HYPERSONIC
SPEED
In aerodynamics, a hypersonic speed is one that is highly supersonic. Since the 1970s, the term
has generally been assumed to refer to speeds of Mach 5 and above.The
precise Mach number at which a craft can be said to be flying
at hypersonic speed varies, since individual physical changes in the
airflow (like molecular dissociation and ionization) occur at different
speeds; these effects collectively become important around Mach 5.
The hypersonic regime is often alternatively defined as speeds where
ramjets do not produce net thrust.
Characteristics
of flow
While the definition of hypersonic
flow can be quite vague and is generally debatable (especially due to the
lack 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. The
peculiarity in hypersonic flows are as follows:
1-Shock layer
2-Aerodynamic heating
3-Entropy layer
4-Real gas effects
5-Low density effects
6-Independence of aerodynamic coefficients with Mach number.
- Small shock stand-off distance
As a body's Mach number increases, the density behind the
shock generated by the body also increases, which corresponds to a
decrease in volume behind the shock wave due to conservation of
mass. Consequently, the distance between the shock and the body decreases
at higher Mach numbers.
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.
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.
High temperatures due to a manifestation of viscous dissipation causenon-equilibrium
chemical flow properties such as vibrational excitationand dissociation and
ionization of molecules resulting in convective and radiative heat-flux.
Classification of Mach regimes
Although "subsonic" and "supersonic" usually refer
to speeds below and above the local speed of sound respectively, aerodynamicists often
use these terms to refer to particular ranges of Mach values. This
occurs because a "transonic regime" exists around M=1 where
approximations of the Navier–Stokes equations used for subsonic design no
longer apply, partly because the flow locally exceeds M=1 even when the
freestream Mach number is below this value. The "supersonic
regime" usually refers to the set of Mach numbers for which
linearised theory may be used; for example, where the (air) flow is not
chemically reacting and where heat transfer between air and vehicle may be
reasonably neglected in calculations. Generally, NASA defines
"high" hypersonic as any Mach number from 10 to 25, and re-entry
speeds as anything greater than Mach 25. Among the aircraft operating in
this regime are the Space Shuttle and (theoretically) various developing
spaceplanes.
Similarity parameters
The categorization of airflow relies on a number of similarity
parameters, which allow the simplification of a nearly infinite number of test
cases into groups of similarity. For transonic and compressible flow, the
Mach and Reynolds numbers alone allow good categorization of many
flow cases.
Hypersonic flows, however, require other similarity parameters. First,
the analytic equations for the oblique shock angle become nearly
independent of Mach number at high (~>10) Mach numbers. Second, the
formation of strong shocks around aerodynamic bodies means that the
freestream Reynolds number is less useful as an estimate of the behavior
of the boundary layer over a body (although it is still important).
Finally, the increased temperature of hypersonic flows mean that real gas
effects become important. For this reason, research in hypersonics is
often referred to as aerothermodynamics, rather than
aerodynamics. The introduction of real gas effects means that more
variables are required to describe the full state of a gas. Whereas a
stationary gas can be described by three variables (pressure, temperature,
adiabatic index), and a moving gas by four (flow velocity), a hot gas in
chemical equilibrium also requires state equations for the chemical
components of the gas, and a gas in nonequilibrium solves those state
equations using time as an extra variable. This means that for a
nonequilibrium flow, something between 10 and 100 variables may be
required to describe the state of the gas at any given time. Additionally,
rarefied hypersonic flows (usually defined as those with a Knudsen number
above 0.1) do not follow the Navier-Stokes equations. Hypersonic
flows are typically categorized by their total energy, expressed as total
enthalpy (MJ/kg), total pressure (kPa-MPa), stagnation pressure (kPa-MPa),
stagnation temperature (K), or flow velocity (km/s). Wallace D. Hayes
developed a similarity parameter, similar to the Whitcomb area rule, which
allowed similar configurations to be compared.
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.
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 ramjets become inefficient, and the upper border
around Mach 10-12.
- 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.
In this regime, diatomic or polyatomic gases (the gases found in
most atmospheres) begin to dissociate as they come into contact with the
bow shock generated by the body. 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.
In this regime the 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 10–12 km/s. Gases in this region
are modeled as non-radiating plasmas.
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.
- Radiation-dominated regime
Above around 12 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:
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.
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