Editing Hypersonic speed (section)

==Characteristics of flow==
[[File:Dopplereffectsourcemovingrightatmach5.0.gif|thumb|Simulation of hypersonic speed (Mach 5)]]
While the definition of hypersonic flow can be quite vague and is generally debatable (especially because of the absence of discontinuity between supersonic and hypersonic flows), a hypersonic flow may be characterized by certain physical phenomena that can no longer be analytically discounted as in supersonic flow.{{cn|date=October 2014}} The peculiarities in hypersonic flows are as follows:{{cn|date=October 2014}}
# Shock layer
# [[Aerodynamic heating]]
# Entropy layer
# Real gas effects
# Low density effects
# Independence of aerodynamic coefficients with Mach number.

===Small shock stand-off distance===
As a body's Mach number increases, the density behind a [[Shock wave#Bow shocks|bow shock]] generated by the body also increases, which corresponds to a decrease in volume behind the shock due to [[conservation of mass]]. Consequently, the distance between the bow shock and the body decreases at higher Mach numbers.<ref>{{Cite journal |last=Shang |first=J. S. |date=2001-01-01 |title=Recent research in magneto-aerodynamics |url=https://www.sciencedirect.com/science/article/pii/S0376042100000154 |journal=Progress in Aerospace Sciences |volume=37 |issue=1 |pages=1–20 |doi=10.1016/S0376-0421(00)00015-4 |bibcode=2001PrAeS..37....1S |issn=0376-0421}}</ref>

===Entropy layer===
As Mach numbers increase, the [[entropy]] change across the shock also increases, which results in a strong [[entropy gradient]] and highly [[vortical]] flow that mixes with the [[boundary layer]].

===Viscous interaction===
A portion of the large [[kinetic energy]] associated with flow at high Mach numbers transforms into [[internal energy]] in the fluid due to viscous effects. The increase in internal energy is realized as an increase in temperature. Since the pressure gradient normal to the flow within a boundary layer is approximately zero for low to moderate hypersonic Mach numbers, the increase of temperature through the boundary layer coincides with a decrease in density. This causes the bottom of the boundary layer to expand, so that the boundary layer over the body grows thicker and can often merge with the shock wave near the body leading edge.{{cn|date=October 2014}}

===High-temperature flow===
High temperatures due to a manifestation of viscous dissipation cause non-equilibrium chemical flow properties such as vibrational excitation and [[dissociation (chemistry)|dissociation]] and [[ionization]] of molecules resulting in [[convection|convective]] and [[Atmospheric reentry#Real (non-equilibrium) gas model|radiative heat-flux]].{{cn|date=October 2014}}