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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.

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. Keywords: Smoothed particle hydrodynamics (SPH), Rotary atomizer, Grooves, Droplet

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. Keywords: Turbulent onset, Key vortical structure, Laminar-turbulent transition, Boundary layer, Streaky structures, Short duration jet, Numerical simulation

An experimental study is performed to investigate the flow structures near the edges of rotating disks with different edge shapes. By changing the rotational speed, the Reynolds number is changed from 277 to 356. It is found that the fluid motion near the sharp edge differs depending on whether a rotating shaft is at the disk surface. A flow developing on the flat side of the disk surface changes its direction toward the curved outer surface on the shaft side, and the flow goes straight outward, leaving the disk on the flat side. The existence of the supporting shaft increases the radial velocity of the flow while decreasing the azimuthal velocity. The effects of the edge shape of the disk on the flow fields are also investigated by changing the shape of the disk edges. Rounded and chamfered edges have no noticeable effect on the azimuthal velocity on the curved outer surface, whereas changing the edge shape enhanced the velocity in the disk-thickness direction. In the FFT analyses of the azimuthal velocity measured at the edge of the curved outer surface when the disk edge is rounded, an increase in power across a range of frequencies is observed. Only in the chamfered edge disk case, a peak in the spectrum of the velocity that corresponds to a wavenumber which appears in the transitional boundary layers is observed.

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.

A wind tunnel experiment was performed to investigate the requirements for providing a suitable environment for easily creating a localized turbulent region in a laminar boundary layer. A combination of a short-duration jet and suctions was used to prepare a potentially unstable region upstream, and another jet was ejected downstream against the region at various timings and different relative spanwise locations. Based on the results, the combination of the potentially unstable region and the short-duration jet promoted the transition to turbulence only under limited conditions, whereas in most cases, it worked negatively. However, the turbulent spot generation was enhanced when the downstream jet was used at the timing and location that enlarged the low-velocity area created upstream. Moreover, the locally disturbed region generated by the combination of the potentially unstable region and the short-duration jet did not directly grow into a turbulent spot; rather, the turbulent spot grew in the region following the disturbed region.

In this study, the receptivity of a flat-plate boundary layer was studied by introducing a thin sheet-type disturbance. An airfoil-shaped device was used to generate a thin disturbance without velocity deficit in which a steady jet was ejected from its trailing edge to the downstream. Despite the absence of strong disturbances in the freestream outside the boundary layer, streaky structures similar to an ordinary bypass transition were generated. They meandered slowly in the spanwise direction where their downstream parts were oscillating in a delayed fashion. Turbulent spots were formed in the further downstream region. Consequently, the energy growth of the low frequency band in the velocity fluctuation spectrum was found to originate from this meandering motion of the streaks, whereas the growth of the middle- and high-frequency bands was attributed to the appearances of the turbulent spots.

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 three-dimensional boundary layer over a rotating disk by solving the full Navier–Stokes equations. Two computational domains with two different azimuthal sizes, $2\unicode[STIX]{x03C0}/68$ and $2\unicode[STIX]{x03C0}/32$, 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 $Re=630$.

A dyad molecule was synthesized, with oligo( p -phenylenevinylene) (OPV) as the electron donor and naphthalene diimide (NDI) as the acceptor. The optical, thermal, and X-ray diffraction measurements of the film structure showed that the OPV and NDI moieties of the dyad crystallized separately in the spin-coated films. This is in striking contrast to the previously reported OPV and C 70 fullerene dyad, in which the introduction of C 70 severely disrupted the OPV packing. The separated crystallization in the OPV–NDI dyad was attributed to the planar shape of the acceptor and the differences in the crystalline structures of planar NDI and rod-like OPV.

