3D contact patch measurement inside rolling tyres

This paper presents a novel method for capturing the 3D profile of the inside of a rolling off-road vehicle tyre at the tyre-road contact region. This method captures the contact region at all times as the vehicle negotiates obstacles. The system uses a pair of inexpensive digital cameras (capable of capturing up to 300 frames per second) and features a purely mechanical stabilisation system to ensure that the cameras capture the contact region at any wheel speed or vehicle acceleration.The captured images are processed using 3D computer vision techniques using an open source computer vision library called OpenCV. Stereo image pairs are used to create clouds of 3D points showing the profile of the inside surface with good accuracy. Various obstacles were traversed with the deformed tyre profile being compared to the undeformed profile. The system improves on current measurement techniques used to measure the contact patch by capturing a large region of the contact patch, providing full 3D surface geometry, as well as remaining centred on the contact patch irrespective of wheel rotation. The system also enables other imaging techniques to be used such as digital image correlation to determine velocity profiles as well as strain measurements.     

Tulip flames: changes in shape of premixed flames propagating in closed tubes

 The experimental results that are the subject of this communication provide high-speed schlieren images of the closed-tube flame shape that has come to be known as the tulip flame. The schlieren images, along with in-chamber pressure records, help demonstrate the effects of chamber length, equivalence ratio, and igniter geometry on formation of the tulip flame. The pressure/time records show distinct features which correlate with flame shape changes during the transition to tulip. The measurements indicate that the basic tulip flame formation is a robust phenomenon that depends on little except the overall geometry of the combustion vessel. Received: 22 April 1997/Accepted: 7 July 1997     

用三维立体摄影测量技术测活猪后腿肉的体积

研发了一高分辨率三维立体摄影测量系统，并成功地应用于扑捉活猪的三雏形状以用于饲养猪的研究。为调查饲料和猪生长的关系，将32头猪分成高Lysine和低Lysine两饲料组观察。连续14周，每周用该系统拍摄猪的图像一次。该系统有三个立体摄影头，分别拍摄猪的侧面、背面和后腿面的立体图像。为重建猪的三维形状用以科学分析，我们研发了图像处理软件，包括相机标定、立体图像匹配和三维形状合成，并提出一种从每周测得的形状中提取每只猪每周后腿肉的体积的算法。实验证明该系统是精确和可靠的。     

3D Deformation Measurement of Soft Material under Indentation Using Improved Diffraction-Assisted Image Correlation

In inverse finite element-based analysis, complete experimental data collection is critical for multi-parameter identification and physical modeling of all kinds of materials. In this paper, diffraction-assisted image correlation (DAIC) is improved and proposed for the deformation measurement of a soft material under indentation with no blind area. A simple and convenient image-based 3D calibration method was developed, and more accurate formulations for 3D displacement measurement based on a more rigorous imaging model were derived. Using the improved DAIC, a newly developed imaging device with indenter-fixed loading and no blind area is proposed that allows 3D displacements of the whole upper surface of a soft silica gel specimen to be retrieved. The experimental results demonstrate that the proposed method is an accurate, efficient and convenient tool with a simple structure for 3D indentation deformation measurement and illustrate its capabilities to capture deformation in indentation tests with tough testing requirements, such as in situ measurement with limited access (high integration level) and dynamic testing (capturing of synchronously stereo images).     

Multi-Scale Stereo-Photogrammetry System for Fractographic Analysis Using Scanning Electron Microscopy

