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The flow and torque mechanisms of an open cross-flow hydraulic turbine in an underflow operation are investigated through experiments and numerical simulations. The unsteady flow field around a hydraulic turbine is measured in an open-circuit water tunnel using phase-averaged particle image velocimetry, and the results are compared with those of a two-dimensional numerical simulation using the volume of fluid method for two-phase flow analysis. The experimental and numerical results of the flow field agree well with each other, suggesting the validity of the two-dimensional numerical simulation for open cross-flow hydraulic turbines. The experimental and numerical results indicate that both the accelerated flows over the downstream blade and through the bottom spacing between the turbine and channel wall yielded the positive torque generation of the cross-flow turbine. The former effect is caused by the stagnation pressure on the concave side of the blade and the formation of accelerated flow on the convex side of the blade, both of which are observed at a low tip-speed ratio. The latter effect of bottom spacing is caused by the converging flow effect of the local velocity between the lower side of the turbine and the channel wall. It is discovered that these flow features improved by decreasing the bottom spacing, and that they contributed to the local torque generation; consequently, the torque performance and efficiency of the cross-flow turbine improved. However, very small bottom spacing may not be effective for improving the efficiency.

The performance of a waterfall-type cross-flow hydraulic turbine was investigated via experiment and numerical simulation. The experiment was carried out using a cross-flow hydraulic turbine operating in a two-dimensional waterfall, and the result was compared with a corresponding numerical result analyzed by the volume of fluid method. The experimental results show that the maximum efficiency reached approximately 60%, higher than that of previous experiments in the literature. It was also confirmed that the recorded experimental efficiency was well reproduced in a two-dimensional numerical simulation, suggesting the validity of the numerical results. Furthermore, flow fields of the turbine were studied based on phase-averaged particle image velocimetry measurement and the numerical simulation. These results indicate that the flow and torque mechanisms of the cross-flow hydraulic turbine studied are explained by stagnation pressure on the concave side of blades and a Coanda-like flow on the convex side of the blades at small angle of blade position, followed by the formation of a high-velocity region within the turbine and flow impingement on the downstream blades at large angle. These flow field variations generated an increased local torque at small angle and a moderate torque at large angle. The improved efficiency was obtained by optimizing the off-axis distance to the waterfall and the tip-speed ratio of the hydraulic turbine, which were examined by a visualization of the flow through the turbine and the local torque distribution with respect to the angle of blade position, respectively.

The secondary flow downstream of a triple elbow layout was studied experimentally and numerically to visualize the flow behavior under swirling inlet flow conditions. The inlet swirling condition was generated by a swirl generator, consisting of a rotary pipe and honeycomb assembly. The experiments were carried out in turbulent water flow condition at Reynolds number Re = 1 × 104 and inlet swirl intensity S = 1. Ultrasonic measurements were taken at four locations downstream of the third elbow. The two-dimensional velocity field of the flow field was measured using the phased array ultrasonic velocity profiler technique to evaluate the flow field with separation. Furthermore, a numerical simulation was performed and its results were compared with the experimental data. The numerical result was obtained by solving three-dimensional, Reynolds-averaged Navier-Stokes equations with the renormalization group k-e turbulence model. The experimental results confirmed that the swirling flow condition modified the size of the separation region downstream of the third elbow. A qualitative comparison between the experimental and CFD simulation results of the averaged velocity field downstream of the third elbow showed similar tendency on reverse flow.

The behavior of a diffusion flame under acoustic excitation was experimentally studied with the aid of simultaneous flow visualizations of the flame combining Mie scattering and schlieren/shadowgraph imaging. The Mie scattering imaging showed the periodic formation of a vortex street along the flame axis, while the schlieren/shadowgraph images indicated the formation of a high-amplitude density fluctuation near the burner exit. Furthermore, mean velocity and temperature along the flame axis were measured for evaluating the flame behavior by acoustic excitation. These experimental results showed that the acoustic excitation led to a density fluctuation near the burner and promoted the flame behavior near the burner. This behavior resulted in enhanced mixing and combustion in the flame near the burner, which grew as the oscillation amplitude of the acoustic excitation increased. In addition, the flame height, oscillation amplitude and luminosity decreased as the acoustic excitation amplitude is highly increased because of the formation of the increased velocity region with high-temperature near the burner associated with the initiation of laminar-to-turbulent transition of the flame.

