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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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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Analysis of velocity interpolation schemes for image deformation methods in PIV

The spatial resolution of PIV can be increased significantly by using an image deformation method (IDM) and very small grid distance (i.e. the final distance between vectors), therefore, also increasing the processing time. By using an interpolation scheme with a good spectral response, in the dense predictor step of the algorithm, it is possible to increase the grid distance without decreasing the spatial resolution therefore decreasing the total processing time.

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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.

An application of Gappy POD

An iterative procedure, based on the proper orthogonal decomposition (POD), first proposed by Everson and Sirovich (J Opt Soc Am A 12(8):1657–1664, 1995) is applied to marred particle image velocimetry (PIV) data of shallow rectangular cavity flow at Mach 0.19, 0.28, 0.38, and 0.55. The procedure estimates the POD modes while simultaneously estimating the missing vectors in the PIV data. The results demonstrate that the absolute difference between the repaired vectors and the original PIV data approaches the experimental uncertainty as the number of included POD modes is increased. The estimation of the dominant POD modes is also shown to converge by examining the subspace spanned by the POD eigenfunctions.

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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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Near-wall hindered Brownian diffusion of nanoparticles examined by three-dimensional ratiometric total internal reflection fluorescence microscopy (3-D R-TIRFM)

A three-dimensional nanoparticle tracking technique using ratiometric total internal reflection fluorescence microscopy (R-TIRFM) is presented to experimentally examine the classic theory on the near-wall hindered Brownian diffusive motion. An evanescent wave field from the total internal reflection of a 488-nm bandwidth argon-ion laser is used to provide a thin illumination field on the order of a few hundred nanometers from the wall. Fluorescence-coated polystyrene spheres of 200±20 nm diameter (specific gravity=1.05) are used as tracers and a novel ratiometric analysis of their images allows the determination of fully three-dimensional particle locations and velocities. The experimental results show good agreement with the lateral hindrance theory, but show discrepancies from the normal hindrance theory. It is conjectured that the discrepancies can be attributed to the additional hindering effects, including electrostatic and electro-osmotic interactions between the negatively charged tracer particles and the glass surface.

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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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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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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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Three-dimensional micro-PTV using deconvolution microscopy

A three-dimensional micro-particle tracking velocimetry (micro-PTV) scheme is presented using a single camera with deconvolution microscopy. This method devises tracking of the line-of-sight (z) flow vectors by correlating the diffraction pattern ring size variations with the defocusing distances of small particle locations. The working principle is based on optical serial sectioning microscopy, or equivalently deconvolution microscopy, that records images of an infinitesimally small particle, and generates a point-spread function of the three-dimensional diffraction patterns. A new image-processing algorithm has also been developed to digitally identify the center locations and measure the radii of the diffraction rings, which allows simultaneous tracking of all three-vector components. The developed PTV technique uses a 40×, 0.75 NA dry objective lens with 500-nm fluorescent seeding particles of SG=1.05, and successfully measures the fully three-dimensional fields flowing over a spherical obstacle snuggly fitted inside a 100 μm × 100 μm micro-channel. The volumetric measurement resolution of the present system is equivalent to a 5.16 μm × 5.16 μm × 5.16 μm cube, and the overall measurement uncertainty for single-point velocity vector detection is estimated to ±7.58%.

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Investigation of the three-dimensional intermediate wake topology for a square cylinder at high Reynolds number

Simultaneous multi-point hotwire measurements are used to investigate the three-dimensional wake topology of a square cylinder at high Reynolds numbers. Wavelet techniques are applied to detect the flow structures and to inquire on the validity or extension of previously proposed low Reynolds number topological models to turbulent wakes. Our results suggest that a flow topological model similar to the horizontal perturbation model proposed by Meiburg and Lasheras (J Fluid Mech 190:1–37, 1988) but with alternate rib cuts in the horizontal plane is plausible for the intermediate wake topology.

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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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Measurements of the minimum elevation of nano-particles by 3D nanoscale tracking using ratiometric evanescent wave imaging

Effect of saline concentration on the minimum elevation of nanoparticles has been examined under the electric double layer interactions with the substrate glass surface. The use of ratiometric total internal reflection fluorescence microscopy (R-TIRFM) allows three-dimensional tracking of nanoparticles in the near-wall region within less than 1 μm from the surface. The measurements of minimum elevation were made for polystyrene fluorescent nanospheres of 100, 250, and 500 nm in radii (SG = 1.05) for the salinity ranging from 0.1 to 10 mM. Special care was taken to insure cleaned surface conditions by elaborate sonication and rinsing of the glass substrate. The laser illumination intensity and duration also had to be carefully examined to minimize photobleaching of the fluorescence emission from particles. It is reported that the minimum elevation decreases with increasing saline concentration and with increasing particle sizes, for the first time experimentally and quantitatively to the authors’ knowledge.

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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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Turbulence statistics from optical whole-field measurements in particle-laden turbulence

Obtaining turbulence statistics in particle-laden flows using optical whole-field measurements is complicated due to the inevitable data loss. The effects of this data loss are first studied using synthetic data and it is shown that the interpolation of missing data leads to biased results for the turbulence spectrum and its derived quantities. It is also shown that the use of overlapping interrogation regions in images with a low image density can lead to biased results due to oversampling. The slotting method is introduced for the processing of particle image velocimetry (PIV) data fields with missing data. Next to this, it is extended to handle unstructured data. Using experimental data obtained by a dual-camera PIV/PTV (particle tracking velocimetry) system in particle-laden grid turbulence, the performance of the new approach is studied. Some preliminary two-phase results are presented to indicate the significant improvement in the statistics, as well as to demonstrate the unique capabilities of the system.An erratum to this article can be found at

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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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A real-time model-based approach for the reconstruction of fluid flows induced by microorganisms

An experiment on living microorganisms is conducted to gain insight into their motion and fluid exchange characteristics. Biocompatible microscopic particle image velocimetry (PIV)-systems are used to capture images of seeded particles in the induced fluid flows. To enhance the abilities of these devices we present a model-based approach for the reconstruction of admissible flow fields from captured images. A priori knowledge of the physical model of the flow is used to iteratively refine a predicted flow field. A physics-based filter operation generates a velocity field that is consistent with the model of incompressible laminar flows described by the Navier–Stokes equations. Interactive steering of the reconstruction process is achieved by exploiting programmable graphics hardware as a co-processor for numerical computations. To validate our method, we estimate velocity vector fields from synthetic image pairs of flow scenarios for which ground truth velocity fields exist and real-world image sequences of the flow induced by sessile microorganisms.

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Measurement of the aerodynamic forces on a small particle attached to a wall

Using operating principles similar to that applied in atomic force microscopes, we have developed a novel measuring method to study the aerodynamic forces, in particular the lift and drag force, acting on a small particle attached to a wall and immersed in a linear shear flow. Results thus far have shown that the system is capable of measuring both the minute aerodynamic lift and drag forces that a particle experiences as a result of the flow.C. Muthanna has also published under the name C. M. Kolera

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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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