## Elements of Radiating Gas Dynamics

As early as in Section 1.1, we noted that manifestations of radiation effects in hightemperature gas dynamic flows are widely diversified. Thus, under the high-altitude flight conditions at velocities U∞ ≤ 10 km/sec the effect of the energy loss for radiation on the main shock layer flow parameters is negligible. However, radiation can turn out to be important from the standpoint of detection of flying objects and in view of its influence on the kinetics of physicochemical processes and electron concentration. At the same time, at high flight velocities, the energy loss due to radiation andvariation of

themain thermodynamic parameters in shock layers can be appreciable and, hence, should be taken into account for spacecraft entering in the atmospheres of different planets (thus, the vehicle velocity at the entry into the Jovian atmosphere can be as high as 50 km/sec). Though the radiative gas dynamics is a wide division of the gas dynamics as a whole,

below we will restrict ourselves only to the presentation of its general model and some examples illustrating the previously mentioned effects.∗

The hot gas radiation is due to the capability of excited molecules or atoms to return spontaneously onto a lower excitation level due to the release of an energy quantum, or photoquenching, for example, at the transition from an n-th electronic level to a lower, m-th level. The reverse photoexcitation effect consists in the excitation of these particles due to the absorption of external light quanta. Since the energetic states between excited levels are discrete, the frequency νnm of the radiated or absorbed quanta, or photons, is strictly determined by the condition hνnm = εn − εm, where h = 6.26 · 10−34 J·sec is the Planck constant, while εn and εm are the level energies. This circumstance generates the well-known line spectrum of atomic gas radiation. More complicated is the molecular radiation mechanism. Each molecule excited on an

n-th electronic level at energy εn possesses also its own vibrational mode with the set of energetic levels εk. Therefore, the energy that can be absorbed or released by a molecule at transition from the n-th electronic level with the k-th vibrational level to them-th electronic level with the l-th vibrational level is equal to

εij = εi − εj = hνij = hνnm + hνkl hνnm = εn − εm, hνkl = εk − εl = εkl (14.1.1)

The n-th and k-th levels taken together will be called level i = n+ k, while the m-th and l-th levels will be called level j = m+ l; the transition itself is called the i − j transition.