[in Japanese]

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In general, the arrangement of tube bundle for heat exchanger has been a equaled one, but by using a tube bundle with non-equal spaced one the flow resistance and heat transfer may be reduced and increased, respectively. In this study, the flow resistance of single tube bundle with non-equal spaces is examined experimentally. The experiment showed that the flow resistance is reduced by non-equal space arrangement under some condition.

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This study proposes a measuring method for individual phase flow rates of gas-liquid two-phase mixtures in a circular pipe. First, the time-variations for pressure are measured at three points around a vortex generator mounted in the pipe under the known flow rates. Secondly the features of the differential pressure fluctuations, such as the mean and the standard deviation, are calculated to construct a database relating the flow rates with the features. Consequently, if the differential pressure fluctuations are measured under unknown flow rates, the individual phase flow rates are successfully identified from the database.

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The specific aim of this work is to develop a new technique to control vortex excitations of a circular cylinder by utilizing interference of a downstream strip-plate in cruciform arrangement. In the previous paper, it is show that two longitudinal vortices, trailing vortex and necklace vortex, shed near the crossing of a fixed circular cylinder/strip-plate system. In this paper, influence of the strip-plate on vortex excitation was investigated by wind tunnel experiment. The strip-plate suppresses Karman vortex excitation effectively compared with a downstream circular cylinder. A wider strip-plate has a larger suppression effect. The trailing vortex induces vortex excitation when gap-to-diameter ratio s/d less than 0.3. When the strip-plate width equals to the upstream circular cylinder diameter, the velocity range for longitudinal vortex excitation is 5 times wider than the case of the downstream circular cylinder. Longitudinal vortex excitation by downstream strip-plate occurs over a wide velocity range, and it can be easily controlled by s/d adjusting.

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We investigate numerically self-sustained oscillating flows over open cavities. The two-dimensional incompressible Navier-Stokes equations are solved using finite difference method. A series of simulations are performed for a variety of cavity length from 1.0 to 3.8 at 0.1 interval. The results show mode switchings among no oscillations, mode II, mode III and wake mode oscillations. The Strouhal number variation is consistent with experimental data. The results of flow fields in the cavity reveal the relationship between the cavity shear layer oscillation modes and recirculating vortices in the cavity. The self-sustained oscillations start when number of recirculating vortices become two. The mode switching form mode II to mode III occurs when number of vortices change from three to four. The wake mode oscillation is similar to that of other experimental and numerical investigations.

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Water is more liable to evaporate than oils since its saturated vapor pressure is higher than the ones of oils. Therefore it is important to prevent erosion due to cavitation bubbles in hydraulic devices using water. In our previous study, we reported that erosion at elbow of pipeline can be reduced by using T-shape elbow. However its mechanism has not been clarified yet. In the present study, we observed cavitating flows in T-shape elbows of different shapes using a digital and high-speed video cameras. The amount of cavity was evaluated from taking images. As a result we confirmed that (1) circulating flow is steadily formed when the length of cavity-trapping region L ≧ 22.5 mm. If L ≦ 15 mm, circulating flow is formed intermittently, or it is not formed, (2) The amount of cavitation bubbles in cavity-trapping region increases as L increases, and (3)there are two uses (a) T-shape elbows can be used to guide cavitation bubbles into the region, in which circulating flow is formed, for defending the downstream of pipeline. (b) Then the elbows can also be used to guide bubbles to the downstream so as to prevent erosion at the elbows.

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It is reported that not only an audible noise but also a low-frequency noise including an infrasound is radiated from a high-speed train toward a wayside open area. Although these noises become larger as the train speed increases, the cause of the low-frequency noise remains to be fully clarified. In this study, we investigated the wayside low-frequency noise at several sites. The investigation results show that the observed low-frequency noise consists of two phenomena; the one is a train passing pressure variation around the nose and tail parts of the train; the other is a low-frequency acoustic pressure wave caused not only by an aerodynamic source around the train but also by a viaduct vibration. The maximum sound pressure level of the low-frequency noise is determined by these phenomena which have different magnitude depending on the train speed, a measurement distance and a type of running section. Furthermore, the characteristic of the measured low-frequency noise in a high-speed region, which is analogous to a line source, shows that the low-frequency noise is mainly related to the aerodynamic sound emitted from all over the train.

