Yu Nishio

Abstract This study numerically investigates an early stage of nonlinear interaction for the better understanding of the onset of nonlinear behaviors. Two-dimensional shear flow is chosen as a canonical flow. When two disturbances of different wavenumbers satisfying no resonance condition are initially given, new components appear one after another while the original disturbances grow. The vorticity budget analysis shows that the beat of the two exciting modes plays an essential role in forming the sum and difference nonlinear components, namely the secondary modes. For the nonlinear interaction mechanism, the high vorticity around the center of the shear layer is locally transported in the transverse direction at specific streamwise sections where the amplitude of the vertical velocity fluctuation becomes relatively larger compared to other sections. The distance between these specific sections corresponds to the wavelength of the beat. The vertically dispersed vorticity will then be convected in the horizontal directions by the mean flow. As a result, several regions of concentrated vorticity appear which eventually develop into vortices. The amplification mechanism is found to be the same for both the primary and secondary modes, though the secondary modes arise from the additional perturbation deriving from the initial perturbation.

<title>Abstract</title> Micro air vehicles (MAVs) have been developed for many fields. The MAVs usually receive strong impact from a velocity change in time or space, and facilities for aerodynamic experiments of MAVs under a gusty environment have been required. The present study has developed a gust wind tunnel to generate unsteady and non-uniform flows. We developed a small wind tunnel with eight multi-fans and a shutter mechanism at the upstream of the test section. We controlled the outputs of the fans independently and obtained a linear shear layer with an error of 5 percent. The velocity gradient of the shear layer was from 5 to 8 s−1. The shutter mechanisms provided a longitudinal gust with the velocity change from 2 m/s to 10 m/s within 0.3 seconds.

<title>Abstract</title> The present study investigates the flow field in a rinsing process of a beverage can numerically and experimentally. The three-dimensional Navier-Stokes equations are solved with a finite volume method along with the volume of fluid (VOF) method for free surface. The beverage can set upside down is transported with a constant velocity and rinsed with a water jet ejected from a nozzle below the can. The case of a can at rest is also simulated. The result shows that the ejected water impinges on the can bottom and spreads along the side surface of the can. Then, as it flows down toward the can mouth, its front surface forms splashes. For the stationary can case, after the jet impinges on the can bottom, it almost evenly spreads over the side surface. The water flows downward and becomes branched flows by fingering. The time average of VOF is calculated to visualize the regions rinsed by water. For the case of a moving can, only the top region of the can is rinsed, and the ratio of the rinsed region drops to 29% from 69% for the stationary case. The computed water surfaces qualitatively agree with the experimental result, but the shape of the front surface, such as splashes and fingerings, cannot be resolved with the simulation.

© 2020, Springer-Verlag GmbH Germany, part of Springer Nature. The present study aims to simulate a collision of two droplets containing immiscible liquids by employing a three- dimensional incompressible smoothed particle hydrodynamics (SPH) method, with models implemented for the computation. The simulation of a head-on collision of two droplets, both of which contain the same glycerol solution, showed that the droplet behavior agrees very well with that observed in an experiment by Qian and Law (J Fluid Mech 331:59–80, 1997. https://doi.org/10.1017/S0022112096003722). The simulation of a head-on collision of a silicone oil droplet and a glycerol solution droplet showed that the droplet behavior agrees fairly well with that observed in the experiment by Planchette et al. (J Fluid Mech 702:5–25, 2012. https://doi.org/10.1017/jfm.2012.94). However, the oil only partially wraps the glycerol solution in the simulation, whereas in the experiment, the oil completely encapsulates the glycerol solution. The shape of the coalesced droplet at the early stage of the collision quantitatively agrees with that observed in the experiment. In the case of an offset collision of a droplet of silicone oil and a droplet of glycerol solution, the two droplets separate again, and small satellite droplets are formed after the collision; this behavior agrees well with that observed experimentally by Planchette et al. (2012). The feasibility of using simulations to estimate droplet liquid composition ratios is also presented. It is shown that the models implemented for the incompressible SPH simulation work reasonably well, and that this method can be a useful tool for studying the droplet collision phenomena.

