Francisco Huera-Huarte

Large-amplitude undulatory swimming near a wall

The propulsive dynamics of a flexible undulating foil in a self-propelled swimming configuration near a wall is studied experimentally. Measurements of the swimming speed and the propulsive force are presented, together with image acquisition of the kinematics of the foil and particle image velocimetry (PIV) in its wake. The presence of the wall enhances the cruising velocity in some cases up to 25% and the thrust by a 45% , for swept angles of 160 and 240°. The physical mechanisms underlying this effect are discussed by studying the vorticity dynamics in the wake of the foil. Proper orthogonal decomposition is applied to the PIV measurements in order to analyse the kinetic energy modal distribution in the flow and to relate it to the propulsion generated by the foil.

DPIV in the wake of a tandem arrangement of two flexible circular cylinders

AbstractVisualization of the wake of a system of two circular cylinders in tandem is presented through Digital Particle Image Velocimetry results in a plane perpendicular to the models’ axes. Both cylinder models have an aspect ratio (length over diameter) of almost 100, a mass ratio (mass divided by mass of displaced fluid) under 2, and they are flexible and free to move in the in-line and cross-flow directions. A supporting structure provided attachment of both models through universal joints at each end and the cylinders were exposed to a uniform flow profile over the lower 45% of their lengths, producing vortex-induced vibrations with wake interactions. The centre to centre separation between the models could be varied and data is shown here for three separations of 2, 3 and 4 diameters.Graphical AbstractGraphical Abstract text

DRAG COEFFICIENTS OF A LONG FLEXIBLE CIRCULAR CYLINDER WITH WAKE INTERFERENCE

Drag coefficients of the downstream cylinder in a tandem arrangement of two models aligned with the flow are shown in this work. The models, which experienced vortex-induced vibrations with wake interference, had an external diameter of 16 mm and a total length of 1.5 m giving an aspect ratio of about 94. More than 400 runs were carried out in a water flume with only the lower 40% of the models exposed to the current. The flow speed was varied up to 0.75 m/s giving Reynolds numbers in the range from 1200 to 12000. A supporting structure. where the models were attached. allowed changes in the centre to centre distances and in the applied top tensions. Separations of up to 4 diameters were tested with tension variations from 15 to l10 N. Reduced velocities based on the fundamental natural frequency reached values up to 16. The mass ratio of the models was around 1.8 and the combined mass-damping parameter about 0.05.

The effect of blade pitch on the flow dynamics inside the rotor of a three-straight-bladed cross-flow turbine

This study analyses the variations in flow dynamics inside the rotor of a three-straight-bladed cross-flow turbine for different blade pitches. A water towing tank facility has been used with a turbine model based on symmetric NACA-0015 profiles with a chord-to-diameter ratio of 0.16. The set-up was especially designed to measure the flow field inside and around the rotor, using planar digital particle image velocimetry. The experiments were made at a constant turbine diameter Reynolds number, R e D , of 6.1 × 104. The forced rotation of the cross-flow turbine using a direct current motor allowed changes in the ratio of the blade tangential speed to the freestream velocity (tip speed ratio), covering the range of 0.7–2.3. Toe-in and excessive toe-out angles have been associated in the past to low performances in this type of turbines. Pitch angle cases with expected low performances have been chosen, namely, β of 8° toe-in and 16° toe-out, as well as a case with a high expected efficiency of 8° toe-out and another with β = 0 ∘ as the reference case. The the goal is to better understand the flow dynamics and the blade–wake interactions inside the rotor of this type of machines, by studying cases with very different expected performances.

On the flow around the node to anti-node transition of a flexible cylinder undergoing vortex-induced vibrations

In this paper, the wake structures behind the node to anti-node transitional region of a flexible cylinder undergoing vortex-induced vibrations are investigated experimentally. The cylinder was towed in a still water tank with Reynolds numbers that produced self-sustained stable 2nd mode of structural vibrations, in the range 1500–2000. The wake of the cylinder was analysed using flow visualisation and digital particle image velocimetry at various planes perpendicular to the axis of the cylinder between the node and the anti-node, as well as at planes parallel to the axis of the cylinder. It was observed that at the anti-node, the wake comprised of the classic vortex shedding. The wake at the node consisted of a pair of counter-rotating vortices that remained attached to the cylinder. A transitional region was observed between the nodal and anti-node regions that exhibited a vortex pinch-off in the vicinity of the cylinder and connectivity further downstream of the cylinder.

