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Two-phase pressure drop in the debris has been studied by many researchers concerning the debris cooling characteristics during a severe accident in a nuclear reactor. However, its flow regime transition of the two-phase flow in the debris has not been well understood, which strongly affects the interfacial drag and the pressure drop. Conventional models for gas-liquid two-phase flow pressure drop have not been established to evaluate interfacial drag accurately. In this study, high-speed imaging of a two-dimensional network model was performed to clarify the effect of flow patterns on interfacial drag and pressure drop. Usually, it would not be easy to visualize such two-phase flow behavior in a randomly packed bed due to the reflection/refraction of light and/or overlapping bubbles, even if the test section is made of transparent materials. Therefore, in this study, a test section, which simulates a two-dimensional network of porous structures, was fabricated to avoid overlapping bubbles. The two-phase flow pattern in the porous structure has been identified by high-speed imaging of the two-dimensional network model. The flow regime map based on the flow pattern visualization results is applied to the pressure drop evaluation and it could reduce the overestimation of experimental values. The experimental results suggested that the interfacial drag term should be modified in the gas-liquid two-phase flow pressure drop model.

The present aim is to reveal the flow past a pipe which is immersed parallel to the mainstream at high Reynolds numbers. In a wind tunnel, we carry out (1) base-pressure measurements, (2) velocity-fluctuation measurements using a hot-wire anemometer and (3) flow visualisations by a smoke-wire method with PIV analyses, where we take consecutive picutures using a high-speed camcorder to obtain quantitative flow-field information such as velocity vector and vorticity. The tested parameter ranges are as follows: Re = 2.0×103 - 1.3×104, d/t = 4.0 - 10.0 and l/t = 1.0 - 10.0, where Re, d, t and l are the Reynolds number, mean diameter, thickness and length of the pipe, respectively. As a result, the Re effects are negligible. The base-suction coefficient -Cpb monotonically decreases with decreasing d/t, or with increasing l/t. We propose a unified formula to predict -Cpb, which are consistent with both a two-dimensional prism and a rod for l/t < 4.0 in addition to a ring. In contrast, the Strouhal number St almost coincides with that for a two-dimensional prism at any l/t, if we can detected any dominant frequencies. In addition, we conduct flow visualisations, and reveal the effects upon axisymmetry of wake. Finally, we classify the flow into three modes based on both periodicity and axisymmetry. Such a modal classification reveals that the enhancement of flow's irregularity corresponds to the decrease of -Cpb.

In the present study, a transition mechanism to explain the change in the motion of a single ascending bubble from linear to zigzag was investigated experimentally. We focused on the relationship between bubble motion (gravity-center motion and surface motion) and hairpin-like vortex motion. First, the bubble motion was visualized using the infrared shadow technique (IST), and the gravity-center motion and surface motion were quantified. Second, the bubble wake structure and its motion were visualized with the laser induced fluorescence using HPTS (LIF/HPTS) method. From the LIF results, position fluctuations of the point of the hairpin-like vortex leg on the bubble rear surface (wake motion) were quantified. From these results, a close relation between the bubble motion and the wake motion was discussed. Furthermore, a relation between the wake motion and the lift force acting on the bubble was discussed. We concluded that the relations between them play an important role in zigzag motion. In addition, we tentatively concluded that bubble shape deformation observed just after the launch was the first trigger for the zigzag motion.

Bubble jet flows are seen in a lot of industrial fields such as power plant, chemical plant and bio-reactor. PIV is very useful tool in order to investigate the bubble's behaviors and flow structures in liquid phase simultaneously. Because it is possible to measure two- or three-dimensional flow structures and the whole flow fields aren't disturbed by measurement instruments. Recently, Dynamic PIV system consisted of high-speed camera and high-repetition laser has been developed. It is possible to capture time-serial images with high temporal resolution. In this paper, a system combined dynamic PIV and IST (Infrared Shadow Technique) with single high-speed camera, a double-pulse laser for illumination of particles and a diode laser for illumination of bubbles is developed in order to capture particles and bubbles simultaneously. Some techniques about image pre-processing to separate particles and bubbles and velocity calculation by PIV/PTV are introduced. POD (Proper Orthogonal Decomposition) is furthermore applied to time-series liquid phase velocity vectors. It is revealed that the effect of spatio smooth filter is obtained by reconstructed velocity vectors in lower spatio mode by POD and it is changed from large vortex structures to small vortex structures as the spatio mode becomes higher.

The aim of this study is to investigate on the flow behavior in the wake behind a circular cylinder by induced jet using dielectric barrier discharge (DBD) plasma. The plasma was created using a wave form with bipolar voltage of 4 kVpp and frequency of 10 kHz, and the plasma actuators were mounted on the circular cylinder. The forward and backward jets for the main flow direction were induced by electrode arrangements of the plasma actuators. The flow field was visualized using a high-speed video camera, and the streamwise and vertical velocities ware calculated by dynamics PIV method at Reynolds number of 1.0×103. We discussed that the vortex motion around the cylinder, Reynolds stress profiles and the half width of wakes by the forward and backward jets. Also, the characteristic frequency of vortex shedding was confirmed from the continuous photographs.

