Hypersonic speed

NASA X-43 at Mach 7

Simulation of hypersonic speed (Mach 5)

In aerodynamics, a hypersonic speed is one that is highly supersonic (even though the origin of the words is the same: "super" is the Latin cognate of the Greek "hyper"). 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

Physical phenomena that cannot be so analytically discounted as they are in supersonic flow may characterize hypersonic flow's definition, which can be vague and is generally debatable (especially because supersonic and hypersonic flows overlap). The peculiarities 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

The density behind the shock that a body generates increases with its Mach number because conservation of mass demands a corresponding to a decrease in volume behind the shock wave and its distance to the body.

Entropy layer

Increasing Mach numbers increases the entropy change across the shock, creating a strong entropy gradient and highly vortical flow that mixes with the boundary layer.

Viscous interaction

Viscosity causes some of the great kinetic energy associated highly supersonic flow to become internal energy in the fluid, thereby increasing its temperature. The pressure gradient normal to the flow within a boundary layer is almost zero for low to moderate hypersonic Mach numbers, increasing the boundary layer's temperature and decreasing the layer's density and thus so expanding the layer's bottom that the boundary layer thickens and often merges with the leading edge's shock wave.

High temperature flow

High temperatures cause such non-equilibrial chemical flow properties as vibrational excitation, dissociation, and ionization of molecules, causing convective and radiative heat-flux.

Classification of Mach regimes

Aerodynamicists use "subsonic" and "supersonic" to refer to Mach number ranges 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 does not.

The "supersonic regime" usually refers to the set of Mach numbers for which linearised theory may be used; e.g., the (air) flow is not chemically reacting, and heat-transfer between air and vehicle may be reasonably neglected.

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 space planes.

In the following table, the "regimes" or "ranges of Mach values" are referred to.

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 (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 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.

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 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.

Dissociated gas

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.

Ionized gas

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 velocities around 10–12 km/s. Gases in this region are modeled as non-radiating plasmas.

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:

1. Optically thin: where the gas does not re-absorb radiation emitted from other parts of the gas
2. Optically thick: where the radiation must be considered as 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.

See also

• Supersonic transport
• Lifting body
• Atmospheric entry

Hypersonic aircraft

• DARPA Falcon Project
• Reaction Engines Skylon, Reaction Engines A2 (design studies)
• HyperSoar (concept)
• Waverider
• X-20 Dyna-Soar, Rockwell X-30 (cancelled)
• Avatar RLV
• Ayaks
• WU-14

Engines

• Rocket engine
• Scramjet
• Reaction Engines SABRE, LAPCAT (design studies)

Missiles

• Shaurya (missile) Hybrid Cruise Missile (Entered Production)
• BrahMos-II Cruise Missile (Under Development)

Other flow regimes

• Subsonic flight
• Transonic
• Supersonic

References

External links

Wikimedia Commons has media related to Hypersonics.

• NASA's Guide to Hypersonics
• Hypersonics Group at Imperial College
• University of Queensland Centre for Hypersonics
• High Speed Flow Group at University of New South Wales