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An experimental system was developed for measuring pressure waves generated by a high-speed train. The system launches a 1/30 scale model of actual train shape and enables measurement with the geometric configurations same as actual situations. Compressed air is used for launching train models, and the air flow in the experimental system is mathematically modeled. This model allows us to determine optimum design parameters of the system for a target velocity and to control a launching velocity by adjusting the pressure of the compressed air. Measurement of the flow in the experimental system shows that the system performs as designed by the mathematical model and is capable of launching a train model at faster than 500 km/h. We measure pressure waves generated by a train moving into a tunnel, and the experimental data agrees well with field measurement data.

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This paper deals with the aerodynamic noise generated by a round bar bended into a circular arc shape in uniform air flow. The bend round bar has a curvature R and attack angle α. In the experiment, we investigated the effects of R or α on the aerodynamic sound generated by bend round bars. The results of the frequency analyses of the aerodynamic sounds show that the aeolian tones are generated by the bend round bar, when α is large, but that when α is small, although the aeolian tones are reduced, the broad band noise of lower frequencies are increased. In order to decrease aeolian tones, we find the appropriate attack angle α.

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In order to clarify the effect of uniform flow turbulence, aerodynamic sound and static pressure fields were measured in a low-noise wind tunnel with an active turbulence generator. The experimental results showed that it is important the eddy scale of the uniform flow rather than turbulent intensity for aerodynamic sound generation. The spanwise coherence length of circular cylinder depended on the scale of incoming flow. Since the spanwise scale decreased in the large-scale turbulent field, as a result, resultant aerodynamic sound also decreased. It indicated that the eddy scale of the uniform flow, L, and the diameter of cylinder, D, were dominant parameters for aerodynamic sound generation in turbulent flow fields. The phase averaged static pressure fields were also measured. Asymmetric vortex street as Karman vortex clearly observed at L/D = 0.4. After that, Karman vortex disappeared when the eddy scale of uniform flow were almost the same as the diameter of the cylinder. The experimental results showed that the level of aerodynamic sound depend on the structure of the wake which depended on the scale of uniform flow.

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In recent years, noise pollution with construction activity is treated as a growing problem. Noise Pollution Regulation Law executes as a result of development of noise pollution. For the purpose, construction machines have been required noise reduction. There are two ways for noise reduction. The one, it is sound source treatment. This way is able to reduce noise by weakening the vibrations of sound source. This way has a dramatic effect on noise reduction, however it is usually difficult to realize because there are many different kinds of sound source. The other one is to take care for propagation path. This way is usually realized to reduce noise by covered with sound proof cover. The biggest benefit in this way is very simple method. In this way, it is very important to calculate and predict sound reduction effectiveness. We have studied a way of analysis on Sound Pressure Level around a soundproof cover by Fluid-Structure Interactions calculation. In this paper the method and results are shown about appropriateness of calculation for acoustic damping phenomenon.

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Influence of the external disturbances on boundary-layer transition was investigated experimentally in two kinds of 2-D wing model. The first series of experiments are conducted to clarify transition process and the characteristic of instability wave on the NACA0015 wing. It is the first step to examine whether sensitivity of the residual turbulence and acoustic noise to N value e^N method is dependent on instability mechanism. In results, the amplification of the traveling disturbance are observed and its characteristics are measured. The second series of experiments are conducted to clarify the influence of surface roughness on the transition, by applying water-repellent coating to NACA63(3)-018 wing. As a result, surface roughness becomes larger, but the transition location does not move, by applying water-repellent coating. This results suggest that maintenance-free technology of natural laminar flow, which is requested from aircraft manufacturing, can be achieved.

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The secondary flow pattern associated with the transition with increasing flow flux in curved rectangular duct flows were experimentally investigated and compared with numerical simulation. The aspect ratio of the experimental duct cross-section is chosen as 1 and 2. The observation has been performed at 180° and 270° from the inlet. It is found that the critical dean numbers obtained experimentally agree with numerical ones very well. We also obtained a sign of occurrence of traveling wave.

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In recent years, commercial software is often used for a design process of fluid machine having a complicated structure. According to this method, the predicted performance was not achieved due to the bad quality solution. In order to authorize the computational accuracy, velocity field with large separation computed by commercial software was compared with measured value by the PIV method. The purpose of this investigation is to show the indication of how to use of the computational method by the commercial software in design process. As the results of this investigation, a quantitative comparison of numerical and experimental result was able to be confirmed in the low Reynolds number area.

