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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 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.

This paper describes the mass transfer characteristics on periodic roughness in a circular pipe and that downstream of an orifice. The mass transfer enhancement ratio of the rough pipe with respect to the smooth pipe are measured by the plaster dissolution method for various combinations of roughness height and wavelength of the periodic roughness, and the influence of roughness on the mass transfer enhancement is examined experimentally. The mass transfer enhancement on periodic roughness is highly increased with increasing the roughness parameter, defined by the roughness height to the wavelength. It is found that the mass transfer enhancement of rough pipe is more than 2 times larger than that of the smooth pipe, which is due to the formation of the local recirculating region over the periodic roughness. On the other hand, minor mass transfer enhancement is observed on the flow downstream of an orifice. The examination of the flow and mass transfer over the periodic roughness and that downstream of orifice is carried out by the numerical simulation with low-Reynolds number k–e model. The result indicates that the formation of local recirculating region on the periodic roughness is the major source of mass transfer enhancement on the rough pipe, while the minor mass transfer enhancement downstream of the orifice is attributed to the effect of separating shear layer originating from the orifice edge on the wall, which is dominant over the local effect of recirculating flow on the periodic roughness downstream of the orifice.

This paper studied ignition and failure mechanisms of hydrogen explosion accident in the container building of Fukushima Daiichi Nuclear Power Plant Unit 1, which occurred a day after the Great East Japan Earthquake in 2011. To understand the mechanism of hydrogen explosion, an unsteady three-dimensional numerical analysis of the flow field in the container building was conducted in real time by assuming that the high-temperature hydrogen gas (500 °C) issued from the head flange of the reactor vessel. The numerical results show that the hydrogen gas spread not only on the 5th floor but also on the 4th floor of the container building covering the stairs. It was found that the hydrogen concentration on the 5th and 4th floors was highly uniform owing to the long real-time accumulation (15 h) of hydrogen, and it spread gradually from the 5th to the 4th floor through the stair. As a result, the hydrogen concentration reached the ignition level on the 4th floor (5%), which could result in the ignition of hydrogen by a tank heater on the 4th floor and the flame propagated to the 5th floor, where fast deflagration and detonation might occur because of the high concentration of hydrogen gas (19%). This scenario is consistent with the observations of failure mechanism after the explosion accident.

To understand the mass transfer mechanism of 90° elbows, a comparative study of mass transfer measurements and oil-flow visualization with image analysis is carried out for long and short 90° elbows in the Reynolds numbers Re = 5 × 104 –2 × 105. The mass transfer distributions are evaluated by using the plaster-dissolution method, while the oil-flow fields are quantitatively visualized by oil-flow technique combined with cross-correlation image analysis, which allows observation of the oil-flow velocity magnitude and direction on the elbow walls. The mass transfer evaluations in the sub-critical Reynolds number indicate an increase in mass transfer distribution along the center wall of the long elbow and a further increase in the downstream of the short elbow. These are highly suppressed on the center wall as the Reynolds number increases, and the mass transfer distributions become almost independent of the elbow curvatures in the post-critical Reynolds number. These observations of mass transfer distributions are well correlated with the oil-flow visualizations with image analysis, which indicate formation of low oil-flow-velocity on the center wall of the long elbow, occurrence of flow separation and reverse flow downstream of short elbow at sub-critical Reynolds number. These features of oil-flow fields on the elbows are highly suppressed and becomes independent of the elbow curvature in the post-critical Reynolds number.

The variations of cloud structure and erosion characteristics of a cavitating jet are experimentally studied by optical and SEM microscope observations, measurement of erosion characteristics and simultaneous shadowgraph imaging combined with the acceleration pulse measurement. The time difference image analysis of the shadowgraphs allows the detection of the near-wall collapsed bubbles in the periodic development of the cavitating jet, while the simultaneous acceleration pulse measurement indicates the generation of acceleration pulses at the same instant of bubble collapse near specimen surface. These results indicate that the generation of acceleration pulses are highly correlated with the collapsed bubbles behind the cloud in the shrinking motion, which triggers the cavitation bubble collapse and leads to the erosion damage on the specimen. It is found from the image analysis that the erosion distributions on the test specimen are well reproduced in the time-difference images of the shadowgraphs, which indicates the sudden intensity growth near the wall due to the cavitation collapse. This result suggests that the collapse of the cavitation bubbles near the specimen surface is correlated with the erosion distribution on the specimen surface in the cavitating jet.

