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Ram Pressure


In physics generally, ram pressure is a pressure exerted on a body that is moving through a fluid medium. It causes a strong drag force to be exerted on the body. For incompressible flow it is given by:

$$P= \frac{1}{2} \rho v^2$$

where $$P$$ is the pressure, $$\rho$$ is the density of the fluid and $$v$$ the relative velocity between the body and the fluid; this is the stagnation pressure without static pressure.

Galactic ram pressure stripping
Within astronomy and astrophysics, Gunn & Gott first suggested that galaxies in a galaxy cluster moving through a hot intracluster medium would experience a pressure of

$$P_r \approx \rho_e v^2$$

where $$P_r$$ is the ram pressure, $$\rho_e$$ the intracluster gas density, and $$v$$ the speed of the galaxy relative to the medium. This pressure can strip gas out of the galaxy where, essentially, the gas is gravitationally bound to the galaxy less strongly than the force from the intracluster medium 'wind' due to the ram pressure. Evidence of this ram pressure stripping can be seen in the image of NGC 4402.

Ram pressure stripping is thought to have profound effects on the evolution of galaxies. As galaxies fall toward the center of a cluster more and more of their gas is stripped out, including the cool, denser gas that is the source of continued star formation. Spiral galaxies that have fallen at least to the core of both the Virgo and Coma clusters have had their gas (neutral hydrogen) depleted in this way and simulations suggest that this process can happen relatively quickly, with 100% depletion occurring in 100 million years to a more gradual few billion years.

Recent radio observation of carbon monoxide (CO) emission from three galaxies (NGC 4330, NGC 4402, and NGC 4522) in the Virgo cluster point to the molecular gas not being stripped but instead being compressed by the ram pressure. Increased Hα emission, a sign of star formation, corresponds to the compressed CO region, suggesting that star formation may be accelerated, at least temporarily, while ram pressure stripping of neutral hydrogen is ongoing.

Ram pressure and atmospheric (re)entry
A meteoroid traveling supersonically through Earth's atmosphere produces a shock wave generated by the extremely rapid compression of air in front of the meteoroid. It is primarily this ram pressure (rather than friction) that heats the air that in turn heats the meteoroid as it flows around it. Harry Julian Allen and Alfred J. Eggers of NACA used an insight about ram pressure to propose the blunt-body concept: a large, blunt body entering the atmosphere creates a boundary layer of compressed air which serves as a buffer between the body surface and the compression-heated air. In other words, kinetic energy is converted into heated air via ram pressure, and that heated air is quickly moved away from object surface with minimal physical interaction, and hence minimal heating of the body. This was counter-intuitive at the time, when sharp, streamlined profiles were assumed to be better. This blunt-body concept was used in e.g. Apollo-era capsules.