The structure of buoyancy-driven exchange flow in a horizontal pipe was investigated experimentally by using brine water as the working fluid. The flow fields were visualized by laser-induced fluorescence (LIF), and the results were analyzed by a proper orthogonal decomposition (POD) to identify the variation of exchange-flow structure in the pipe. The LIF visualization enabled the investigation of the inflow and the purging conditions of the exchange flow in a horizontal pipe, which agreed closely without hysteresis, while they shifted to a higher Froude number regime in comparison with the vertical pipe conditions reported in the literature. The POD analysis of the exchange flow indicates that the fluctuating concentration level increased in the lower POD modes, owing to the periodic fluctuation of the exchange flow. The fluctuating concentration in the lowest POD mode increased at a lower Froude number, which reflects the periodic fluctuation of the interface on the lower wall and on both sides of the exchange flow. At higher Froude numbers, the exchange flow was suppressed and the fluctuating concentration level in the lower POD modes decreased. It was also found that the fluctuating velocity measured by particle image velocimetry (PIV) was highly magnified at the interface of the exchange flow of high Reynolds number, where the velocity gradient was highly generated owing to the interaction with the wall boundary.

In this study, flow structures and mixing performance in a blade-free planetary mixer, which combines rotation and revolution motions inside a cylindrical vessel, are numerically investigated. Flow fields in the mixer vessel are simulated in a single rotating reference frame with various revolution speeds and a fixed rotation speed. The mixing process is investigated by a Lagrangian particle tracking method and the mixing performance is evaluated based on particle concentration. The results of the numerical simulations show that a vortical flow with an axis inclined with respect to the rotation axis of the vessel is generated by the combined influence of the rotation and revolution motions. The flow structure and vortical flow intensity vary as a function of the precession rate, which is the ratio of the revolution speed to rotation speed. The mixing performance of the blade-free planetary mixer is found to be maximum at aspecific precession rate.

In this study, the erosion pattern, pits, and shockwave formation of a cavitating jet were experimentally observed to gain an understanding of the erosion mechanism of a cavitating jet discharging from a cavitator nozzle into a still water environment. The erosion pattern and formation of pits were visualized by direct imaging on the eroded material surface, while the shockwave initiation points were detected using the cross-schlieren visualization method, combined with two orthogonal high-speed observations near the wall. The experimental results indicated that the radial distributions of the erosion depth, number of pits, and shockwave initiation points were highly correlated. These results provided direct evidence of the cavitation erosion caused by the formation of pits that resulted from the shockwave generated by the periodic cloud collapse event near the wall. The results also demonstrated that the growth of the cavitation erosion volume is highly correlated with the number of pits formed rather than the diameters of the pits.

The mechanism of laminar mixing in a planetary mixer of cylindrical container in precession was studied experimentally using three different types of flow measurement techniques, the measurement of mixing from particle dispersion, cross-sectional velocity measurement by particle image velocimetry (PIV), and the three-dimensional trajectory observation by stereo imaging. These visualization studies reveal that the laminar mixing performance was improved in a certain range of precession rate (the ratio of revolution to rotation), whereas the three-dimensional flow structure promoted the laminar mixing in the container. Furthermore, it was found that the three-dimensional tracer trajectory highly fluctuated in the cylindrical container at the optimum precession rate. This was caused by the occurrence of chaotic advection in the cylindrical container by the influence of precession. The results were supported by the numerical simulations of laminar mixing in the planetary mixer reported in the literature.

The erosion behavior of a cavitating jet on groove roughness was investigated experimentally using mass-loss characteristics, scanning electron microscopy (SEM) observation, time-resolved shadowgraph, and schlieren flow visualizations. The wall morphology of the cavitating-jet erosion on the groove roughness indicated an increased mass loss, which was highly increased along the groove rather than across the groove. Furthermore, increased erosion pits were observed on the groove bottom along the grooves. The shadowgraph imaging of the cavitating jet on the rough wall showed noncircular cavitation bubble distributions along and across the grooves, which corresponds to the increased number of cavitation bubbles along the grooves and the decreased number of bubbles across the grooves. This result is consistent with the erosion morphology of the groove roughness. Schlieren imaging indicated that the frequency and intensity fluctuation of the shockwave formation did not change significantly on the groove roughness along and across the grooves. The findings in the study show that the increased erosion mechanism on groove roughness is caused by the increased number of impulsive forces and the shockwave focusing effect on the groove bottom.

