Wikipedia:Reference desk/Archives/Science/2022 September 26

= September 26 =

If an airliner windshield disintegrates at cruise
There'd be an approximately Mach 0.85 wind going backwards and a wind from the ~6-8 kilofoot cabin going forwards. What would the initial net wind velocity on the pilots' seats be? What's the Reynolds number of an airline cockpit who's windshield suddenly disappeared? Will it be more like neat streamlines bending around obstacles like the seats or full of eddies? Will there be supersonic shock waves? Sagittarian Milky Way (talk) 13:14, 26 September 2022 (UTC)
 * See chicken gun. Not exactly what you are looking for but there may be some leads. 41.23.55.195 (talk) 13:26, 26 September 2022 (UTC)
 * See British Airways Flight 5390 for an example of uncontrolled decompression. 41.23.55.195 (talk) 13:44, 26 September 2022 (UTC)


 * I'm going to regret asking this, but what on earth is a "6-8 kilofoot cabin"? Yet another obscure imperial/customary unit? Obviously not a cabin 6000 foot long.... (The metric prefix with imperial unit is hurting my head...) Fgf10 (talk) 15:02, 26 September 2022 (UTC)
 * A measure of altitude it seems, so I translate that as "a cabin at an altitude of 6,000–8,000 ft". Mikenorton (talk) 15:59, 26 September 2022 (UTC)
 * Cabin altitude. Sagittarian Milky Way (talk) 17:28, 26 September 2022 (UTC)


 * Under the assumption that the airliner has a velocity of 0.85 relative to the air (not in addition to any high-altitude wind), there should be no supersonic shock waves. Almost no commercial aircraft ever went supersonic.
 * For the Reynolds number of the air flow within the open cockpit, one would need to look up air density and temperature at that altitude, then find out what the dynamic viscosity of air is under those conditions... or just find out that the NASA website has an applet to play with and hope it’s accurate. Obviously I followed in the footsteps of my ancestors onto the shoulders of giants (or some other metaphor that says laziness is the mother of efficiency) and "found" 6.106. "Much greater than 1" was never in doubt, the only question is whether we are in the territory of the drag crisis, to which the answer is a resounding "maybe".
 * It is certain that there will be all kinds of interesting turbulent vortices, because while "cockpit" is rather well-optimized to keep the streamlines smooth, "cockpit minus a part of it leaving sharp edges" definitely is not. Regarding the characteristics of those turbulent vortices... well depending on how much you want to know, it can be either "we have no idea and it’s still an area of active research" or "we have it all figured out since Kolmogorov".
 * Be sure to give a read to the very nice article at energy cascade. I make a habit of whining that our technical articles are impossible to understand (without attempting to fix them of course), but this time I’m going to give barnstars to the two main contributors. Tigraan Click here for my talk page ("private" contact) 16:59, 26 September 2022 (UTC)
 * Wouldn't an anemometer show fast outflow at first? In space Mach 1 decompression outflow is possible if I'm not mistaken, in the lower stratosphere I don't know. Transonic suggests supersonic shock waves as air is sped up trying to go around objects like the seats. I don't understand how air can move supersonically instead of immediately piling up into a shock wave, or how the shock waves of subsonic superjumbo jets are (I think?) inaudible from the ground when Concorde was half the wing area, twice the height and a third the air density. Sagittarian Milky Way (talk) 19:24, 26 September 2022 (UTC)
 * "A sonic boom is a sound associated with shock waves created when an object travels through the air faster than the speed of sound." Opening sentence from Sonic boom. Subsonic jets do not generate shock waves. Martin of Sheffield (talk) 19:56, 26 September 2022 (UTC)
 * I thought so but: File:Transonic flow patterns.svg. Transonic flow patterns.svg.]] Sagittarian Milky Way (talk) 20:59, 26 September 2022 (UTC)
 * I'm pretty sure that the designers ensure that these conditions do not occur. A shock wave is a serious structural issue, as well as causing serious problems with the regulators.  See Concorde for the sort of problems that a (percieved) sonic boom could cause. Martin of Sheffield (talk) 22:15, 26 September 2022 (UTC)
 * That must be it then, they use good design to get critical Mach very close to 1 (it must be over 0.92 on a 747-400 as that is its maximum operating Mach). Sagittarian Milky Way (talk) 23:24, 26 September 2022 (UTC)
 * Sorry that is far too generic a statement. The flow speed varies around an aircraft or duct, and you can get shock waves forming locally when the local speed of flow reaches m1. Typically this is seen at flow speeds >0.3m around tight radii. Greglocock (talk) 22:29, 26 September 2022 (UTC)
 * Our article on stagnation pressure gives a formula for stagnation pressure in a compressible flow. It tells us that, even assuming the broken window was facing straight to the front, the pressure from the air ramming into the cockpit will be no more than 60% above the outside pressure. At cruise altitude, the initial pressure of the cabin will be higher. So initially, the air will rush out. After a while there will just be a turbulent flow with equal amounts of air going in and out.
 * The idea behind supersonic shockwaves is that they go faster than the speed of sound relative to the air ahead of the shockwave, but slower than that relative to the air behind the shockwave (skipping some details on oblique shockwaves; this applies to all kinds of waves). In case of a transonic aircraft, the shockwaves can only exist along the backsides of areas where the flow speed exceeds the speed of sound, so the shockwaves don't propagate all the way to the ground. Supersonic aircraft with a subsonic groundspeed (the speed of sound decreases as you go up, or maybe it has a headwind) don't cause a sonic boom on the ground either. PiusImpavidus (talk) 09:30, 27 September 2022 (UTC)