Experimental study of parametric subharmonic instability for internal plane waves

Baptiste Bourget (1) , Thierry Dauxois (1) , Sylvain Joubaud (1) , Philippe Odier (1)

1 Phys-ENS - Laboratoire de Physique de l'ENS Lyon

Internal waves are believed to be of primary importance as they affect ocean mixing and energy transport. Several processes can lead to the breaking of internal waves and they usually involve non linear interactions between waves. In this work, we study experimentally the parametric subharmonic instability (PSI), which provides an efficient mechanism to transfer energy from large to smaller scales. It corresponds to the destabilization of a primary plane wave and the spontaneous emission of two secondary waves, of lower frequencies and different wave vectors. Using a time-frequency analysis , we observe the time evolution of the secondary waves, thus measuring the growth rate of the instability. In addition, a Hilbert transform method allows us to measure the different wave vectors. We compare these measurements with theoretical predictions, and study the dependence of the instability with primary wave frequency and amplitude , revealing a possible effect of the confinement due to the finite size of the beam, on the selection of the unstable mode.

Domaines: Dynamique des Fluides [physics.flu-dyn]; Mécanique des fluides [physics.class-ph]; Océan, Atmosphère; Mécanique statistique [cond-mat.stat-mech]

Baptiste Bourget, Thierry Dauxois, Sylvain Joubaud, Philippe Odier. Experimental study of parametric subharmonic instability for internal plane waves. Journal of Fluid Mechanics Digital Archive, 2013, 723, pp.1-20. ⟨10.1017/jfm.2013.78⟩. ⟨hal-01139422⟩