As for cavitating jets injected from an orifice nozzle of inner diameter 0.15 mm at pressure 10-200 MPa into still water at atmospheric pressure, we conducted experiments of flow visualization of free jets with high-speed CCD camera at 50×104 fps and erosion tests of aluminum specimens. From these experiments, the following results were obtained. In erosion tests of cavitating jets, it is known that there are two peaks in mass loss curves, and besides, the peak of the diameter of eroded area was found to exist between two mass loss peaks. The distance may be suited to cleaning material and peening on metal for large damaged area and controlling erosion effect by adjusting injection pressure. The mass loss of downstream peak owing to collapses of cavitation clouds was highest at certain pressure. A high pressure pulse wave was generated 1-5 μs before the volume of cavitation clouds becomes smallest, the incidence frequency was of the order of 10 kHz, and the propagation speed of pressure wave was 1000-1100 m/s. Cavitation noise and erosion could be caused by these behaviors of cavitation clouds.

In order to clarify the generation mechanism of aerodynamic sound in cavity flows, aerodynamic sound and flow fields were simultaneously measured with a microphone and a high-speed flow visualization system. The effects of acoustic resonance on the flow and acoustic field were investigated by changing the cavity depth and free stream velocity. An expansion wave was found to be generated when a vortex collided with the downstream edge. This mechanism was independent of the oscillation mode of the cavity tone. Based on this generation mechanism, a new formula for the prediction of the frequency of the cavity tone was proposed. Also the effects of the ratio of the cavity length to the incoming boundary layer thickness on the cavity tone were investigated. As a result, it was shown that the oscillation mode became higher as the boundary layer thickness became thinner in comparison with the cavity length.

The flow structures in a coaxial jet with axisymmetric and helical instability modes for a comparatively low Reynolds number were investigated. The flow visualization and the measurements of velocities were carried out in an open water tank. In addition, three-dimensional numerical simulation of a coaxial jet was also performed using the commercial CFD software FLUENT 6.3. It was confirmed that the helical vortex was shed for the range of ratios of velocity of the inner to that of the outer jet from 0.5 to 1.0. Two characteristic flow regimes, i.e., axisymmetric and helical instability modes, were simulated in the flow field. For the coaxial jet with helical instability, the axial velocity along the centerline of the jet decreased more than that of the coaxial jet with axisymmetric instability. The axial velocity fluctuation at the centerline of the jet was small near the nozzle exit. However, the radial velocity fluctuation at that location increased. The convection velocity of vortices in the inner shear layer was larger than that for the outer shear layer. The convection velocity of vortices with the helical instability was slightly larger than that of the vortices with the axisymmetric instability. Consequently, the variation of velocity fluctuations and the convection velocity was associated with the arrangement of the vortex street in the shear layer.

Creep tests, flow birefringence measurements, and flow visualization experiments in shear flows between parallel plates were carried out for aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) with excess of NaSal. The mol concentration of CTAB was fixed to 0.030 mol/l and the concentration ratio of NaSal to CTAB was varied. The creep tests indicate that it requires long time to develop micellar network structures for low to moderate concentrations at low shear stresses. Under conditions of shear rate at which the shear-thickening property in viscosity is observed in a flow curve, periodical changes in flow birefringence and shear stress occur and these frequencies are almost the same as that of the emergency of white turbidity observed in the flow visualization. These results indicate that relatively large structures of micellar network are induced by the shear flow and repeatedly appear and disappear.

Experiment has been performed to clarify the structure of aerodynamic noise sources generated in a production vehicle. The details of noise sources were measured from the outside and inside of vehicle by applying noise source identification techniques. The noise sources show high level in the regions of bonnet head, front underbody, side window and rear window. Moreover, the noise sources are dominated by that from underbody in the low frequency, and side window in the high frequency, respectively. The velocity dependency of aerodynamic noise was analyzed based on Lighthill-Curle analogy. As a result, it is found that the noise sources around vehicle are mainly dipole sources.

In this paper, the flow behavior of viscoelastic fluid in asymmetric planar contraction geometry is studied by flow visualization and PIV measurement. Experiments are carried out for three channels of different contraction widths. The experimental results indicate that the flow field in the contraction channel is characterized by the Weissenberg number to Newtonian-like flow, transitional flow and vortex formation flow. It is found that the vortex formation flow occurs over the critical Weissenberg number, which is independent of the Reynolds number. However, the critical Weissenberg number decreases with increasing the contraction width due to the influence of velocity acceleration in the entry channel in the upstream of the contraction.