If the flow is still subsonic downstream of the area change, the characteristics starting there will point upstream and will reach the area change.
Miles (1950) studied the matter of the ' compressibility correction' for wings oscillating sinusoidally in subsonic flow.
However, this condition is not exclusively a supersonic one, since it can also be met if the ion flows are individually subsonic.
The associated variation in the (subsonic) proton speed also corresponds to a compression (up < 1), while the (subsonic) electrons are accelerated.
The grey zone highlights the supersonic regime, the white zone indicates the subsonic region.
Based on all the above data it appears that only the axisymmetric instability and acoustic wave modes are allowed in subsonic resonant impinging jets.
The subsonic case (figure 12a) is asymmetric, indicating possible pressure communication across the potential core.
Now, close to the stagnation point the speeds are subsonic, and all compressibility effects are due to thermal variations in the fluid properties.
