In this paper the improved Background Oriented Schlieren technique called CBOS (Colored Background Oriented Schlieren) is described and used to reconstruct density fields of three-dimensional flows. The Background Oriented Schlieren technique allows to measure the light deflection caused by density gradients in a compressible flow. For this purpose the distortion of the image of a background pattern observed through the flow is used. In order to increase the performance of the conventional Background Oriented Schlieren technique, the monochromatic background is replaced by a colored dot pattern. The different colors are treated separately using suitable correlation algorithms. Therefore, the precision and the spatial resolution can be highly increased. The CBOS technique is explained and applied to the measurement of the flow field around a free-flight space model. These flow conditions lead to very strong shocks in the front region of the model. Therefore a special arrangement of the different colored dot patterns in the background allows astigmatism in the region with high density gradients to be overcome. An algebraic reconstruction technique (ART), taking into account forward and backward projections, is then used to reconstruct the density field of such flows from CBOS measurements. It could be shown that the accuracy and the spatial resolution of the CBOS technique allows us to obtain a reliable reconstruction of the density field. Especially for complex flows the distribution of the density helps considerably to understand the flow phenomena.
The Background Oriented Schlieren (BOS) technique is one of the novel measurement techniques and its application range is very wide. The principle of BOS is similar to that of the conventional schlieren technique, it exploits the bending of light ray caused by a refractive-index change corresponding to the density change in the medium. The BOS technique allows the quantitative measurement of density with very simple experimental setup and proper image analysis. Only a background and a digital camera are required for the experiment, so that even the real scale experiments can be realized. In recent years, the development of the high-speed camera is remarkable and so many high-speed phenomena can now be captured. To realize the precise measurement with BOS technique using high-speed camera, higher resolution (larger number of pixels) is desirable.
In this paper, with a technical support from Nobby Tech Ltd., a 4K high-speed camera (4096 × 2160 pixels) is applied to the BOS measurement of the lateral jet/cross flow interaction filed in the supersonic wind tunnel test as a trial of the quantitative density measurement with higher resolution. The measurement system consists of a 4K high-speed camera and a pulsed laser for background illumination. A telecentric optical system is also employed to improve the spatial resolution of the measurement. The measurement results of BOS technique up to 1000 fps with higher resolution are discussed.
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.
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.
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.
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