3D holographic PIV with a forward-scattering laser sheet and stereoscopic analysis

 A new 3D PIV system combining holography and stereoscopic PIV is presented. The double pulsed holographic recording relies on the forward scattering of particles in the laser sheet. The holographic images of the particles are used for a stereoscopic PIV analysis. An imaging system with a rightangle prism is used to acquire a stereoscopic pair of images. The application of the system to the vortex flow from an inclined delta wing shows the prospects and limitations of the technique. Received: 23 December 1996/Accepted: 1 June 1997     

Pseudo-schlieren CT measurement of three-dimensional flow phenomena on shock waves and vortices discharged from open ends

1A pseudo-schlieren technique is applied to the interferometric computed tomography (CT) measurement of three-dimensional (3-D) shock waves discharged from a square open end and a pair of circular open ends in a shock tube experiment. The experiment is performed for incident shock Mach numbers of 2.0 and 2.2 in nitrogen gas under supersonic post shock flow conditions at the open end. The 3-D density-gradient distributions are evaluated from the CT data of the 3-D density distributions, and are depicted in gray-scale CT images of the gradient magnitude and in pseudo-color CT images of the gradient component. The resultant pseudo-schlieren CT images clearly illustrate the 3-D flow features of shock waves, contact surfaces, and the other sharp density fronts. Their image characteristics and meaning in gas dynamics are discussed in comparison with the pseudo-color images of the density. We demonstrate that the pseudo-schlieren CT technique is a useful tool for studying 3-D problems in shock dynamics. Communicated by K. Takayama PACS 47.32.cd; 47.40.Nm; 47.80.jk     

Measurement of velocity fields in ball mills

This paper describes a computer vision technique used in measuring the velocity field related to the motion of balls in a ball mill. A camera combined with two color flashes facing an experimental ball mill is used to take pairs of pictures by triggering the flashes in sequence with a time interval Δt. In order to ensure consistency, the pictures are of different colors and in a single frame. The ball positions in each image are detected by image-processing techniques, and the velocity is then the displacement of each ball between two successive images divided by Δt. The measurement error is estimated by measuring the displacements from a pair of images when the balls remain stationary.     

Stereoscopic imaging and reconstruction of the 3D geometry of flame surfaces

Stereoscopic imaging and reconstruction of turbulent impinging diffusion flames have been demonstrated. The 3D geometry of the flame surface is reconstructed from any pair of stereo images through digital image processing and computer vision. Only one camera is required to capture the pair of stereo images due to the use of a stereo adapter. As a result the pair of flame images is captured on a single CCD chip. They are then post-processed to produce a 3D geometry of the flame. The 3D results and surface area calculations demonstrate the potential of stereo visualisation, data extraction and the validity of the proposed methodology.     

Visualizing surface strain distribution of facial skin using stereovision

An experimental technique has been developed for measuring and visualizing strain distribution on facial skin. A stereovision technique based on digital image correlation is employed for obtaining the displacement distribution on the human face. Time-variation of the movement of the facial skin surface is obtained from consecutive images obtained using a pair of high-speed cameras. The strains on the facial skin surface are then obtained from the measured displacements. The performance of the developed system is demonstrated by applying it to the measurement of the strain on facial skin during the production of sound. Results show that the strains on facial skin can be visualized. Further discussion on the relationship between the creation of wrinkles and strains is possible with the help of the developed system.     

Simultaneous concentration and velocity field measurements in a shock-accelerated mixing layer

A novel technique to obtain simultaneous velocity and concentration measurements is applied to the Richtmyer–Meshkov instability. After acceleration by a Mach 2.2 shock wave, the interface between the two gases develops into a turbulent mixing layer. A time-separated pair of acetone planar laser-induced fluorescence images are processed to yield concentration and, through application of the Advection-Corrected Correlation Image Velocimetry technique, velocity fields. This is the first application of this technique to shock-accelerated flows. We show that when applied to numerical simulations, this technique reproduces the velocity field to a similar quality as particle image velocimetry. When applied to the turbulent mixing layer of the experiments, information about the Reynolds number and anisotropy of the flow is obtained.     