Current systems for photogrammetry analysis rely mainly on two-dimensional visualization methods, particularly Scanning Electron Microscopy (SEM). The absence of three-dimensional information prevents the determination of important quantitative features such as local roughness and precludes a deeper comprehension of the failure mechanisms. This paper describes a new multi-scale stereo-photogrammetry system for inspection of fracture surfaces based on SEM images. The system facilitates the reconstruction of complete 3D fracture surfaces and provides interactive visualization of the multi-scale structure, thus offering better insight into fracture surfaces at different levels of detail. In particular, a new method has been developed for geometric reconstruction of a 3D textured mesh from SEM stereo images. The mesh is represented as a 3D geometric multi-resolution structure. The sampled images are represented in the form of a multi-scale hierarchical textured structure. Thus, the global shape of the sample is represented by a 3D mesh, while its micro details are represented by textured data. This multi-scale and hierarchical structure allows interactive multi-scale navigation of the 3D textured mesh. The Regions of Interest (ROI) can actually be inspected interactively at different scales by means of optical or digital zooming. Thus, the digital model can be visualized and the behavior of the 3D material can be analyzed interactively. The contributions of this research include: (a) a new 3D multi-scale reconstruction method for SEM stereo images; (b) a new visualization module for multi-scale inspection, modeling and analysis of micro-structures for a variety of materials; and (c) 3D insight into and better understanding of fracture phenomena for material micro-structures. The feasibility of the proposed method is demonstrated on samples of different materials, and a performance analysis is applied on the resulting multi-scale model. The roughness calculation was verified against roughness calculation applied to the optical profilometer.     

On the Hysteresis Phenomenon of Turbulent Lifted Diffusion Methane Flame

This paper reports an experimental investigation on the flow characteristics upstream of a lifted turbulent diffusion flame in the presence of a coflow. Three fuel nozzles made of a long pipe with different outlet geometry were examined. One pair of these nozzles has the same orifice diameter but different normalized lip thickness, and another pair has the same normalized lip thickness but different orifice diameter. The strength of the co-airflow was also varied to assess its impact on the liftoff height of the jet diffusion flame. Previously published studies reported the existence of a hysteresis phenomenon in the liftoff height of a turbulent diffusion flame in the presence of a high co-airflow. That is, as the fuel velocity decreases, the lifted flame base would first move upstream (toward the burner) to a local minima followed by a downstream movement before its reattachment. The results of the present study, however, showed that such a phenomenon does not appear for a fuel pipe having a very small lip thickness. The present results also revealed that in the hysteresis region, the flame base sits where the turbulence intensity experiences its local maxima in the upcoming unburnt mixture. This corroborates the premixed stability theory which is based on turbulence intensity. Based on this, a correlation was found between the flame liftoff height in the hysteresis region and the fuel and co-airflow velocity at the nozzle exit. This relationship predicts successfully the liftoff height trend as a function of the fuel jet and co-airflow velocity and nozzle geometry. Away from the hysteresis region, however, the flame base location tends more toward the outside of the local turbulence intensity maxima. This indicates the limitations of the premixed stability theory in predicting the flame behavior in this region where the effect of the flow large-scale structures becomes important.     

3D-CT(Computer Tomography) Measurement of an Instantaneous Density Distribution of Turbulent Flames with a Multi-Directional Quantitative Schlieren Camera (Reconstructions of High-Speed Premixed Burner Flames with Different Flow Velocities)

In order to provide a suitable technique for 3D observation of high speed turbulent flames, non-scanning 3D-CT(Computer Tomography) technique using a multi-directional quantitative schlieren system with flash light source, is proposed for instantaneous density distribution of unsteady premixed flames. This “schlieren 3D-CT” is based on (i)simultaneous acquisition of flash-light schlieren images taken from numerous directions, and (ii) 3D-CT reconstruction of the images by an appropriate CT algorithm. In this paper, first, as a preliminary research, 3D-CT reconstruction of non-axisymmetric steady flame is made with a single-directional quantitative schlieren system. Next, with custom-made 20 directional schlieren camera, instantaneous density distributions of a high-speed turbulent flames of nozzle exit velocities of 8.0 and 10.0 m/s has been CT-reconstructed. The 3D-views of the reconstructed flame front shape clearly give the information of the flame structure with fine scale corrugations. Based on the distributions, area-enlargement rates of the flame front area are derived, and investigated.     

