Study of force-dependent and time-dependent transition of secondary flow in a rotating straight channel by the lattice Boltzmann method

A numerical study using the lattice Boltzmann method has been carried out for flow through a rotating straight channel with a rectangular cross section. With different forces applied, the secondary flow exhibits two-cell states, four-cell states or six-cell states at a range of low rotational Reynolds number, however, within which only the two-cell states have been commonly reported. In addition to the force-dependent flow transition, a time-dependent flow transition of the secondary flow among two-cell states, four-cell states and six-cell states is also discovered during flow development. These newly found flow transitions and their regulations by force application have been analyzed. Based on numbers of case studies, it is found that a dimensionless number, the ratio of the driving pressure gradient to the centrifugal force, regulates such flow transitions. This study not only releases new phenomena of flow transition, but also indicates new applications in flow control, particle separation and heat transfer.     

Computational modeling of MR flow imaging by the lattice Boltzmann method and Bloch equation

In this work, a computational model of magnetic resonance (MR) flow imaging is proposed. The first model component provides fluid dynamics maps by applying the lattice Boltzmann method. The second one uses the flow maps and couples MR imaging (MRI) modeling with a new magnetization transport algorithm based on the Eulerian coordinate approach. MRI modeling is based on the discrete time solution of the Bloch equation by analytical local magnetization transformations (exponential scaling and rotations).     

Role of apparent diffusion coefficient values for the differentiation of viable and necrotic areas of breast cancer and its potential utility to guide voxel positioning for MRS in the absence of dynamic contrast-enhanced MRI data

We carried out retrospective analysis of apparent diffusion coefficient (ADC) values in 48 infiltrating ductal breast cancer patients who had dynamic contrast-enhanced magnetic resonance imaging (DCEMRI; Group I) and in 53 patients (Group II) for whom DCEMRI data were not available. Twenty-three patients of Group I showed no necrosis (Group Ia), while in 25 patients, both viable (nonnecrotic) and necrotic tumor areas (Group Ib) were observed on DCEMRI. T1-weighted, fat-suppressed and short inversion recovery images were used to identify the viable and necrotic tumor areas in Group II patients, and necrosis was not seen in 11 patients (Group IIa), while 42 (Group IIb) showed both viable and necrotic tumor areas. The ADCs of the necrotic area of Group Ib (1.79±0.30 ×10(-3) mm(2)/s) and Group IIb (1.83±0.40 ×10(-3) mm(2)/s) patients were similar and significantly higher (P<.01) compared to the ADCs of the viable tumor area of Group Ia (0.96±0.21 ×10(-3) mm(2)/s) and Group IIa (0.90±0.17 ×10(-3) mm(2)/s) patients. Proton MR spectroscopy (MRS) data were also available in these patients, and the ADC values were retrospectively determined from the voxel from which MR spectrum was obtained. These values were compared with the ADC obtained for the viable and necrotic areas of the tumor. ADC of the MRS voxel was similar to that obtained for the viable tumor area in patients of both groups. This interesting observation reveals the potential utility of using ADC values to identify viable tumor area for positioning of voxel for MRS in the absence of DCEMRI data.     

On minimum aspect ratio for duct flow facilities and the role of side walls in generating secondary flows

To the surprise of some of our colleagues, we recently recommended aspect ratios of at least 24 (instead of accepted values over last few decades ranging from 5 to 12) to minimise effects of side walls in turbulent duct flow experiments, in order to approximate the two-dimensional channel flow. Here we compile available results from hydraulics and civil engineering literature, where this was already documented in the 1980s. This is of great importance due to the large amount of computational studies (mainly direct numerical simulations, DNSs) for spanwise-periodic turbulent channel flows, and the extreme complexity of constructing a fully developed duct flow facility with aspect ratio of 24 for high Reynolds numbers with adequate probe resolution. Results from this non-traditional literature for the turbulence community are compared to our recent database of DNS of turbulent duct flows with aspect ratios ranging from 1 to 18 at Re_{τ, c} values of 180 and 330, leading to very good agreement between their experimental and our computational results at these low Reynolds numbers. The DNS results also reveal the complexity of a multitude of streamwise vortical structures in addition to the secondary corner flows (which extend up to z ? 5h). These time-dependent and meandering streamwise structures are located at the core of the duct and scale with its half-height. Comparisons of these structures with the vortical motions found in spanwise-periodic channels reveal similitudes in their time-averages and the same rate of decay of their mean kinetic energy ～ T^{? 1}_A, with T_A being the averaging time. However, differences between the two flows are identified and ideas for their future analysis are proposed.     

