Accurate measurements of particle number density along with particle diameters and velocities are strongly required both in academic and industrial fields. A new imaging technique, through the evaluation of the effective depth of field of a camera, is developed using standard solid particles with constant diameters. To measure the effective depth of field for a wide range of particle diameters, three optical setups, named microscale, mesoscale, and macroscale setups, are used for the diameters of 50 μm - 201 μm, 201 μm - 3.97 mm, and 3.97 mm - 15 mm, respectively. The measured effective depth of field is further applied to measure the size dependence of the number density of entrained bubbles by breaking waves in a wind-wave tank. The results show that the slopes of the number density of the entrained bubbles in the experiments corresponded to those measured by a phase Doppler particle analyzer under 500 μm, and was -5 over 500 μm in both fresh and salt waters. This emphasizes that the present imaging technique can measure the diameters and particle number density with high precision and is important for measurements of droplets, bubbles, and solid particles with a wide range of diameters.

In this study, density-driven natural convection in porous media associated with Rayleigh–Taylor instability was visualized by X-ray computed tomography to investigate the effect of the thickness of the diffusing interface on convection. The thickness of the interface was changed by molecular diffusion with time, and the effective diffusivity in a porous medium was estimated. Compared with the thick interface, for the thin interface, many fine fingers formed and extended rapidly in a vertical direction. The onset time of natural convection increased proportionally with the thickness of the interface, being correlated with Rayleigh number and Péclet number. For the thinner initial interface, the finger number density increased more rapidly after onset and reached a higher value. Next, we discussed the mass transport in Rayleigh–Taylor convection to show how dispersion affects mass transport based on finger extension velocity and concentration in fingers. Increasing the interface thickness delayed the onset of convection, while the finger extension velocity remained the same. The reduced finger extension velocity changed nonlinearly with the Péclet number, reflecting the effect of dispersion. High transverse dispersion and longitudinal dispersion quickly reduced finger density. Transverse dispersion between ascending and descending fingers decreased the density; the density decreased linearly along the finger on both sides of the symmetric plane. As a result, the Sherwood number was proportional to the Rayleigh number, whereas the coefficient changed nonlinearly with Péclet number because of dispersion, reflecting the nonlinear dependences of the reduced velocity and the reduced density difference on Péclet number.

Control of noise generation at an airfoil trailing-edge was conducted using a plasma actuator for an NACA0012 airfoil with an angle of attack of -2° at a chord Reynolds number Re = 1.6×10⁵, where the boundary-layer instability on the pressure side was responsible for the generation of the tonal trailing-edge noise. A thin electrode was installed uniformly in the spanwise direction at the maximum wing thickness location. The actuator was operated in pulsed (burst) mode to excite linear disturbances other than the naturally growing one artificially, and the responses of flow and tonal sound were examined. The naturally radiated tonal trailing-edge noise was found to be replaced by the weaker tonal sound at the bursting frequency when the bursting frequency was near that of the natural tone. It was also demonstrated that when the actuation frequency was far from that of original (natural) sound, the boundary-layer transition was dominated by naturally unstable broadband disturbances, leading to complete suppression of the tonal trailing-edge noise.

In recent years, as a new application technology of jet pumps, attention is paid to a method of installing a jet pump at the bottom of a contaminated lake or a river to remove pollutants and promote purification. The large flow rate that can be generated by the jet pump supplies dissolved oxygen in the water to the stagnant area. As a result, underwater bacteria are activated, and contaminants are decomposed and removed by the activated bacteria. In this research, we aimed to pursue the structure and operating characteristics of a jet pump that consumes less power and can generate jet flow with larger flow rate. The jet pump used in this experiment consists of one or more primary nozzles and a secondary nozzle for flow amplification and is installed at the bottom of a large tank. By measuring the flow velocity distribution blown out from the secondary nozzle, the flow rate flowing out from the secondary nozzle is obtained, and the flow rate amplification factor which is the ratio with the injection flow rate of the primary nozzle can be calculated. In this research, the performance difference between a system consisting of one primary nozzle and a system consisting of two or more primary nozzles was experimentally investigated. As a result, knowledge on the optimum supply system of the primary jet was obtained.

