The influence of higher harmonics on vortex pairing in an axisymmetric mixing layer

Strong forcing was used to produce vortex pairing in a submerged axisymmetric water jet. Phase-averaged hot-wire measurements were combined with phase-averaged flow visualization to identify the relevant nonlinear interactions. The leading resonant interaction was not a subharmonic resonance. Instead, it was a triad resonance involving the subharmonic, the fundamental and the 3/2 harmonic. The profound influence of higher harmonics on the amplification of the fundamental and subharmonic was demonstrated in a systematic way by successive truncation of the Fourier series representation of the excitation waveform.