What is it about?
A fluid in motion exerts extra pressure on obstacles, called ram pressure. Not all of the momentum of the fluid is converted into ram pressure, because the fluid's mass does not all come to a dead stop in front of the obstacle. The ratio of ram pressure to the fluid's momentum depends on how fast the fluid is going, as characterized by its Mach number, the ratio of the flow speed to the speed of sound. Engineers have been measuring ram pressure in rivers, wind tunnels, and airplanes starting in 1732, with Henri Pitot's invention of an inexpensive device with no moving parts, now called the pitot-static tube. Unfortunately, in the early 1970s, astronomers began assuming all of a fluid's momentum is converted into ram pressure. This article reviews the correct formulae for subsonic and supersonic flow.
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Why is it important?
Astronomers have been widely making two mistakes regarding ram pressure since the 1970s: i) a systematic overestimation of its strength, and ii) a fundamental violation of conservation of mass in their derivation. Both issues are straightforward to correct.
Perspectives
The one-dimensional mass fallacy discussed in this article is a fundamental violation of conservation of mass, to the same degree that a patent application for a perpetual motion machine is a fundamental violation of the first and/or second law of thermodynamics. Fortunately, in this case, the ensuing errors have traditionally been tolerable in most astronomical applications.
Professor Timothy E. Dowling
University of Louisville
Read the Original
This page is a summary of: Ram pressure in astronomy and engineering, Proceedings of the Royal Society A Mathematical Physical and Engineering Sciences, February 2023, Royal Society Publishing,
DOI: 10.1098/rspa.2022.0504.
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