© 2019 The Authors. Published by Elsevier Ltd on behalf of The Chinese Society of Theoretical and Applied Mechanics Numerical simulations of the liquid flow scattering from rotary atomizers are performed using an incompressible smoothed particle hydrodynamics (SPH) method. The influence of grooves at the edges of the atomizers on the formation of ligaments and droplets is investigated changing the numbers and shapes of the grooves. As a result, it is found that small droplets are likely to be generated when the number of grooves is large and the depth of grooves is deep. It is also found that the grooves work more effectively in bell-cup atomizers than in disk type atomizers.

© 2019 The Japan Society of Fluid Mechanics and IOP Publishing Ltd. The flapping motion of a permeable square-shaped flag is studied using an incompressible smoothed particle hydrodynamics method together with a newly proposed permeable flag model. Wind tunnel experiments are also performed, and the results are compared. The present model is successful in simulating two characteristic states depending on the uniform velocity, namely the 'stretched-straight state' and the 'flapping state.' Both the oscillation periods and the tail-end trajectories quantitatively agree with the experiments. However, the bistable region, which appears in the experiments, is not seen in the computational results. Except for the high-permeability case, the flags oscillate two dimensionally and no flow separation is observed irrespective of the permeability conditions. This might be due the flexibility of the flag in which the flag can change its shape in response to the low fluid pressure acting on the surface. A pair of longitudinal vortices rotating in opposite directions is generated at both spanwise ends of the flapping flag. Their strengths vary in accordance with the instantaneous lift force. When the flag is impermeable, these longitudinal vortices are connected to the spanwise vortex shed from the trailing edge and deform into a V-shaped vortex downstream. Conversely, the longitudinal vortices generated from a permeable flag simply move downstream without reconnecting. In addition, it is found that fluid permeation takes place at the location where the work by the fluid on the flag is concentrated.

© 2018 Flow structures created during the intake stroke of an engine were investigated by means of multi-planar particle image velocimetry (PIV). A unique water-analogue engine model has been developed, where all essential parts and parameters, such as the cylinder head, valve timing, piston geometry and motion, etc. can easily be modified. Two cylinder heads with geometrically different inlet ports were investigated and experiments were performed with both moving and fixed valves. Three-dimensional visualisations of the flow field, mode decomposition through proper orthogonal decomposition, circulation as well as classical statistics were obtained and evaluated in order to gain an understanding of the flow structures, i.e. tumble and swirl, created by the two cylinder heads. It was clearly shown that one of the cylinder heads created a strong swirling motion in the cylinder. Three different fixed valve positions were investigated and the fully opened valve gave the strongest large-scale structures, whereas with smaller openings a larger amount of the kinetic energy was converted into small-scale turbulence. Results showed a more organised and stable flow field consisting of a well-defined swirl motion occupying the whole cylinder at the end of the intake stroke when the valves were fixed at the highest position. The moving valve case gave results similar to the fully open case but with slightly higher turbulence. Cycle-to-cycle variations were found to be less pronounced for these two cases as compared to the smaller fixed valve lifts. The second cylinder head showed a flow field that was more turbulent and much less coherent. Statistical analysis showed that this had a direct effect on cyclic variations in the flow where this head showed more profound variations.

Copyright © 2019 ASME. A tube-type gas burner consists of a straight tube with a slit along it and discharges an air-gas mixture through the slit to produce a flame. The flow velocity from the slit depends on the pressure in the tube and the pressure loss at the slit, and it varies in the longitudinal direction of the tube. The resulting uneven flame degrades the quality of the burner. In this study, we develop a one-dimensional theoretical model of the flow in a tube with a slit. To validate the result of the theoretical model, we also conduct experiments and numerical simulations for the same flow field. We applied this theoretical model to a flow in a tube, 1 m length, 40 mm in diameter, with a slit 2.5 mm wide. The end of the tube is closed. We also discuss the effect of the length of the burner on the unevenness.

