Swimming of a ludion in a stratified sea

We describe and model experimental results on the dynamics of a ‘ludion’ – a neutrally buoyant body – immersed in a layer of stably stratified salt water. By oscillating a piston inside a cylinder communicating with a narrow (in one of its horizontal dimensions) vessel containing the stably stratified layer of salt water, it is easy to periodically vary the hydrostatic pressure of the fluid. The ludion or Cartesian diver, initially positioned at its equilibrium height and free to move horizontally, can then oscillate vertically when forced by the pressure oscillations. Depending on the ratio of the forcing frequency to the Brunt–Väisälä frequency of the stratified fluid, the ludion can emit its own internal gravity waves that we measure by a classical particle image velocimetry technique. Our experimental results describe first the resonance of the vertical motions of the ludion when excited at different frequencies. A theoretical oscillator model is then derived taking into account added mass and added friction coefficients and its predictions are compared with the experimental data. Then, for the larger oscillation amplitudes, we observe and describe a bifurcation towards free horizontal motions. Although the internal gravity wave frequencies are affected by the Doppler shift induced by the horizontal displacement velocities, it seems that, contrary to surface waves associated with Couder walkers (Couder et al. Nature, vol. 437, 2005, p. 238) they are not the cause of the horizontal swimming. This does not, however, exclude possible interactions between the ludion and internal gravity waves and possible hydrodynamic quantum analogies to be explored in the future.

Numerical Investigating on Representativeness of Tracers in PIV Model Test of Dredged Slurry Treated by Vacuum Preloading

The vacuum preloading method is commonly adopted for improving the soft ground that the embankment of the railway line is laid on. The PIV (Particle Image Velocimetry) technique is a powerful tool in observing the formation of the soil column, a phenomenon that is unique to the dredged slurry when treated by vacuum preloading. However, it is not clear to what extent the motions of the slurry particles can be represented by the PIV tracers. In this paper, a mesoscopic model has been established by using the CFD-DEM method to reproduce the vacuum consolidation process of the slurry, in which the PVD (Prefabricated Vertical Drain) membrane, the slurry particles, and the tracers are described by the DEM, and the pore water is governed by the CFD method. Eight computational cases that can cover a broad range of material parameters governing the PIV model tests on the dredged slurry have been designed and studied by the established model. The representativeness of the PIV tracer is evaluated by comparing the statistic displacement of the tracer to that of the slurry particles. It is found that for the commonly used tracer, the carbon powder, can reliably represent the particle motions of the slurry since the difference in displacements of the tracer and the slurry particles is smaller than 6.5% if the diameter ratio between the tracer and the slurry particle is within 1.8.

Experimental and Numerical Study of Multiple Jets Impinging a Step Surface

Multiple jet impingement is a widely implemented convective process for enhancing heat transfer over target surfaces. Depending on the engineering application, the impinging plate can have different configurations. However, the increased complexity of the surface induces complicated thermal behaviors that must be analyzed. In that sense, this study consisted of the experimental and numerical analysis of multiple jets impinging on a step surface. A particle image velocimetry technique was applied to measure velocity fields, while a heat flux sensor was mounted on the surface to determine the heat transfer. Numerical simulations, for both flat and non-flat plates, were conducted in ANSYS FLUENT applying the SST k-ω model, and experimental results were used to validate the model. Three surface configurations were analyzed, flat, 1 D, and 2 D steps, and the results show an increase in the average Nusselt number compared with the flat plate, 9% and 20%, respectively. This increase was mainly due to the intensification of the flow turbulence induced by the step. Numerical results were in good agreement with the experiments, but the heat transfer was slightly underpredicted for the 2 D step case due to the difficulty of predicting with accuracy the velocity field near the step.

Pull-Out Mechanism of Horizontal and Inclined Plate Anchors in Normally Consolidated Clay

As most existing experimental studies on plate anchors were carried out in uniform clay, a centrifuge model study is presented in this paper to investigate the pull-out behaviour of plate anchors in normally consolidated clay, which is not uncommon in offshore seabed. Horizontal and inclined anchors with different embedment depths and aspect ratios (length to width) are considered. The soil movement pattern around the plate anchor is evaluated from high-resolution photographs taken during the tests employing the Particle Image Velocimetry technique. The separation mechanism at the plate-soil interface is hence identified. The significant contribution of suction towards the ultimate pull-out capacity of a plate anchor is quantified by monitoring the soil resistance and the pore pressure beneath the anchor base under undrained condition. By comparing the pull-out responses of horizontal and inclined anchors, the effect of anchor inclination on the anchor capacity and failure mechanism is evaluated.