Impact of hindered Brownian diffusion on the accuracy of particle-image velocimetry using evanescent-wave illumination

The nano-particle image velocimetry technique uses evanescent-wave illumination generated by total internal reflection at the wall to excite colloidal neutrally buoyant fluorescent tracer particles. The displacement of these particles over time as they are convected by the flow then gives the flow velocity components tangential to the wall. Since the extent of the illumination region normal to the wall is comparable to the particle diameter, a major source of error in this technique is particle mismatch within a pair of images due to Brownian diffusion causing a particle to move in to or out of the illuminated region. The brightness (proportional to the amount of imaged fluorescence) and size of individual particle images in nPIV data are discussed. A sequence of artificial nPIV images are generated for a known uniform velocity field with the particles subject to hindered Brownian diffusion. The velocity fields calculated from these artificial images are compared with the known velocity field to determine the effect of Brownian diffusion-induced particle mismatch on nPIV accuracy. A similar analysis is carried out for experimental nPIV images. The results provide design guidance for experimental measurements using the nPIV technique.     

Velocity field and surfactant concentration measurement techniques for free-surface flows

 Noninvasive measurement techniques were developed to obtain the surfactant concentration and the velocity field of a vortex pair interacting with a contaminated free surface. The optical method of second-harmonic generation (SHG) was utilized to determine the time-varying concentration of a surfactant at a point on the surface, and the established technique of digital particle image velocimetry (DPIV) was used for the measurement of the velocity field. The evolution of the initially uniform, insoluble monolayer along with the vorticity field are described. One of the present limitations is the temporal resolution of the concentration measurements which is of order 5 Hz. The extension of the SHG technique to high Reynolds number, turbulent flows is discussed. Received: 24 January 1996 / Accepted: 17 July 1996     

A cross-correlation technique for velocity field extraction from particulate visualization

A rapid time series of photographs of the horizontal cross-sections of several y ⁺ locations were taken of a turbulent open-channel water flow with Re _d = 3,900. A pair of photographic images were obtained with a time difference of 1.3 v/u ² at each y ⁺ locations. The pictures were digitized into 8 bit data with a spatial resolution of 2.5 viscous scales. Instead of identifying discrete particles, a variable interval spatial correlation technique was used to extract the velocity components. With this technique, two-dimensional spatial cross-correlations of the illumination intensities were taken between a pair of picture images. The correlations were taken over small areas and the peak of the correlation coefficients were used to obtain the convection velocity yielding the u and w components of velocity. Some statistical properties were calculated and are shown to be comparable with previous data. Spatial correlations of the velocity components revealed some unique characteristics related to the structure of turbulence.     

Evolution of vortex structure in boundary layer transition induced by roughness elements

The particle image velocimetry (PIV) and hydrogen-bubble visualization technique are used to investigate the flat-plate boundary layer transition induced by an array of roughness elements. The streamwise evolutions of the mean and fluctuation velocity are analyzed, and the critical Reynolds number Re _k,c is determined between 339 and 443 under current experimental setup. The hairpin vortices shed from supercritical roughness elements are visualized by swirling strength, in which two pairs of counter-rotating vortices can be observed: one vortex pair is the manifestation of the neck of the hairpin vortices shed from the top of the roughness; the other vortex pair, which originates from the lower part of the roughness, comes from the streamwise vortices that are constantly perturbed by the hairpin vortex shedding.     

