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Instantaneous three-dimensional (3D) density distributions of a shock-cell structure of perfectly and imperfectly expanded supersonic microjets escaping into an ambient space are measured. For the 3D observation of supersonic microjets, non-scanning 3D computerized tomography (CT) technique using a 20-directional quantitative schlieren optical system with flashlight source is employed for simultaneous schlieren photography. The 3D density distributions data of the microjets are obtained by 3D-CT reconstruction of the projection’s images using maximum likelihood-expectation maximization. Axisymmetric convergent-divergent (Laval) circular and square micro nozzles with operating nozzle pressure ratio 5.0, 4.5, 4.0, 3.67, and 3.5 have been studied. This study examines perfectly expanded, overexpanded, and underexpanded supersonic microjets issued from micro nozzles with fully expanded jet Mach numbers Mj ranging from 1.47 - 1.71, where the design Mach number is Md = 1.5. A complex phenomenon for free square microjets called axis switching is clearly observed with two types “upright” and “diagonal” of “cross-shaped”. The initial axis-switching is 45° within the first shock-cell range. In addition, from the symmetry and diagonal views of square microjets for the first shock-cells, two different patterns of shock waves are viewed. The shock-cell spacing and supersonic core length for all nozzle pressure ratios are investigated and reported.

This report deals with introducing two new techniques based on a novel concept of complex brightness gradient in quantitative schlieren images, “inverse process” and “multi-path integration” for image-noise reduction. Noise in schlieren images affects the projections (density thickness) images of computerized tomography (CT). One spot noise in the schlieren image appears in a line shape in the density thickness image. Noise effect like an infectious disease spreads from a noisy pixel to the next pixel in the direction of single-path integration. On the one hand, the noise in the schlieren image reduces the quality of the image and quantitative analysis and is undesirable; on the other it is unavoidable. Therefore, the importance of proper noise reduction techniques seems essential and tangible. In the present report, a novel technique “multi-path integration” is proposed for noise reduction in projections images of CT. Multi-path integration is required the schlieren brightness gradient in two orthogonal directions. The 20-directional quantitative schlieren optical system presents only images of schlieren brightness in the horizontal gradient and another 20-directional optical system seems necessary to obtain vertical schlieren brightness gradient, simultaneously. Using the “inverse process”, a new technique enables us to obtain vertical schlieren brightness gradient from horizontal experimental data without the necessity of a new optical system and can be used for obtaining any optional directions of schlieren brightness gradient.

The three-dimensional density field of a supersonic microjet issued from a square convergent nozzle with a cross-sectional area of 1 mm × 1 mm at the exit is measured by the Mach-Zehnder interferometer with the finite fringe setting. The experiment is performed at a nozzle pressure ratio of 4.0 to produce underexpanded free jets. Interferogram analyses for reconstructing the density field of the microjet are performed using the convolution back projection (CBP) algorithm in which the analysis of the phase shift of the deformed fringe relative to the background fringe is carried out by the Fourier-transform method. A flow topology of the microjet is clarified with various visual representations.

The Mach–Zehnder interferometer with the finite fringe method is applied for the first time to diagnose the three-dimensional density field of a shock-containing microjet issued from a convergent nozzle with an inner diameter of 1 mm at the exit. Experiments are performed at a nozzle pressure ratio of 4.0 to produce an underexpanded free jet with a Mach disk in the first shock-cell where the Reynolds number, based upon the diameter and flow properties at the nozzle exit, is turbulence model for a quantitative mutual comparison between the simulation and the experiment. The detailed variation of the density field associated with the Mach disk, the slip streams and the outer shear layers near the jet boundaries are successfully captured. Furthermore, the density profile along the jet centreline obtained by the present experiment is quantitatively compared with those from prior quantitative visualization studies, such as rainbow schlieren deflectometry, background oriented schlieren, moire schlieren and Mach–Zehnder interferometer. The three-dimensional contour map coloured by the magnitude of the density gradient vector inside the microjet reveals the flow topology of the near-field shock systems and elucidates the spatial variation of the shock strength.