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This paper presents a new RANS based model for bypass transition using an intermittency transport equation. The intermittency transport equation proposed in this study is the modification of Cho and Chung model with respect to the base equation, diffusion term and empirical model constants. The intermittent behavior of the transitional flow is incorporated into the computation by modifying the eddy viscosity μ_1, obtained from a turbulence model. Wilcox low Reynolds κ-ω turbulence model is employed to calculate the eddy viscosity and others turbulent quantities. This model is designated for the prediction of flow transition under the influence of freestream turbulence and pressure gradient. For validation, the current transition model is applied to the benchmark experiments of flat plate test cases of ERCOFTAC series and to predictions of a modern low pressure turbine experiment. All test cases show well agreements with the experiments and thus it implies, the new approach has a potential as a predictive tool.

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The purpose of this paper is to investigate the stress at the separation point in laminar flow from an analysis of velocity distribution obtained by PIV method. For the purpose of measurement of fluid velocity near the wall, density and size of smoke particle in air were better than those of additives in mineral oil or of scattering particles seeded in tap water. The fluid on the stream line starting from the point of separation accelerated in a narrow area with large tensile and shearing stress. This result means that huge stress is produced around at the point of separation by flow.

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The pressure pulsation for a centrifugal blower piping system with the discharge opening below the water surface has been investigated. The experiments are carried out using the centrifugal blower facility with a plenum chamber, and the pressure in the pipe line is measured for various parameters such as the number of revolution of the blower, the opening of the throttle valve, and the depth of the discharge opening. A one-dimensional model is proposed to predict the transient response of the compression system, and the results were compared with the experimental results. It is clear that the amplitude and the period of the pressure fluctuation in the plenum chamber increase as the depth increases. The diameter of the air ejection hole influences the pressure fluctuation. The results obtained by the calculation model agree approximately with those obtained by experiments.

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The flow field in a centrifugal compressor is highly three-dimensional and complicated by the strong secondary flows. In order to improve the performance and operating range, it is indispensable to elucidate the three-dimensional flow phenomena. In this paper, the flow fields in a centrifugal compressor impeller with vaneless diffuser have been investigated by numerical flow analysis. Three-dimensional separated and vortical flow fields were analyzed by Reynolds-averaged Navier-Stokes simulations (RANS) using an unfactored implicit upwind relaxation scheme. As the turbulence model k-ω two-equation model was employed. To elucidate the complicated flow fields, vortex structures were identified by the critical point theory. From numerical analysis results , it was found that the tip leakage vortex breakdown occurred and the leakage vortex of the full-blade interacts with the splitter-blade leading edge at the operating point of maximum pressure rise. This phenomenon causes the large loss and significant impact on the total performance characteristics.

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Effects of the rotor tip clearance on the performance and the stall phenomena at near-stall conditions in an axial flow compressor rotor have been investigated experimentally. For two different tip clearances, the internal flow structure and the unsteady flow behavior was analyzed by measuring the wall pressure on the casing and the three-dimensional velocity field downstream of the rotor. It is found that the stall inception originates from the separation of blade suction surface boundary layer for the small tip clearance and from the breakdown of tip leakage vortex for the large tip clearance.

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In this research, protein was used to consider the mechanism of surfactant adsorption on a single bubble interface. In order to change the property of a bubble interface, protein was added in purified water, and the bubble motion was visualized using a high-speed video camera. Trajectories, bubbles velocity and aspect ratios of the bubbles were calculated. In the protein solution, the single bubbles showed the motion similar to that observed in a solution of surface active surfactant such as 1-pentanol; attenuation in the high frequency shape deformation, an increase in the aspect ratio, a decrease in vertical velocity, and an increase in horizontal velocity. From these results, it was considered that protein molecules were absorbed on a bubble interface, and played a role similar to surface-active surfactant.

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An inducer is installed upstream of a main impeller to obtain high suction performance. In rocket engines, the turbopump inducer, which is actually a high-precision, high-speed, sophisticated rotating impeller, is one of the key components. In this article, the relation between impeller's suction specific speed and inducer's specific speed is discussed by classical similarity consideration with focusing on the design of quiet inducer.

