No abstract

An underwater force sensor for internal waves is presented. Using this sensor, we measure forces at a location near the surface of a fluid. The sensor performs a point measurement with a high temporal resolution. We perform measurements in a density-stratified fluid contained in a trapezoidal basin. By shaking this basin longitudinally, internal gravity waves are generated. Controlling the frequency with which the basin oscillates, these waves propagate toward a wave attractor whose shape varies from complicated to rectangular. We measure the force exerted by these waves on a plate that is partially submerged into the fluid. We observe the formation and decay of wave attractors. When a surface reflection of a wave attractor is near our sensor we measure (relatively) strong forces. We confirm our findings with simulations. We observe an asymmetry in the direction of the force. This asymmetry leads to a net force and could imply the driving of a mean flow. Graphical Abstract

A new optical method is applied for the quantitative measurement of the density field of two-dimensional, stratified or homogeneous, transparent fluids in a laboratory set-up. A crucial new step is the angle at which we place a camera to view the experimental set-up. This step is motivated by a fallacy observed when applying our method in a classical configuration, in which the camera's optical axis is perpendicular to the flat surface of a fluid container. Application of this method is illustrated by the optical determination of static density fields of linearly and nonlinearly stratified fluids, as well as of multi-layered fluids. The method is validated by comparing with density profiles obtained from probe measurements of conductivity and temperature. Our method yields similar density and buoyancy profiles as the probe while also providing a whole field measurement, without disturbing the fluid and allowing the determination of dynamical density fields.