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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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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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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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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Quantitative planar Raman imaging through a spectrograph: visualisation of a supersonic wedge flow

Planar Raman imaging through a spectrograph is demonstrated as a diagnostic tool for quantitative flow visualisation of internal supersonic wedge flow. A dedicated Bayesian deconvolution filter is used to remove the spectral structure that is introduced by the spectrograph. The 2D density field is determined with ca. 10% precision using average images over 6,000 laser pulses, down to 0.5 mm from the surface of the wedge. Direct interpretations of Raman intensities provide more precise density data than indirect interpretations based on shock geometry in 2D inviscid flow.

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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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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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Density stratified environments: the double-tank method

We present an alternative method of producing density stratifications in the laboratory based on the ‘double-tank’ method proposed by Oster (Sci Am 213:70–76, 1965). We refer to Oster’s method as the ‘forced-drain’ approach, as the volume flow rates between connecting tanks are controlled by mechanical pumps. We first determine the range of density profiles that may be established with the forced-drain approach other than the linear stratification predicted by Oster. The dimensionless density stratification is expressed analytically as a function of three ratios: the volume flow rate ratio n, the ratio of the initial liquid volumes λ and the ratio of the initial densities ψ. We then propose a method which does not require pumps to control the volume flow rates but instead allows the connecting tanks to drain freely under gravity. This is referred to as the ‘free-drain’ approach. We derive an expression for the density stratification produced and compare our predictions with saline stratifications established in the laboratory using the ‘free-drain’ extension of Oster’s method. To assist in the practical application of our results we plot the region of parameter space that yield concave/convex or linear density profiles for both forced-drain and free-drain approaches. The free-drain approach allows the experimentalist to produce a broad range of density profiles by varying the initial liquid depths, cross-sectional and drain opening areas of the tanks. One advantage over the original Oster approach is that density profiles with an inflexion point can now be established.

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A pragmatic approach for deriving constitutive equations endowed with pom–pom attributes

The most important rheological and mathematical features of the pom–pom model are presently used to compare and improve other constitutive models such as the Giesekus and Phan-Thien–Tanner models. A pragmatic methodology is selected that allows derivation of simple constitutive equations, which are suited to possible software implementation. Alterations to the double convected pom–pom, Phan-Thien–Tanner and Giesekus models are proposed and assessed in rheometric flows by comparing model predictions to experimental data.

XPIV–Multi-plane stereoscopic particle image velocimetry

We introduce the three-dimensional measurement technique (XPIV) based on a Particle Image Velocimetry (PIV) system. The technique provides three-dimensional and statistically significant velocity data. The main principle of the technique lies in the combination of defocus, stereoscopic and multi-plane illumination concepts. Preliminary results of the turbulent boundary layer in a flume are presented. The quality of the velocity data is evaluated by using the velocity profiles and relative turbulent intensity of the boundary layer. The analysis indicates that the XPIV is a reliable experimental tool for three-dimensional fluid velocity measurements.More information at:

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Investigation of internal pressure gradients generated in electrokinetic flows with axial conductivity gradients

Field amplified sample stacking (FASS) is used to increase sample concentrations in electrokinetic flows. The technique uses conductivity gradients to establish a non-uniform electric field that accumulates ions within a conductivity gradient, and can be readily integrated with capillary electrophoresis. Conductivity gradients also cause gradients in near-wall electroosmotic flow velocities. These velocity gradients generate internal pressure gradients that drive secondary, dispersive flows. This dispersion leads to a significant reduction in the efficiency of sample stacking. This paper presents an experimental investigation of internally generated pressure gradients in FASS using micron-resolution particle image velocimetry (μPIV). We measure velocity fields of particles seeded into an electrokinetic FASS flow field in a glass microchannel with a single buffer–buffer interface. μPIV allows for the direct quantification of local, instantaneous pressure gradients by analyzing the curvature of velocity profiles. Measurements show internally generated pressure-driven velocities on the order of 1mm/s for a typical applied electric field of 100 V/cm and a conductivity ratio of 10. A one-dimensional (1D) analytical model for the temporal development of the internal pressure gradient generation is proposed which is useful in estimating general trends in flow dynamics.

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Spatial resolution of the Stereo PIV technique

A theoretical analysis of the spatial resolution in terms of modulation transfer function of the Stereo PIV technique with and without the correction of the misalignment error is performed, and the results show that some wavelengths of the flow field can be significantly dephased and modulated. A performance assessment has been conducted with both synthetic and real images and shows a good agreement with the theoretical analysis. The reconstruction of the three-dimensional displacement field is achieved using both the methods proposed by Soloff et al. (Meas Sci Technol 8:1441–1454, 1997) and by Willert (Meas Sci Technol 8:1465–1479, 1997).

