Non-standard mixing in stars

One of the most powerful tests of the stellar evolution theory is analysis of stars' atmospheric chemical composition. It has shown that some non-standard mixing different from ordinary convection, semiconvection and convective overshooting seems to occur in stellar interiors. In the present study Zahn's rotationally induced turbulent diffusion is assumed to be responsible for such kind of mixing. We compare results of our evolutionary calculations with available observational data for massive main-sequence stars and red giants in globular clusters.     

Sensitivity of Low Frequency Oscillations to Updated Solar Models

A large number of acoustic frequencies have already been detected, leading to a "seismic" model of the Sun rather close to the actual standard solar models. The core however is not yet well constrained by these observations and frequencies of low degree, low frequency modes which penetrate deeply into the solar core are needed. We present here a study on the sensitivity of low degree low frequency (50 - 900 μHz) modes to the structure of the solar interior, in order to help their detection and identification in the low frequency spectrum observed by SoHO experiments like VIRGO and GOLF. The frequencies of p and g modes have been computed for a set of solar models with updated physics (Morel et al., 1997). We analyze their sensitivity to solar parameters like age and metallicity, and to various physical processes, like convective core overshoot and mass loss during the beginning of solar evolution. This revised version was published online in June 2006 with corrections to the Cover Date.     

叶轮机转子内流动的三维跨声Navier-Stokes方程组数值计算

应用有限体积法及Baldwin-Lomax代数紊流模型求解叶轮机转子内流动的三维可压缩非定常薄层Navier-Stokes方程组,计算一个绕X轴等转速旋转的转子内流场问题,用当地时间步长方法来加速收敛。采用隐式算法对对流通量进行求解,而扩散通量采用显式算法。采用高阶TVD下的Roe格式计算对流通量,采用中心差分格式计算扩散通量。关于守恒变量Q的非线性方程组,采用牛顿迭代方法求解,线化后的方程用对称的Gauss-Seidle方法求其渐近解。应用该算法模拟了转子NASARotor67流场,与试验做了大量的对比研究,包括性能、等马赫线以及总压和总温等等。在最高效率点附近计算与试验比较接近,但在靠近失速点时计算与试验略有差别。      

The gravity-darkening of the main-sequence components of spectral types A,F and G in detached close binary systems

From analysis of the photometric ellipticity effect in seven well-understood detached close binary systems, empirical values of the exponent α of gravity-darkening have been practically deduced for eleven main-sequence components of spectral types A, F and G which should cover the range of structural change (from radiative to convective) in stellar atmospheres. The result indicate that values of the exponent diminish gradually with decreasing effective temperatures from α ～ 1.0 for radiative atmospheres with T > 8500 K to α = 0.2 ～ 0.3 for convective atmospheres with T < 6500 K, in spite of some uncertainty in the reflection correction process.     

Who is who in Algol-land ? - Part II -

In part I (De Greve and Packet) we have investigated the occurrence of reversed phases of mass-transfer during Case A evolution in close binaries. If the initial period of a system is shorter than 1–2 days (Early Case A) the reversed phase starts before core hydrogen exhaustion of the gainer (part I). This type of evolution is characterized by at least two phases of slow mass-transfer. We have computed the evolution of four Early Case A systems with initial masses of the loser equal to 3 Mo and 5 Mo. These four systems start mass-exchange when Xc of the primary has decreased to 0.525 (75% of its initial value). They all experience two phases of slow mass-transfer. We find that both phases have about the same duration for all systems. The mass ratios are clearly distinct, being closer to unity during the first phase. In the Hertzsprung-Russell, mass-radius and mass-luminosity diagrams both components remain close to the main-sequence band during slow mass-transfer. Evolution as an Algol is ended when both components overflow their outer critical surface after a second reversal of the mass-transfer. Observed Algol systems evolving in Early Case A are scarce. A search thruogh the catalogue by Giuricin et al. gives us the following candidates: X Tri, SX Aur and V Pup. Based on their mass ratios, SX Aur can tentatively be assigned to the first phase of slow mass transfer and X Tri to the second phase. For V Pup (which is more massive) this choice can not be made with certainty.     