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The present authors have conducted observations of separating oil flows for years and have discovered a phenomenon that an infinitesimal cavity suddenly emerges on the wall close to the point of separation. The new-born cavity grows and splits releasing a lot of minute bubbles to downstream. The present paper intends to answer the following question presumably given to the discovery by traditional cavitation researchers dealing with water flow; does the same phenomenon occur in water flows with separation? Through observations of water flows in rectangular and cylindrical constrictions using a super-high speed video camera and a long distance microscope, it has been confirmed that a cavity is suddenly born near the separation point in water flows as well. In other words, the same mechanism of cavity generation at the point of separation commonly exists both in water flows and oil ones.

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In Electro Chemical Machining (ECM) process, generation of air bubbles and the flow separation during processing complicate the flow behavior of electrolyte and decrease the shape accuracy of the product. It is highly important to understand the influence of air bubbles on the separated flow. In this study, the flow characteristics of vertical upward gas-liquid two-phase flow with millimeter-scale bubbles and separation in a rectangular channel were examined. Bubbles were fed to the liquid phase from stainless-steel needles with inside diameter of 0.1 mm. Flow separation is generated by a square rod. The liquid velocity was measured using the PIV and the bubble shape was detected by the shadow image technique. It was found that mean velocity profiles of the bubbly flow were steep near the wall and mainstream was flat in the main stream. Furthermore, the reattachment length was reduced by increasing the void fraction.

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We carried out numerical simulations of 2D unsteady cavitating flow in three-blade and four-blade cascades under the various operating conditions of inflow angle and cavitation number. The results showed that steady cavitating flow, which has the steadily distributed cavity on the blades, transited to unsteady states as inflow angle increases or cavitation number decreases. Consequently, it was found that the cavitating flow can be classified into steady or unsteady cavitation phenomena according to the operating condition regardless of the number of the blades. However, the patterns of unsteady cavitating flow in three-blade and four-blade cascades are not corresponding, such as rotating cavitation and alternative cavitation.

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Characteristics of the surrounding liquid motion of single rising bubbles were experimentally investigated. Three kinds of bubbles were generated and highly controlled by bubble generator. The motion of a bubble was captured by high-speed video camera using LEDs. In addition, surrounding liquid motion was visualized by PIV measurement. From PIV result, vorticity distribution and standard deviation of liquid-phase velocity were calculated. As a result, as increasing the bubble radius, the area of disturbance region increases. In addition, the shape oscillation of zigzagging bubble is a dominant factor for the standard deviation of the velocity fluctuation of surrounding liquid motion. On the contrary, the magnitude of the liquid disturbance induced by bubble increases as decreasing the bubble motion.

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It is known that ring-shape cavitation clouds are formed when a cavitating water jet impinges on solid wall. The ring-shape cavitation clouds cause ring-shape erosion distribution. In this study, we use a narrow channel in order to observe the sectional view of cavitating jet. We observe collapsing behaviors of cavitation clouds and formation process of cavitation clouds spreading on solid wall in order to connect directly collapsing behavior of cavitation clouds with erosion distribution. As a result, two types of bubble collapse behavior can be observed at least. The one is a collapse of cavitation clouds around jet center and the other is a collapse of cavitation clouds spreading on the impinging wall.

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Flow fields at the rotor tip region in a low-speed axial-flow fan were experimentally and computationally investigated to clarify the flow mechanism behind a modal stall inception pattern. The behavior of tip leakage flow at the modal stall inception was different from that at the spike stall inception. The interface between the incoming flow, tip leakage flow, and end-wall backflow did not parallel to the rotor leading edge plane. Leading edge separation occurred when the tip leakage vortex broke down. This separation grew and formed three-dimensional separation vortex, which stood on the suction side near the leading edge. We concluded from these results that this three-dimensional separation vortex influenced on the initiation of the modal disturbance.

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Two kinds of passive control devices are successfully combined for suppression of unstable flow at small flow rates in a centrifugal blower. The one is the inlet ring groove arrangement and the other is the low solidity cascade diffuser. By applying the inlet ring groove arrangement, the tip leakage flow and the reverse flow in the inducer is sucked through the ring groove located near the inducer tip throat, and the flow incidence was effectively reduced. The pressure rise at the impeller exit was successfully improved by the ring groove arrangement, but the pressure rise in the vaneless diffuser was decreased. The CFD analysis showed that the tangential component of absolute velocity at the impeller exit was decreased. Then, the decreased centrifugal force must be a main reason why the pressure rise in the diffuser became small. By applying the low solidity cascade diffuser as well as the ring groove arrangement, the blower pressure rise was improved around 6%, however, the surge margin was deteriorated a little.

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