Measurement of Three-Dimensional Temperature Field of Flickering Premixed Flame with and without Coflow

The three-dimensional (3D) temperature field of the flickering flame with and without coflow can be measured using the flame reaction technique combined with tomographic reconstruction. This combined experimental technique facilitates the non-intrusive measurement of the unsteady 3D temperature field of a premixed methane/air flame. The target flame visualization, which was achieved by the flame reaction of sodium in the supplied mists of sodium chloride solution and line-of-sight intensity images of the flame, was transformed into the temperature field using calibration with the sodium D-line reversal method combined with imaging from six CCD cameras located around the flame. The uncertainty in tomographic temperature measurement was confirmed for the steady axisymmetric flame under the influence of strong coflow. Tomographic temperature measurements were applied to the flickering flame with and without coflow, and the results were analyzed using proper orthogonal decomposition (POD) to understand the unsteady behavior of the temperature field of the flickering flame. The flickering energy was found to be dominant in the first two POD modes. Flame flickering with and without coflow was found to be dominant in the axisymmetric and non-axisymmetric modes, respectively. The characteristics of the flickering flame with and without coflow are discussed in this paper, based on spectrum analysis. The results suggest that the structure of the flickering flame is highly modified by the presence of even a small magnitude of coflow.     

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.     

Estimation of three-dimensional flame surface densities from planar images in turbulent premixed combustion

Turbulence motions are, by nature, three-dimensional while planar imaging techniques, widely used in turbulent combustion, give only access to two-dimensional information. For example, to extract flame surface densities, a key ingredient of some turbulent combustion models, from planar images implicitly assumes an instantaneously two-dimensional flow, neglecting the unresolved flame front wrinkling. The objective here is to estimate flame surface densities from two-dimensional measurements assuming that (1) the flow is statistically two dimensional; (2) the measuring plane is a plane of symmetry of the mean flow, either by translation (homogeneous third direction as in slot burners for example) or by rotation (axi-symmetrical flows such as jets) and (3) flame movements in transverse directions are similar. The unknown flame front wrinkling is then modelled from known quantities. An excellent agreement is achieved against direct numerical simulation (DNS) data where all three-dimensional quantities are known, but validations in other conditions (larger DNS, experiments) are required.     

Development of 3D Pocket Tracking Algorithm from Volumetric Measured Turbulent Flames

The flame pocket formation, including reactant pocket, product pocket, soot pocket, and fluid parcel, is a common phenomenon in turbulent combustion occurred as a response of the flame to flow straining and shearing. Understanding pocket behavior is vital to study the flames in such a regime. This work addresses the research need to experimentally measure and track multiple flame pockets in 3D. For this purpose, volumetric measurements were performed to measure the high-speed turbulent flame structure at 15 kHz based on emission tomography. With the 3D flame structures, a new tracking algorithm was developed to identify and track the multiple flame pockets simultaneously in 3D. The instantaneously tracked 3D flame pockets enabled the extraction of key properties of pocket dynamics, including the favorable formation location, 3D3C movement speed, and pocket expanding/shrinking speed. The developed methods were evidently able to resolve the detailed behavior of flame pockets in highly turbulent flames.

     

Volumetric correlation PIV: a new technique for 3D velocity vector field measurement

A method is proposed that allows three-dimensional (3D) two-component measurements to be made by means of particle image velocimetry (PIV) in any volume illuminated over a finite thickness. The method is based on decomposing the cross-correlation function into various contributions at different depths. Because the technique is based on 3D decomposition of the correlation function and not reconstruction of particle images, there is no limit to particle seeding density as experienced by 3D particle tracking algorithms such as defocusing PIV and tomographic PIV. Correlations from different depths are differentiated by the variation in point spread function of the lens used to image the measurement volume over that range of depths. A number of examples are demonstrated by use of synthetic images which simulate micro-PIV (μPIV) experiments. These examples vary from the trivial case of Couette flow (linear variation of one velocity component over depth) to a general case where both velocity components vary by different complex functions over the depth. A final validation—the measurement of a parabolic velocity profile over the depth of a microchannel flow—is presented. The same method could also be applied using a thick light sheet in macro-scale PIV and in a stereo configuration for 3D three-component PIV.     

