Determine whether higher-frequency wave-attractor signals are propagating waves

Determine whether the higher-frequency signals at nondimensional frequencies Ω₃ ≈ 0.85 and Ω₄ ≈ 1.0 observed by Scolan (2013) and Brouzet (2016b) in internal-wave-attractor experiments correspond to propagating internal waves.

Background

The paper compares its higher-frequency signals, generated through interactions involving the primary and secondary internal waves, with signals reported in earlier internal-wave-attractor experiments. Scolan (2013) and Brouzet (2016b) observed spectral peaks near Ω₃ = 0.85 and Ω₄ = 1.0, but their analyses focused primarily on the fundamental triad comprising the primary wave and its two secondary waves.

Because the earlier studies did not characterize the spatial structure or dispersion-relation properties of these additional spectral peaks, it remains unresolved whether they represent freely propagating internal waves or non-propagating interaction products. Resolving this issue would clarify whether the signals reported in wave-attractor geometries are physically analogous to the non-propagating higher-frequency disturbances identified in the present experiments.

References

However, as both \citet{Scolan2013} and \citet{Brouzet2016b} --- who used the same geometry and forcing frequency --- focused their analysis on the $\Omega_{0,1,2}$ triad, it remains unclear whether $\Omega_{3,4}$ corresponded to propagating waves.

— Experimental observations of sub-threshold triadic resonances in internal wave beams  (2609.26689 - Irvine et al., 22 Sep 2026) in Section 5.1, “Relationship to prior work on wave attractors and surface waves”