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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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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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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Rheological characterization of asphalt in a temperature-gradient combinatorial squeeze-flow setup

A new technique for parallel rheological characterization of asphalt in a combinatorial squeeze-flow array is described. The basis of the technique is a device that is capable of subjecting multiple samples simultaneously to constant volume (Type B) squeeze flow with application of a temperature gradient. The time-dependent sample dimensions, which are calculated from digital images taken through the transparent top plate, are used to derive the flow curves. The results obtained using the combinatorial setup compared favorably with those obtained using conventional parallel-plate torsional flow in a commercial rheometer. With the existing setup, the accessible shear rate range is limited to about one decade at a single temperature.

Scanning Electron Microscopy for Quantitative Small and Large Deformation Measurements Part I: SEM Imaging at Magnifications from 200 to 10,000

A series of baseline displacement measurements have been obtained using 2D Digital Image Correlation (2D-DIC) and images from Scanning Electron Microscopes (SEM). Direct correlation of subsets from a reference image to subsets in a series of uncorrected images is used to identify the presence of non-stationary step-changes in the measured displacements. Using image time integration and recently developed approaches to correct residual drift and spatial distortions in recorded images, results clearly indicate that the corrected SEM images can be used to extract deformations with displacement accuracy of ±0.02 pixels (1 nm at magnification of 10,000) and mean value strain measurements that are consistent with independent estimates and have point-to-point strain variability of ±1.5 × 10⁻⁴.

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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Surfactant turbulent drag reduction in an enclosed rotating disk apparatus

An enclosed rotating disk apparatus (RDA) with rotational speed up to 5,500 rpm and with temperature control from −5 to 55°C was designed to screen the turbulent drag reducing effectiveness of small samples of newly synthesized drag reducing additives. First, the rotating disk was calibrated with water using both logarithmic and power law models. Then experiments were carried out to measure the frictional torque reduction for a drag reducing aqueous cationic surfactant system (5 mM Ethoquad O12 with 12.5 mM sodium salicylate) over a range of Re. The maximum drag reduction at 30°C was about 47% at Re = 1.90 × 10⁶. For the first time, results with the RDA were compared with those in a circular pipe flow system. They showed similar trends indicating it is a useful screening device for small samples, giving conservative estimates of surfactant effectiveness compared with pipe flow.

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A New Shear-Compression-Specimen for Determining Quasistatic and Dynamic Polymer Properties

A new design of the shear compression specimen (SCS) for investigating the viscoelastic shear response of polymers is presented. The specimen consists of a polymer gage section with two metal ends that remain essentially rigid during deformation. Two closed-form analytic models are developed to predict the average stress and strain in the gage section from the deformation-load histories. This new SCS design and its analytic models are thoroughly evaluated via laboratory measurements and numerical simulations. These simulations show that the deformations in the gage section are more uniform than in the original design, and the distribution of the average shear stress and strain are highly homogenous. The simulation results yield good agreement with those of closed-form analytic results and the experiments demonstrate that the new SCS geometry and its analytic models are as reliable as other commonly employed specimens. It can also generate higher strain rates under usual loading conditions because of its smaller specimen gage length. The need for care in specimen preparation is also discussed in detail as illuminated by the experimental and simulation results.

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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Global measurement of water waves by Fourier transform profilometry

In this paper, we present an optical profilometric technique that allows for single-shot global measurement of free-surface deformations. This system consists of a high-resolution system composed of a videoprojector and a digital camera. A fringe pattern of known characteristics is projected onto the free surface and its image is registered by the camera. The deformed fringe pattern arising from the surface deformations is later compared to the undeformed (reference) one, leading to a phase map from which the free surface can be reconstructed. Particularly, we are able to project wavelength-controlled sinusoidal fringe patterns, which considerably increase the overall performance of the technique and the quality of the reconstruction compared to that obtained with a Ronchi grating. In comparison to other profilometric techniques, it allows for single-shot non-intrusive measurement of surface deformations over large areas. In particular, our measurement system and analysis technique is able to measure free surface deformations with sharp slopes up to 10 with a 0.2 mm vertical resolution over an interrogation window of size 450 × 300 mm² sampled on approximately 6.1 × 10⁶ measurement points. Some illustrative examples of the application of this measuring system to fluid dynamics problems are presented.