Hydrodynamic Forces and DPIV on a Towed Pitching Foil

More than 90% of the thrust generated by thunniform swimmers is known to be produced by the oscillation of their caudal fin, and the rest by their caudal peduncle. We have designed an experiment in which we can mimic, in a simplified manner, the kinematics of swimmers that mainly use their caudal fin for propulsion. The set-up consists of a rectangular foil attached to a shaft that is controlled by a stepper motor, and the whole assembly can be towed in still water at different controllable speeds. With this system we can study the effect of different types of pitching on the hydrodynamic loads and the performance of the propulsion system. By changing the type of foil, the effects of the flexibility in the propulsion can also be analysed. Hydrodynamic loads were measured with a 6-axes balance, and the flow structures were investigated using a Digital Particle Image Velocimetry (DPIV). Loads and DPIV velocity fields were acquired synchronously.© 2014 ASME

Some observations on the flow physics of paddle racquets

Paddle, padel or platform tennis is a rapidly growing sport derived from tennis, but played in smaller courts using a solid racquet, instead of a strung one. Most of the racquet manufacturers drill holes in the head arguing improved aerodynamic performances. The question is, if almost all manufacturers drill aerodynamic holes, are they optimally distributed from the fluid dynamics point of view? In this work, we use a generic racquet model to analyse what is the effect of the holes on the drag forces suffered by the racquet and how these forces are related to the wakes seen by the racquet during its motion. A porosity parameter is defined that takes into account not only the porosity itself but also how it is distributed within the racquet’s head. Digital particle image velocimetry is used to quantify the flow field around the racquet, allowing the identification of different wake topologies that can be related to different drag performances.

CFD Simulations on the Vortex-Induced Vibrations of a Flexible Cylinder With Wake Interference

In this work, CFD computations showing the dynamic response of a long flexible cylinder subject to a stepped current immersed in the wake of another cylinder are presented. These two cylinders are placed upstream in tandem configuration, where the flexible cylinder is excited by vortex shedding mechanisms. This work completes from the computational point of view, the research started 2 years ago with experiments conducted at the E.T.S.I. Navales towing tank of the Technical University of Madrid. The flexible cylinder studied is 3 m long having an external diameter of 16 mm. A combination of two codes that simulate the fluid-structure interaction phenomenon was used to obtain the velocity and pressure fields and also to measure the deformation of the cylinder at the same points where the strain gauges where placed during the experiment. This code communicates a finite volume (FV) software that solves the Navier-Stokes equations and reports the shear and pressure fields on the flexible cylinder to a second finite element (FEM) code that is able to compute stresses and deformations. Deformations are reported back to the first fluid solver in order to compute the next time step. In the experiments, only the 65% length of the cylinders were under the water surface, consequently a VOF technique was used to simulate the free surface separation between air and water. The numerical stability of these two combined codes is one of the most delicate aspects of the simulation. Taking into account that the upstream cylinder was orders of magnitude more rigid than the downstream one, we considered the upstream cylinder as stationary and consequently having no role during the FEM calculation. Boundary conditions for the flexible cylinder where such that they should imitate the universal joints used in the experiments. The fundamental natural frequencies of oscillation were monitored and compared to the towing tank experiments.Copyright

Towing Tank Experiments on the Vortex-Induced Vibrations of a Flexible Cylinder With Wake Interference

We describe recent results showing the dynamic response, excited by vortex shedding, of a long flexible cylinder subject to a stepped current immersed in the wake of another cylinder, placed upstream in tandem configuration. Experiments were conducted at the E.T.S.I. Navales towing tank of the Technical University of Madrid during March 2012. The tank is 80 m long with a cross-section of 4 × 2.5 m. A supporting structure was designed in order to provide support for a 3 m long cylinder with an external diameter of 16 mm. The cylinder was instrumented with strain gauges providing curvature measurements in the in-line and the cross-flow directions at 11 locations along its length. Tension and drag forces were also measured at both ends of the model. For these experiments, the upstream rigid cylinder was made stationary by fixing it at both ends, and it was located at different centre to centre distances.More than 200 runs were conducted, with its lower 65% length under the water free surface, connected to the structure by means of universal joints. The supporting structure allowed to configure different top end conditions and to apply different top tensions. Tests were conducted with speeds up to 1.4 m/s. The cylinder had a low flexural stiffness of 6.04 Nm2 and low mass ratio of 2.7. Fundamental natural frequencies were in the range from about 2.3 to 6.2 Hz, and the cylinder responded in modes up to the third cross-flow.Copyright

Towing Tank Experiments on the Vortex-Induced Vibrations of a Long Flexible Cylinder in a Stepped Current

We describe recent results showing the dynamic response, excited by vortex shedding, of a long flexible cylinder subject to a stepped current. The experiments were conducted at the Naval Architecture Department towing tank of the Technical University of Madrid (UPM) during March 2012. The tank is 100 m long with a cross-section of 3.8 × 2.5 m, and it is able to deliver speeds over 4 m/s. A supporting structure was designed in order to provide support for a 3 m long cylinder with an external diameter of 19 mm. The cylinder was instrumented with strain gauges providing curvature measurements in the in-line and the cross-flow directions at 11 locations along its length. Tension and drag forces were also measured at both ends of the model. More than 50 runs were conducted with the cylinder being placed vertically having its lower 65% length under the water free surface, connected to the structure by means of universal joints. The supporting structure allowed to configure different top end conditions and to apply different top tensions. Tests were conducted for Reynolds numbers as high as 34000. The cylinder had a low flexural stiffness and very low mass ratio m* of 0.67. Fundamental natural frequencies were in the range from about 4 to 7.9 Hz, and the cylinder responded in modes up to the third cross-flow. In this article we will describe the experiments and the instrumentation used, the modal tests conducted and the results obtained during the experiments.Copyright