Copyright © 2019 ASME. Motion of liquid pouring from a beverage can is numerically studied. A liquid is poured from a can which is rotated at a prescribed angular speed. The flow is simulated by solving the unsteady three-dimensional Navier-Stokes equations. An experiment under the same condition is also carried out to validate the computational result. The result shows that, when the can is tipped, the liquid flows over the lid of the can and is once obstructed by the rim of the lid. The numerical result is in good agreement with the experimental result. The effect of condensation formed on a can surface is also considered. The effect of condensation is taken into account by adjusting a contact angle. The liquid pouring from a can trickles down along the can body. The computation reproduces these experimental observations.

© 2019 The Authors. Published by Elsevier Ltd on behalf of The Chinese Society of Theoretical and Applied Mechanics A numerical simulation is performed to find out a key vortical structure in the laminar-turbulent transition. A low-speed streak is generated inside a laminar boundary layer using an isolated cuboid roughness, aimed at providing an environment unstable to outer disturbances. Then, a short duration jet is issued into the boundary layer. When the jet velocity is low, some vortices appear in the boundary layer, but the transition of the boundary layer does not take place. However, when the jet velocity exceeds a certain threshold, two vortices newly appear above the elongated legs of a V-shaped vortex and only one of them is stretched and survives. After that, vortices are generated one after another around the survived one. By comparing the decayed and the survived vortices, it is found that the difference in their heights is the key characteristic which leads to the transition.

© 2019, Isfahan University of Technology. In this study, attempts to suppress numerical viscosity in incompressible smoothed particle hydrodynamics (SPH) computations are reported. Two-dimensional computations are performed for inviscid and viscous flows to evaluate the effects of numerical viscosity suppression. The first approach is to reduce numerical viscosity at the wall by considering only the wall-normal components of the forces between fluid particles and wall particles. The second approach is to reduce numerical viscosity within the flow field by employing elliptic kernel functions whose major axes are aligned with the local mean flow direction. It is found that special treatment of the wall radically reduces the numerical wall friction. Using an elliptic kernel function is found to work reasonably well in reducing numerical viscosity.

© 2018, The Author(s). Abstract: The in-cylinder flow prior to combustion is considered to be one of the most important aspects controlling the combustion process in an engine. More specifically, the large-scale structures present in the cylinder, so-called tumble and swirl, before compression are believed to play a major role into the mixing and combustion processes. Their development during the intake stroke and their final strength depend mainly (but not only) on the inlet port design. In the present study, the turbulent large-scale structures during the intake stroke are investigated in a unique water-analogue engine where inlet ports and valve timings can easily be configured and tested. The flow field in the cylinder volume is reconstructed through multi-planar stereoscopic particle image velocimetry measurements which reveal a wealth of vortical structures during the stroke’s various phases. The aim of the present paper is to present and show results from a unique setup which can serve as a test bench for optimisation of inlet port designs to obtain a desired vortical pattern in the cylinder after the intake stroke is finished. This setup can simulate the intake stroke in a much more realistic way as compared to a through-flow setup with a fixed valve lift. Graphical Abstract: [Figure not available: see fulltext.].

© 2018 Cambridge University Press. Direct numerical simulations are carried out to investigate the role of the turbulent region in a self-sustaining system with a spiral vortex structure in the threedimensional boundary layer over a rotating disk by solving the full Navier-Stokes equations. Two computational domains with two different azimuthal sizes, 2π/68 and 2p=32, are used to deal with different initially dominant wavenumbers. An artificial disturbance is introduced by short-duration strong suction and blowing on the disk surface. After the flow field reaches a steady state, a turbulent region forms downstream of Re=640. The turbulent region is then removed using two methods: A sponge region, and application of a slip condition at the wall. In both cases, the turbulent region disappears, leaving the spiral vortex structure upstream unaffected. The results suggest that the downstream turbulent region is not related to the velocity fluctuations that grow by the global instability. In addition, when the area where the slip condition is applied is changed from Re > 630 to Re > 610, the velocity fluctuations decay. The results indicate that the vibration source of the velocity fluctuations which grow by the global instability is located between Re=611 and Re=630.

