Masanori Ota

Computed-tomographic density measurement of supersonic flow field by colored-grid background oriented schlieren (CGBOS) technique

The background oriented schlieren (BOS) technique is one of the visualization techniques that enable the quantitative measurement of density information in the flow field with very simple experimental setup. The principle of BOS is similar to the conventional schlieren technique, which exploits the bending of light caused by refractive index change corresponding to density change in the medium and both techniques are sensible to density gradient. In this report we propose colored-grid background oriented schlieren (CGBOS) technique. The experiments were carried out in a supersonic wind tunnel of test section size 0.6 × 0.6 m2 at JAXA-ISAS. A colored-grid pattern was used as background image and density gradient in vertical and horizontal direction was obtained. Computed tomographic reconstructions of 3D density information of the supersonic flow field around an asymmetric body from multi-directional CGBOS images were examined.

Three-Dimensional Density Measurement of Supersonic and Axisymmetric Flow Field by Colored Grid Background Oriented Schlieren (CGBOS) Technique

The background oriented schlieren (BOS) technique is one of the visualization techniques that enable the quantitative measurement of density information in the flow field with very simple experimental setup. BOS requires only a background and a digital still camera and it can realize the quantitative measurement of density. In this report we propose the colored grid background image for BOS technique (CGBOS). The experiments were carried out in the 0.6 m x 0.6 m test section of supersonic wind tunnel at JAXA-ISAS. The measurement setup consisted of metal halide ramps, a colored grid background image and a digital still camera. A colored grid pattern was used as background image and density gradient in vertical and horizontal direction was obtained. The measurement result and prospect of CGBOS technique are reported.

CARS Measurement of Vibrational/Rotational Temperatures with Total Radiation Visualization behind Strong Shock Waves of 5–7 km/s

In the development of aerospace technology the design of space vehicles is important in phase of reentry flight. The space vehicles reenter into the atmosphere with range of 6–8 km/s. 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. The experimental data for re‐entry analyses, however, have remained in classical level. Recent development of optical instruments enables us to have novel approach of diagnostics to the re‐entry problems. We employ the CARS (Coherent Anti‐Stokes Raman Spectroscopy) method for measurement of real gas temperatures of N2 with radiation of the strong shock wave. The CARS signal can be acquired even in the strong radiation area behind the strong shock waves. In addition, we try to use the CCD camera to obtain 2D images of total radiation simultaneously. The strong shock wave in front of the reentering space vehicles is experimentally r...

Propagation Characteristics of the Shock Wave in Small Diameter Tubes at Atmospheric Initial Driven Pressure

Recently, the micro-shock waves have attracted attention of researchers in several fields of science. It is well known that the shear stress and the heat transfer between a test gas and a wall lead to significant deviations from the normal theory of a shock wave propagating in a small diameter shock tube[1][2]. In our previous research, we developed a diaphragmless driver section using a rubber valve, and estimated the valve opening characteristics[3]. In addition, the shock wave measurements in the driver pressure range from 0.1 to 0.2 MPa were made in 1 and 3 mm inner diameter shock tubes with rubber valves, while the shock wave measurements in small diameter tubes were performed at a relatively low driver pressure[4][5][6]. At relatively high driver pressures over 0.5 MPa, shock wave measurements using the diaphragmless driver section with the rubber valve are very difficult to perform, because of the structural properties of the section. The technique of two pistons for diaphragmless driver section, which was invented and applied by Oguchi, et al.[7] and Maeno, et al.[8]. Its technique is successfully used especially in the larger diameter shock tube and initial pressure ratio, and has some accomplishments[9][10]. They concluded that, the diaphragmless driver section with two pistons is capable of producing the shock waves and is very convenient at the driver pressure range from 0.3 to 0.9 MPa, and there are some advantages over the convential shock tube as high reproducibility, and so on. However, there are no reports about applying this technique for generation of the shock wave in the small diameter tube. In this study, we simultaneously measured the velocities of shock waves and the density ratios across the shock wave, generated by originally developed diaphragmless driver section with two pistons, propagating in 2 and 3 mm inner diameter tubes by using laser differential interferometer.

