This study is an extension of our earlier work, which examined the effectiveness of using an elliptic tip to control the side force acting on an ogive cylinder. In that study, only one tip was considered, and thus the effect of tip eccentricity on the side force was not known. In the present study, we examine another elliptic tip of a smaller eccentricity to get an insight into how tip eccentricity affects the local and overall side force distribution. Our measurements show that, although the smaller eccentricity tip has a side force distribution similar to that of the larger eccentricity tip, there are some major differences in their flow characteristics. For example, the larger eccentricity tip is found to reduce the onset angle of attack and delay the disappearance of the side force to a higher angle of attack. Furthermore, when α 60 deg, only the lower eccentricity tip displays a hysteresis effect in its side force distribution. To the best of our knowledge, this phenomenon has not been observed on an elliptic tip before, even though a similar phenomenon has been observed on a conical body with a rounded tip when the cone was subjected to unsteady bleeding
Selectivity remains the biggest challenge for metal oxide semiconductor (MOS)-based gas sensors, especially for inactive gases such as CH4. In this work, the core-shell structured ZnO/Pd@ZIF-8 was fabricated via the self-templated method and the sensor based on the ternary hybrid showed excellent CH4 selectivity against NH3, CO and NO2. ZIF-8 was critical for the selectivity enhancement via dual filtering effects: a) size-induced sieving, where NO2 with a larger kinetic diameter of 4.5Å can be hindered by ZIF-8 with an effective aperture size of 4.0-4.2 Å; b) polarity-induced sieving, where NH3 (2.6Å) and CO (3.76Å) with larger polarity showed stronger interaction with ZIF-8 and their diffusion is therefore retarded. Therefore, the selectivity of nonpolar CH4 (3.8Å) is improved. The present insight about the dual filtering effects of ZIF-8 may provide a new insight to improve MOS sensing selectivity by rational designing MOF structures.
Employing electron paramagnetic resonance (EPR) and excitation and photoluminescence (PL) spectra, changes of the local structure of Gd3+ ions were investigated for the CaF2 crystals containing 0.00015, 0.17, 1.22, 5.75 at% Gd ions, respectively. The obtained spin Hamiltonian parameters of the cubic configuration Gd3+ monomer are g = 1.9862, B 4 = -2.3153 ± 0.015 MHz and B 6 = -0.0005 ± 0.001 MHz. The peak-to-peak width of the EPR lines of the cubic configuration Gd3+ is significantly broadened and partially quenched with the increase of the Gd3+ concentration, which indicates that the exchange and dipole interactions between Gd3+-Gd3+ are enhanced. From theoretical calculations, combined with experimentally measured angular dependence, we found that for the 1.22 at% Gd:CaF2 crystals, Gd3+ remains essentially cubic, and the majority of the Gd3+ ions are distributed as a dimer conformation along the [110] direction and at a distance of about 7.7 Å.
In this work, two synthesis routes were applied to investigate the effect of citrates on the construction of the ZnO structure. Well-dispersed ZnO plates with (001) facet highly exposed were prepared via one-step hydrothermal route, while ZnO nanoparticles were obtained via two-step route. In one-step route, citrates were added before the formation of Zn(OH)2 precursor, while citrates were added after the formation of Zn(OH)2. For the first time, the interaction between citrates and the Zn(OH)2 precursor was investigated and citrates that participated in the formation of Zn(OH)2 were the main cause for (001) facet exposed structure construction. A growth mechanism about the formation of ZnO plates in the presence of citrates was proposed. The as-prepared ZnO plates showed enhanced photocatalytic activity for the degradation of methylene blue (MB).
The flow development past a rotating circular cylinder in a viscous fluid is investigated. A new diffusion-vortex method is employed and extended to calculate the flow problem. The variations with time of the pressure and vorticity distribution, the lift and drag forces exerted by the fluid on the cylinder surface, are determined for the selected values of the Reynolds number and rotation rate, and compared with the published experimental results. The results show a variety of unusual and complex flow patterns around the rotating cylinder.
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