© 2018 American Physical Society. Numerical simulations are carried out to discover the flow structure that plays an important role in the laminar-turbulent transition process of a boundary layer on a flat plate. The boundary layer is destabilized by ejecting a short-duration jet from a hole in the surface. When the jet velocity is set to 20% of the uniform-flow velocity, a laminar-turbulent transition takes place, whereas in the 18% case, the disturbances created by the jet decay downstream. It is found that in both cases, hairpin vortices are generated; however, these first-generation hairpins do not directly cause the transition. Only in the 20% case does a new hairpin vortex of a different shape with wider distance between the legs appear. The new hairpin grows with time and evokes the generation of vortical structures one after another around it, turning the flow turbulent. It is found that the difference between the two cases is whether or not one of the first-generation hairpin vortices gets connected with the nearby longitudinal vortices. Only when the connection is successful is the new hairpin vortex with wider distance between the legs created. For each of several cases tested with changing jet-ejecting conditions, no difference is found in the importance of the role of the hairpin structure. Therefore, we conclude that the hairpin vortex with widespread legs is a key structure in the transition to turbulence.

© 2017 Author(s). The laminar-turbulent transition of a rotating-disk flow dominated by global instability is studied by solving the full Navier-Stokes equations in direct numerical simulations. A flow field in the 2π/32 region is computed using a periodic boundary condition. The flow field is disturbed in two ways. In the first case, a disturbance is introduced at the Reynolds number, Re ≈ 600, while in the second case, a disturbance is introduced at Re ≈ 650. In both cases, wall-normal short-duration suction and blowing are used to disturb the flow field. When a disturbance is added upstream at Re ≈ 600, the wavenumber 64 component becomes dominant when the flow reaches a steady state, whereas when a disturbance is added downstream at Re ≈ 650, the wavenumber 96 component becomes prominent. The transition points are different between the two cases. In addition, in both cases, the distances between neighboring spiral vortices are quite the same when measured at the locations where the turbulence begins.

© Published under licence by IOP Publishing Ltd. Stretching vortices whose sizes are in the inertial subrange of a homogeneous isotropic turbulence are picked up, and the geometric relations with the neighboring vortices whose scales are twice larger are studied. Hierarchical vortices are extracted using a Fourier band-pass filter, and each extracted vortex is reconstructed as a set of short cylindrical segments along the vortex axis to discuss the vortex interactions. As a result, it is shown that the directions of larger vortices near the segments of the fast stretching vortices tend to be orthogonal to the direction of the stretching segments, and the locations of the larger vortices that contribute most to the stretching of smaller vortex segments are likely to be found in the direction with the relative angle of 45° from the axes of the stretching vortex segments. And, the vortices with the second highest contributions tend to be in the directions 45° from the stretching segments' axes and orthogonal to the directions of the highest contributing vortices.

Three-dimensional numerical simulation of the flow around a leading edge of a flat plate when vortices with their axes normal to the flat plate surface is carried out to investigate the leading-edge receptivity to the vortical disturbances. It is shown that the vertical vortices outside the boundary layer are not titled and deformed, contradicting to what was reported in previous studies. It is revealed that the streamwise structures, which are dominant in the boundary layer, are formed due to the velocity field induced by the wall-normal vortices outside the boundary layer. It also is found that neighboring pair of vortices side-by-side in the spanwise direction are connected to each other very close to the wall.

This paper describes the analysis of flows around a straight wing vertical axis wind turbine. Hotwire measurements in two-dimensional wind tunnel and numerical simulations are conducted. The rotor has two blades, whose airfoil is NACA0012 and the chord length is 0.1 of the rotor diameter. Reynolds number based on the uniform velocity and rotor diameter is 8.0 × 10 4. The tip speed ratio is 2.0. Incompressible Navier-Stokes equation is used for numerical simulation. A rotating coordinate system, which rotates at the same speed of the rotor, is employed. Both the experimental and numerical results are compared. As a result, the velocity fields show good agreements at each rotation angle, and the aerodynamic torque varies according to the flow condition, especially the flow separation. © 2011 The Japan Society of Mechanical Engineers.