Attractive internal wave patterns

This paper gives background information for the fluid dynamics video on internal wave motion in a trapezoidal tank.Comment: 2 pg, movie at two resolutions _low(Low-resolution) and _hr(High-resolution

Attractive internal wave patterns

This paper gives background information for the fluid dynamics video on internal wave motion in a trapezoidal tank.Comment: 2 pg, movie at two resolutions _low(Low-resolution) and _hr(High-resolution

Zero absolute vorticity plane Couette flow as an hydrodynamic representation of quantum energy states under perpendicular magnetic field

Here we extend the Madelung transformation of the Schr\"{o}dinger equation into a fluid-like form to include the influence of an external electromagnetic field on a charged particle. The vorticity of the Madelung fluid is then in the opposite direction to the imposed magnetic field and equal in magnitude to the cyclotron angular frequency. When the particle motion is confined to a plane, perpendicular to an imposed magnetic field, the equivalent flow dynamics is that of zero absolute vorticity obtained in a quasi 2D rotating frame, where the cyclotron frequency plays a role equivalent to that of the Coriolis frequency in a rotating frame. We show how the Landau levels and the extended modes in the integer quantum Hall effect are all mapped into such zero absolute vorticity-like plane Couette flows, where the latter exhibit a geostrophic-like balance between the magnetic force and the gradients of the quantum (Bohm) potential and the electric force.Comment: 17 pages, 3 figures; preprint version here; published in Physics of Fluid

Imminent reversal of the residual flow through the Marsdiep tidal inlet into the Dutch Wadden Sea based on multiyear ferry-borne acoustic Doppler current profiler (ADCP) observations

The Dutch Wadden Sea is a UN World Heritage Site connected to the North Sea by multiple tidal inlets. Although there are strong tidal currents flowing through these inlets, the magnitude and direction of the residual circulation in the western Dutch Wadden Sea is important for sediment, salinity and nutrient balances. We found that the direction of this residual flow is reversing. This residual circulation has been the subject of various studies since the 1970s, in which substantially different net volume fluxes were presented. Differences in tidal conditions in the main inlets, tidal rectification and meteorology were identified as driving mechanisms. Here we analysed almost 13 years of acoustic Doppler current profiler (ADCP) observations collected on the ferry crossing the Marsdiep tidal inlet in the Dutch Wadden Sea since 2009. The results are combined with earlier investigations covering the period 1998–2009. We find a significant trend in the magnitude of the residual volume flux, with decreasing export to the North Sea and with occasional imports observed in recent years. We hypothesise that this trend is related predominantly to changes in tides in the North Sea, which are caused by increased strength and duration of stratification in response to global warming. With warming projected to continue, we expect the residual flow in the Marsdiep to continue to reverse to full inflow within the current decade, with potential knock-on effects for the sediment budget and ecosystem of the western Wadden Sea.</p

Patchiness in internal tidal beams

Results are presented from measurements on internal tides and near-inertial motions, obtained using deep-towed acoustic Doppler current profilers along a single transect over the continental slope in the Bay of Biscay and, in another experiment, over a flank of Great Meteor Seamount in the Canary Basin. Each measurement lasted two days and involved repeated passage of the same track, making it possible to extract by harmonic analysis the semidiurnal (and, over Great Meteor Seamount, also the combined diurnal/near-inertial) signal. In the Bay of Biscay, the transect covered by the towing was sufficiently long to follow the internal semidiurnal tidal beam for large-scale stratification well beyond its detachment from the continental slope. Here, large-scale stratification is computed from CTD-observations over vertical scales O(100 m). Remarkably, the beam is much more distinct in its phase field, which is coherent throughout, than in its amplitude, which shows a lot of patchiness and small-scale near-horizontal layering. The hypothesis is put forward that this may be associated with the interaction between internal waves and variations in space and time of stratification. In part, it may be attributable to aliased, weaker, near-inertial beams that are more horizontal for large-scale stratification. The diurnal/inertial and, to a lesser extent, semidiurnal signals over Great Meteor Seamount show the same phenomenon, but here co-phase and coamplitude bands are more distinctly nearly horizontal, indicative of near-horizontal energy propagation at all frequencies investigated

Whole-field density measurements by digital image correlation

A novel application of Synthetic Schlieren in a laboratory set-up yields a quantitative measurement of the density field of two-dimensional, stratified or homogeneous, transparent fluids in a laboratory set-up using a single camera. This application obtains local values of the density without the need for tomographic reconstruction algorithms that require images taken from different directions through the fluid nor does the application require regularization. This is achieved by placing the camera at a large oblique angle with respect to the experimental set-up. This step is motivated by a fallacy observed when applying ray tracing in a classical configuration, in which the camera’s optical axis is perpendicular to the flat surface of a fluid container. The application is illustrated by the optical determination of static density fields of linearly and nonlinearly stratified fluids, as well as of multi-layered fluids. The application is validated by comparing with density profiles obtained from probe measurements of conductivity and temperature. Our application yields similar density and density gradient profiles as the probe while also providing a whole-field measurement without disturbing the fluid, and allowing the determination of dynamical density fields

Excitation of inertial modes in a closed grid turbulence experiment under rotation

We report an experimental study of the decay of grid-generated turbulence in a confined geometry submitted to a global rotation. Turbulence is generated by rapidly towing a grid in a parallelepipedic water tank. The velocity fields of a large number of independent decays are measured in a vertical plane parallel to the rotation axis using a corotating Particle Image Velocimetry system. We first show that, when a "simple" grid is used, a significant amount of the kinetic energy (typically 50%) is stored in a reproducible flow composed of resonant inertial modes. The spatial structure of those inertial modes, extracted by band-pass filtering, is found compatible with the numerical results of Maas [Fluid Dyn. Res. 33, 373 (2003)]. The possible coupling between these modes and turbulence suggests that turbulence cannot be considered as freely decaying in this configuration. Finally, we demonstrate that these inertial modes may be significantly reduced (down to 15% of the total energy) by adding a set of inner tanks attached to the grid. This suggests that it is possible to produce an effectively freely decaying rotating turbulence in a confined geometry

In situ measurement of wave attractor induced forces

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: [Figure not available: see fulltext.