This paper presents a numerical analysis of liquid droplet impingement (LDI) erosion in the pipeline of nuclear/fossil power plants. The numerical method is based on combining a Eulerian/Lagrangian computational fluid dynamics (CFD) model with an erosion model to consider various aspects of LDI erosion, such as the influence of the droplet velocity, diameter, number density, impingement angle, material hardness, liquid-film thickness, and erosion depth on the wall-thinning rate. A numerical analysis of the erosion depth distribution was carried out for the LDI erosion of a bent pipe downstream of an orifice at the Onagawa power plant incident in 2007. The results were compared with existing erosion models, and the variations in the peak erosion depths were examined. The present model results showed reasonable agreement with the prototype results at the location and the erosion depth distribution of the bent pipe. The comparison showed the importance of the liquid film and erosion depth to predicting the LDI erosion characteristics.

This paper describes an experimental study on the wall-thinning rate in deep erosion by liquid droplet impingement (LDI) in a pipeline for application to nuclear/fossil power plant. The experiment is carried out in a spray jet apparatus, which allows the evaluation of local wall-thinning rate by the LDI erosion. The surface contour of erosion and the wall-thinning rate are measured and the observation by scanning electron microscope (SEM) is carried out in this experiment. The experimental result indicates that the wall-thinning rate is highly attenuated and the macro structure on the erosion surface grows with an increase in the erosion depth, which is due to the influence of the liquid film over the erosion surface. The erosion model for predicting the wall-thinning rate in deep erosion is proposed by introducing the attenuation factor with a function of erosion depth. The introduction of attenuation factor with liquid-film effect shows a better correlation with the experimental data, and the accuracy of correlation is improved by a factor of 2.

The influence of swirling flow on mass and momentum transfer downstream of a pipe with elbow and orifice is studied experimentally. The experiments are carried out in the pipe at Reynolds number Re = 3 × 10 4 for various magnitudes of swirl intensity up to 0.9 driven by the rotary swirler. The mass transfer coefficient and the cross-sectional velocity field downstream of the orifice are measured by the plaster dissolution method and the stereo PIV, respectively. It is found that the mass transfer coefficient and the turbulent energy increases gradually with increasing the swirl intensity, while they become non-axisymmetric downstream of the orifice for the swirl intensity larger than 0.6 at the inlet of the elbow, which is followed by the saturation at higher swirl intensity. The maximum mass transfer coefficient is found to have a peak at one diameter downstream of the orifice and it reaches 6 times larger value than that of the straight pipe without swirling flow. The behavior of the maximum mass transfer coefficient with respect to the swirl intensity is similarly observed in the near-wall turbulent energy.

In this paper, the measurement of an aerodynamic sound source for a semi-circular cylinder in a uniform flow is described using Particle Image Velocimetry (PIV). This experimental technique is based on vortex sound theory, where the time derivative of vorticity is evaluated with the aid of two sets of standard PIV systems. The experimental results indicate that the sound source for the semi-circular cylinder is located around the shear layer near the edge of the semi-circular cylinder. The sound source intensity and the area are reduced in the semi-circular cylinder compared with those of a circular cylinder. This result indicates that the aerodynamic sound of the semi- circular cylinder is smaller than that of the circular cylinder, which supports the microphone measurement result.

Aeolian tone from a semi-circular cylinder in a uniform flow is studied experimentally for various angles of attack. It is found that the peak sound spectrum of the Aeolian tone from the semi-circular cylinder is smaller than that from the circular cylinder and the lowest sound is observed around the zero angle of attack. This is due to the reduction in the fluctuating lift force on the semi-circular cylinder compared to that of the circular cylinder. This result suggests the validity of the analogy between the sound pressure level and the fluctuating lift force on a semi-circular cylinder in a stream. The flow visualization study also supports these results.

