Study of vortex in flow fields induced by surface dielectric barrier discharge actuator at low pressure based on Q criterion

Flow fields induced by a surface dielectric barrier discharge actuator at low pressure of 7 kPa are measured by particle image velocimetry.The distribution of local vortices in the flow field is revealed by the Q criterion.The reason for the generation of vortices is analyzed and the influence of pulse frequency and duty cycle on vortices is studied.The results show that the Q criterion can reveal the small-scale vortices,which cannot be indicated by the streamline.The direction transition zone where the induced jet moves from the vertical to the tangential and the shear layer between the jet and stationary air are prone to the generation of strong vortices.The influence of pulse frequency on vortices is not obvious,but the variation of duty cycle can significantly affect the strength and distribution of vortices.     

Topological evolution of laminar juncture flows under different critical parameters

Horseshoe vortex topological structure has been studied extensively in the past, traditional “saddle of separation” and new “attachment saddle point” topologies found in literature both have theoretical basis and experimental and computational evidences for support. The laminar incompressible juncture flows at low Reynolds numbers especially are observed to have new topology. Studies concerning the existence of the new topology though found in literature, the topological evolution and its dependency on various critical flow parameters require further investigation. A Particle Image Velocimetry based analysis is carried out to observe the effect of aspect ratio, δ*/D and shape of the obstacle on laminar horseshoe vortex topology for small obstacles. Rise in aspect ratio evolves the topology from the traditional to new for all the cases observed. The circular cross section obstacles are found more apt to having the new topology compared to square cross sections. It is noted that the sweeping effect of the fluid above the vortex system in which horseshoe vortex is immersed plays a critical role in this evolution. Topological evolution is observed not only in the most upstream singular point region of horseshoe vortex system but also in the corner region. The corner vortex topology evolves from the traditional type to new one before the topological evolution of the most upstream singular point, resulting in a new topological pattern of the laminar juncture flows “separation-attachment combination”. The study may help extend the understanding of the three-dimensional boundary layer separation phenomenon.     

Magnetization reversal in asymmetric trilayer dots: effect of the interlayer magnetostatic coupling

The spin structure and magnetization reversal in Co/insulator/Fe trilayer nanodots are investigated by micromagnetic simulations. The vortex and C-state are found and the magnetization reversal is dominated by the shape asymmetry of the dots, which is produced by cutting off a fraction of the circular dot. The vortex chirality is thus controlled by the magnetic field direction. On the other hand, the magnetostatic interaction between the top and bottom magnetic layers has interesting influence on the dot reversal process, where the magnetocrystalline anisotropy direction of the Co layer is allowed to vary within the layer plane. The combined effect of these two aspects is discussed on the base of dot coercivity, remanent magnetization, vortex nucleation and annihilation, and the bias of the Fe layer hysteresis loop. While leading to a new S-state in circle dots, the interlayer interaction facilitates the formation of C-state in asymmetric dots, which reduces the vortex nucleation field. The bias effect of all dots is decreased with the deviation of the Co layer easy axis from the field direction. Unlike the circle and semicircle dots, the field range of the vortex state in other asymmetric dots increases with the angle between the cutting direction and the Co layer anisotropy. Additionally, vortex ranges in less asymmetric dots even larger than that in the circle dots are evidenced unexpectedly. Therefore, the control of the vortex chirality and enhancement of the vortex range are found simultaneously.     

水平旋流内消能泄洪洞结构应力分析研究

根据水平旋流内消能泄洪洞水力特性,旋转水流在水平旋流洞内形成的非均匀内水压力沿程变幅大,各部位结构受力差异大,不便于结构设计.该文利用有限元软件对水平旋流洞典型断面进行有限元分析,计算其应力分布,结合加筋和运行期放空的情况进行对比,分析其应力的分布特点和影响因素.结果表明,水平旋流洞末端的阻塞在减缓水平洞流速的同时,会引起局部应力增大,应考虑加环向钢筋等措施;当环向钢筋不能有效降低结构应力值时,可考虑设置钢衬或采用高标号混凝土,既可以提高隧洞的结构抗力,又可以起到防止和减轻空蚀的作用.     

Numerical Simulation of Fluxes Generated by Inhomogeneities of the Underlying Surface over the Jinta Oasis in Northwestern China

Using land-use types derived from satellite remote sensing data collected by the EOS Moderate Resolution Imaging Spectroradiometer (EOS/MODIS),the mesoscale and turbulent fluxes generated by inhomogeneities of the underlying surface over the Jinta Oasis,northwestern China,were simulated using the Regional Atmospheric Modeling System (RAMS4.4).The results indicate that mesoscale circulation generated by land-surface inhomogeneities over the Jinta Oasis is more important than turbulence.Vertical heat fluxes and water vapor are transported to higher levels by mesoscale circulation.Mesoscale circulation also produces mesoscale synoptic systems and prevents water vapor over the oasis from running off.Mesoscale circulation transports moisture to higher atmospheric levels as the land-surface moisture over the oasis increases,favoring the formation of clouds,which sometimes leads to rainfall.Large-scale wind speed has a significant impact on mesoscale heat fluxes.During the active phase of mesoscale circulation,the stronger large-scale winds are associated with small mesoscale fluxes; however,background wind seems to intensify the turbulent sensible heat flux and turbulent latent heat flux.If the area of oasis is enlarged properly,mesoscale circulation will be able to transport moisture to higher levels,favoring the formation of rainfall in the oasis and protecting its "cold island" effect.The impact of irrigation on rainfall is important,and increasing irrigation across the oasis is necessary to protect the oasis.     

