In recent years, running speed of the trains of conventional lines becomes faster with improving vehicle and rail performance. At the high-speed range compression wave is formed when a high speed train enters a tunnel. This compression wave propagates in the tunnel at the speed of sound. This propagated wave is called "tunnel pressure wave". In some cases, when the station of conventional lines is located in the tunnel, problems such as breaking the window glass have been reported by the tunnel pressure wave at the station. Though the research on pressure wave inside the tunnel of the Shinkansen has been widely studied in connection with "tunnel micro-pressure wave” problems, the number of research reports on the operating speed of conventional lines(130~160km/h) is insufficient. In this study we focused on Hokuhoku line which has maximum operating speed of conventional lines in Japan (160km/h), and we performed the experiment on the gradient of the pressure wave by using diaphragmless driver acceleration system, small train nose model, and tunnel model of the limited express of Hokuhoku line. We have performed the pressure-time variation measurement on the tunnel model, including a station model or signal crossing station [SCS] model. As the thpical train model, we used Streamline-type or Gangway-type for train nose geometry. We have obtained pressure gradient data on several running conditions and observed the temporal .behavior in the tunnel pressure wave. As a result, we clarified large difference in pressure gradient with the train nose geometry and with the cross-sectional area of the tunnel.
Coherent Anti-Stokes Raman Spectroscopy (CARS) method is commonly used for measuring molecular structure or
condition. In the aerospace technology, this method is applies to measure the temperature in thermic fluid with relatively
long time duration of millisecond or sub millisecond. On the other hand, vibrational/rotational temperatures behind
hypervelocity shock wave are important for heat-shield design in phase of reentry flight. The non-equilibrium flow with
radiative heating from strongly shocked air ahead of the vehicles plays an important role on the heat flux to the wall
surface structure as well as convective heating. In this paper CARS method is applied to measure the
vibrational/rotational temperature of N2 behind hypervelocity shock wave. The strong shock wave in front of the
reentering space vehicles can be experimentally realigned by free-piston, double-diaphragm shock tube with low density
test gas. However CARS measurement is difficult for our experiment. Our measurement needs very short pulse which
order of nanosecond and high power laser for CARS method. It is due to our measurement object is the momentary
phenomena which velocity is 7km/s. In addition the observation section is low density test gas, and there is the strong
background light behind the shock wave. So we employ the CARS method with high power, order of 1J/pulse, and very
short pulse (10ns) laser. By using this laser the CARS signal can be acquired even in the strong radiation area. Also we
simultaneously try to use the CCD camera to obtain total radiation with CARS method.
We set up an experimental apparatus to investigate the micro shock wave with high experimental efficiency. We have
performed the micro shock wave visualization with schlieren method by using laser induced plasma, which generated by
a focusing pulsed CO2 laser. The propagation and reflection of the shock waves are investigated. Additionally, a
numerical simulation for compressible and inviscid flow is also performed to understand the propagation characteristics.
As a consequence, our experimental apparatus is very convenient to simulate the explosion phenomena. Moreover, the
result from a numerical simulation shows an agreement with the experimental results.
The cryogenic liquid with vapor bubbles is regarded as phase-changing and unsteady field with heat and mass transfer phenomena. The cryogenic liquid has a characteristic feature of the small latent heat, surface tension, and viscosity, as compared with those of normal temperature water. As the cryogenic laser processing technology is still under development and research, there have been few reports on laser-matter interactions, for example, on micro/nano particle production. This paper firstly deals with behavior of a cavitation bubble induced by a pulsed YAG laser in liquid nitrogen. The interaction of the bubble with the solid wall has been studied by flow visualization, and, furthermore, the laser-particle processing in liquid nitrogen has been studied. As our research on cryogenic laser-submicron particle processing is in the first stage of experiment, the paper concentrates mainly on the microscopic observation of the laser-processed holes on Al surface and small particles.
The behavior of Vapor bubbles in cryogenic liquid is regarded as a cryogenic and phase-changing flow field, where instability of bubble surface becomes larger than those in normal temperature liquid as water or oil, since the cryogenic liquid has characteristic feature of small latent heat, surface tension, and viscosity. The cavitation phenomena in cryogenic temperature range are regarded as vacitation in the liquid of near-boiling point. The cryogenic cavitation, however, have a significant influence on solid surfaces due to their weakness in cryogenic range. In this paper, shock waves discharged from a pulse-laser induced bubble and behavior of the bubble are experimentally investigated. Pulsed YAG laser is used to produce a bubble in cryogenic liquid nitrogen, and shock waves are visualized by using a digital still camera with schlieren method.
