Imaging and correlation used for guided laser lithotripsy

High-speed imaging of phenomena akin to laser lithotripsy is performed. A new method for guiding laser light for the stone destruction is proposed. This is based on a combination of signals obtained from the correlation of fluorescent and ballistic images.     

Imaging of a convective field in a rectangular cavity using interferometry, schlieren and shadowgraph

The present study is concerned with the quantitative imaging of buoyancy-driven convection in a fluid medium that is confined in a horizontal differentially heated rectangular cavity. The horizontal surfaces of the cavity provide a temperature difference, for initiating convection in the fluid. The vertical side walls are thermally insulated. Three imaging techniques, namely laser interferometry, schlieren, and shadowgraph have been utilized. Experiments have been conducted in a cavity of length 447 mm and 32 mm vertical height. The cavity is square in cross-section, and the imaging direction is parallel to its longer side. Convection in air and water have been investigated. Temperature differences in the range of 5–50 K for air and 3–10 K for water have been employed in the experiments. Quantities of interest are the temperature profiles in unsteadiness in the thermal field. At lower temperature differences across the fluid region, temperatures as recorded by interferometry and schlieren are in good agreement with each other. Further, they match the numerical predictions, as well as correlations available in the literature. Imaging based on shadowgraph is not as satisfactory at lower temperature differences. At larger cavity temperature differences, the shadowgraph images become clear enough for quantitative analysis, but the flow becomes time-dependent. The three techniques reveal similar trends in terms of the spatial distribution of temperature gradients and the time scales of unsteadiness. The schlieren and shadowgraph are more suitable for high gradients and interferometry is suitable for low gradients and all these three techniques are not flow visualization tools alone but are appropriate for quantitative imaging of thermal field.     

应用激光全息术研究轴针式喷嘴燃油喷雾特性

本文应用调Q红宝石激光全息照相技术,对轴针喷嘴的燃油喷雾特性进行了研究,证实了喷雾初始段内部存在一个空心的内区,并在广安博之的孔式喷雾分裂长度公式的基础上,提出了应用于轴针喷嘴喷雾的准分裂长度半经验公式,同时还对粒径的空间分布特性进行了测定分析.     

Velocity measurements based on shadowgraph-like image correlations in a cavitating micro-channel flow

Cavitation is generally known for its drawbacks (noise, vibration, damage). However, it may play a beneficial role in the particular case of fuel injection, by enhancing atomization processes or reducing nozzle fouling. Studying cavitation in real injection configuration is therefore of great interest, yet tricky because of high pressure, high speed velocity, small dimensions and lack of optical access for instance. In this paper, the authors proposed a simplified and transparent 2D micro-channel (200–400 μm), supplied with test oil at lower pressure (6 MPa), allowing the use of non-intrusive and accurate optical measurement techniques. A shadowgraph-like imaging arrangement is presented. It makes it possible to visualize vapour formations as well as density gradients (refractive index gradients) in the liquid phase, including scrambled grey-level structures connected to turbulence. This optical technique has been already discussed in a previous paper (Mauger et al., 2012), together with a Schlieren and an interferometric imaging technique. In this paper, the grey-level structures connected with turbulence are considered more specifically to derive information on flow velocity. The grey-level structure displacement is visualized through couples of images recorded within a very short time delay (about 300 ns). At first, space and space–time correlation functions are calculated to characterize the evolution of grey-level structures. Space–time correlations provide structure velocity that slightly under-estimates the real flow velocity deduced from flowmeter measurements. Since the grey-level structures remain correlated in time, a second velocity measurement method is applied. An image correlation algorithm similar to those currently used in Particle Image Velocimetry (PIV) is used to extract velocity information, without seeding particles. In addition to the mean velocity of grey-level structures, this second method provides structure velocity fluctuations. In particular, an increase in structure velocity fluctuations is observed at the channel outlet for a critical normalized length of vapour cavities equals to 40–50%, as expected for the real flow velocity fluctuations. The present study is completed by a parametric study on channel height and oil temperature. It is concluded that none of them significantly impact the critical normalized length for which the fluctuation increase is observed, even though the magnitude of these fluctuations is larger for the higher channel.     

Simultaneous Shadow, Schlieren and Interferometric Visualization of Compressible Flows

For detailed investigations of processes and phenomena in the flow of compressible fluids, it is sometimes necessary to apply more than just one flow visualization technique as each method has its own characteristic strengths and weaknesses. In the case of flows with a low degree of repeatability, it may become mandatory to perform these multiple visualizations within the same experiment at identical or at least almost identical instants. This paper describes how two or more density-sensitive visualization techniques can be coupled in order to obtain simultaneously the distribution of density and its gradient and/or its second derivative in a flow field. The resulting optical systems are more complex than a conventional single visualization apparatus, but they can provide an unprecedented wealth of information about the flow field. By applying multiple visualization techniques, the inherent shortcomings of each individual method can be overcome and the risk of overlooking or misinterpreting certain flow features is reduced.     