Direct measurement of local instantaneous laminar burning velocity by a new PIV algorithm

This paper presents a new experimental approach using PIV technique to measure the local instantaneous laminar burning velocity of a stretched premixed flame. Up to now, from experimental techniques, this physical property was only accessible in average and the instantaneous interactions of flame with flow structures, mixture variations and walls could not be considered. In the present work, the local burning velocity is measured as the difference between the local flame speed and the local fresh gas velocity at the entrance of the flame zone. Two original methods are proposed to deduce these quantities from pair of particle images. The local flame speed is measured from the distance between two successive flame positions. For the flame localization, a new extraction tool combined with a filtering technique is proposed to access to the flame front coordinates with sub-pixel accuracy. The local fresh gas velocity near the flame front is extracted from the maximum of the normal velocity profile, located within 1 mm ahead of the flame front. To achieve the required spatial resolution, a new algorithm based on adaptive interrogation window scheme has been developed by taking into account the flow and flame front topologies. The accuracy and reliability of our developments have been evaluated from two complementary approaches based, respectively, on synthetic images of particle and on the well-established configuration of outwardly propagating spherical flames. In the last part of the paper, an illustration of the potentials of our new approach is shown in the case of a laminar flame propagating through a stratified mixture.     

Digital interferometry for flow visualization

Digital holographic interferometry is a hybrid optical-digital technique for determining the phase of an interferogram. This technique improves the accuracy of interferometric measurement of fluid properties and enhances the utility of interferometric flow visualization. Displays of the interferometric phase produce excellent images of weakly refracting two-dimensional flows and can be used to produce integral projection images of three dimensional flows which differ from and complement schlieren and shadowgraph images. The technique is explained herein and examples of its use in both continuous wave and pulsed interferometry are presented.This work was presented in part at the 1985 Optical Society of America Annual Meeting     

An extension of digital PIV-processing to double-exposed images

 In this paper digital processing techniques for PIV (Partical Image Velocimetry) using double-exposed particle images have been studied. It has been found that a pattern matching technique is significantly superior to the traditional autocorrelation method in the case that a large particle displacement between the double exposures is present on the image. In PIV using double-exposed images, the image shifting technique is usually used to solve the directional ambiguity problem. The performance of PIV using autocorrelation technique is dependent on the flow speed and the amount of image shift applied. This dependence, for example, causes a difficulty of autocorrelation in flows close to a solid boundary. The present study shows that a pattern matching technique eliminates such a difficulty. At the same signal-to-noise ratio, the pattern matching techndique has a better spatial resolution than that of autocorrelation. In concert with the pattern matching technique, PID (Particle Image Distortion) can be applied to double-exposed images, further improving the reliability and accuracy of velocity estimates of PIV in the presence of large velocity gradients. Generally speaking, PIP-matching and PID extend the validity of PIV using double-exposed images. The total processing time required by the PIV using the pattern matching technique and one PID iteration is of the same order as that required by the PIV using autocorrelation. Received: 7 July 1995 / Accepted: 11 September 1997     

Limits and accuracy of the Stereo-LFC PIV technique and its application to flows of industrial interest

Particle image velocimetry with local field correction (LFC PIV) has been tested in the past to obtain two components of velocity in a two dimensional domain (2D2C). When compared to conventional correlation based algorithms, this advanced technique has shown improvements in three important aspects: robustness, resolution and ability to cope with large displacements gradients. A further step in the development of PIV algorithms consists in the combination of LFC with the stereo technique, which is able to obtain three components of velocity in a plane (2D3C PIV). In this work this combination is implemented and its performance is evaluated carrying out the following two different tasks:

Measurement of velocity and velocity derivatives based on pattern tracking in 3D LIF images

Pattern tracking in consecutive 3D LIF images based on least squares matching (LSM) of grey levels has been developed recently for velocity and velocity gradient measurements. The shortcomings of this method are clearly shown. The present article presents an improvement on this method by introducing a local multi-patch (LMP) technique through the LSM approach. The method is validated using the flow field of a turbulent channel flow obtained by direct numerical simulation (DNS) and a synthetic image with grey-level patterns. The results show that LMP matching allows the determination of the velocity and the velocity gradient fields with high accuracy including the second derivatives. Measurements of a round non-buoyant jet are presented which demonstrate the good performance of the method when applied under laboratory conditions. This method can also be applied on two-dimensional images provided that the flow is strictly two-dimensional. Received: 7 July 1999/Accepted: 13 November 1999     