Numerical and Experimental Study of Aerodynamic Performances of a Morphing Micro Air Vehicle

The present work is focused on the investigation of the aerodynamic performances of a novedous bioinspired morphing Micro Air Vehicle (MAV) with an adaptive wing structure geometry. For this purpose, a numerical study of Computational Fluid Dynamics (CFD) implemented by Ansys Fluent 15.0 was performed in order to obtain insight about the aerodynamic effect of wing structure deformation when morphing devices are used, and its influence on the global aerodynamic parameters related with aircraft performances. On the other hand, an experimental study using the Particle Image Velocimetry technique and balance measurements in a Low-Speed Wind Tunnel was conducted to obtain experimental information about performances measured to establish a comparison between both, experimental and numerical results. The Micro Air Vehicle (MAV) presents a Zimmerman wing with an Eppler 61 airfoil. Three different wing configurations according to curvature and thickness variations and for all angles of attack have been studied. A comparative analysis based on aerodynamic features is performed by an assessment of lift coefficient (CL), total aerodynamic drag coefficient (CD) and aerodynamic efficiency as lift/drag ratio (CL/CD) in order to conclude the best wing configuration in terms of aerodynamic performance.

Fluid Dynamics Behind a Circular Cylinder Embedded with an Active Flapping Jet Actuator

The effects of an active flapping jet actuator on the wake flow dynamics behind a circular cylinder in wind tunnel tests were investigated. An active flapping jet actuator was embedded in the cylinder in advance to invoke a spontaneous flapping jet into the cylinder's wake. The experiment, which was performed in a wind tunnel with a Reynolds number of Re = 1.99 × 104, was based on the oncoming wind speed, cylinder diameter, and kinematic viscosity of the air at the laboratory's temperature. The flow field structures behind the cylinder model with different dimensionless jet momentum coefficients, Cu, were obtained using the high-speed particle image velocimetry technique. The proper orthogonal decomposition (POD) method was used to represent the variation of the POD mode energy, mode coefficients, and the reconstructed spreading vorticity. The dynamic temporal evolution and time-averaged results in the near wake region of the cylinder with and without active flapping-jet control were calculated and analyzed to illustrate the rich phenomena produced by, and the control effect of, the flapping jet. For Cu values up to 0.0554, the periodic vortex shedding was pushed to farther wakes. Meanwhile, the time-averaged wake changed considerably, and the distributions of the turbulent kinetic energy and Reynolds shear stress decreased significantly. A data-driven dynamic mode decomposition method was used to extract the coherent structure of the wake of the cylinder embedded with the flapping jet actuator. The Strouhal number of the main mode of the Cu = 0.0865 case was different from the natural case.

Flow Field Measurement of Laboratory-Scaled Cross-Flow Hydrokinetic Turbines: Part I—The Near-Wake of a Single Turbine

Recent developments in marine hydrokinetic (MHK) technology have put the cross-flow (often vertical-axis) turbines at the forefront. MHK devices offer alternative solutions for clean marine energy generation as a replacement for traditional hydraulic turbines such as the Francis, Kaplan, and Pelton. Following previous power measurements of laboratory-scaled cross-flow hydrokinetic turbines in different configurations, this article presents studies of the water flow field immediately behind the turbines. Two independent turbines, which operated at an average diameter-based Reynolds number of approximately 0.2×105, were driven by a stepper motor at various speeds in a closed circuit water tunnel with a constant freestream velocity of 0.316 m/s. The wakes produced by the three NACA0012 blades of each turbine were recorded with a monoscopic particle image velocimetry technique and analyzed. The flow structures with velocity, vorticity, and kinetic energy fields were correlated with the turbine power production and are discussed herein. Each flow field was decomposed into the time averaged, periodic, and random components for all the cases. The results indicate the key to refining the existed turbine design for enhancement of its power production and serve as a baseline for future comparison with twin turbines in counter-rotating configurations.

A new experimental set-up to study the shear strength of snow-mortar interfaces

<p>The progressive failure of a snow layer deposited on a stiff substrate is at the base of the comprehension of several physical processes that can be found both in natural and artificial conditions. For instance, glide avalanches often originate from the reduction of the basal friction between the snowpack and the underlying ground due to the presence of liquid water film or depth hoar at the snow-ground interface. Moreover, the interaction between snow and construction materials relates to many other applications such as the study of new and more efficient snow removal techniques, the safety of travelers along snow covered roads, the snow redistribution from roofs and buildings, etc. </p><p>Despite this large number of application fields, laboratory investigations are still limited. We performed cold room tests on artificially made snow-mortar interface specimens through a direct shear test device. The effects of confinement pressure, temperature and dry snow hardness (due to sintering times) were taken into account. The tests were carried out in displacement-controlled conditions in order to study the entire failure process at the interface and the following irreversible sliding. The results show some interesting and encouraging aspects for understanding the shear strength of the interface. From a micromechanical point of view we recorded the tests with a high-definition video camera and analyzed the data with the Particle Image Velocimetry technique to obtain the motion fields on the external side of the specimens. Here, we present and discuss some preliminary results of the experimental activity and suggest some future implementations and further developments of the studied topic.       </p>