The influence of elbow curvature on the flow field of a 90° elbow was studied experimentally to understand the flow and turbulence structure in the elbow using planar and stereo particle image velocimetry (PIV) measurements. Three elbows with different radius to diameter ratios (1.0, 1.2, and 1.5) were investigated in the Reynolds number range of Re = (3–10) × 104. The experimental results showed that the mean velocity decreased and turbulence intensities increased along the inner wall in the second half of the elbow, and similar observations were made in all of the elbows. However, variations in these quantities increased with decreasing radius ratio. This was caused mainly by the occurrence of flow separation on the inner wall, near the elbow outlet, for smaller radius ratios. Similarly, flow separation on the inner wall was promoted at lower Reynolds numbers. Cross-sectional velocity field measurements at the elbow outlet using stereo PIV showed the presence of a pair of counter-rotating vortices, which increased in mean velocity and turbulence energy on the inner wall with decreasing radius ratios of the elbow. This result was well reproduced in the contour of the first proper orthogonal decomposition (POD) mode. In the POD analysis, opposite signs of the high POD mode were formed on both sides of the inner wall and the peak of the POD mode increased with decreasing radius ratio.

Damping of the impact pressure from liquid droplet impingement (LDI) on a wet wall was studied by numerical simulation and experiment. The numerical simulation was carried out for the impact of an axisymmetric spherical droplet on a wet wall using a compressible form of the Euler equations combined with the stiffened gas equation. The impact pressures on the wall were highly damped by the influence of the liquid film prevailing over the wet wall, and the damping effect was formulated as a function of the liquid-film thickness to droplet diameter. The physical mechanism of the liquid-film damping effect is due to the two-stage compression during LDI and its weakening by the diffraction of the shock wave propagated in the liquid film. In order to understand the liquid-film damping effect obtained from the numerical simulation, experiments on LDI erosion on a wet wall were carried out for various liquid temperatures, which generated a thinner liquid film on the wall at higher temperatures by the viscous effect. The experimental results indicated that the LDI erosion rate increased with rising liquid temperatures, which corresponds to the erosion-rate growth at thinner liquid-film thicknesses. This result is consistent with the liquid-film damping effect obtained from the numerical simulation.

The effects of air co-flow and equivalence ratio on flickering of a partially premixed flame of methane/air were experimentally studied for various combinations of burner diameters, fuel/co-flow velocities, and equivalence ratios of methane/air fuel. A scaling law was proposed by introducing the corrections made to the Froude number, equivalence ratio, and co-flow velocity ratio to describe the flickering characteristics of the flame in the co-flow. It is found that the corrected Strouhal number of the partially premixed flame increases with an increase in the co-flow at higher equivalence ratios with a frequency jump, whereas the value is still higher without the frequency jump at lower equivalence ratios. The oscillation amplitude weakens with an increase in the corrected co-flow velocity ratio at high equivalence ratios, which reduces at low equivalence ratios. The corrected critical co-flow velocity ratio used to suppress the flickering is found to be independent of the equivalence ratio in this scaling law. The proper orthogonal decomposition (POD) analysis of the flame indicates that the random flickering motion at high equivalence ratios is weakened and the fluctuating energy distribution of the lower POD modes is modified when the equivalence ratio is low. These changes in the flame structure are due to the disappearance of the clip-off flame in the partially premixed flickering flame at low equivalence ratios in the co-flow.

In this study, the mechanism of cavitation erosion in the cavitating jet discharging from a cavitator nozzle into a still water environment was examined through simultaneous observations of cloud collapse behavior and shock wave formation with and without wall. The simultaneous observations of the cloud collapse and shock wave formation were conducted by time-resolved shadowgraph and laser schlieren methods, respectively, while the impulsive signal on the wall was synchronously detected by an acceleration sensor. The time-resolved shadowgraph and laser schlieren images indicated that some shock waves were occurred at the instant of cloud collapse during the periodic behavior of the cavitating jet, which results in the impulsive forces on the wall. To a large extent, the shock waves originated from the near-wall region, and they induced the impulsive forces on the wall; in the case without wall, shock wave formation was observed in a similar distance from the nozzle with a broad distribution from the origin along the jet centerline. These results indicate that the cavitating-jet erosion occurs due to the cloud collapse near the wall, where some shock waves are generated and results in the erosion of the wall.

