Pure tetrahedral quadruple systems with index two

An oriented tetrahedron defined on four vertices is a set of four cyclic triples with the property that any ordered pair of vertices is contained in exactly one of the cyclic triples. A tetrahedral quadruple system of order $n$ with index $\lambda$, denoted by ${\text{TQS}}_{\lambda }\left(n\right)$, is a pair $\left(X,\mathrm{?}\right)$, where $X$ is an $n$‐set and $\mathrm{?}$ is a set of oriented tetrahedra (blocks) such that every cyclic triple on $X$ is contained in exactly $\lambda$ members of $\mathrm{?}$. A ${\text{TQS}}_{\lambda }\left(n\right)$ is pure if there do not exist two blocks with the same vertex set. When $\lambda =1$, the spectrum of a pure TQS $\left(n\right)$ has been completely determined by Ji. In this paper, we show that there exists a pure ${\text{TQS}}_2\left(n\right)$ if and only if $n\equiv 1,2\left(\mathrm{mod}3\right)$ and $n\ge 7$. A corollary is that a simple ${\text{QS}}_4\left(n\right)$ also exists if and only if $n\equiv 1,2\left(\mathrm{mod}3\right)$ and $n\ge 7$.     

轴流风扇叶片端导叶作用的研究

本文采用数值方法研究了叶片端导叶对轴流风扇性能的影响。通过与普通开式轴流风扇比较,分析了叶片端导叶对内部流动作用的机理.数值计算结果表明:叶片端导叶的安装位置将影响轴流风扇气动效率,安装叶片端导叶不能提高风扇静压升,但是在压力面安装时能有效地减小风扇叶顶泄漏流与主流的掺混损失;在设计流量下,压力面安装叶片端导叶使泄漏涡的作用范围较小,涡核更靠近吸力面;吸力面安装叶片端导叶弱化了泄漏涡的强度但没有减小泄漏涡的作用范围。     

基于网络的风机防喘振控制系统设计

针对某钢厂转炉煤气加压站大风机电机采用工频加变频的恒压闭环运行方式,变频风机极易发生喘振的现象,为了确保大风机稳定、安全的运行,在风机喘振的预防和控制方法上进行改进。采用两台风机同步变频调速与最小极值流量法的模糊PID控制相结合的防喘振控制方法,同步变频调速控制策略有效地预防喘振的发生,最小极值流量法的模糊PID控制策略最大限度的扩大了风机的运行区间,通过MATLAB软件仿真,证明模糊PID的动态响应速度和鲁棒性较常规PID有较大的改良。控制系统通过工业网络完成现场设备和远程的通讯,使控制系统达到高水平的智能自动化。     

数值模拟不等距叶片对贯流风机的影响

贯流风机的通过频率(BPF)是其重要的噪声频率.降低BPF噪声可以降低基频处的声压级,其中一种方法是采用不等距叶片.本文采用3种叶片距分布形式,采用realizable k-ε两方程和大涡模拟(LES)湍流模型模拟了风机的内流场,计算线性欧拉方程(LEE)中声源项得到声源位置及强度,采用基于Lighthill声类比的FW-H积分方程获得了叶轮和蜗舌处偶极子型的离散噪声频谱.比较了不同叶片距对风机性能,噪声特别是BPF噪声的影响.计算结果表明在对性能影响较小的情况下,不等距叶轮可降低BPF噪声和总A声级噪声.     

开式轴流风扇气动噪声预测

本文采用LES/FW-H的匹配方法,研究了开式轴流风扇内部旋涡流动特征及其与叶片表面干涉引起的气动噪声之间的联系,同时进行了远场噪声预测,探讨了叶轮不同表面辐射噪声时的频谱分布特征.研究结果表明,开式轴流风扇吸力面附近形成的叶尖涡和前缘分离涡在吸力面叶片表面相应位置形成大压力波动,形成主要噪声源;叶片吸力面的辐射噪声可以通过改善吸力面附近的旋涡流动来降低;低速轴流叶轮由叶轮壁面辐射的噪声以宽频成分为主.     

降低轴流风机噪声的两种方法

本文对轴流风机的噪声源进行了理论和实验分析,对降低风机噪声的方法进行了总结。在此基础上,研究了降低风机噪声的两种新方法——叶片不等距分布方法和叶片穿孔方法。理论分析和实验结果表明,这两种方法为降低轴流风机噪声提供了新的途径。     

一个跨音风扇级转/静干扰流动的时间精确模拟

本文采用双时间步法,较为方便地将孤立叶排定常流场求解拓广为转／静干扰非定常流场时间精确求解,而且该流场时间精确求解方法收敛速度较作者最初工作有较大提高．文章通过对一跨音风扇级非定常流场时间精确模拟,对叶轮机非定常流动建立了一定理解．     

Base-cover paracompactness

Call a topological space base-cover paracompact if has an open base such that every cover of contains a locally finite subcover. A subspace of the Sorgenfrey line is base-cover paracompact if and only if it is . The countable sequential fan is not base-cover paracompact. A paracompact space is locally compact if and only if its product with every compact space is base-cover paracompact.

     

Convex polytopes all of whose reverse lexicographic initial ideals are squarefree

A compressed polytope is an integral convex polytope any of whose reverse lexicographic initial ideals is squarefree. A sufficient condition for a -polytope to be compressed will be presented. One of its immediate consequences is that the class of compressed -polytopes includes (i) hypersimplices, (ii) order polytopes of finite partially ordered sets, and (iii) stable polytopes of perfect graphs.

     

On Unimodular Rows over Polynomial Rings

Let A be a commutative ring and n 3 a positive integer. In this paper, we consider unimodular rows (f₁(x),..., f_n(x)) over A[x]. We prove that, if the row of the leading coefficients of f_i(x) is unimodular over A and a A, then there exists E_n(A[x]) such that (f₁(x),...,f_n(x)) = (f₁(a),...,f_n(a)). Also, if A is a Noetherian ring with finite Krull dimension and the row of leading coefficients satisfies the same condition, then we give a bound for the length of in terms of elementary transvections.Partially supported by INTAS 93–436EXT and DFG–RFBR grant No. 96–01–00092G.2000 Mathematics Subject Classification: 19A13, 19B14, 13C10, 13B25, 13F20