Two-dimensional critical nozzle flows at low Reynolds numbers are visualized by the rainbow schlieren deflectometry. Experiments have been performed in a region of overexpanded nozzle flow. The variation of the shock structure against the back pressure ratio can be clearly visible with color gradation. Static pressure rises due to the shock-induced flow separation are compared with the previous theories. The unsteady characteristics of overexpanded critical nozzle flows at low Reynolds numbers are quantitatively and qualitatively visualized using laser schlieren and Mach-Zehnder interferometer systems combined with a high-speed digital camera. It was found that an oscillating normal shock wave appears inside the nozzle, and that the shock wave has a specified dominant frequency. Also the time-history of the oscillating shock wave is obtained from both the systems and compared with each other.

The focusing schlieren PIV technique is being done to investigate the velocity vector, and it is considered as the useful measurement technique in the compressible flow. In this study, the LED light sources of a focusing schlieren system was used to visualize the flow fields of helium jets. And the flow vector was estimated by using PIV software as the first step. The system was constructed by an extended light source, a Fresnel lens, source and cutoff grids, focusing lens, and a CCD camera. The three source grids were prepared. The light source was designed by consisting of four LEDs. This system was used to visualize free jets from the convergent nozzle. The helium gas was used as the test gas to obtain large density gradient in low-speed flows. As the result, the density gradient could be seen though the condition was low speed. And the flow field from the nozzle was visualized by the LEDs as the extended light source. The jet at the focal plane could be seen clear, and the flow vector due to the turbulence was seen by the PIV analysis.

The rainbow schlieren deflectometry has been combined with the computed tomography to obtain three-dimensional density fields of shock containing free jets and we call the method the schlieren CT. Experiments on the schlieren CT have been performed at a nozzle pressure ratio of 4.0 by using an axisymmetric convergent nozzle with an inner diameter of 10 mm at the exit where the nozzle was operated at an underexpanded condition. Multidirectional rainbow schlieren pictures of an underexpanded sonic jet can be acquired by rotating the nozzle about its longitudinal axis in equal angular intervals and the three-dimensional density fields are reconstructed by the schlieren CT. The validity of the schlieren CT is verified by a comparison with the density fields reconstructed by the Abel inversion method. As a result, it is found that excellent quantitative agreement is reached between the three-dimensional jet density fields reconstructed from both methods.

The slightly underexpanded free jet issued from a supersonic micro nozzle with a design Mach number of 1.5 and a square shape of 1 mm × 1 mm at the nozzle exit has been experimentally investigated using the rainbow schlieren deflectometry combined with the computed tomography where the nozzle operating pressure ratio is held constant at 4.0. Density fields in the free jet obtained from the rainbow schlieren deflectometry are the vortex sheet theory proposed by Tam. It is shown that there is good quantitative agreement between experiment and theory. It is shown that the rainbow schlieren deflectometry is a useful tool for studying the three-dimensional structure of shock containing free jets from micro-scale supersonic nozzles.

The rainbow schlieren deflectometry is effective in studying quantitatively the density fields in shock-containing free jets at high precision and high spatial resolution. However, there has been no practical application of rainbow schlieren deflectometry for shock trains in a confined duct. Therefore, in the present study, the rainbow schlieren deflectometry is applied to the flow field including a shock train in a constant-area duct where just upstream of the shock train the freestream Mach number is 1.34, the unit Reynolds number is 5.39 × 107 m-1, and the boundary layer displacement thickness is 0.149 mm. As a result, a two- dimensional density field of the shock train is for the first time quantitatively displayed and the fine structure of the shock train is illustrated as a color gradation representation.

We have developed new firefighting hoses and compared them to conventional hoses, measuring their pressure loss when water alone or mixed water passed through. As a result, it is found that the new hoses have lower pressure loss than conventional hoses, and fewer kinks and they are more durable.

This paper analyzes the correctly-expanded supersonic jet from a convergent-divergent axisymmetric nozzle by using numerical simulation of turbulent flow. And the calculated density distributions in this flow are compared with the present experimental data using rainbow schlieren deflectometry. The value of the density from the experimental data agrees well with the results calculated by this simulation. Therefore, the present method of the measurement using rainbow schlieren deflectometry is useful for the measurement of the density of the correctly-expanded supersonic jet.

