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A. del Val et al.
derived from other measurements and often based on the assumption of an isentropic
flow expansion. However, this is a severe assumption which is violated as soon as
non-equilibrium effects are present in the flow due to the irreversibilities introduced.
Non-equilibrium effects easily occur in hypersonic wind tunnels having stagnation
temperatures large enough to excite the vibrational modes of the gas species. A
dedicated free stream static pressure probe can be used to directly calibrate the free
stream conditions in the chamber.
Stagnation probes are set up to obtain measurements of the heat flux and pressure
that are mainly used to derive the total enthalpy of the free stream based on a
series of assumptions about the flow in the facility. Optical diagnostics have been
also developed for these facilities as a mean to obtain qualitative information about
different flow features of interest concerning transition on the boundary layer and
shock formation among others [22].
16.2.2.2 Shock Layer Radiation
In addition to dissociation and ionization, collisional excitation, which comprises
the phenomenon of excitation of atoms and molecules by means of collisional
energy, may lead to thermal radiation. Excited atoms and molecules can spontaneously emit photons and drop to lower energy states. The radiant energy can then
be absorbed by other atoms and molecules in the flowfield, causing particles to jump
to higher energy levels, or directly absorbed by the surface of the vehicle.
Emission and absorption from bound levels to other bound levels is referred to
as bound-bound radiation. The emission and absorption spectra of bound-bound
processes is highly oscillatory in nature due to the many transitions between discrete
internal energy levels of atoms and molecules. For sufficiently energetic atoms
and molecules, absorption of a photon may lead to dissociation or ionization.
In particular, the photo-dissociation of O 2 and photo-ionization of N and O are
common in air. These processes are called bound-free since the particles in question
begin the process bound to one another and are separate or free at the end. The
reverse processes are likewise termed free-bound. Finally, free electrons may also
contribute to radiation. As an electron passes through the electric field of another
charged particle, it may undergo a deceleration and emit a photon with energy
equal to the difference between the kinetic energy of the electron before and after
the collision. This process is known as Bremsstrahlung from the German words
bremsen for to brake and strahlung for radiation. Bremsstrahlung is also called freefree radiation, since transitions occur between two unbound electrons. Bound-free
and free-free processes exhibit a continuous spectrum, since the energy transitions
are not limited to discrete jumps. Figure 16.6 gives illustrations of the different
processes.
To experimentally assess the relative importance of these radiative processes,
hypersonic wind tunnels can use optical diagnostics to detect the emission or
absorption spectra in different parts of the flowfield, especially the post-shock
region. The data gathered through spectrometers is used to validate radiation models
A. del Val et al.
derived from other measurements and often based on the assumption of an isentropic
flow expansion. However, this is a severe assumption which is violated as soon as
non-equilibrium effects are present in the flow due to the irreversibilities introduced.
Non-equilibrium effects easily occur in hypersonic wind tunnels having stagnation
temperatures large enough to excite the vibrational modes of the gas species. A
dedicated free stream static pressure probe can be used to directly calibrate the free
stream conditions in the chamber.
Stagnation probes are set up to obtain measurements of the heat flux and pressure
that are mainly used to derive the total enthalpy of the free stream based on a
series of assumptions about the flow in the facility. Optical diagnostics have been
also developed for these facilities as a mean to obtain qualitative information about
different flow features of interest concerning transition on the boundary layer and
shock formation among others [22].
16.2.2.2 Shock Layer Radiation
In addition to dissociation and ionization, collisional excitation, which comprises
the phenomenon of excitation of atoms and molecules by means of collisional
energy, may lead to thermal radiation. Excited atoms and molecules can spontaneously emit photons and drop to lower energy states. The radiant energy can then
be absorbed by other atoms and molecules in the flowfield, causing particles to jump
to higher energy levels, or directly absorbed by the surface of the vehicle.
Emission and absorption from bound levels to other bound levels is referred to
as bound-bound radiation. The emission and absorption spectra of bound-bound
processes is highly oscillatory in nature due to the many transitions between discrete
internal energy levels of atoms and molecules. For sufficiently energetic atoms
and molecules, absorption of a photon may lead to dissociation or ionization.
In particular, the photo-dissociation of O 2 and photo-ionization of N and O are
common in air. These processes are called bound-free since the particles in question
begin the process bound to one another and are separate or free at the end. The
reverse processes are likewise termed free-bound. Finally, free electrons may also
contribute to radiation. As an electron passes through the electric field of another
charged particle, it may undergo a deceleration and emit a photon with energy
equal to the difference between the kinetic energy of the electron before and after
the collision. This process is known as Bremsstrahlung from the German words
bremsen for to brake and strahlung for radiation. Bremsstrahlung is also called freefree radiation, since transitions occur between two unbound electrons. Bound-free
and free-free processes exhibit a continuous spectrum, since the energy transitions
are not limited to discrete jumps. Figure 16.6 gives illustrations of the different
processes.
To experimentally assess the relative importance of these radiative processes,
hypersonic wind tunnels can use optical diagnostics to detect the emission or
absorption spectra in different parts of the flowfield, especially the post-shock
region. The data gathered through spectrometers is used to validate radiation models