The present experimental study deals with a pulsatile turbulent flow simulating the exhaust flow of an automotive engine. In the experiments, a four-cylinder engine is used as an exhaust-flow generator to realize flow conditions close to those in an engine environment. Particle image velocimetry (PIV) measurements visualize the flow field in an S-shaped double-bend duct at a Reynolds number of 48,000 and a Womersley number of 70.9. Stereo PIV, which is classified as a two-dimensional three-component measurement, is conducted in the duct cross sections located downstream of the bends. The stereo PIV system is synchronized with the engine operation to enable phase-locked measurements at particular phases, and the phase-averaged results show the large-scale vortical structures and the duct axial velocity distribution. Downstream of the first bend, the secondary flow consists of vortices that circulate as Dean-type vortices. Downstream of the second bend, by contrast, vortices that circulate in opposite directions to the Dean-type vortices, so-called Lyne-type vortices, form in the core of the cross section. These secondary flows persist without significant changes in their large-scale vortical structures over time. Time-resolved PIV is conducted to track the temporal evolution of the flow in the bend planes. The results show that the flow reverses locally along the inner wall of the bends during flow deceleration. Snapshot proper orthogonal decomposition (POD) is used on the time-resolved PIV data to extract the significant flow structure from the instantaneous field. We propose POD as a good post-processing tool for the instantaneous data of pulsatile cases.

Immiscible displacement processes have been investigated for decades to understand their applications, such as geosequestration of CO₂ and enhanced oil recovery, in detail. In this study, the effect of buoyancy on fingering growth activity during a drainage process was investigated under the competitive influence of buoyancy, capillary, and viscous forces. A packed bed of glass microbeads was used as a porous medium for immiscible fluid pairing of silicon oil and water as a non-wetting phase (NWP) and wetting phase (WP), respectively. The time lapse 3D structure of finger development was visualized for a Bond number Bo range of 3.84 × 10^-4 to 3.45 × 10^-3 and a capillary number Ca range of 4.30 × 10^-9 to 4.30 × 10^-7 using an X-ray CT scanner. Three instability regimes—capillary fingering, transition, and gravity fingering enhanced by buoyancy forces—were successfully observed. The crossover from capillary fingering to gravity fingering was observed at Bo values between 1.53 × 10^-3 and 3.45 × 10^-3. The gravity fingering structure was observed at Bo = 3.45 × 10^-3 and Ca = 4.30 × 10^-7. Under gravity fingering conditions, the pressure of the NWP to displace the WP is highest at the tip of the most advanced finger because the buoyancy force is relatively higher than the capillary force. Consequently, massive new invasions were induced near the tip of the most advanced finger. As a result, the fingers vertically extended with a streak-like structure, and the diameter of the fingers was as small as single-pore sizes. The NWP saturation at breakthrough was low and fluctuated in the z direction. The WP was merely trapped as an isolated cluster. Under capillary fingering conditions (Bo < 1.53 × 10^-3), the fingers not only grew near the tip of the most advanced finger but also grew far below the tip. Some fingers extended to link pores with a small diameter of approximately single-pore size and developed in a horizontal direction, i.e., in the negative z direction as well as in the z direction. The size of trapped WP clusters was distributed from single-pore size to large size, connecting several pores. A skewed distribution of saturation was shown at the breakthrough for low Ca because of competition between capillary fingering and gravity fingering. At high Ca, the pressure gradient established along the z direction as a result of viscous shear force resulted in a gradual decrease in the saturation profile in the z direction.

The high efficiency cooling of pistons in internal combustion engines by oil jets has recently received widespread attention. To date, curved circular oil jet nozzles have been commonly employed. However, although liquid jets from straight circular nozzles have been studied, there are few reports regarding the characteristics of liquid jets injected by curved circular nozzles. The present work therefore examined the characteristics of oil jets, such as jet width and interfacial fluctuations, following injection via either a 90° curved circular nozzle or straight circular nozzle into a stationary atmosphere. Oil jet parameters were assessed at Reynolds numbers from 1000 to 3000 and the jet interfaces were visualized using a background illumination method in conjunction with a high-speed camera. The oil jet injected from the curved nozzle exhibits more complex behavior compared with that from the straight nozzle. The cross section of the jet from the curved nozzle perpendicular to the direction of movement gradually expanded from a circular shape to an elliptical shape in the downstream direction. The side corresponding to the long diameter of this elliptical jet shape was also observed to switch depending on the Reynolds number. At the inner and outer side interfaces, the outer side interface became larger than the inner side interface at the Reynolds number of 1500 or less. However, the inner side interface became greater than the outer side interface at the Reynolds number of 2000 or more. The effects of the velocity distribution following the bend in the curved nozzle on oil jet behavior were examined by stereoscopic particle image velocimetry. The nozzle internal flow evidently generated movement from the outer side toward the inner side at the Reynolds number of 2000, while flow toward the inner side was dominant when the Reynolds number was 2500.