The structure of the moist air sonic jet issued from axisymmetric convergent nozzles has been experimentally investigated. Static pressures along the nozzle axis for a range of relative humidity from  = 0.35 to 0.98 have been measured by the through-tube system installed in the indraft supersonic wind tunnel. Also, the Reynolds averaged Navier-Stokes equation with the Menter's k-SST model for dry air sonic jets ( = 0) has been numerically solved. Effects of the relative humidity on the centerline static pressures in the moist air sonic jets are clarified.

The present study describes an experimental work to investigate the effect of a nozzle exit reflector on a supersonic jet that is discharged from a convergent–divergent nozzle with a design Mach number of 2.0. An annular reflector is installed at the nozzle exit and its diameter is varied. A high-quality spark schlieren optical system is used to visualize detailed jet structures with and without the reflector. Impact pressure measurement using a pitot probe is also carried out to quantify the reflector’s effect on the supersonic jet which is in the range from an over-expanded to a moderately under-expanded state. The results obtained show that for over-expanded jets, the reflector substantially increases the jet spreading rate and reduces the supersonic length of the jet, compared with moderately under-expanded jets. The reflector’s effect appears more significant in imperfectly expanded jets that have strong shock cell structures, but is negligible in correctly expanded jet.

The present study describes a new technique for the reduction of supersonic jet noise using a control wire device that is placed into the supersonic jet stream. The control wire device is composed of long cylinders with a very small diameter and its location is varied to investigate the control effectiveness of supersonic jet noise. The jet pressure ratio is varied to obtain the supersonic jets which are operated in a wide range of over-expanded to moderately under-expanded conditions. A high-quality Schlieren optical system is used to visualize the flow field of supersonic jets both with and without the control wire device. In order to quantify the control effect of the wire device on a supersonic jet, pressure measurements are also accomplished. Acoustic measurements are performed to obtain the overall sound pressure level and noise spectra. The results obtained show that the present wire device effectively breaks the shock-cell structure, reduces the shock strength, and consequently leads to a substantial suppression of supersonic jet noise. The location of the control wire device is an important factor in reducing the supersonic jet noise. The present wire device suppresses the screech tones and the broadband shock-associated noise as well as the overall sound pressure level, when it is placed at a location smaller than three times the exit diameter of nozzle in the downstream of the nozzle exit. For over-expanded jets, the noise control effectiveness of the wire device appears more significant, compared to under-expanded jets.

This paper describes an experimental work to investigate the effect of a reflector on supersonic jet noise radiated from a convergent-divergent nozzle with a design Mach number 2.0. In the present study, a metal reflector and reflectors made of three different sound-absorbing materials (grass wool and polyurethane foam) were employed, and the reflector size was varied. Acoustic measurement is carried out to obtain the acoustic characteristics such as frequency, amplitude of screech tone and overall sound pressure level (OASPL). A high-quality schlieren optical system is used to visualize the detailed structure of supersonic jet. The results obtained show that the acoustic characteristics of supersonic jet noise are strongly dependent upon the jet pressure ratio and the reflector size. It is also found that the reflector with sound-absorbing material reduces the screech tone amplitude by about 5–13dB and the overall sound pressure levels by about 2–5dB, compared with those of the metal reflector.

Under certain conditions, supersonic jet issuing from a supersonic nozzle invariably generates supersonic jet noise, which consists of three principal components ; the turbulent mixing noise, the broadband shock-associated noise, and the screech tones. In present study, it was experimentally investigated to the effect of nozzle lip thickness on the characteristics of supersonic jet noise. The convergent-divergent nozzle of design Mach number 2.0 was used in experiment. With two different cases in the nozzle-lip thickness, pressure ratio was varied in the range between 2.0 and 12.0. Acoustic measurements were conducted by microphones in an anechoic room, and the major structures of the supersonic jets were visualized by a Schlieren optical system for the purpose of the effect of nozzle lip thickness. The measured results show that the characteristics of supersonic jet noise, such as overall sound pressure level and screech frequency, depend upon the thickness of nozzle lip.

