Experimental investigation of the flow around a radially vibrating circular cylinder

This work aims to develop a process for controlling a cylinder wake, especially the von Karman vortex street, in such way so as to drastically reduce the drag coefficient. A new technique for influencing the cylinder wake is proposed in the present experimental study. The flow around a circular cylinder is perturbed by temporarily changing the cylinder diameter. Experiments have been performed for Reynolds numbers in the range Re=9,500 to Re=31,500. Three values of the controlling frequencies are considered: fs₁=0.41, fs₂=0.54 and fs₃=0.73, in addition to the stationary case corresponding to a non-deformable cylinder, fs₀=0. The visualisation flow shows that the pulsing motion of the cylinder walls greatly influences both the near and far wake dynamics. A decrease of the drag is expected.

Microscale Synthetic Schlieren

We develop the axisymmetric Synthetic Schlieren technique to study the wake of a microscale sphere settling through a density stratification. A video-microscope was used to magnify and image apparent displacements of a micron-sized random-dot pattern. Due to the nature of the wake, density gradient perturbations in the horizontal greatly exceed those in the vertical, requiring modification of previously developed axisymmetric techniques. We present results for 780 and 383 μm spheres, and describe the limiting role of noise in the system for a 157 μm sphere. This technique can be instrumental in understanding a range of ecological and environmental oceanic processes on the microscale.

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Time-resolved reconstruction of the full velocity field around a dynamically-scaled flapping wing

The understanding of the physics of flapping flight has long been limited due to the obvious experimental difficulties in studying the flow field around real insects. In this study the time-dependent three-dimensional velocity field around a flapping wing was measured quantitatively for the first time. This was done using a dynamically-scaled wing moving in mineral oil in a pattern based on the kinematics obtained from real insects. The periodic flow is very reproducible, due to the relatively low Reynolds number and precise control of the wing. This repeatability was used to reconstruct the full evolving flow field around the wing from separate stereoscopic particle image velocimetry measurements for a number of spanwise planes and time steps. Typical results for two cases (an impulsive start and a simplified flapping pattern) are reported. Visualizations of the obtained data confirm the general picture of the leading-edge vortex that has been reported in recent publications, but allow a refinement of the detailed structure: rather than a single strand of vorticity, we find a stable pair of counter-rotating structures. We show that the data can also be used for quantitative studies, such as lift and drag prediction.

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Calibration of an oscillating hot-wire anemometer for bidirectional velocity measurements

A novel compact low-frequency oscillating hot-wire (OHW) anemometer is calibrated in a custom-built wind tunnel. Laser Doppler anemometry is used for reference velocity measurements, phase-locked with the oscillating wire. Three probe designs are calibrated, examining the influence of prong shape on the wake contamination. Results for two oscillation amplitudes and several frequencies are discussed. Through non-dimensional analysis, the optimum probe design and operating parameters are extracted. The OHW features a maximum measurable negative velocity of −1.0 m/s which is comparable to existing oscillating and flying hot-wire anemometers. The compact OHW can be applied to reversing flow in confined geometries such as flow in exhaust systems.

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A random synthetic jet array driven turbulence tank

We measure the flow above an array of randomly driven, upward-facing synthetic jets used to generate turbulence beneath a free surface. Compared to grid stirred tanks (GSTs), this system offers smaller mean flows at equivalent turbulent Reynolds numbers with fewer moving parts.

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DPIV study of the effect of a gable roof on the flow structure around a surface-mounted cubic obstacle

The present paper reports a thorough comparison of the turbulent flow characteristics exhibited by a cubic surface-mounted obstacle and a simple geometric variant (gable roof of 30° roof pitch). The measurements supporting this study were obtained by the use of a 2D-DPIV system. Significant differences in the large-scale vortical structures and turbulent kinetic energy fields implied drastic consequences with respect to the advective and turbulent dispersive characteristics of the flow at roof and ground levels.

Fluid dynamics analysis of channel flow geometries for materials characterization in microfluidic devices

The fluid dynamics of geometries for liquid state materials characterization in microfluidic devices are investigated. Numerical simulation together with flow classification criteria are used to explore combinations of geometry and boundary conditions for which the flow type can be adjusted between simple shear and extension, while providing adequate flow strength and a stable environment for material observation. Two classes of flow geometries are identified. Both make use of opposing, laterally offset fluid streams that produce a stagnation point in the center of the geometry. In the first class, the flow type is manipulated by changing parameters inherent to the base geometry. This first case serves as a basis for identifying a second class in which the flow type is manipulated by changing the pressure boundary conditions, while keeping the geometry constant.Electronic Supplementary Material is available for this article if you access the article at .