Grid Method for Microscale Discontinuous Deformation Measurement

The objective of this paper is to explore both grid method and Digital Image Correlation (DIC) technique for microscale and discontinuous displacement measurements, such as those associated with crack tips. First, the principle of the grid method is revisited. The grid method and DIC technique are then applied to computer generated images to calculate the displacement field around crack tips. Finally, the grid method is applied to actual experimental images of fracture tests which are conducted inside a Scanning Electron Microscope (SEM) chamber. A new technique is developed to generate microscale pattern that is suitable for both grid method and DIC technique. The displacement fields calculated from grid method are compared with those from DIC technique to identify the strengths and weaknesses of each technique for the microscale and discontinuous displacement measurements. It has been determined that grid method can obtain data closer to the discontinuity than DIC; however, DIC produces smoother displacement fields at the far field. Using this new pattern generation technique, both grid method and DIC technique can be applied to the fracture test at the microscale to complement with each other to achieve the best experiment results.     

Digital particle velocimetry technique for free-surface boundary layer measurements: Application to vortex pair interactions

A variation of the digital particle image velocimetry (DPIV) technique was developed for the measurement of velocity at a free surface for low Froude number flows. The two-step process involves first determining the location of the free surface in the digital images of the seeded flow using the fast Fourier transform-based method of surface elevation mapping (SEM), which takes advantage of total internal reflection at the interface. The boundary-fitted DPIV code positions the interrogation windows below the computed location of the interface to allow for extrapolation of interfacial velocities. This technique was designed specifically to handle large surface-parallel vorticity which can occur when the Reynolds number is large and surface-active materials are present. The SEM technique was verified on capillary-gravity waves and the full boundary-fitted DPIV technique was applied to the interaction of vortex pairs with a free surface covered by an insoluble monolayer. The local rise and fall of the free surface as well as the passage and return of a contamination front was clearly observed in the DPIV data. Received: 20 June 1999/Accepted: 27 November 2000     

A rubidium magnetometer using small vapour cells for homogeneity measurements

Summary A magnetometer is described, which uses the optical pumping technique in rubidium. This apparatus is of special small design to measure the homogeneity of a magnetic field produced by a pair of large coils in Helmholtz position. Some of the results are discussed in connection with the dimensions of the cells and the intensity of the high frequency field.     

Hydrodynamic interaction between a pair of bubbles ascending in shear-thinning inelastic fluids

The interaction of two bubbles rising in shear-thinning inelastic fluids was studied. The experimental results were complemented by numerical simulations conducted with the arbitrary Lagrangian–Eulerian technique. Different initial alignments of the bubble pair were considered. Similarities and differences with the Newtonian fluids were found. The most noticeable difference is the so-called drafting–kissing–tumbling (DKT) process: for the case of bubbles rising in thinning fluids, the tumbling phase does not occur and the pair tends to form a stable doublet. The DKT process is also influenced by the amount of inertia and deformability of the individual bubbles and the initial angle between them. The experimental and numerical results suggest that the thinning wake formed behind the bubbles plays an important role in the speed of the pair and the formation of clusters in thinning fluids.     

The interferometric strain rosette technique

An interferometric strain rosette can be used to measure three in-plane strains. The strain rosette consists of three microindentations produced on an object surface. Upon illuminating the indentations with laser light, each pair of indentations acts as a two-point source generating a pair of Young's interference fringe patterns. When the object is deformed, the distance between the indentations is altered. By measuring the change of spacing of the Young's fringes, the strain in the direction of the separation of the indentations is measured. Three indentations are at the vertexes of an equilateral triangle to constitute a strain rosette. Each pair of the microindentations enables measurement of an axial strain in the indentation separation. The rosette measures simultaneously three in-plane strains in the directions of the triangular sides. As three in-plane strains are measured, the in-plane shear and two normal strains can be found. Compared with a resistance strain rosette, the interferometric strain rosette has great features such as non-contacting and a short gage length. In addition, the interferometric strain rosette can measure large elastoplastic strains and is applicable to measurements at elevated temperatures. The theory with experimental evaluation is presented. Measurement sensitivity of the technique is discussed. Potential applications and limitations of the technique are to be described as well.