Critical conditions for the group speed of propagation of internal gravity waves

Four experimentally observed unstable and resonant regimes of generation of internal waves by a moving or oscillating cylinder are considered. Two of them can be treated as a manifestation of the critical-layer effect, but for the group rather than for the phase speed of propagation of small perturbations, one regime can be regarded as a manifestation of the effect of compaction of the energy of two waves, and one more regime admits both of the indicated treatments.