Geometrical relationships and interactions among hierarchical structures in homogeneous isotropic turbulence are investigated by identifying individual vortices of three different scales in the inertial subrange. Fourier bandpass filters are employed to extract the vortices, and each extracted vortex is replaced by a group of vortex segments with a cylindrical shape. The axes of neighboring vortices tend to be anti-parallel when the distance between them is equivalent to their diameter. Further, results indicate that a vortex is most strongly stretched by vortices twice the scale irrespective of the Reynolds number and the vortex scales. Then, the geometric relationships between the vortices being stretched and those that cause the stretching are analyzed, and the stretching is found to be particularly strong when the two vortices are orthogonal to each other.

A wind tunnel experiment was performed to investigate the suitable conditions in which a localized turbulent region can easily be generated in a flat-plate boundary layer. An artificial disturbance of zero-mass flux was introduced upstream using a combination of a short-duration jet and suctions to prepare a potentially unstable environment. The disturbed region by itself decayed downstream. Another jet was then ejected downstream at several different timings and two different spanwise locations relative to the passage of the locally disturbed region to promote transition to turbulence. Although the jet was too weak to trigger the turbulence transition by itself, an isolated turbulent region, the so-called turbulent spot, was generated when ejected against the disturbed region. The optimum conditions were found when the jet was ejected between the high- and low-speed areas of the convecting unstable region.

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.

Low-band-gap dyad molecules containing diketopyrrolopyrole (DPP) chromophore and hexathiophene were designed and synthesized in combination with C60 and C70 derivatives as the electron acceptors. The effect of alkyl side chains in the donor parts on film morphology, thermal properties, and optical properties were investigated by X-ray diffraction analysis, differential scanning calorimetry, UV–vis absorption spectroscopy, and fluorescence spectroscopy. The donor part with a shorter alkyl side chain showed better packing of the π-plane and an enhancement of absorption in the longer-wavelength region, which led to higher JSC of organic solar cells. The use of C70 as the electron acceptor led to stronger absorption in the visible region, resulting in improved external quantum efficiency and JSC compared with those of the C60 derivatives. The single-component solar cell of the dyad with the methyl side chains and C70 derivative showed the power conversion efficiency of 0.84% with a relatively high JSC of 4.4 mA cm−2 under AM 1.5 illumination (100 mW cm−2).

The streamwise interfaces of an isolated turbulent region in a flat plate boundary layer are investigated by performing a wind tunnel experiment. A bimorph-type piezoceramic actuator is used to generate a trapezoidal turbulent region whose spanwise width is larger than those of a typical turbulent spot. The experimental results show that the traveling speeds of the leading and trailing ends of the turbulent region are almost same as those of the turbulent spots with an arrow-head shape. This result indicate that the spanwise width have little influence to the streamwise growth of an isolated turbulent region. It is also found that the traveling speed of the trailing end of the turbulent region exceeds that of local flow velocity in the near-wall region, which is η < 1.3, and vice versa. This result shows that a relaminarization is in progress in the near-wall region, where the turbulent fluid is moving across the trailing end turning laminar. The critical wall-normal distance of η = 1.3 corresponds to about 10 in the wall unit (y+), suggesting that the relaminarization process is taking place in the viscous sublayer.

The streamwise interface of an isolated turbulent region in a laminar flat plate boundary layer is investigated by performing a wind tunnel experiment. In the experiment, bimorph-type piezoceramic actuators are used to generate a trapezoidal-shaped turbulent region that has a wide interface in the spanwise direction. With the use of the piezoceramic actuators, the interference of the spanwise interface with the streamwise interface of the turbulent region is eliminated. Therefore, the streamwise interface can be considered as an independent interface. The experimental results show that the rms value of the velocity fluctuation and the turbulent fluctuation rapidly changes at the leading edge of the turbulent region. On the other hand, the rms value of the velocity fluctuation remains high after the turbulent fluctuation disappears at the trailing edge of the turbulent region. It is also shown that the traveling speed of the trailing edge of the turbulent region is faster than the ensemble-averaged streamwise velocity at the trailing edge of the turbulent region near the wall (η≤1.3). In addition, the velocity profile at the trailing edge of the turbulent region indicates the existence of the acceleration of the turbulent fluid near the wall. From the above results, it is found that the inverse transition occurs at the trailing edge of the turbulent region near the wall due to the acceleration of the turbulent fluid.