Two- and three-dimensional flows in nearly rectangular cavities driven by collinear motion of two facing walls

Two- and three-dimensional flows in nearly cuboidal cavities are investigated experimentally. A tight cavity is formed in the gap between two long and parallel cylinders of large radii by adding rigid top, bottom, and end walls. The cross-section perpendicular to the axes of the cylinders is nearly rectangular with aspect ratio Γ. The axial aspect ratio Λ > 10 is large to suppress end-wall effects. The fluid motion is driven by independent and steady rotation of the cylinders about their axes which defines two Reynolds numbers Re _1,2. Stability boundaries of the nearly two-dimensional steady flow have been determined as functions of Re _1,2 for Γ = 0.76 and Γ = 1. Up to six different three-dimensional supercritical modes have been identified. The critical thresholds for the onset of most of the three-dimensional modes, three of which have been observed for the first time, agree well with corresponding linear-stability calculations. Particular attention is paid to the flow for Γ = 1 under symmetric and parallel wall motion. In that case the basic flow consists of two mirror symmetric counter-rotating parallel vortices. They become modulated in span-wise direction as the driving increases. Detailed LDV measurements of the supercritical three-dimensional velocity field and the bifurcation show an excellent agreement with numerical simulations.

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Analysis of the wake–mixing-layer interaction using multiple plane PIV and 3D classical POD

The strong interaction between turbulent structures arising from a plane mixing layer impinging on a circular cylinder is studied. This complex flow has been investigated by a set-up called dual-plane PIV that uses two 2D PIV (two-dimensional particle image velocimetry) planes acquired simultaneously. This approach allowed us to apply a 3D-POD (three-dimensional proper orthogonal decomposition) treatment. The first POD modes show the main footprint of the flow configuration, which comprises oblique structures associated with the action of the mixing layer on the near wake. The present study suggests, by analogy, that this phenomenon behaves like the dislocation observed in uniform wake flows.

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A distributed parameter integral model for predicting the thermal performance characteristics of single- and multi-wall radiation heat shield systems

Heat shielding has become an increasingly necessary means for protecting temperature-sensitive components from direct exposure to thermal radiation from high temperature sources. A simple but comprehensive distributed parameter integral model has been developed for predicting the temperature distribution of the shield and the protected component for a variety of heat shield systems. The integral model presented here is seen to be more accurate than lumped models, and can be computed with much greater speed than that required for numerical models.

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Experimental study of the behavior of a valveless impedance pump

When a fluid-filled flexible tube is connected to tubing of different impedance, a net flow in either direction can be induced by periodically pinching the flexible section asymmetrically from the ends. We have experimentally demonstrated a variety of conditions under which pumping occurs; including changes in actuator position, size and pinching frequency, transmural pressure, systemic resistance and materials. Data collected includes dynamic pressure and flow-rate measurements at the inlet and outlet of the pump and ultrasound imaging of the tube walls. The net flow rate is highly sensitive to pinching frequency. The pump does not require a closed loop and can sustain a pressure head. We have also shown that a flexible, yet inelastic material is a sufficient condition for impedance-driven flow. A micro-scaled version of the pump was simultaneously tested demonstrating the feasibility of a miniature design.

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An erratum to this article is avaliable at .

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Turbulence measurements in a swirling pipe flow

This paper reports laser-Doppler measurements of the mean flow and turbulence stresses in a swirling pipe flow. Experiments were carried out under well-controlled laboratory conditions in a refractive index-matched pipe flow facility. The results show pronounced asymmetry in mean and fluctuating quantities during the downstream decay of the swirl. Experimental data reveal that the swirl significantly modifies the anisotropy of turbulence and that it can induce explosive growth of the turbulent kinetic energy during its decay. Anisotropy invariant mapping of the turbulent stresses shows that the additional flow deformation imposed by initially strong swirling motion forces turbulence in the core region to tend towards the isotropic two-component state. When turbulence reaches this limiting state it induces rapid production of turbulent kinetic energy during the swirl decay.

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Impingement cooling for modern combustors: experimental analysis of heat transfer and effectiveness

This paper presents results obtained from a wide-ranging experimental investigation into impingement cooling from multiple jet arrays which reproduced real LPP combustor liner geometries. The work was performed during the four years of the European project LOPOCOTEP. Two sparse, staggered impingement arrays were examined in detail and each case was compared with data from the literature relating to the same configuration and jet Reynolds numbers. As a result of this study, it has been possible to obtain detailed data about local distribution of the heat-transfer coefficient and spanwise row-averaged effectiveness, by using a new method which combined transient and steady-state thermochromic liquid-crystal (TLC) techniques. It was found that the data obtained in this work were in good agreement with results presented in the literature. This study shows that measured, row-by-row effectiveness values can be usefully employed in a preliminary design stage. Some data relating to hole-discharge coefficients are also presented.