A study of erosion mechanism during the liquid droplet impingement (LDI) on a rough surface was conducted using both numerical simulation and experiments. The numerical simulation was carried out for the LDI on single groove roughness utilizing the two-phase full Eulerian approach based on the high-resolution finite volume method to understand the evolution of the droplet deformation and the wall pressure distribution. The numerical results for the LDI on a large groove roughness showed that the first and the second impacts occurred close to the first contact location of the droplet on the wall, which was similar to the LDI on a smooth surface. The largest impact occurred by the third impact at the groove bottom, which was caused by the side-jet focusing and droplet focusing generated from the contact edge of the droplet. It was also found that the maximum wall pressure at the groove bottom increased with the increase in the groove roughness parameter that represented the geometrical ratio of the groove depth to the droplet diameter. In addition to the numerical studies, the erosion behavior of LDI on single groove roughness was observed experimentally using a Scanning Electron Microscope (SEM) after spray jet impingement on an aluminum specimen with various groove roughness parameter. The experimental results showed that the LDI erosion on the large groove roughness started from the groove bottom, while the LDI erosion on the small groove roughness started from the groove edge and the groove bottom. These experimental findings are well correlated with the maximum wall pressure distributions obtained from the numerical results.

In order to understand the flow structure of a flickering flame, time-resolved observations are carried out of the velocity and density fields in a diffusion flame in co-flow using particle image velocimetry (PIV) and shadowgraph imaging, respectively. The PIV measurement combined with invalid velocity vector analysis using proper orthogonal decomposition (POD) allows for the high spatial resolution measurement of the velocity field in the flame, while the shadowgraph images show the high-temperature contour and soot formation of the flame. These experimental results indicate a periodic variation in the flow field and soot structure of the flickering flame. The observation of the velocity field shows that the periodic inflow and outflow are generated in the outer layer of the flame near the burner, which promotes the flow acceleration in the vertical direction leading to clip-off behavior. It is found that the vorticity contour evaluated from the velocity field matches well with the high-temperature contour observed in the shadowgraph images. Further examination of the flow structure is carried out using the first two POD modes, which demonstrate the presence of counter-rotating vortices in the outer flame contributing to stretching of the flame due to the clip-off motion in the flame.

Liquid droplet impingement (LDI) erosion on a rough surface was studied experimentally in the initial stage of erosion using spray-jet impingement on various levels of sandpaper roughness and single groove roughness. The experimental results using an aluminum specimen indicated that the incubation period decreased and the erosion rate increased with increasing the relative roughness, defined as the ratio of roughness height to droplet diameter, in a range of small relative roughness. The growth of the erosion rate in a large relative roughness is almost constant for the sandpaper roughness with increasing the relative roughness, while the erosion rate for the groove roughness decreased at large relative roughness. Therefore, the erosion rate of the groove roughness indicated a peak at small relative roughness. This might be due to the influence of smooth surface surrounding the single groove roughness. A scanning electron microscope (SEM) observation for the small relative roughness of groove showed that erosion was started on the sidewall of the grooves and was followed by the penetration of erosion perpendicular to the groove. The erosion initiation points shifted from the sidewall to the bottom of the groove with increasing the relative roughness, which might be due to the effective impact angle variation across the groove.

Mass and momentum transfer characteristics in a 90° elbow with a radius to pipe diameter ratio of 1.5 are studied experimentally with the aid of the plaster dissolution method, planar velocity measurement by particle image velocimetry (PIV), and surface flow visualization in the Reynolds number (Re) range of 5 × 104 to 20 × 104. The experimental results indicate that the most significant change in mass transfer distribution occurs on the inner wall of the elbow. The mass transfer coefficient increases along the centerline of the first half of the elbow and decreases in the second half with increasing Reynolds number. The near-wall velocity measurements by PIV show that the flow accelerates on the first half of the inner wall and decelerates on the second half, which contributes to the growth of the turbulent intensities on the second half of the inner wall. The surface flow visualization indicates that the secondary flow is weak on the inner wall of the elbow with higher Reynolds number. These results show that the mass transfer characteristics change on the inner wall of the elbow with increase in the Reynolds number, even when it is larger than Re = 5 × 104.

