This galaxy is moving in the upper-left direction through a dense medium. Shown is a composite of visible (Hubble Space Telescope) and X-ray (Chandra X-ray Observatory) observations. The jellyfish morphology of Galaxy ESO 137-001 provides a striking example of ram pressure stripping. In extragalactic astronomy, ram pressure stripping explains several morphological details observed when galaxies and the intergalactic medium are in relative motion, as illustrated in figure 1.įigure 1. In space physics, the standoff distance of a planetary magnetic field in a stellar wind is typically modelled as a balance between fluid stagnation pressure and magnetic pressure, with stagnation pressure equal to the sum of thermal (static) pressure and ram pressure. In engineering, ram pressure is often called dynamic pressure or impact pressure depending on whether the flow is incompressible or compressible. The concept is ubiquitous across all science and engineering fields that deal with fluid dynamics. Ram pressure, P r, is the difference between the pressure in a fluid at a downstream stagnation point and the lower pressure upstream where the flow is characterized by a Mach number, Ma > 0, where Ma is the local ratio of flow speed to sound speed. This article seeks to raise awareness of this issue, and to review the determination of Sp for subsonic and supersonic flow. Unfortunately, by the early 1970s, in astronomy ram pressure was defined to be the momentum flux and Sp was fixed to be unity and forgotten as a parameter. This relationship may be expressed as P r = Sp ρ u 2, where Sp is the dimensionless Spreiter number, which ranges between 0.5 and 0.88 for a monatomic gas, depending on the upstream Mach number, Ma.
Up through the mid-1960s, across both astronomy and engineering the ram pressure of a moving gas and its momentum flux, ρ u 2, where ρ and u are the upstream mass density and flow speed, were properly treated as related but distinct quantities. On aeroplanes and in wind tunnels, it is measured with a pitot-static tube, an inexpensive device with no moving parts that was invented in 1732. In astronomy, it is used to calculate the interaction of stellar winds with planets and to quantify the effects of ram pressure stripping. The ram pressure of a moving fluid, P r, is the rise in pressure at a stagnation point relative to the upstream pressure.
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