A study of the SMC cluster NGC 330

We analyze the properties of the SMC cluster NGC 330 on the basis of a new CMD and of published effective temperatures and bolometric magnitudes for a smaller sample of stars (Caloi et al 1993, Stothers & Chin 1992a,b) to check for the kind of mixing taking place in massive stars. The existence of a few stars in the so-called HR gap led Stothers & Chin (1993) to claim that only models with semiconvective mixing and OPAL could match the HR diagram, whereas induced Caloi et al (1993) to conclude that neither semiconvective nor overshoot models are suited. The careful analysis of this cluster with new stellar models indicates that 1) models with convective overshoot and models with semiconvection can both explain the gross features of the HR diagram, 2) models with overshoot ought to be preferred.     

Who is who in Algol-land ? - Part II -

In part I (De Greve and Packet) we have investigated the occurrence of reversed phases of mass-transfer during Case A evolution in close binaries. If the initial period of a system is shorter than 1–2 days (Early Case A) the reversed phase starts before core hydrogen exhaustion of the gainer (part I). This type of evolution is characterized by at least two phases of slow mass-transfer.We have computed the evolution of four Early Case A systems with initial masses of the loser equal to 3 Mo and 5 Mo. These four systems start mass-exchange when Xc of the primary has decreased to 0.525 (75% of its initial value). They all experience two phases of slow mass-transfer.We find that both phases have about the same duration for all systems. The mass ratios are clearly distinct, being closer to unity during the first phase. In the Hertzsprung-Russell, mass-radius and mass-luminosity diagrams both components remain close to the main-sequence band during slow mass-transfer. Evolution as an Algol is ended when both components overflow their outer critical surface after a second reversal of the mass-transfer.Observed Algol systems evolving in Early Case A are scarce. A search thruogh the catalogue by Giuricin et al. gives us the following candidates: X Tri, SX Aur and V Pup. Based on their mass ratios, SX Aur can tentatively be assigned to the first phase of slow mass transfer and X Tri to the second phase. For V Pup (which is more massive) this choice can not be made with certainty.     

基于高阶WENO格式的旋翼非定常涡流场数值模拟

为提高直升机旋翼黏性涡流场计算流体力学（CFD）的计算精度及降低数值耗散,发展了一套基于五阶加权本质无振荡（WENO）格式的旋翼非定常涡流场数值计算方法。采用运动嵌套网格方法生成围绕前飞状态旋翼的网格系统,以Navier-Stokes方程作为主控方程,湍流模型采用一方程Spalart-Allmaras模型。为了提高旋翼流场中对涡的形成、演化等发展过程的模拟精度,采用高间断分辨率的Roe-WENO格式计算对流通量。对状态变量的插值,选取适当的加权因子,通过多个重构模板的凸组合来构造五阶WENO格式,在交界面附近获得具有五阶精度的状态变量。为了提高计算效率,采用高效的隐式LU-SGS（Lower-Upper Symmetric Gauss-Seidel）推进方法和并行计算策略。最后,运用该方法分别对悬停状态下C-T（Caradonna-Tung）旋翼和UH-60A旋翼以及前飞状态下C-T旋翼和SA349/2旋翼的涡流场及气动特性进行了数值模拟,并将数值结果与传统二阶精度格式的计算值进行了对比。结果表明：在计算时间增长10%～20%的前提下,五阶WENO格式能够捕捉到更精确的涡流场特性,反映了五阶WENO格式在计算旋翼非定常涡流场时具有更高的计算精度与更低的数值耗散特性。     

用多重网格法模拟三维流动

采用多重网格法模拟三维流动。控制方程在三维直角或圆柱坐标系中通过有限体积方法进行离散 ,SIMPLE算法对速度与压力进行耦合 ,亚松弛策略提高离散方程的收敛速度。在压力方程的求解过程中 ,界面速度应用由Rhie、Chow和Majumdar等人提出的动量方程插值法来剔除不合理的压力波动。三维流动算例表明采用多重网格法模拟三维流动可以得到良好的计算精度和收敛性。     

Two-dimensional viscous vortex flow around a circular cylinder

A code based on a vortex method for simulating viscous vortex flow around a circular cylinder has been developed. The original method to obtain the no-through and no-slip condition introduced here is described in detail. The diffusive part of the vorticity transport equation is treated using a deterministic method based on the solution of the heat equation. Results have been obtained and are presented concerning the vorticity field, the velocity field, the evolution of drag and lift coefficients and the Strouhal number.     