Properties and characteristics of P(VDF/TrFE) transducers manufactured by a solution casting method for use in the MHz-range ultrasound in air

Highly effective piezoelectric polymer transducers operating in air at high frequencies have been successfully made by casting a solution of ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) P(VDF/TrFE) directly on a backing metal plate, and their performance has been evaluated. By utilizing this method, it has been possible to develop the three kinds of transducers that operate respectively at 4, 6 and 10 MHz in air. For precise evaluation of the performance of the P(VDF/TrFE) transducers, the absorption loss in air was measured up to 10 MHz. It was confirmed that the empirical formula obtained from the measured absorption values in air at high frequencies was in alignment with its theoretical value. In addition, a high lateral resolution acoustic image of a ROM-Chip (amplitude-image) at 6 MHz in air was successfully displayed using an air coupled concave type P(VDF/TrFE) transducer by bonding an epoxy adhesive.     

中小学门口道路上学期间的一个元胞自动机模型

在已有交通流道路瓶颈、交织区和混合交通流研究的基础上,建立一个中小学门口道路区段上学期间的元胞自动机模型,对上学期间的道路交通流特性进行了模拟,对驾私家车送学生上学比例、乘校车上学比例和交警现场指挥对道路交通流的影响进行实验分析.模拟结果表明减少驾车送学生上学比例、增加乘校车学生比例、辅助以交警现场指挥,可保证交通安全,从而达到抑制交通拥堵、减少车辆总通行时间提高道路实际通行能力的目的.     

Instability threshold of rippled carbon nanotube nanotweezers in the low vacuum gas flow incorporating Dirichlet and Neumann modes of Casimir energy

The aim of this research work is to address the influences of dispersion forces and rippled configuration on the instability threshold of carbon nanotube (CNT) based nanotweezers. To this end, the Dirichlet and Neumann modes of Casimir force arisen from the electric and magnetic energies is developed for cylinder–cylinder geometry. Moreover, the CNTs rippling deformation which experimentally revealed is included in the Euler-Bernoulli beam model to modify the governing equations. The differential quadrature method (DQM) in conjunction with the 4th-order Runge-Kutta algorithm is employed to numerically simulate the non-linear partial differential equations. It is interestingly demonstrated that these phenomena remarkably affect the electromechanical behavior of nanotweezers fabricated from CNTs. By taking the rippling configuration and Casimir attraction between tubes into account, the pull-in voltage decreases. On the other hand, when the gas damping effect due to low vacuum environment is taken into consideration, the pull-in value increases. The accuracy of the present modeling is compared with those experimentally published in the literature, giving excellent results.     

A complex first-principles study of the L2,3-edge XANES spectra and crystal field effects for divalent 3d ions in cubic ZnS

A detailed first-principles analysis of the L_2,3 X-ray absorption near edge structure (XANES) spectra, crystal field strength 10Dq, covalent effects and molecular orbitals (MO) position for all divalent 3d ions from Sc²⁺ to Cu²⁺ in cubic ZnS is performed in the present paper. The calculations were done in the framework of the first-principles fully relativistic discrete variational multi-electron (DVME) based on numerical solution of the Dirac equation with the local density approximation. As a result of the performed calculations, the L_2,3 XANES spectra for all considered ions were calculated and assigned in terms of the electron configurations involved into the absorption transitions; fairly good agreement with available experimental data for Ti²⁺, Mn²⁺, Fe²⁺, Co²⁺ and Ni²⁺ is demonstrated. Experimental XANES spectra for Sc²⁺, V²⁺, Cr²⁺, Cu²⁺ not reported previously were also calculated for the sake of completeness of the present study to enable a systematic analysis of all calculated results for the whole series considered. It was shown that the L₃ and L₂ bands shift to the higher energies on increasing a 3d ion atomic number. In addition, the separation between the L₃ and L₂ bands, the crystal field strength 10Dq and mixture between the 3d ion and sulfur wave functions increase along the considered series, from Sc²⁺ to Cu²⁺. On the other hand, all orbitals of the 3d ions systematically lower down in the same direction. The above formulated trends were confirmed by the experimental data on the crystal field splittings and nephelauxetic effect.