Editing Hypersonic speed (section)

==Regimes==
Hypersonic flow can be approximately separated into a number of regimes. The selection of these regimes is rough, due to the blurring of the boundaries where a particular effect can be found.{{cn|date=October 2014}}

===Perfect gas===
In this regime, the gas can be regarded as an [[ideal gas]]. Flow in this regime is still Mach number dependent. Simulations start to depend on the use of a constant-temperature wall, rather than the adiabatic wall typically used at lower speeds. The lower border of this region is around Mach 5, where [[ramjet]]s become inefficient, and the upper border around Mach 10–12.{{cn|date=October 2014}}

===Two-temperature ideal gas===
This is a subset of the perfect gas regime, where the gas can be considered chemically perfect, but the rotational and vibrational temperatures of the gas must be considered separately, leading to two temperature models. See particularly the modeling of supersonic nozzles, where vibrational freezing becomes important.{{cn|date=October 2014}}

===Dissociated gas===
In this regime, diatomic or polyatomic gases (the gases found in most atmospheres) begin to [[dissociation (chemistry)|dissociate]] as they come into contact with the [[Shock wave|bow shock]] generated by the body. [[catalysis|Surface catalysis]] plays a role in the calculation of surface heating, meaning that the type of surface material also has an effect on the flow. The lower border of this regime is where any component of a gas mixture first begins to dissociate in the stagnation point of a flow (which for nitrogen is around 2000 K). At the upper border of this regime, the effects of [[ionization]] start to have an effect on the flow.{{cn|date=October 2014}}

===Ionized gas===
In this regime the [[ionization|ionized]] electron population of the stagnated flow becomes significant, and the electrons must be modeled separately. Often the electron temperature is handled separately from the temperature of the remaining gas components. This region occurs for freestream flow velocities around 3–4&nbsp;km/s. Gases in this region are modeled as non-radiating [[Plasma (physics)|plasmas]].{{cn|date=October 2014}}

===Radiation-dominated regime===
Above around 12&nbsp;km/s, the heat transfer to a vehicle changes from being conductively dominated to radiatively dominated. The modeling of gases in this regime is split into two classes:{{cn|date=October 2014}}
#[[Optical depth|Optically thin]]: where the gas does not re-absorb radiation emitted from other parts of the gas
#Optically thick: where the radiation must be considered a separate source of energy.
The modeling of optically thick gases is extremely difficult, since, due to the calculation of the radiation at each point, the computation load theoretically expands exponentially as the number of points considered increases.