A rainbow schlieren system with a spatial resolution of 20 m is employed to determine the hue and density profiles in shock-containing jet from a cylindrical nozzle with a field of view of 100 mm diameter. Experiments have been conducted with the operating pressure ratio equal to 5.0 which corresponds to the range of underexpanded sonic jet. Underexpanded sonic jet structure can be clearly observed by schlieren pictures with horizontal and vertical rainbow filters. As a result, rainbow schlieren images show jet boundaries, expansion waves, barrel shock waves, Mach disks, and reflected shock waves by continuous color gradation. Also, the rainbow schlieren image reveals that the Mach disk is concave facing downstream in shape. Density profiles at any jet cross-section can be efficiently inferred from the Abel inversion of the schlieren images. The hue and density distributions along the jet centerline and radial density profile elucidate flow properties of shock-containing jet structure quantitatively. Furthermore, an analytical model to predict flow properties at the exit plane of the cylindrical nozzle is proposed and compared with the results from the rainbow schlieren.

The pressure sensitive paint (PSP) has become a powerful tool for the surface pressure measurements in the windtunnel testing. This diagnostic method, however, is limited to the time-averaged measurements so far, because the temporal behavior of PSP has not been well understood. Present paper describes the experimental investigations on the time response of three pressure sensitive paints, PtOEP, H2TFPP and PtTFPP. The coating on which the PSPs are painted is a commercial porous silica thin-layer chromatography (TLC) plate. The sudden change in pressure is provided by a shock tube, and the luminescence change due to the pressure rise by an incident shock wave is detected by a photomultiplier. The results indicate that the characteristic response time is found to differ among the PSPs tested, and the shortest characteristic time is on the order of ten microseconds. The effect of the sudden change in gas temperature on the luminescence intensity is also investigated by using the contact surface of the shock tube flow, and it is found that the luminescence intensity depends only on the wall temperature and is affected little by the gas temperature.

The interaction between a normal shock wave and a boundary layer along a wall surface in internal compressible flows causes a very complicated flow. When the shock is strong enough to separate the boundary layer, the shock is bifurcated and one or more shocks appear downstream of the bifurcated shock. A series of shocks thus formed, called “shock train”, is followed by an adverse pressure gradient region, if the duct is long enough. Thus the effect of the interaction extends over a great distance. The flow is decelerated from supersonic to subsonic through the whole interaction region. In this sense, the interaction region including the shock train in it is referred to as “pseudo-shock” in the present paper, as Crocco called it. The shock train and pseudo-shock strongly affect the performance and efficiency of various flow devices. In the present review some fundamental characteristics of the shock train and pseudo-shock are first described. Some simple predictions are made to simulate these very complicated phenomena. Pseudo-shocks appearing in various flow devices are explained. Control methods of the pseudo-shocks are also described. Finally, the current understanding of self-excited oscillation of pseudo-shock is reviewed.

A fundamental experiment of the temperature characterisitc of the pressure sensitive paint (PSP) to a continuous change in pressure is performed. As the results, for fixed temperature, as the pressure decreases, the paint's brightness increases and for high temperature the brightness decreases. Also, as temperature increases, relative intensity decreases, and relative sensitivity to pressure changes increases. Furthermore, the characteristic of the response of the PSP to a step change in pressure is presented and the time constant is estimated.

An overexpanded flow in a two-dimensional supersonic nozzle with 10 degree half-angle has been experimentally investigated with schlieren observation and wall static pressure measurement, and the experimental results have been compared with the one-dimensional inviscid theory. The characteristics of nozzle flow for various pressure ratios across the nozzle are made clear The range of pressure ratio for the overexpanded flow by the inviscid theory is different from the present experimental results. This is due to the fact that the pseudo-shock is formed in real flows instead of the normal shock.

The effect of a small protuberance in a boundary layer on the structure of a λ-type pseudo-shock wave was experimentally investigated. The Mach number upstream of the pseudo-shock is 1.80. When the head of the pseudo-shock is located at the protuberance, the pseudo-shock is affected most and it consists of a series of oblique shock waves. In such a flow state, the wavy variation of the pressure distribution along the duct axis is much smaller in comparison with a pseudo-shock wave in the case of no protuberance.