高精度ENO格式在射流数值模拟中的应用

本文分析了ENO格式的特点，并应用于全NS方程的迁移项和压力顶，模拟了轴对称和三维射流。首先对轴对称自由射流为算例比较了高阶TVD格式和ENO格式，证实了ENO格式捕获复杂激波波系的能力；其次计算了四喷管湍射流干扰流场，给出了相应的计算结果；最后计算了四股射流流入圆管内的流场，给出了物理量沿壁面和截面的分布。     

旋翼桨尖涡生成及演化机理的高精度数值研究

为了细致捕捉直升机旋翼桨尖涡的生成和演化过程,建立了一个基于高精度网格和高阶通量计算格式的旋翼桨尖涡计算流体力学(CFD)求解方法。在该方法中,流场求解选取旋转坐标系下的Navier-Stokes方程为控制方程;空间离散采用迎风Roe格式,并采用低耗散的5阶WENO(Weighted Essentially Non-Osciltatory)格式进行对流通量的计算;时间推进则采用双时间法,在伪时间步上使用隐式LU-SGS(Lower-Upper Symmetric Gauss-Seidel)推进格式;应用嵌套网格方法实现桨叶网格和背景网格的数据交换。应用所建立的方法对悬停状态的旋翼桨尖涡流场进行了高精度模拟,在桨叶网格上精细地捕捉到了桨尖涡的具体生成过程,在背景网格上捕捉到了更多圈数的桨尖涡尾迹,并对桨尖涡的演化机理进行了深入研究。结果表明:建立的高精度数值方法能够有效地对旋翼桨尖涡的生成和演化过程进行细致模拟;悬停状态下旋翼桨尖气流在上下表面压力梯度的作用下经历了边界层增厚、逐渐卷起形成涡核以及最终脱离桨叶形成桨尖涡的过程。     

Recent Evidence for Convection in Sunspot Penumbrae

Whereas penumbral models during the last 15 years have been successful in explaining Evershed flows and magnetic field inclination variations in terms of flux tubes, the lack of contact between these models and a convective process needed to explain the penumbral radiative heat flux has been disturbing. We report on recent observational and theoretical evidence that challenge flux tube interpretations and conclude that the origin of penumbral filamentary structure is overturning convection.     

Advanced lithium ion battery charger

A lithium ion battery charger has been developed for four and eight cell batteries or multiples thereof. This charger has the advantage over those using commercial lithium ion charging chips in that the individual cells are allowed to be taper charged at their upper charging voltage rather than be cutoff when all cells of the string have reached the upper charging voltage limit. Since 30-60% of the capacity of lithium ion cells may be restored during the taper charge, this charger has a distinct benefit of fully charging lithium ion batteries by restoring all of the available capacity to all of its cells     

A spectroscopic analysis of B stars in the SMC cluster NGC 330

Medium resolution (2A/px) but high s/n spectra of approximately twenty of the brightest blue stars in the young open cluster NGC 330 in the SMC have been analyzed in order to determine their atmospheric parameters and the evolutionary status. Stellar parameters are determined by comparison with LTE and NLTE model atmosphere calculations and an HR diagram constructed. Luminosities of the sample stars lie in the range 4.0L _*/L )<5.0 and spectral types between O9 and late-B. The stars in our sample appear to define 4 groups: main-sequence B-stars (B2-B4), B-supergiants (B4) in a blue-loop phase of evolution, a small number of blue stragglers (O9-B0 near main-sequence stars) and a group of luminous giants (B1-B2) which reside in the so-called post main-sequence gap of the HR diagram. Furthermore, we have confirmed spectroscopically the very high incidence of Be stars in this cluster. Finally the almost complete absence of metal lines (at this resolution) is in keeping with the expected very low metallicity of the SMC.     