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 × 104. 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.

Controlling the pn-type behavior of a semiconductor such as silicon by adding an extremely small quantity of an impurity (doping) is a central part of inorganic semiconductor electronics since the 20th century. Recent progress in the doping of organic semiconductors strongly suggests the advent of a new era of doped organic semiconductors. Here, the principles and effects of doping at the level of parts per million (ppm) in organic semiconductor films and single crystals are described, including descriptions of complete pn-control, doping sensitization, ppm doping using an extremely low-speed deposition technique reaching 10-9 nm s-1 , and emerging ppm-level doping effects, such as trap filling, majority carriers, homojunction formation, and decreased mobility, as well as ppm-level doping effects in organic single crystals measured by the Hall effect, which shows a doping efficiency of 24%. The Wannier excitonic doping of organic single crystals possessing band conduction and the defect science of organic single crystals related to carrier trapping and scattering are introduced as a new scientific field. The dawn of organic single-crystal electronics is also discussed.

The interface characteristics in a millimeter-scale channel are investigated in order to find a technique to speed up fluid mixing. The channel is fabricated on an acrylic-resin plate, where a cavity is attached in the downstream region of a T-shaped conduit. The mixing effect is evaluated using a blue dye and a colorless liquid, which are alternately injected into the channel by two syringe pumps. The important factors for highly efficient mixing under the combination of alternate inflow and cavity are investigated. Two-dimensional and three-dimensional numerical simulations are also performed, and the results are compared those of the experiment. It is confirmed that the fractal dimension and interface length inside a cavity can be used as indicators to predict the mixing level in the downstream region, although they are not perfect. The importance of focusing on both the stretching rate of the interface and the concentration gradient where the stretching of the interface occurs is presented.

The structure at a spanwise edge of spreading turbulent region is studied in a wind tunnel experiment using conditional-sampling and ensemble-averaging techniques. A turbulent region is created by locally tripping the laminar boundary layer using two bi-morph piezoelectric actuators. The turbulent region protrudes into the laminar region at the middle height of the boundary layer. The results show that when the turbulent region spreads, a pair of streamwise vortices exists near the interface on the turbulent side, and the flow induced by the vortex pair pushes the turbulent fluid into the laminar region. An experiment in which a thin flat plate is inserted into the boundary layer reveals that the spanwise spreading of the turbulent region can be effectively suppressed by destroying these vortical structures.

This paper describes the effect of a thin plate as a static controlling device to suppress cavity noise when it is inserted vertically into a cavity. Experiments using small and large wind tunnels are carried out to invistigate the size effect of the cavity model. From the experimental results, it is shown that there are two optimum plate locations for the noise reduction. One is near the upstream edge and the other is just in front of the downstream edge. By comparing the experiments at small and large wind tunnels, it is found that in the large wind tunnel, noise reduction is not so effective as in the small one.

Energy levels of the first monolayer are manipulated at donor/acceptor interfaces in planar heterojunction organic photovoltaics by using molecular self-organization. A "cascade" energy landscape allows thermal-activation-free charge generation by photoirradiation, destabilizes the energy of the interfacial charge-transfer state, and suppresses bimolecular charge recombination, resulting in a higher open-circuit voltage and fill factor.

A passive control of aerodynamic noise generated by a laminar separating flow over a cavity is carried out in a wind tunnel test and by numerical simulation, using a thin plate inserted into the cavity as a passive flow-controlling device. In the experiment, a noise suppression effect up to 14 dB is achieved by placing a plate in a proper location. It is shown that a plate of smaller vertical size is more effective. In the numerical simulation, it is shown that the location where the shear layer rolls up and the path of vortices are both very sensitive to plate position.