This paper deals with a trial experiment of decomposition of environmental gas R-12 by the pulsed TEA CO2 laser. Nowadays refrigerant R-12 and other hydro-chlorofluorocarbon gases are strongly prohibited to produce, as these gases have both strong ozone-depleting effects and green-house effects. The gases of already produced by huge amount should be decomposed as fast as possible by suitable technical methods. Along with the conventional kiln furnace of cement, arc discharge and the HG discharge are good methods for the freon decomposition. Both methods, however, have the weakness of electrode damages (arcing) or low-pressure operation (HF discharge). High power CO2 laser seems to have good properties for such decomposition with favorable wavelength for the absorption. In our small-scale experiment of gas decomposition a pulsed TEA CO2 laser of several joules is utilized to produce the plasma in R-12 flow channel of glass tube. The withdrawal of decomposed gases is performed by Ca alkalized water. The deposit mass is measured, and powder X-ray diffraction measurement is carried out on the deposit powder. The possibility of our laser gas decomposition is discussed.
When space vehicles reenter onto the atmosphere with velocity over 10 km/s, radiative heating from the shocked air ahead of the vehicle plays an important role on the heat flux to the wall surface as well as convective heating. So far, spectroscopic study has been developed for temperature measurement of radiation behind strong shock waves. In this paper, CARS method (Coherent Anti-Stokes Raman Spectroscopy) for radiation behind strong shock waves in gases has been applied. The CARS measurement system consists of a YAG laser, a dye laser, and high-sensitivity CCD camera systems. The preliminary tests have been performed for the purpose of detecting CARS signal from strong shock waves with velocity range over 5km/s.
Three-dimensional flow phenomena have been observed in a shock tube experiment for shock waves and vortices by using an interferometric CT (Computed Tomography) technique with a N2 pulse laser. A model with small duct, which has a pair of circular open ends, is introduced in a test section of diaphragmless shock tube, and can be rotated around its central axis to change the observation angle. The projection image of density distribution for each observation angle is obtained by using a fixed Mach-Zehnder interferometer. Three-dimensional density distribution is reconstructed from these projection images. The shock Mach number is 2.3 in nitrogen gas of 19.4kPa initial pressure at the exits of the open ends. The resultant 3-D density flow fields are illustrated by several imaging technique to clarify 3-D features of shock waves, vortices, and their mutual interactions. A computational fluid dynamics (CFD) simulation is also applied to the 3-D flow fields. The CFD results can represent density and another properties in flow fields, and these properties are useful for identifying the phenomena. The mutual validation between the experimental CT density results and these CFD results is discussed. Three-dimensional features of flow fields are investigated in detail by analyzing the experimental CT results with CFD results.
This paper describes some applications of image-intensified CCD camera systems to high-speed photography of radiation behind strong shock waves in low-density air to get two- dimensional images of total radiation and spatial-resolved spectra in shock tube experiments. The resultant images of total radiation are shown for incident shock waves and shock reflections on a wedge. Some two-dimensional flow features are clarified for these shock wave phenomena. A couple of CCD camera systems are applied to simultaneous observation of the total radiation and its spatial-resolved spectra along the tube axis, giving us a clear understanding for the spatial relationship between them.
An interferometric CT technique is developed to observe three- dimensional phenomena in shock tube experiments, and is applied to investigate three-dimensional features of shock waves and vortices discharged from a square open end. A small duct model is introduced in the test section of the shock tube, and is rotated around its central axis to change the observation angle in order to obtain the finite-fringe interferograms from multi-directions. The CT images of the density distribution are obtained with good quality by carefully selecting the projection data within the limited ranges of the incident shock Mach number Mi and the frontal shock position. The rotation angle is changed from 0 degree to 90 degrees in an interval of 5 degrees. For Mi equals 1.50, the three-dimensional nature of the distortion of vortices and the shape of secondary shock wave are clearly illustrated by pseudo-color images of the density distribution and isopycnic surfaces. The CFD images exhibit a good agreement with the CT images.
Some modification of the YBaCuO phase analysis method using the Raman spectra is proposed. It is based not only on the analysis of the corresponding Raman band maxim positions, as usually, but on the analysis of the integral contours of these bands as well. The 400-500 cm-1 region of Raman spectra of the YBaCuO single crystals was studied for this purpose. The proposed method was realized in original PC-program, which allows fulfilling an express, non- destructive and local phase control for HT SC samples.
Shock waves, which are generated by pulsed discharges in excimer lasers, cause a complicated time history of density of laser medium, and successively causing arcing and non- homogeneous excitation in the laser cavity. To clarify the characteristics of the generation and propagation of shock waves by these strong pulse-discharges in an excimer laser, experimental study and numerical analysis using a TVD scheme and Grid-distortion Splitting Method have been carried out. The shock waves are visualized using a CCD color schlieren technique. Numerical calculation by Yee's symmetric TVD scheme is performed for the conditions that corresponds to the experiments. The initial conditions of our calculations are determined by measured results by the laser schlieren method, and from schlieren photographs. The propagation and attenuation of the shock waves in our experimental operation are also visualized from the numerical results and compared to the experimental results.