The onset of exchange flow and the flow structure in a combined buoyancy and forced pipe flow are experimentally studied at Reynolds number up to 7000. The experiment is carried out in a vertical pipe flow of water issuing into a still brine water environment of various concentrations and flow rates. The critical Froude number for the onset of exchange flow is evaluated by the laser-induced fluorescence technique and the velocity fields near the pipe exit are measured by the particle-image velocimetry. The critical Froude number in the laminar regime increases with increasing the Reynolds number and the onset of inflow and the purging of the exchange flow are deviated due to the hysteresis effect, while it is not observed in the turbulent regime. It is found that the laminar exchange flow is characterized by the large-scale structure and the turbulent exchange flow is featured by the small-scale structure near the wall. The enhancement of velocity fluctuation near the pipe exit is clearly observed in the laminar regime due to the occurrence of exchange flow, while it is limited to the near-wall region in the turbulent regime.

This short communication is based on a previous experimental study of liquid droplet impingement (LDI) erosion of groove roughness published in Wear 432–433 (2019). In the present study, further experiments were performed using a spray-jet apparatus to understand the erosion mechanism of triple and quadruple grooves using a scanning electron microscope. It was found that the erosion behavior of triple grooves is similar to that of double grooves, as reported in the previous article. However, the erosion behavior of the quadruple grooves is different from that of the double grooves owing to the presence of less eroded groove peaks far from the surrounding smooth walls. This is due to the weakened side-jet effect from the surrounding smooth walls on the isolated groove peak erosion. Furthermore, the role of the central smooth wall between two single grooves was studied, and a decreased erosion was observed with an increase in the length of the smooth wall. These experimental results explain the importance of the surrounding smooth walls on the erosion behavior of grooves.

Unsteady turbulence structures in and downstream of a short elbow (radius to pipe diameter ratio 1.0) were studied experimentally for sub-to-post-critical Reynolds numbers (Re = 1 × 105 to 3 × 105). Measurement of the velocity field was carried out using particle image velocimetry (PIV) combined with proper orthogonal decomposition (POD) analysis. The mean velocity, turbulence intensity, power spectrum, spatial variation of POD modes, and time variation of instantaneous velocity and vorticity fields were evaluated. The experimental results indicate that a separation region forms around the elbow exit, which is almost independent of the Reynolds number for the post-critical Reynolds numbers. The power spectrum analysis indicates that the periodic oscillation is observed at Strouhal number 0.5, and that it originates from interaction of the secondary flow in the elbow and the unsteady separating flow behavior near the exit of the inner elbow wall. Furthermore, POD analysis was conducted to clarify the variation of POD modes in the elbow flow. It was found that the periodic velocity fluctuation is generated by the oscillation of the flow in spanwise direction downstream of the elbow, which consists of a secondary flow structure directed to the inner elbow wall, and a streamwise longitudinal structure along the centerline of the elbow. The POD reconstructed velocity and vorticity fields indicate periodic oscillation of the entire secondary flow arising from unsteady oscillation of a pair of vortices in the cross section of the elbow; this condition prevails in and downstream of the elbow.

This paper presents an experimental study on erosion initiation via liquid droplet impingement on smooth and rough aluminum (A1070) walls. Experiments were conducted using a spray jet apparatus with three different orifice diameters at various nozzle pressures, which facilitated the variation of droplet characteristics, such as droplet velocity and diameter, which were measured using particle image velocimetry and a shadowgraph imaging technique, respectively. Experimental results are presented for the incubation period based on the spray flow rate, droplet diameter, impact velocity, and surface roughness. It was determined that the incubation period decreases with increasing impact velocity and surface roughness, while droplet diameter has a relatively minor effect on the incubation period. Furthermore, the incubation period based on the impact velocity and surface roughness is defined by an empirical equation for prediction. Scanning electron microscopy observations of erosion initiation on the rough walls reveal that erosion damage was initiated along crystal grain boundaries on the smooth wall, but much larger pits were generated on the rough surface based on the stress concentration effects of the roughness.

Aerodynamic noise reduction of a circular cylinder using longitudinal grooves was experimentally investigated by measuring noise and velocity fields with a microphone and particle image velocimetry, respectively. Experiments were performed in an acoustic wind tunnel operating at subcritical Reynolds numbers, Re = (2–6) ​× ​104. The results indicate that the aerodynamic noise from a circular cylinder decreased by 8 and 5 ​dB compared with a smooth circular cylinder when the ratio of groove-depth to cylinder-diameter is set to 0.010 and 0.017, respectively. Velocity field measurements suggested a downstream shift in flow separation on the circular cylinder and a corresponding decrease in turbulence intensities near the wake of the grooved circular cylinder. Furthermore, the spanwise correlation of streamwise velocity fluctuations decreased near the wake of the grooved circular cylinder. These results demonstrated that aerodynamic noise reduction and variation in turbulence structure occurred in circular cylinders with longitudinal grooves at subcritical Reynolds numbers.