To elucidate effects of boundary layers on flow choking phenomena through a convergent nozzle, a rectangular convergent nozzle composed of a convergent part of 10 mm followed by a straight duct of 100 mm with a height of 5 mm is used in the present experiments. The nozzle has a constant width of 36 mm over the entire length. A conventional schlieren system with tricolor filter and knife edge is employed to observe the detail of choking mechanism. The measurements of mass flow rates through the nozzle and of wall static pressures are performed to estimate the choking criteria. As a result, color schlieren pictures indicate a nearly normal shock wave just upstream of the nozzle exit and Mach waves from the nozzle wall surfaces for low back pressures. Also, the momentum integral equation for the compressible turbulent boundary layer is solved to reveal a flow mechanism from subsonic to supersonic in a convergent duct with thick boundary layers.

A quantitative visual study of an underexpanded sonic jet issuing from an axisymmetric convergent nozzle by the rainbow schlieren deflectometry technique has been conducted to reveal a complex flowfield containing a Mach disk as well as Prandtl Meyer compression and expansion fans in the jet. A numerical simulation of underexpanded sonic jets has also been carried out to estimate the density flowfield inferred from the rainbow schlieren measurements. The structure of the underexpanded sonic jet from the rainbow schlieren measurements are compared with those from the numerical simulation.

A visual study of an overexpanded supersonic jet issuing from an axisymmetric convergent-divergent nozzle by the rainbow schlieren deflectometry technique has been conducted to reveal the shock containing jet flow field quantitatively. Rainbow schlieren images of the jet have been taken by a schlieren system with a rainbow filter which has a continuous hue distribution. The density contour plot of the jet flow is determined by using the image analysis based on the Abel inversion of experimental data of transverse displacement at the cut-off plane. Leading and rear shocks as well as a Mach disk in the jet flow are clearly visible in the density contour plot of the jet. Also, the density ratio across the Mach disk obtained from the experiment shows an excellent agreement with the theoretical value across a normal shock wave quantitatively.

The oscillatory characteristics of a pseudo-shock wave with a Mach number of 1. 75 in a rectangular straight duct are experimentally investigated by static pressure measurements both on the duct axis and on the wall surface. The aspects of the pressure oscillation in the pseudo-shock region are analyzed from the treatment based on the intermittent characteristics, root-mean square, probability density function and cross correlation of the pressure signals. The correlation between the oscillatory characteristics and the flow mechanism of the pseudo-shock wave is also discussed in detail in the last section of the present paper.

A quantitative visual study of a correctly-expanded supersonic jet of a Mach number of 1.6 by the rainbow schlieren deflectometry has been conducted to reveal the jet flow field quantitatively. Rainbow schlieren images of the jet have been taken by a schlieren system with a rainbow filter which has a continuous hue distribution. The density contour plot of the jet flow is determined by using the image analysis based on the Abel inversion of experimental data of transverse displacement at the cut-off plane. Also, Pitot probe measurements at a jet cross-section have been carried out to obtain density profiles. The radial distribution of the density from the schlieren quantitatively agrees well with that from the Pitot probe.

The interaction of a normal shock wave with a turbulent boundary layer at a low Mach number in a square straight duct is investigated experimentally and theoretically. Wall static pressures have been measured in the interaction region and visualization by the color schlieren system has been performed to observe a variation of the shock structure by the interaction. It is shown that a weak normal shock wave interacts with a turbulent boundary layer to form a shock train and pseudo-shock wave for a large blockage ratio. An analytical flow model to estimate a characteristic property across the interaction is proposed and the analytical results are compared with the present experimental ones.

A numerical study of underexpanded axisymmetric supersonic free jet issuing from supersonic nozzles having semi divergence angle of 0 deg and 7.5 deg at the nozzle exit was made using TVD finite difference scheme. The effect of nozzle exit angle on Mach disc position and Mach disc diameter was investigated numerically, and numerical results have been compared with schlieren observation. Favorable agreement was obtained between numerical results and schlieren pictures.