Supersonic liquid jets: Their generation and shock wave characteristics

The generation of high-speed liquid (water and diesel fuel) jets in the supersonic range using a vertical single-stage powder gun is described. The effect of projectile velocity and mass on the jet velocity is investigated experimentally. Jet exit velocities for a set of nozzle inner profiles (e.g. straight cone with different cone angles, exponential, hyperbolic etc.) are compared. The optimum condition to achieve the maximum jet velocity and hence better atomization and mixing is then determined. The visual images of supersonic diesel fuel jets (velocity about 2000 m/s) were obtained by the shadowgraph method. This provides better understanding of each stage of the generation of the jets and makes the study of their characteristics and the potential for auto-ignition possible. In the experiments, a pressure relief section has been used to minimize the compressed air wave ahead of the projectile. To clarify the processes inside the section, additional experiments have been performed with the use of the shadowgraph method, showing the projectile travelling inside and leaving the pressure relief section at a velocity of about 1100 m/s. Received 23 January 2001 / Accepted 2 July 2001     

An evaluation of two classes of shock hydrodynamic codes

The accuracy of four industrial shock hydrodynamics codes for blast environments in baffled systems is evaluated based on the shadowgraph data of Reichenbach and Kuhl (1992,3). Both problems involve a planar shock passing through a baffled channel. The numerical methods employed in these codes are representative of two classes, namely, the set of high-resolution schemes advanced in the 1980's, and the classical finite-difference schemes from the late 1960's. The four codes are: (1) the AMR code based on the higher-order Godunov scheme with adaptive grids, (2) the FEM-FCT code based on the flux-corrected transport scheme with unstructured grids, (3) and (4) the finite-difference based HULL and SHARC codes with fixed grids. From the comparisons of these calculations it is concluded that the high-resolution schemes: (1) calculate sharper shocks and sharper density profiles across vortices, (2) predict shear layer rollup forming coherent structures in the spiral vortices immediately downstream of every baffle, and (3) predict development of inviscid instabilities from these shear layers that, upon interaction with the reverberating shocks in the system, quickly become ‘turbulent’. The finite-difference codes predict essentially laminar behavior for the shear layers. Comparisons with shadowgraph data suggest that both classes of codes are able to predict shock reflections and diffractions in the baffled systems. The high-resolution codes give better agreement in the spiral vortices and the shear layers. As expected, turbulent flow features involving highly dissipative flow fields are not predicted by the high-resolution codes. Received March 5, 1995 / Accepted June 20, 1995     

菲涅尔透镜光学系统在弹道测量中的应用

目前,间接阴影照相系统多数采用光学透镜或凹面反射镜组成锥形光路或平行光路,但向大口径推广时受到加工难度及造价的限制。而菲涅尔透镜则以其加工方便、口径极易扩大且价格低廉的特点,显示了其它光学系统无法比拟的优点。本文介绍菲涅尔透镜的基本原理及目前在弹道流场显示中已开拓的几种用途;分析了加工误差对光学质量的影响。大量的拍摄结果表明,清晰度满足要求。同时亦指出了菲涅尔透镜的使用局限性。     

Optical measurement and scaling of blasts from gram-range explosive charges

Laboratory-scale experiments with gram-range explosive charges are presented. Optical shadowgraphy and high-speed digital imaging are used to measure the explosive-driven shock-wave position as a function of time. From this, shock Mach number-versus-distance from the explosion center can be found. These data then yield the peak overpressure and duration, which are the key parameters in determining the potential damage from an explosion as well as the TNT equivalent of the explosive. Piezoelectric pressure gage measurements of overpressure duration at various distances from the explosive charges compare well with theoretical calculations. A scaling analysis yields an approach to relate the gram-range blast to a large-scale blast from the same or different explosives. This approach is particularly suited to determining the properties and behavior of exotic explosives like triacetone triperoxide (TATP). Results agree with previous observations that the concept of a single TNT equivalence value is inadequate to fully describe an explosive yield, rather TNT equivalence factor and overpressure duration should be presented as functions of radius.      

用阴影法观察水声模型的声场

我们建立了一套用于显示模拟海底反射声场的光学系统。实验水声模型是由水-玻璃粉-玻璃-铝组成的三层模拟海底结构,声源是一个管状换能器,其轴线平行于分界面。实验用这套系统记录了从水下三个界面反射的六组波的声场图像。并且分辨出了它们各自的传播路径。从声场图像也可以得出水-玻璃粉界面、玻璃粉-玻璃界面的面波声速,玻璃粉和玻璃中的体波声速以及它们的厚度,结果与预先测出的参数符合。