Time-resolved observations of pit formation and cloud structure in a cavitating jet are carried out to understand the erosion mechanism of a cavitating jet issuing from a converging and diverging nozzle in a still water environment. Pit formation is detected by a sensor made of gold film (0.2 μm in thickness) glued to a transparent glass plate, and the number and size of the pits are evaluated from digital image analysis. High-speed camera shadowgraph imaging allows observation of the cloud structure in the cavitating jet, which captures the cavitation cloud collapse combined with the time-difference analysis. The radial distribution of pits is in close agreement with the erosion depth distribution evaluated from the weight-loss profiles of an aluminum specimen in a cavitating jet. The pit distribution in the cloud collapse is well reproduced in the time-difference analysis. Furthermore, simultaneous observations of the pit and cloud structures show that pits are formed on the wall at the instant of cloud collapses during the periodic behavior of the cavitating jet.

Mass and momentum transfer characteristics in and downstream of a 90° elbow are studied experimentally with the aid of the plaster dissolution method, surface flow visualization in the elbow and the cross-sectional velocity field measurement by stereo particle image velocimetry (SPIV) downstream of the elbow. The experiments are carried out for the elbow with the radius to diameter ratio 1.5 at moderate Reynolds number Re = 5 × 10 4 . The mass transfer measurements in the elbow indicate that the major change of mass transfer coefficient is observed along the inner wall of the elbow, where low mass transfer coefficient is found in the first half of the inner wall and it increases abruptly in the second half of the elbow, which is followed by a gradual recovery of the mass transfer in the downstream, while the mass transfer on other wall does not change so much with that of the straight pipe. These features are mainly due to the flow acceleration in the first half of the inner wall and the following flow separation in the second half of the inner wall, where the high turbulent energy production is expected from the SPIV measurement downstream of elbow. These results indicate that the mass transfer coefficient in and downstream of the elbow is highly modified by the flow separation and secondary flow in the elbow, which may generate the high turbulent energy production in the second half of the inner wall in the elbow.

The exchange flows in a buoyancy driven forced flow in vertical pipes of circular and square cross-sections were studied experimentally and the flow structure was examined by proper orthogonal decomposition (POD) analysis. The experiments were carried out in the vertical pipes issuing into a still brine water environment and the flow fields were visualized by the laser-induced fluorescence (LIF) technique. The flow visualization study showed that the critical inflow condition was deviated from the purging boundary due to the influence of hysteresis, and they are shifted to the high Froude numbers in the square pipe compared to the circular one. The POD analysis of the exchange flow indicates that the fluctuating energy is increased in the lower POD modes due to the formation of large-scale structure of the exchange flow. It is found from the analysis that the exchange flow in the low Reynolds number is promoted at the corners of the cross section in the square pipe, while it is randomly distributed in the circular pipe. This corresponds to an increased critical Froude number in the square pipe than that of the circular pipe, which is due to the presence of exchange flow through the corners. With increasing the Reynolds numbers, the flow becomes turbulent and the scale of the exchange flow is decreased and restricted to the near-wall region, while the location of the exchange flow becomes random along the pipe wall irrespective of the cross-sectional shape of the pipes.

In order to understand the mechanisms of non-axisymmetric pipe-wall thinning in the Mihama nuclear power plant accident of 2004, a numerical simulation of pipe-wall thinning in an elbow-orifice pipeline with swirling flow was carried out based on mass and momentum transfer equations with a standard k - e model of turbulence at a Reynolds number of 5.8 × 10 6 . The pipe-wall thinning was evaluated on the basis of the mass transfer coefficient using the diffusion coefficient of ferrous iron in bulk water flow. The numerical simulation revealed that swirling flow with a certain level of swirl intensity generates a spiral motion downstream of the elbow and sustains a longer distance than expected from an elbow flow without swirl. This type of non-axisymmetrical flow triggers strongly biased flow at the orifice, which results in non-axisymmetric pipe-wall thinning downstream of the orifice.