rises, Kn p approaches 1. Collisions begin to occur and the flow becomes disturbed
near the axis of symmetry of the system. A further increase leads to the formation of
shocks on either side of the plane of symmetry. Incipient penetration and mixing
occurs. As the density increases further such that Kn p < < 1, there is essentially no
mixing between the gas from the two plumes. They are separated from each other.
Furthermore, significant back flow to the inert surface between the jets occurs.
Gas dynamics simulations using Direct Simulation Monte Carlo show these
different penetration regimes extremely well. In Fig. 3.27, we can see examples of
the change in the interaction. Here the distance between the sources is constant but
the distance is referred to the mean free path as given in the caption. At the top, the
flow is rarefied whereas the bottom example has a much higher source strength with
a small mean free path. On the left, the temperature is shown colour-coded while the
Mach number within the flow is given by the contours. The degree of penetration can
be seen on the right. Here the number of molecules from the left hand source at each
position in the domain is colour-coded from 0 to 100%. One can see how the
interaction develops as the Knudsen penetration number decreases from top to
Fig. 3.26 The strength of the interaction between two plumes of gas is described via the Knudsen
penetration number. The different regimes are illustrated. From Marschall et al. (2020), modified
from Dankert and Koppenwallner (1984)
230
3 Gas Emissions Near the Nucleus
near the axis of symmetry of the system. A further increase leads to the formation of
shocks on either side of the plane of symmetry. Incipient penetration and mixing
occurs. As the density increases further such that Kn p < < 1, there is essentially no
mixing between the gas from the two plumes. They are separated from each other.
Furthermore, significant back flow to the inert surface between the jets occurs.
Gas dynamics simulations using Direct Simulation Monte Carlo show these
different penetration regimes extremely well. In Fig. 3.27, we can see examples of
the change in the interaction. Here the distance between the sources is constant but
the distance is referred to the mean free path as given in the caption. At the top, the
flow is rarefied whereas the bottom example has a much higher source strength with
a small mean free path. On the left, the temperature is shown colour-coded while the
Mach number within the flow is given by the contours. The degree of penetration can
be seen on the right. Here the number of molecules from the left hand source at each
position in the domain is colour-coded from 0 to 100%. One can see how the
interaction develops as the Knudsen penetration number decreases from top to
Fig. 3.26 The strength of the interaction between two plumes of gas is described via the Knudsen
penetration number. The different regimes are illustrated. From Marschall et al. (2020), modified
from Dankert and Koppenwallner (1984)
230
3 Gas Emissions Near the Nucleus