钢包浇注过程的数学物理模拟

采用CFD商用软件FLUENT,选取VOF气-液两相流模型和标准k-ε湍流模型对60t钢包浇注过程进行了数值模拟和物理模拟,利用水口流量和液面高度随时间的变化曲线分析了浇注过程中旋涡的产生及变化规律,得到了产生旋涡时自由液面的速度场,并与水模拟实验结果进行了比较.研究结果表明,水口直径及其水口位置对旋涡影响的数值计算结...     

基于多普勒谱特征的飞机尾涡识别

为解决飞机尾涡威胁飞行安全以及限制机场容量的难题,提出了一种基于多普勒谱特征的尾涡识别算法,以提高尾涡检测性能。构建了飞机尾涡回波多普勒谱模型,分析得出其多普勒谱具有对称性、展宽性以及反比性等重要特征。依据上述谱特征,给出了尾涡识别算法的设计流程,包括回波数据预处理、多普勒谱特征提取和特征门限判决。以空客A-340为例...     

PetFMM—A dynamically load‐balancing parallel fast multipole library

Fast algorithms for the computation of N‐body problems can be broadly classified into mesh‐based interpolation methods, and hierarchical or multiresolution methods. To this latter class belongs the well‐known fast multipole method (FMM ), which offers ??(N) complexity. The FMM is a complex algorithm, and the programming difficulty associated with it has arguably diminished its impact, being a barrier for adoption. This paper presents an extensible parallel library for N‐body interactions utilizing the FMM algorithm. A prominent feature of this library is that it is designed to be extensible, with a view to unifying efforts involving many algorithms based on the same principles as the FMM and enabling easy development of scientific application codes. The paper also details an exhaustive model for the computation of tree‐based N‐body algorithms in parallel, including both work estimates and communications estimates. With this model, we are able to implement a method to provide automatic, a priori load balancing of the parallel execution, achieving optimal distribution of the computational work among processors and minimal inter‐processor communications. Using a client application that performs the calculation of velocity induced by N vortex particles in two dimensions, ample verification and testing of the library was performed. Strong scaling results are presented with 10 million particles on up to 256 processors, including both speedup and parallel efficiency. The largest problem size that has been run with the P etFMM library at this point was 64 million particles in 64 processors. The library is currently able to achieve over 85% parallel efficiency for 64 processes. The performance study, computational model, and application demonstrations presented in this paper are limited to 2D. However, the software architecture was designed to make an extension of this work to 3D straightforward, as the framework is templated over the dimension. The software library is open source under the PETS c license, even less restrictive than the BSD license; this guarantees the maximum impact to the scientific community and encourages peer‐based collaboration for the extensions and applications. Copyright © 2010 John Wiley & Sons, Ltd.     

基于管土耦合的海底管跨涡激疲劳分析程序

由于海流冲刷,海底管线在服役期间会在某些管段形成管跨,管跨的涡激振动是引发海底管线疲劳失效的主要因素之一。基于Visual Basic环境开发海底管跨涡激疲劳分析程序,利用所封装的ANSYS软件建立管土耦合非线性管跨有限元模型,依据Miner线性累积损伤理论,采用S-N曲线法计算管跨在海流作用下的疲劳损伤,并与SHEAR7软件的计算结果进行对比验证,开发的疲劳分析程序具有较高的精度和工程应用价值。     

超音速喷嘴涡流管气体分离性能的数值模拟与实验

 建立了超音速喷嘴涡流管的简化几何模型,利用描述两相双组分超音速冷凝流动的 Eulerian 双流体三维湍流模型,以含湿空气为介质对装置内部的自发凝结过程进行数值模拟。设计制造相应的装置,建立实验平台,对装置的含湿气体分离性能进行实验研究。通过模拟和实验2种手段研究了影响超音速喷嘴涡流管气体分离性能的参数及其影响规律。结果表明,气流在喷嘴内部达到超音速流动,使得水蒸气自发凝结成液滴,为混合气体分离提供了先决条件;热端管内气体自旋产生的离心加速度可以达到重力加速度的6×10⁵倍,为气、液分离提供了必要条件;降低进出口压力之比和冷流率,增大入口相对湿度和长径比可以提高装置的气体分离性能,安装阻涡器对性能有不利影响。因此,采用超音速喷嘴涡流管实现混合气体中重组分的脱除是可行的。