates would close at different times, even if they closed simultaneously due to relativistic effects. 67.233.34.199 (talk) 17:22, 13 December 2017 (UTC)
 * Note to anyone who would answer this question: do not link in your answer to a copyright violation. Tigraan Click here to contact me 17:27, 13 December 2017 (UTC)
 * If someone is looking for a youtube video, how would they watch it if there's no link posted? ←Baseball Bugs What's up, Doc? carrots→ 17:47, 13 December 2017 (UTC)
 * Presumably by not hyperlinking it, thus cleverly committing the crime of the century. Matt Deres (talk) 19:58, 13 December 2017 (UTC)
 * Or maybe by some clever ruse, such as inserting blank spaces into the URL. ←Baseball Bugs What's up, Doc? carrots→ 06:59, 14 December 2017 (UTC)
 * Not all Youtube videos are copyright violations, but for those who are, WMF Legal says we should not knowingly and intentionally [direct] others to a site that violates copyright. If you think that's an overbroad interpretation of statute/jurisprudence, call the WMF; if you think that's the US law that is stupid, call your congressman. Tigraan Click here to contact me 13:35, 15 December 2017 (UTC)
 * If you think anyone can answer a question about a youtube entry without providing a link to the youtube entry, call the Logic Police. ←Baseball Bugs What's up, Doc? carrots→ 13:42, 15 December 2017 (UTC)
 * The logic police just told me that "providing a link to [a] youtube entry" and "providing a link to a copyright violation" are not equivalent sentences, and neither implies the other. Tigraan Click here to contact me 16:42, 17 December 2017 (UTC)
 * So why did you bring it up? ←Baseball Bugs What's up, Doc? carrots→ 17:56, 17 December 2017 (UTC)


 * Ladder paradox is the name of the general idea. Sagittarian Milky Way (talk) 01:49, 14 December 2017 (UTC)
 * Or check out Time dilation and search for "time dilation" in youtube. --Kharon (talk) 03:03, 14 December 2017 (UTC)

Found it, thanks everyone
The "time dilation" + "train" keywords helped me find the video thanks! 67.233.34.199 (talk) 08:04, 14 December 2017 (UTC)

also no copyright intended (no copyright issue imo) but can someone tell me what "hatted" means? I read the term on Steve_Baker's talk page (an old friend) he got me introduced to bitcoin because he worked at a bitcoin company 18 months ago. 67.233.34.199 (talk) 08:05, 14 December 2017 (UTC)
 * See: "Hidden Archive Top" -- by coincidence, the next topic is "hatted". → [gone] 107.15.152.93 (talk) 09:10, 14 December 2017 (UTC) 2606:A000:4C0C:E200:2D25:CC2:ECEA:860B (talk) 03:57, 15 December 2017 (UTC)