In this study, the interaction of two adjacent diffusion flames (dual flame) were experimentally investigated using the direct imaging method and particle image velocimetry (PIV) measurements for varied distances between the two flames under the influence of the co-flow. The direct imaging method indicates the presence of various synchronization modes, such as merging, in-phase synchronization, amplitude death, anti-phase synchronization, and desynchronization, as the distance between the flames was increased. The dual flame synchronization was characterized using an image analysis of the time-series variation of flame heights summarized by the mean flame height, root-mean-square of the flame height, Strouhal number, and the cross-correlation coefficient of flame heights against the burner distance. Furthermore, the unsteady velocity fields of the synchronized flames were measured using PIV combined with the proper orthogonal decomposition (POD) analysis to extract the flow structure of the dual flame. The experimental results indicated that the in-phase mode is characterized by the formation of symmetrical vortices, whereas the anti-phase mode features the formation of asymmetrical vortices inside and outside of the flame. The POD analysis demonstrated higher fluctuating energy in the anti-phase mode than in the in-phase mode, which suggested the formation of a highly organized flame structure in the anti-phase mode.

The mass transfer behavior on sinusoidal wavy walls with respect to the smooth wall are studied both experimentally and numerically, and the influences of the height to wavelength ratio and Reynolds number are investigated. The mass transfer measurements are conducted by the plaster dissolution method on sinusoidal wavy walls of the height to wavelength ratios smaller than 0.1 at Reynolds numbers ranging from 3 × 104 to 2 × 105. The experimental results indicate that the mass transfer increase rate grows with the increase of the height to wavelength ratio, while it decreases with the increase of the Reynolds number. The peak mass transfer increase was observed on the leeward of the trough irrespective of the height to wavelength ratio, while the peak position changes with the height to wavelength ratio and Reynolds number. These experimental results of mass transfer increase rate were well reproduced by the numerical simulations using the low-Reynolds number k-e model. The results show the mechanisms of the mass transfer increase by sinusoidal wavy walls, and it decreases with increasing Reynolds numbers, which are caused by the reduction of the local recirculation region and turbulence over the wavy walls.

This paper describes the experimental study on the liquid droplet impingement erosion of metal materials to understand the influence of material hardness on the erosion rate. The experiment is carried out using a water spray jet apparatus with a condition of relatively thin liquid film thickness. The metal materials tested are pure aluminum, aluminum alloy, brass, mild steel, carbon steel and stainless steel. The liquid droplets considered are 30 ± 5 μm in volume average diameter of water, which is the same order of droplet diameter in the actual pipeline in nuclear/fossil power plants. In order to understand the influence of material hardness on the liquid droplet impingement erosion, the scanning electron microscope (SEM) observation on the eroded surface and the measurement of erosion rate are carried out in the terminal stage of erosion. The experimental results indicate that the erosion rates are expressed by the droplet velocity, volume flux, Vickers hardness and the liquid film thickness, which are fundamentals of the liquid droplet impingement erosion. The empirical formula shows that the power index for droplet velocity dependency is found to be 7 with a scattering from 5 to 9 depending on the materials, while the power index for Vickers hardness dependency is found as −4.5.

The non-axisymmetric wall-thinning leading to a pipeline break is studied in a scaled model experiment in a water tunnel. The pipeline consists of an elbow, an orifice and a straight pipe between them. The measurement of velocity field is carried out downstream of the orifice using stereo PIV and the spatial correlation of velocity fluctuations is analyzed by snapshot Proper Orthogonal Decomposition (POD), while the mass transfer coefficient is measured by a benzoic acid dissolution method. These measurements are carried out at Reynolds number Re = 3 × 104 with and without swirl. It is found that the non-axisymmetric flow and mass transfer is found downstream of the orifice due to the combined influence of the secondary flow in the elbow and the swirling flow. The POD analysis indicates that non-axisymmetric velocity field is generated in the first two POD modes, which suggests the structural change in the velocity field downstream of the orifice, such as the non-axisymmetric growth of the velocity and turbulent energy in the lower POD modes. It is also found that the near-wall turbulent energy distribution is correlated with the measurement of mass transfer coefficient.