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Identification of vortex pairs in aircraft wakes from sectional velocity data

The dynamics of multiple-vortex wake systems behind aircraft endangering air traffic can be assessed also from physical modelling. Large-scale laboratory investigations of multiple-vortex systems have been performed in a free-flight laboratory and in a water towing tank. Specialized PIV measurements provide time-resolved flow velocity fields normal to the wake axis. The applicability of various ∇u-based vortex identification schemes to planar velocity data is addressed and demonstrated for unequal-strength co- and counter-rotating vortex pairs. Large vortices shed off the wing tips and flaps are identified employing a ∇u-based criterion. Their cooperative mechanisms of generation and decay are evidenced from iso-surfaces of squared swirling strength and from further characteristic vortex parameters.

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Unsteady air–water flow measurements in sudden open channel flows

Measurements of air–water flow properties are reasonably simple in steady flows, but not so in unsteady flows. Some studies investigated periodic flows in which instantaneous data were averaged over several cycles. During the present work, new unsteady air–water flow measurements were performed in sudden open channel flow surges. Unsteady air–water flow measurements were performed in the wave front with an array of resistivity probes. The results demonstrated quantitatively strong aeration of the leading edge in terms of void fractions, bubble count rates and specific interface areas. Experimental results highlighted that this strongly aerated region was relatively short: i.e. typically 0.3 to 0.5 m long. Measurements of air and water chord sizes highlighted a wide range of bubble and droplet sizes. Time-variations of air–water flow structure were observed.

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Selective seeding for micro-PIV

A novel seeding method for microscale particle image velocimetry (micro-PIV) is presented. The method relies on selective seeding of a thin fluid layer within an otherwise particle-free flow. In analogy to the laser sheet in macroscale PIV, the generated particle sheet defines both the depth and the position of the measurement plane, independent of the details of the optical setup. Selectively seeded micro-PIV is applied to measure the instantaneous velocity field in a microchannel with a depth-wise resolution 20% below the estimated optical measurement depth of the micro-PIV system. In principle, a measurement depth corresponding to the diameter of the tracer particles may be achieved.

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Variational optical flow estimation for particle image velocimetry

We introduce a novel class of algorithms for evaluating PIV image pairs. The mathematical basis is a continuous variational formulation for globally estimating the optical flow vector fields over the whole image. This class of approaches has been known in the field of image processing and computer vision for more than two decades but apparently has not been applied to PIV image pairs so far. We pay particular attention to a multi-scale representation of the image data so as to cope with the quite specific signal structure of particle image pairs. The experimental evaluation shows that a prototypical variational approach competes in noisy real-world scenarios with three alternative approaches especially designed for PIV-sequence evaluation. We outline the potential of the variational method for further developments.The publications of the CVGPR Group are listed under .

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The optimal drop shape for vortices generated by drop impacts: the effect of surfactants on the drop surface

Subsurface vortices are frequently created when a falling drop strikes a flat water surface. Prior work has demonstrated that the shape of the drop at the point of impact is critical in determining how deep or how fast the resulting vortex will penetrate into the water bulk. In the present study, the details of this phenomena are explored by using surfactants to vary surface tension. Specifically, Triton X-100 monolayers are created on the surface of the drop, and on the flat water surface. The results of these experiments suggest that there is no single optimal drop shape resulting in best vortex penetration. Rather, the data suggest that the optimal shape depends on the surface tension of the falling drop. An attempt is made to reconcile contradictory results in the literature using this result.

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Simultaneous stress and birefringence measurements during uniaxial elongation of polystyrene melts with narrow molecular weight distribution

Tensile stress and flow-induced birefringence have been measured during uniaxial elongation at a constant strain rate of two polystyrene melts with narrow molecular weight distribution. For both melts, the stress- optical rule (SOR) is found to be fulfilled upto a critical stress of 2.7 MPa, independent of strain rate and temperature. Estimation of the Rouse times of the melts, from both the zero-shear viscosity and the dynamic-shear moduli at high frequency, shows that the violation of the SOR occurs when the strain rate multiplied by the Rouse time of the melt exceeds by approximately 3. The presented results indicate that in contrast to current predictions of molecular theories, the regime of extensional thinning observed by Bach et al. (2003) extends well beyond the onset of failure of the SOR, and therefore the onset of chain stretch in the non-Gaussian regime.

Aging of an unstable w/o gel emulsion with a nonionic surfactant

We study an unstable highly concentrated emulsion of water droplets in oil with a nonionic surfactant. A technique of light diffusion coupled to a rheometer allows simultaneous measurement of average droplet radius and emulsion shear elastic modulus during time. Over the studied range of volume fraction (from 71 to 95%), we show that Princen and Kiss’ (J Colloid Interface Sci 112:427–437, 1986) model does not apply. A dimensional analysis based on the hypothesis of dominant van der Waals forces is proposed for nonionic surfactants, which is in good agreement with experimental data. We also show that the measured average droplet volume increases linearly with time and that the coalescence rate strongly depends on the volume fraction in relation with different topological conformations of droplets.