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Label-free visualization of microfluidic mixture concentration fields using a surface plasmon resonance (spr) reflectance imaging

A label-free visualization technique based on surface plasmon resonance (SPR) illumination sensing is applied for the nonintrusive and real-time mapping of microscale mixture concentration fields. The key idea is that the SPR reflectance sensitively varies with the refractive index of the near-wall region of the test mixture fluid contacting the metal (Au) layer, of tuned 47.5 nm thickness. The Fresnel equation, based on Kretschmann’s theory, correlates the SPR reflectance with the refractive index, or dielectric constant, of the test medium, and then, the measured refractive index correlates with the mixture concentration. An example application is presented for the case of ethanol penetrating into water contained in a micro-channel with a rectangular cross-section of 91 μm wide and 50 μm high. The measurement sensitivity, uncertainties and detection limitations of the implemented SPR imaging sensor are carefully examined for its potential as a nonintrusive means of microscale concentration field mapping.

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Coupled Strain and Fresnel Response of Photoelastic Coatings at Oblique Incidence

This paper presents a theoretical model and corresponding experimental results of the oblique-incidence response of a luminescent photoelastic coating (LPC). LPCs use a luminescent dye that both partially preserves the stress-modified polarization state and provides high emission signal strength at oblique surface orientations. These characteristics enable the technique to acquire full-field strain separated measurements and principal strain directions, potentially on complex three-dimensional geometries, without the use of supplemental experimental or analytical techniques. Results of a single-layer LPC on a disk in diametral compression are presented to assess a theoretical model and evaluate the measurement sensitivity.

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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Reduction of primary freeze-drying time by electric field induced ice nucleus formation

The potential of a high electric field was utilized to induce ice nucleus formation in aqueous solutions. Using this technique it was possible to reduce the primary drying time during lyophilization. Samples of 10% (w/v) hydroxyethylstarch (HES) solution were frozen at a constant rate of −1 K/min, while nucleation was initiated at temperatures of −1.5, −4.5 and −8.5°C. In contrast, spontaneous nucleation was observed in a range between −11.5 and −17.1°C. Electrically induced nucleus formation has proved to be a reliable method to start crystallization at a desired temperature. Continuous measurement of the weight allowed to determine the drying rate and to detect at which time primary drying was completed. The drying time and the drying rate were found to be strongly dependent on the nucleation temperature during freezing. A relation between the nucleation temperature, the structure of the frozen samples and the drying times could be established.

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A model of a subcritical Joule–Thomson cryocooler with condensation inside the recuperator

To develop a tool for predicting of heat and mass transfer in Joule–Thomson cryocoolers working at subcritical pressures, we study a counter flow heat exchanger with condensation by employing the integral method. The effects of inlet pressure and working fluid are predicted. We also show that there is an optimal value of the enthalpy difference along the heat exchanger for which its length is minimal.

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Scanning Electron Microscopy for Quantitative Small and Large Deformation Measurements Part II: Experimental Validation for Magnifications from 200 to 10,000

A combination of drift distortion removal and spatial distortion removal are performed to correct Scanning Electron Microscope (SEM) images at both ×200 and ×10,000 magnification. Using multiple, time-spaced images and in-plane rigid body motions to extract the relative displacement field throughout the imaging process, results from numerical simulations clearly demonstrate that the correction procedures successfully remove both drift and spatial distortions with errors on the order of ±0.02 pixels. A series of 2D translation and tensile loading experiments are performed in an SEM for magnifications at ×200 and ×10,000, where both the drift and spatial distortion removal methods described above are applied to correct the digital images and improve the accuracy of measurements obtained using 2D-DIC. Results from translation and loading experiments indicate that (a) the fully corrected displacement components have nearly random variability with standard deviation of 0.02 pixels (≈25 nm at ×200 and ≈0.5 nm at ×10,000) in each displacement component and (b) the measured strain fields are unbiased and in excellent agreement with expected results, with a spatial resolution of 43 pixels (≈54 μm at ×200 and ≈1.1 μm at ×10,000) and a standard deviation on the order of 6 × 10⁻⁵ for each component.

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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Particle tracking velocimetry and accelerometry (PTVA) measurements applied to quasi-two-dimensional multi-scale flows

We have developed and validated a new adaptive method, particle tracking velocimetry and accelerometry (PTVA), to measure velocity and acceleration from the post-processing of particle tracking (PT) data. This method is shown to be more accurate than non-adaptive methods based on PT: errors are about six times smaller on velocity measurements and about four times smaller on acceleration ones. We apply this method to a turbulent-like flow generated and controlled in the laboratory. Taking advantage of the Eulerian repeatability of our multi-scale laminar flow, we are able to extract the acceleration field, a, and all terms of Navier–Stokes equation. To complete this we extract u·a and ∇·a fields. We finally compare the probability density function of the acceleration components of our turbulent-like flow with one of the highly turbulent flows and show that they are similar. The quality of these PTVA results and their robustness (in particular to local convection) are extremely encouraging. This method allows access to a deeper insight into the physic of turbulent-like flows and its high accuracy may apply to a broader range of flows.

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