Nous présentons un nouveau type de batteur à houle générant des ondes à contrecourant imaginé à l'Institut Pprime de Poitiers sur la base d'une idée de Jean-Marc Mougenot. Ce générateur de vagues est composé d'un double seuil : le premier permettant de réguler la hauteur d'eau moyenne, le second, muni d'un moteur linéaire, de générer les ondes. Nous utilisons un canal hydraulique équipé de sondes acoustiques permettant des mesures locales et non-intrusives de la hauteur d'eau. Dans ce travail, nous explorons différentes configurations afin de vérifier la linéarité des ondes sinusoïdales générées. Nous regardons l'effet du courant sur les ondes générées en faisant varier le débit et la hauteur d'eau asymptotique, et nous traçons des fonctions de transfert présentant le rapport entre l'amplitude de la houle et l'amplitude de déplacement mécanique, en fonction de la fréquence du batteur.
We report on observations made on a run of transcritical flows over an obstacle in a narrow channel. Downstream from the obstacle, the flows decelerate from supercritical to subcritical, typically with an undulation on the subcritical side (known in hydrodynamics as an undular hydraulic jump). In the Analogue Gravity context, this transition corresponds to a white-hole horizon. Free surface deformations are analyzed, mainly via the two-point correlation function which shows the presence of a checkerboard pattern in the vicinity of the undulation. In non-gated flows where the white-hole horizon occurs far downstream from the obstacle, this checkerboard pattern is shown to be due to low-frequency fluctuations associated with slow longitudinal movement of the undulation. It can thus be considered as an artifact due to a time-varying background. In gated flows, however, the undulation is typically "attached" to the obstacle, and the fluctuations associated with its movement are strongly suppressed. In this case, the observed correlation pattern is likely due to a stochastic ensemble of surface waves, scattering on a background that is essentially stationary.
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