|WikiProject Physics / Fluid Dynamics||(Rated Start-class, Low-importance)|
|WikiProject Aviation||(Rated C-class)|
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This article was automatically assessed because at least one WikiProject had rated the article as start, and the rating on other projects was brought up to start class. BetacommandBot 10:05, 10 November 2007 (UTC)
The article says that all flow around a black hole is transonic. This seems counterintuitive, so I looked into the cited sources and a few others by the same author. Most are written in more math than English, so it's very difficult for me to parse through, but from what I gather there are regions of transonic flow which occur between subsonic and supersonic flow, but the entire flow in the accretion disc is not transonic. According to Chakrabarti (Accretion and Winds around Galactic an Extragalactic Black Holes), "For black-hole accretion, the flow passes through the [event] horizon with the velocity of light, and therefore it must remain supersonic."
The first confusing thing I see is that we should explain how we can even define the speed of sound in the vacuum of space (where no one can hear you scream), and how that speed is very dependent upon temperature and, to a lesser degree, pressure. I kind of get it myself, but have yet to see anything that gives an actual speed. These are just defined as areas of shockwave formation. And, although the presence of a shockwave indicates a transition from sub to supersonic (or visa-versa), this does not take into account that both the object and the flow may be both moving well beyond supersonic speeds, relative to some third reference point, and the shockwave is only transonic relative to the two interacting with each other. (There's no way to determine the absolute speed of a celestial body, because there is no truly stationary reference-frame by which to compare it.) A clear example in English would help.
The flow in the disc is very complex too, apparently, as both angular velocity and radial velocity can be drastically different in different parts of the disc. Pressure can be very high in some areas and almost nonexistent in others (the closest thing possible to total vacuum) and viscous forces generate lots of heat through torque and shear (not to mention turbulence and convection). The inner part spins much faster than the outer part, radiating energy outwards (because matter can only enter the horizon after losing enough of its energy). This is as best as I can interpret it, but someone who really understands the math really should be the one the clarify this section and put it into layman's terms. If anyone feels up to the task it would be appreciated. Zaereth (talk) 00:57, 28 August 2018 (UTC)