Experimental investigation of laser-induced bubble dynamics near elastic/soft material in distilled water

This study deals with an experimental investigation of the dynamics of laser-induced single bubble near the rigid material plate and near the elastic/soft material plate in the distilled water at room temperature under atmospheric pressure. A pulsed Nd:YAG laser was focused into the distilled water to make plasma and single bubble. The bubble repeated expanding and shrinking motion several times, and then collapsed. This behavior occurred on the sub-millisecond timescale. The solid wall near the bubble makes an asymmetric flow field. Many experiments on the behavior of laser-induced bubble near the rigid material have been reported. The bubble near the solid wall moves toward the rigid wall during its shrinking and rebounding process. The behavior of laser-induced bubble near the soft material, however, has not been well clarified. The soft material such as body tissue can deform and influence the behavior of the laser-induced bubble. Since the high peak power laser has been applied in the field of bioengineering and medical treatment, it is of great importance to clarify the effect of the soft material near the laser-induced cavitation bubble. In this research the behavior of laser-induced bubble near the elastic/soft material was visualized with schlieren method and investigated.

Nonlinear CARS measurement of nitrogen vibrational and rotational temperatures behind hypervelocity strong shock wave

The hypervelocity strong shock waves are generated, when the space vehicles reenter the atmosphere from space. Behind the shock wave radiative and non-equilibrium flow is generated in front of the surface of the space vehicle. Many studies have been reported to investigate the phenomena for the aerospace exploit and reentry. The research information and data on the high temperature flows have been available to the rational heatproof design of the space vehicles. Recent development of measurement techniques with laser systems and photo-electronics now enables us to investigate the hypervelocity phenomena with greatly advanced accuracy. In this research strong shock waves are generated in low-density gas to simulate the reentry range gas flow with a free-piston double-diaphragm shock tube, and CARS (Coherent Anti-stokes Raman Spectroscopy) measurement method is applied to the hypervelocity flows behind the shock waves, where spectral signals of high space/time resolution are acquired. The CARS system consists of YAG and dye lasers, a spectroscope, and a CCD camera system. We obtain the CARS signal spectrum data by this special time-resolving experiment, and the vibrational and rotational temperatures of N2 are determined by fitting between the experimental spectroscopic profile data and theoretically estimated spectroscopic data.

A measurement of 3D flow field induced by shock waves using interferometric CT method and numerical analysis

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.

Background Oriented Schlieren (BOS) measurement in supersonic flow with 4K high-speed camera

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.

Behavior of the Shock Wave Propagating in the Small-Diameter Tubes

Recently, the micro-shock waves have attracted attention from researchers in several fields of science. It is well known that the shear stress and the heat transfer between a test gas and a wall lead to significant deviations from the normal theory of a shock wave propagating in a small-diameter shock tube [1, 2]. In our first research, we developed a diaphragmless driver section using a rubber valve, and estimated the valve opening characteristics as the first step towards clarifying the shock wave characteristics propagating in the small-diameter tube [3]. As a second step, the shock wave measurements in small-diameter shock tubes were performed at a relatively low driver pressure by using the driver section with rubber valve [4–6]. However, at relatively high driver pressures over 0.5 MPa, shock wave measurements using the diaphragmless driver section with rubber valve are very difficult to perform, because of the structural properties of the rubber valve.

Investigation of poppet valve vibration with cavitation

Abstract The poppet valve is a popular component in hydraulic systems, but it is also well known as trouble maker because it may occasionally induce unpredictable vibration. In former previous studies it has been found that cavitation is an important reason for this kind of vibration, but the causal mechanism between the vibration and cavitation is unclear. In this study, we developed a visualisation experiment system, in which we can observe and analyse the dynamic relationship among the displacement of the poppet, the cavitation quantity and the pressures around the poppet in a visualisation experiment. Based on the observation of the experimental phenomena and data analysis, we propose a hypothesis of a mechanism that can explain how cavitation influences the vibration of the poppet valve.