三维超声速混合层内标量混合的大涡模拟

对三维对流马赫数0.62的超声速混合层流动的标量混合进行大涡模拟, 控制方程对流项采用五阶精度的WENO格式求解, 小尺度涡的作用采用一方程LDKM亚格子模型处理, 过滤后的组分方程中的亚格子组分对流通量采用梯度扩散模化.模拟得到了混合层流场大尺度拟序结构以及标量场的演化过程, 研究表明标量混合过程受混合层内涡系演化所控制, 标量场具有明显的三维特征.模拟得到的速度、组分及其脉动的统计时均结果和实验结果相符较好.      

应用TVD格式分离求解不可压N-S流动

提出一种新的将连续方程与动量方程分离求解的TVD引用方法。对动量方程中的对流项采用三阶精度、迎风TVD格式离散, 粘性项和压力项采用中心差分格式离散。压力采用压力替代法求解。计算所用的网格是具有交错网格主要特点的修正非交错网格, 并采用一种压力修正法和G-S中心对称迭代法加速收敛。给出了二维管流和二维空穴流动在不同状态下的计算结果, 并与其它解法的计算结果或精确解进行比较。结果表明, TVD格式不仅能用于不可压流数值计算, 而且还能得到比非TVD差分法精度更高的结果。      

Origin of the Solar Wind: Theory

A theory is presented for the origin of the solar wind, which is based on the behavior of the magnetic field of the Sun. The magnetic field of the Sun can be considered as having two distinct components: Open magnetic flux in which the field lines remain attached to the Sun and are dragged outward into the heliosphere with the solar wind. Closed magnetic flux in which the field remains entirely attached to the Sun, and forms loops and active regions in the solar corona. It is argued that the total open flux should tend to be constant in time, since it can be destroyed only if open flux of opposite polarity reconnect, a process that may be unlikely since the open flux is ordered into large-scale regions of uniform polarity. The behavior of open flux is thus governed by its motion on the solar surface. The motion may be due primarily to a diffusive process that results from open field lines reconnecting with randomly oriented closed loops, and also due to the usual convective motions on the solar surface such as differential rotation. The diffusion process needs to be described by a diffusion equation appropriate for transport by an external medium, which is different from the usual diffusion coefficient used in energetic particle transport. The loops required for the diffusion have been identified in recent observations of the Sun, and have properties, both in size and composition, consistent with their use in the model. The diffusive process, in which reconnection occurs between open field lines and loops, is responsible for the input of mass and energy into the solar wind. This revised version was published online in August 2006 with corrections to the Cover Date.     

超声速空气介质自由扩散混合流涡的演化

在不同谐波扰动促发方式下,观察二维超声速空气/空气自由扩散混合流中涡的形成及演化.考虑含空气组分扩散效应的可压缩Navier-Stokes方程,对流项采用3阶迎风紧致格式离散,输运项采用6阶对称紧致格式离散,非定常时间推进采用3阶紧致存储显式Runge-Kutta方法.在无相差情形下,获得了大尺度基频涡结构的饱和、一次对并、二次对并、三涡对并等现象.在Ma_c=0.3低对流Mach数下,基频涡较饱满,但流向尺度较小.受空气真实气体特性影响,在Ma_c=0.8高对流Mach数下未发现涡/小激波结构.      

On the evolution of secondary components in massive close binary systems

For the evolution of the secondary component of a massive close binary system, it is generally assumed that the mass accretion during core H-burning simply leads to its rejuvenation, i.e. that it evolves like a normal main sequence star with a mass corresponding to its mass after the accretion ceased. We reinvestigate this problem in the framework of a time-dependent semiconvection theory. We find that the process of adaptation of the convective core size to the new (larger) stellar mass may not be completed until core hydrogen depletion, i.e. no rejuvenation occurs. The resulting secondaries show strong differences compared to single stars of same mass.