Breakdown process of the Klebanoff mode and origin of turbulence are experimentally investigated in detail. An acoustic excitation using a 2D roughness attached on the wall are used for T S wave generation. Piezo-ceramic actuators are used to introduce the Klebanoff mode fluctuation. It is shown that the turbulence region starts from two locations, one is beneath the head of a A vortex and the other is the high-shear layer accompanying by the A vortex.

An experimental attempt to control oblique waves in the transitional boundary layer of a flat plate using small pieces of thin piezo-ceramic actuators attached on the wall surface is carried out. In addition, whether piezo-ceramic actuators are appropriate as a transition-controlling device is examined. The basic idea is to generate waves whose sign are opposite of the incoming oblique waves. Two rows of piezo actuators are used in the experiment. The upstream row is used to generate the intruding waves and the downstream row is used to generate the intercepting waves. From the experimental results, it is shown that the piezo-ceramic actuators are capable of reducing the amplitude of the oblique waves. It is also shown that the strength of waves after control can be estimated as a linear addition of the waves from the upstream and downstream rows of actuators.

Interaction between a vortex ring and shear layer vortices was simulated by the three-dimensional vortex blob method. The purpose of this research is clarifying interaction between a vortex ring and shear layer vortices, and is also the basic research of active control of the shear layer by introducing the self-propelling vortex ring from outside of the shear layer. Interaction of the vortex ring is effective in enhancing the local growth of the shear layer. Numerical simulation was performed in order to clarify the circulation ratio between the vortex ring and the shear layer vortex tube which promote the interaction between a vortex ring and shear layer vortices most. In the case of circulation ratio ΓR/ΓF of 1.0, shear layer vortices and the vortex ring interact strongly. Large vortices were generated by amalgamation between the shear layer vortices and the vortex ring. The vorticity thickness attains a maximum when circulation ratio ΓR/ΓF is 1.0. However, enhancement of the growth of the shear layer saturated when the ratio exceeds about 1.5. This is because the vortex ring which has large circulation passes through the shear layer.

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.

The standard technique to separately and simultaneously determine the carrier concentration per unit volume (N, cm-3 ) and the mobility (μ) of doped inorganic single crystals is to measure the Hall effect. However, this technique has not been reported for bulk-doped organic single crystals. Here, the Hall effect in bulk-doped single-crystal organic semiconductors is measured. A key feature of this work is the ultraslow co-deposition technique, which reaches as low as 10-9 nm s-1 and enables us to dope homoepitaxial organic single crystals with acceptors at extremely low concentrations of 1 ppm. Both the hole concentration per unit volume (N, cm-3 ) and the Hall mobility (μH ) of bulk-doped rubrene single crystals, which have a band-like nature, are systematically observed. It is found that these rubrene single crystals have (i) a high ionization rate and (ii) scattering effects because of lattice disturbances, which are peculiar to this organic single crystal.

The purpose of this paper is to propose a turbulence model for the three-dimensional vortex blob method. This model uses a modified version of the core-spreading method introduced by Leonard and Chua (1989) in the spirit of the Smagorinsky type of eddy viscosity. The diffusion of vorticity by the molecular viscosity is represented by the core spreading whose rate is approximated by that of a rectilinear viscous vortex tube. The linear combination of the two viscosities yields a Lagrangian turbulence model, which includes the effects of Reynolds number. We apply each model to an impulsively started round jet forced by two helical disturbances rotating in counter directions and compare the vortical structures of the jet among the models to show that the combined model can be a reasonable turbulence model.

An attempt to control an unsteady flow separation from a flat plate surface using a jet ejection is carried out. The unsteady flow is artificially generated by periodically moving a curved ceiling up and down at a constant frequency. The controlling system consists of a jet ejecting device on the upstream side and tufts attached to the surface on the downstream side. The movements of the tufts are monitored by a USB camera to watch for a flow separation. Once a separation is detected, a jet is ejected into the boundary layer through a slot in the plate. The jet ejection causes the separation region to shift downstream and become smaller. In addition, it is revealed that the control effect can be enhanced by stirring the fluid, generating turbulence, before it is ejected into the boundary layer.