Experimental criteria for the determination of fractal parameters of premixed turbulent flames

The influence of spatial resolution, digitization noise, the number of records used for averaging, and the method of analysis on the determination of the fractal parameters of a high Damköhler number, methane/air, premixed, turbulent stagnation-point flame are investigated in this paper. The flow exit velocity was 5 m/s and the turbulent Reynolds number was 70 based on a integral scale of 3 mm and a turbulent intensity of 7%. The light source was a copper vapor laser which delivered 20 nsecs, 5 mJ pulses at 4 kHz and the tomographic cross-sections of the flame were recorded by a high speed movie camera. The spatial resolution of the images is 155 × 121 m/pixel with a field of view of 50 × 65 mm. The stepping caliper technique for obtaining the fractal parameters is found to give the clearest indication of the cutoffs and the effects of noise. It is necessary to ensemble average the results from more than 25 statistically independent images to reduce sufficiently the scatter in the fractal parameters. The effects of reduced spatial resolution on fractal plots are estimated by artificial degradation of the resolution of the digitized flame boundaries. The effect of pixel resolution, an apparent increase in flame length below the inner scale rolloff, appears in the fractal plots when the measurent scale is less than approximately twice the pixel resolution. Although a clearer determination of fractal parameters is obtained by local averaging of the flame boundaries which removes digitization noise, at low spatial resolution this technique can reduce the fractal dimension. The degree of fractal isotropy of the flame surface can have a significant effect on the estimation of the flame surface area and hence burning rate from two-dimensional images. To estimate this isotropy a determination of the outer cutoff is required and three-dimensional measurements are probably also necessary.     

Comparison of Flow and Transport Experiments on 3D Printed Micromodels with Direct Numerical Simulations

Understanding pore-scale flow and transport processes is important for understanding flow and transport within rocks on a larger scale. Flow experiments on small-scale micromodels can be used to experimentally investigate pore-scale flow. Current manufacturing methods of micromodels are costly and time consuming. 3D printing is an alternative method for the production of micromodels. We have been able to visualise small-scale, single-phase flow and transport processes within a 3D printed micromodel using a custom-built visualisation cell. Results have been compared with the same experiments run on a micromodel with the same geometry made from polymethyl methacrylate (PMMA, also known as Perspex). Numerical simulations of the experiments indicate that differences in experimental results between the 3D printed micromodel and the Perspex micromodel may be due to variability in print geometry and surface properties between the samples. 3D printing technology looks promising as a micromodel manufacturing method; however, further work is needed to improve the accuracy and quality of 3D printed models in terms of geometry and surface roughness.

     

Three-dimensional Voronoï imaging methods for the measurement of near-wall particulate flows

A set of stereoscopic imaging techniques is proposed for the measurement of rapidly flowing dispersions of opaque particles observed near a transparent wall. The methods exploit projective geometry and the Voronoï diagram. They rely on purely geometrical principles to reconstruct 3D particle positions, concentrations, and velocities. The methods are able to handle position and motion ambiguities, as well as particle-occlusion effects, difficulties that are common in the case of dense dispersions of many identical particles. Fluidization cell experiments allow validation of the concentration estimates. A mature debris-flow experimental run is then chosen to test the particle-tracking algorithm. The Voronoï stereo methods are found to perform well in both cases, and to present significant advantages over monocular imaging measurements.     

Three-dimensional temporally resolved measurements of turbulence–flame interactions using orthogonal-plane cinema-stereoscopic PIV

A new orthogonal-plane cinema-stereoscopic particle image velocimetry (OPCS-PIV) diagnostic has been used to measure the dynamics of three-dimensional turbulence–flame interactions. The diagnostic employed two orthogonal PIV planes, with one aligned perpendicular and one aligned parallel to the streamwise flow direction. In the plane normal to the flow, temporally resolved slices of the nine-component velocity gradient tensor were determined using Taylor’s hypothesis. Volumetric reconstruction of the 3D turbulence was performed using these slices. The PIV plane parallel to the streamwise flow direction was then used to measure the evolution of the turbulence; the path and strength of 3D turbulent structures as they interacted with the flame were determined from their image in this second plane. Structures of both vorticity and strain-rate magnitude were extracted from the flow. The geometry of these structures agreed well with predictions from direct numerical simulations. The interaction of turbulent structures with the flame also was observed. In three dimensions, these interactions had complex geometries that could not be reflected in either planar measurements or simple flame–vortex configurations.     

