Ram pressure

In the simple case when the relative velocity is normal to the surface, and momentum is fully transferred to the object, the ram pressure becomes The Eulerian form of the Cauchy momentum equation for a fluid is[1] for isotropic pressure

at a point is thus (using Einstein notation): Substituting the conservation of mass, expressed as this is equivalent to using the product rule and the Kronecker delta

is and the momentum per second it carries into the body is equal to the ram pressure term.

This discussion can be extended to 'drag' forces; if all matter incident upon a surface transfers all its momentum to the volume, this is equivalent (in terms of momentum transfer) to the matter entering the volume (the context above).

"[8] Ram pressure stripping is thought to have profound effects on the evolution of galaxies.

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[9] and simulations suggest that this process can happen relatively quickly, with 100% depletion occurring in 100 million years[10] to a more gradual few billion years.

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.

[12] More recently, it has been shown that ram pressure can also lead to the removal of gas in isolated dwarf galaxies that plunge through the cosmic web (the so-called cosmic web stripping process).

[13] Although the typical overdensity within the cosmic web is significantly lower than that found in the environment of galaxy clusters, the high relative speed between a dwarf and the cosmic web renders ram pressure efficient.

[16] Meteoroids enter Earth's atmosphere from outer space traveling at hypersonic speeds of at least 11 km/s (7 mi/s) and often much faster.

Despite moving through the rarified upper reaches of Earth's atmosphere the immense speed at which a meteor travels nevertheless rapidly compresses the air in its path, creating a shock wave.

This is not due to friction, rather it is simply a consequence of many molecules and atoms being made to occupy a smaller space than formerly.

Ram pressure and the very high temperatures it causes are the reasons few meteors make it all the way to the ground and most simply burn up or are ablated into tiny fragments.

Large meteoroids do not explode in the sense of chemical or nuclear explosives.

Rather, at a critical moment in its atmospheric entry the enormous ram pressure experienced by the leading face of the meteoroid converts the body's immense momentum into a force blowing it apart over a nearly instantaneous span of time.

This occurs when fine tendrils of superheated air force their way into cracks and faults in the leading face's surface.

Once this high pressure plasma gains entry to the meteoroid's interior it exerts tremendous force on the body's internal structure.

This occurs because the superheated air now exerts its force over a much larger surface area, as when the wind suddenly fills a sail.

This sudden rise in the force exerted on the meteoroid overwhelms the body's structural integrity and it begins to break up.

The breakup of the meteoroid yields an even larger total surface area for the superheated air to act upon and a cycle of amplification rapidly occurs.

[18] 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.