In this paper laser applications to fluid dynamical problems are presented. Firstly as for the recent research on cavitations, pulsed-laser-induced cavitation bubble in liquid nitrogen is studied. The bubble is produced by focused and pulsed irradiation of second harmonics of YAG laser in the cryostat. The dynamics of laser-induced bubble is visualized by high-speed shadowgraphs and schlieren photographs by an image-converter camera (Imacon-790). Bubble and solid wall interactions are also investigated. Based on the results obtained, a novel laser surface processing technology using the pulse-laser-induced cavitation bubbles is secondly proposed. The possibility of cold material surface processing by produced cavitation bubble is discussed including the cryogenic range. Furthermore, discussing by the fundamental results of the experiment of laser-gas molecular absorption, the possibility of decomposition of environmental gases by strong CW CO2 laser irradiation is also studied. Freon 12, 113, and other environmental gases including SF6 are very tough to be decomposed, and they break effectively the ozone molecules at high altitude above the Earth, or they heat up the earth. The wavelength range of the infrared laser is suitable for the molecular absorption to increase their temperature to be ionized. The possibility and trial experiments are discussed.
Studies on some laser interactions to fluid materials are presented. Firstly, pulsed-laser-induced cavitation bubble in liquid nitrogen is investigated by YAG laser irradiation. High speed schlieren photographs by Imacon-790 are taken to observe the laser-induced bubble dynamics. Bubble and solid wall interactions are also investigated. Based on the results obtained, a novel laser surface processing technology using the pulse-laser-induced cavitation bubbles is secondly proposed. Furthermore, discussing by the fundamental results of the experiment of laser-gas molecular absorption, the possibility of decomposition of environmental gases by CW CO2 laser irradiation is also studied.
A composite material including optical fibers is submitted to the mechanical effects induced by a high-power carbon-dioxide laser radiation in order to test its behavior when undergoing repetitive stress. The experiments pointed out that such a new material could be used to monitor the stress to which it is submitted.
This paper describes an investigation on the behavior of laser induced vapor bubble in cryogenic liquid nitrogen. The bubble is produced by the pulse ruby laser beam focused in the special cryostat. The dynamics of the laser-induced bubble is visualized by high speed photography. The pressure pulse signals from the bubble motion in the liquid nitrogen are measured under equilibrium or nonequilibrium conditions. Furthermore a numerical study is also performed on the dynamics of single spherical bubble in cryogenic liquid under the conditions corresponding to our experiment. Among the results the rapid growth of bubble and its rebounds have been visualized even in near-equilibrium conditions. Calculated results are compared with the experimented data.
To clarify the characteristics of the generation and propagation of shock waves generated by pulse discharges in an excimer laser, numerical simulation using a TVD scheme and a grid- distortion splitting method is carried out. The calculations are conducted in conditions corresponding to our visualization experiments and high-frequency operations. The propagation and attenuation of the shock waves in the high-frequency operation are visualized from the numerical results.
High repetition rate excimer lasers are expected for wide industrial application. The power of excimer laser, however, decreases rapidly in a higher repetition rate operation. Shock or acoustic waves, which are caused by the periodic pulse discharge, may limit the repetition rate of an excimer laser up to 2.5 kHz. Such waves cause inhomogeneity of gas density in the discharge region of the excimer laser. In high repetition rate operation this inhomogeneity remains at the next discharge. Arcing may be generated by this inhomogeneity and the homogeneous excitation of the laser gas is obstructed. Although these phenomena have been reported, the research for the effects of shock waves has remained insufficient. And the relation between these shock waves and discharge phenomena has not been clarified. To resolve this problem, we developed a scaling model chamber of a UV preionized excimer laser cavity with windows for flow visualization. We report the first result by using this model and Schlieren technique in a pure helium gas case. In our experiment three types of shock waves are found in the discharge cavity. Those shock waves are generated from the boundary of the main discharge area, from sparking pin gaps, and from the main electrode surfaces. In this study we focus on the effect of xenon gas on the generation and the propagation of shock waves. Components of the Xe-Cl excimer laser gas are helium, xenon, and hydrogen chloride. In those gases xenon has the largest molecular weight of 131.29. So we conclude xenon plays an important role in the shock wave propagation and in discharge phenomenon.
The discharge effects on gas flow lasers have been treated from both aspects of the activating features to the laser performance and of demerits by electrical heating and disturbances in the laser medium. The approach has been from two types of studies, namely on the continuous discharge in CO2 supersonic mixing electric discharge laser (EDL) and on the pulsed discharge in excimer laser. For CO2 supersonic mixing EDL, small scale experiments have been performed on the small signal gain coefficient and on the laser power, together with the numerical analysis by using the quasi-one-dimensional and vibrationally nonequilibrium equation system. Discharge effect is included by solving the Boltzmann equation for electron energy distribution function, and power extraction analysis also is carried out. By comparing the experimental results, various characteristics have been clarified on this type of supersonic discharge laser. As for the pulsed discharge, flow visualization experiments have been conducted in the model cavity of excimer laser, along with a numerical calculation on the one- dimensional Euler equation system by TVD approach. There have been three types of waves in the laser cavity, and Mach numbers of horizontally propagating main waves have been discussed from both numerical and experimental aspects.
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