[en] Fundamental studies on the drag reduction of the circular cylinder having dimple were conducted by the measurement of the fluid force acting on the cylinder and by the flow visualization around the cylinder. The drag coefficients were changed by the shape and the space for the arrangement of the dimple. The drag of the cylinder was reduced about 50% by the proper arrangement of the dimple. The flow field around the cylinder having dimple, which was the minimum drag, was visualized by the hydrogen bubble technique. In this case, the separation points were moved rearward and the wake region was small in comparison with the cylinder having no dimple

[en] Vanadium-oxide nanotubes, synthesized through hydrothermal treatment of a vanadium oxide-alkylamine composite, were studied by means of electron spin resonance (ESR). The ESR spectra were composed of a broad Lorentzian line superposed by a hyperfine structure, from which the uniaxial g-tensor and the hyperfine coupling tensor were obtained both for the nanotube and the composite. The eight-line hyperfine splitting structure appears to indicate that the 3d electrons of the V⁴⁺ nuclei interact with only one vanadium nucleus and, thus, are strongly localized. The hyperfine splitting in the nanotubes was much greater than it was in the V₂O₅ single crystal, which is apparently ascribable to the curvature of the vanadium-oxide layers.

[en] In superconducting thin films as well as in non-ideal single crystals the distribution of magnetic flux quanta is affected by existing defects. This interplay between repulsive vortex-vortex interaction and attractive pinning ends up in a strongly disordered vortex arrangement. Visualization of flux lines with magnetic force microscopy (MFM) followed by the quantitative analysis of data offers direct insight into pinning mechanism on a local scale. We have performed quantitative analysis of MFM images of two superconductors: single crystalline BaFe_2-xCo_xAs₂ and NbN thin films. Statistical evaluation of the images allows us to conclude about the existence of a short range hexagonal order in the FeAs crystal with strong pinning sites. Evaluation of the measured signal in NbN thin films gives us an information about the local pinning force as well as the magnetic penetration depth.

[en] The discovery of quantum oscillations in the high-temperature superconductor YBa₂Cu₃O_6.5 above its critical magnetic field showed that the concepts of Fermi liquid theory are applicable to the strongly correlated electron system in the underdoped cuprates. However, the observation of small electron pockets in these experiments puts a new question as such pockets are incompatible with band structure calculations and the Fermi surface topology found in angle-resolved photoemission at zero magnetic field. Our recent neutron scattering experiments on YBa₂Cu₃O_6.45 reveal an enhancement of the static incommensurate magnetic signal at low temperatures when applying an external magnetic field. This field-enhanced magnetic order may lead to a Fermi surface reconstruction and thus provide a natural explanation for the unexpected Fermi surface topology observed in the quantum oscillation experiments at high magnetic fields.

[en] Using low temperature magnetic force microscopy (MFM) we imaged the vortex distribution in a slightly overdoped BaFe_2-xCo_xAs₂ (x=0.19, T_c=23 K) superconducting single crystal. Our local method reveals that at low fields (3 mT und 6 mT) the superconducting flux lines arrange in a vortex glass phase with only a short-range order. This denotes a presence of pinning centres which prohibit a formation of the ordered Abrikosov lattice. From the statistical processing of MFM data we have extracted the radial correlation length ζ of the vortex lattice and have established the hexagonal local lattice symmetry. It reveals that the visualized vortex distribution can be treated as a disordered triangular lattice. Moreover, the isothermal magnetization loops measured at various temperatures in a wide field range up to 14 T exhibit the ''fishtail'' effect that is known to be related to the vortex pinning and to the crossover between two different regimes of the vortex lattice.

[en] We performed a combined study of the slightly underdoped novel iron pnictide superconductor Ba_1-xK_xFe₂As₂ by means of X-ray powder diffraction, neutron scattering, muon spin rotation (μSR), and magnetic force microscopy (MFM). Commensurate static magnetic order sets in below T_m ∝70 K as inferred from the emergence of the magnetic (1 0 3) reflection in the neutron scattering data and from the observation of damped oscillations in the zero-field-μSR asymmetry. Transverse-field μSR below T_c shows a coexistence of magnetically ordered and non-magnetic states, which is also confirmed by MFM imaging. This coexistence could be explained by electronic phase separation into antiferromagnetic and superconducting/normal state regions on a scale of several tens of nanometers indicating that such mesoscopic phase separation can be considered an intrinsic property of some iron pnictide superconductors.

[en] The fittings such as diffusers and reducers are essential flow elements in a piping system, which play a role to connect the different sized pipes or to branch a part of main stream to other piping. However, their central role is to convert the dynamic energy (velocity) of a flow into the static energy (pressure), or vice versa. When designing a piping system, the pressure drop calculation is very important due to the estimation of power consumption for maintaining a required flowrate. For this reason, many coefficient values of the standard sized fitting products are usually provided by manufacturers or by engineering handbooks. In this study, some analytical calculations were performed to find the optimum angles of non-standard reducers and diffusers based on a dimensionless aspect ratio (length/diameter). The results showed that the optimum angles decreased exponentially as the aspect ratio increased and vice versa. Especially, the optimum angle of a diffuser converged to about 5° when the aspect ratio is over 10. Furthermore, these results were verified through the CFD calculations and the previous reported data.