**Titre/Title :**

Viscous effects in gravity currents and internal wave beams

**Contact :**

Chantal Staquet

**Résumé/Abstract :**

The subject of the talk is related with experimental studies on the influence of viscous effects on propagation of gravity currents (lock-exchange problem) and radiation of internal waves by rectilinear and circular oscillations of a circular cylinder in a uniformly stratified fluid.

In the first part of the presentation we discuss the influence of viscous dissipation on the propagation speed of gravitycurrents at moderate values of a gravity Reynolds number. Two cases are considered: gravity currents propagating along a rigid boundary and intrusive gravity currents. For gravity currents, a semi-empirical formula for the front propagation speed derived from simple energy arguments is shown to capture well the effect of flow deceleration because of viscous dissipation. For intrusions, the propagation speed is shown to agree with the one predicted for energy-conserving virtually inviscid flows, which implies that the losses due to vorticity generation and mixing at the liquid–liquid interface play only a minor role in the total balance of energy.

In the second part of the presentation we discuss the experimental results for internal-wave radiation by large-amplitude circular and

rectilinear oscillations of a circular cylinder. Synthetic schlierentechnique is used for quantitative analysis of the internal-wave parameters. It is shown that at small oscillation amplitudes the wave pattern observed for circularoscillations is in good agreement with linear theory: internal waves are radiated in the wave beams passing through the first and third quadrants of a Cartesian coordinate system for clockwise direction of the cylinder motion, and the intensity of these waves is twice the intensity measured for ’St.Andrew cross’ waves generated by purely horizontal or vertical oscillations of the same frequency and amplitude. As the amplitude of circular oscillations increases, significant non-linear effects are observed: i) a strong density-gradient ’zero-frequency’ disturbance is generated, and ii) a region of intense fluid stirring is formed around the cylinder serving as an additional wave-making and dissipative mechanism that essentially changes the shape of wave envelopes and the intensity of wave motions. Analysis of spatiotemporal images reveals essentially different scenarios of transient effects in the cases of circular and rectilinear

oscillations.