In-hole and mainflow velocity measurements of low-momentum jets in crossflow emanating from short holes

Discrete hole film cooling utilizes jet-in-crossflow geometry where the jet is supplied through a short hole which may be pitched relative to the main flow. Typically, the velocity ratio is near one. Under these conditions, the mean flow structure of the jet/mainstream interaction may be strongly affected by the characteristics of the flow within the hole. Magnetic resonance velocimetry (MRV) is used to measure the 3-dimensional mean velocity field for various jets in crossflow with short holes of varied inclination angles and blowing ratios typically of gas turbine applications. Novel measurements of the flow within inclined feed holes are captured using MRV. Secondary flows within the hole are found to be strongly dependent on the inclination of the hole. The traditional counter-rotating vortex pair is observed in the mainstream, as well as high levels of wall-normal vorticity. The 3D vorticity field is used to modify traditional jet-in-crossflow vortex ring theory to apply to low-momentum jets which remain attached to the ejection surface.     

A comparative study of flame-wall interaction and flame-cooling air interaction

This paper presents a three-dimensional (3D) direct numerical simulation (DNS) study of flame-wall interaction (FWI) and flame-cooling air interaction (FCAI). A preheated, methane/air mixture enters a channel with constant temperature walls, where the top wall is effusion cooled. An imposed vertical hot sheet near the inlet creates two flame branches interacting with the top and bottom walls. The flame is observed to be leaner in the region where it interacts with the effusion cooling jets. In this region, the flame is longer and features reduced CO mass fraction. The fluctuations in the heat release rate (HRR) and CO mass fraction are also relatively small near the top wall. Near the bottom wall, finger-like flame structures are formed due to the interaction of turbulent vortices with the flame surface. These flame structures initially move away from the wall as they propagate further downstream before eventually collapsing at the wall. This leads to the creation of regions of high wall heat flux and CO. While analysis of the CO thermochemical state shows a complex picture near the bottom wall, two-dimensional (2D) manifolds can be identified near the top wall. Therefore, a framework to estimate CO mass fraction due to FCAI based on 1D freely-propagating flame solutions is proposed showing a good agreement with the DNS results.     

基于真实细观结构的准脆性材料三维建模及应用初探

采用连续切片的方法获取准脆性材料的表面图像,利用数字图像处理技术检测材料的细观结构并进行矢量化。通过一种简单的变换,将每一切片矢量化的细观结构转换成单层的三维结构,然后将这些切片连续的细观结构逐层叠加,形成整个试件的三维真实细观结构,并针对准脆性材料图像的特点,编制了能够批量处理数字图像并进行细观结构矢量化的程序,建立了与有限元三维网格模型之间的数据接口,模型数据可直接导入岩石三维破裂过程分析RFPA3D系统中,研究真实细观结构对准脆性材料破坏力学行为的影响。以颗粒材料为例,分析了在单轴受压情况下的三维空间裂纹的产生及扩展过程,计算结果显示颗粒分布与界面显著影响材料的破裂模式。     

Three-dimensional displacement measurements using digital image correlation and photogrammic analysis

The application of digital image correlation and stereoscopic principles is used to determine three-dimensional displacements. Two pairs of stereo images of a speckled surface before and after deformation are digitized and correlated to determine the three-dimensional displacements. The images are interpolated so as to account for subpixel displacements. A sequential decision technique and a coarsefine search are employed to increase computer efficiency and decrease run time. Very accurate results are obtained, expecially when the magnification is increased. The effect of camera tilt is shown to be negligible. Theory and experimental verification are presented.