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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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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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Local mass transfer measurements in an inclined enclosure at high Rayleigh number

An electrochemical technique is used to study local mass transfer coefficients on surfaces of inclined enclosures over the range 1.1×10⁴ < Ra_H < 1.4×10¹⁰ for a nominal Schmidt number of 2280. Scaling with gcos instead of g in the Rayleigh number correlates the data well at low angles of inclination; however, as either the aspect ratio or the angle of inclination increase, the longitudinal density stratification causes the data to deviate from a power law scaling.

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.

A laser-induced fluorescence measurement for aqueous fluid flows with improved temperature sensitivity

This paper presents temperature-sensitive laser-induced fluorescence measurements of Fluorescein 27 dissolved in aqueous solutions. We show that Fluorescein 27, dissolved in water and excited by a 532-nm Nd:YAG laser pulse, yields improved temperature sensitivity over traditional organic dyes such as Rhodamine B. The high temperature sensitivity of Fluorescein 27 when excited at 532 nm is due primarily to a temperature-dependent shift of the absorption spectrum to longer wavelengths for increased temperatures. The linearity of the fluorescence signal with respect to the incident laser intensity and dye concentration is reported. In addition, Fluorescein 27 dissolved in an aqueous solution remains photo-stable for >10⁵ laser pulses at both ambient and high temperatures (T > 60°C) when excited with low-irradiance laser pulses. Finally, we demonstrate that using a dual tracer (or ratiometric) technique in which the fluorescence from Fluorescein 27 and another dye (e.g., Rhodamine B or Kiton Red 620) are detected following the 532 nm excitation results in a significantly enhanced temperature sensitivity over a single tracer measurement and previously reported dual tracer methods. Such temperature sensitivity is useful in multi-dimensional temperature imaging and temporally resolved measurements.

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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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Attenuation technique for measuring sediment displacement levels

A technique for obtaining accurate, high (spatial) resolution measurements of sediment redeposition levels is described. In certain regimes, the method may also be employed to provide measurements of sediment layer thickness as a function of time. The method uses a uniform light source placed beneath the layer, consisting of transparent particles, so that the intensity of light at a point on the surface of the layer can be related to the depth of particles at that point. A set of experiments, using the impact of a vortex ring with a glass ballotini particle layer as the resuspension mechanism, are described to test and illustrate the technique.

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The generation and quantitative visualization of breaking internal waves

New techniques for the generation and quantitative visualization of breaking progressive internal waves are presented. Laboratory techniques applicable to general stratified flow experiments are also demonstrated. The planar laser-induced fluorescence (PLIF) technique is used to produce calibrated images of the wave breaking process, and the details of the PLIF measurements are described in terms of the necessary corrections and considerations for the application of PLIF to stratified flows. Results of the flow visualization and wave generation techniques are presented, which show that the nature of internal wave breaking is strongly dependent on the type of breaking internal wave considered.

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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 physical decomposition of the stress tensor for complex flows

Traditionally, the components of the stress with respect to a relevant coordinate system are used for the purpose of stress visualisation and interpretation. A case for using a flow dependent measure to interpret and visualise stress is made for two dimensional flow, together with a suggestion for extending the idea to three dimensions. The method is illustrated for Newtonian and Oldroyd B fluids in both the eccentrically rotating cylinder and flow past a cylinder benchmark problems. In the context of a generalised Newtonian fluid, the relation between the flow-dependent stress measure to other field variables under certain flow conditions, is examined and is indicative of its importance in complex flow.