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. Lavrent'ev Institute of Hydrodynamics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 41, No. 4, pp. 12–20, July–August, 2000.