静脉血流动力学模型基本波的相互作用北大核心CSCD

静脉系统是心血管系统的重要组成部分.脉搏波在血液流动中有着突出的重要性.本文主要研究静脉血流动力学模型基本波的相互作用.血流动力学模型是2×2严格双曲型方程组,其基本波包括疏散波和激波,属于血液流动中的脉搏波.基本波相互作用后血管截面面积和血流速度发生相应的变化.     

Shock waves and turbulence in a hypersonic inlet

A numerical investigation for an axisymmetric hypersonic turbulent inlet flow field of a perfect gas is presented for a three-shock configuration consisting of a biconic and a cowl. An upwind parabolized Navier-Stokes solver based on Roe's scheme is used to compute an oncoming flow Mach numberM =8, temperatureT =216 K, and pressureP =5.5293×10³ N/m². In order to assess the flow quantities, the interaction between shock and turbulence, and the inlet efficiency, three different flow calculations — laminar, turbulent with incompressible and compressible two-equationk- turbulence models — have been performed in this work.Computational results show that turbulence is markedly enhanced across an oblique shock with step-like increases in turbulence kinetic energy and dissipation rate. This enhancement is at the expense of the mean kinetic energy of the flow. Therefore, the velocity behind the shock is smaller in turbulent flow and hence the shock becomes stronger. The entropy increase through a shock is caused not only by the amplification of random molecular motion, but also by the enhancement of the chaotic turbulent flow motion. However, only the compressiblek- turbulence model can properly predict a decrease in turbulence length scale across a shock. Our numerical simulation reveals that the incompressiblek- turbulence model exaggerates the interaction between shock and turbulence with turbulence kinetic energy and dissipation rate remaining high and almost undissipated far beyond the shock region. It is shown that proper modeling of turbulence is essential for a realistic prediction of hypersonic inlet flowfield. The performed study shows that the viscous effect is not restricted in the boundary layer but extends into the main flow behind a shock wave. The loss of the available energy in the inlet performance therefore needs to be determined from the shock-turbulence interaction. The present study predicts that the inlet efficiency becomes relatively lower when turbulence is taken into account.     

Geometric indices and the Alexander polynomial of a knot

It is well-known that any Laurent polynomial satisfying and is the Alexander polynomial of a knot in . We show that can be realized by a knot which has the following properties simultaneously: (i) tunnel number 1; (ii) bridge index 3; and (iii) unknotting number 1.

     

The mixed valent tellurate SrTe3O8: Electronic lone pair effect of Te

A novel mixed valent tellurium oxide, SrTe₃O₈, has been synthesized and its crystal structure was determined ab initio from powder X-ray diffraction data. This oxide, which crystallizes in a tetragonal unit-cell, P4₂/m space group, with very close a and c cell parameters (6.8257(1) and 6.7603(1) Å, respectively), exhibits a very original structure built up of corner-sharing TeO₆ (Te⁶⁺) octahedra and Te₂O₈ (Te⁴⁺) twin-pyramidal units. The latter ones form [Te₃O₈]_∞ chains running along the [001] and the [110] directions. Besides the four sided tunnels where the Sr²⁺ cations are located, there are very large four sided tunnels running along the c-axis which are obstructed by the electronic lone pairs of the Te⁴⁺ cations.     

Shock-wave equation-of-state studies at Los Alamos

A history of the shock-wave equation-of-state (EOS) studies at Los Alamos is given. Particular emphasis is placed on the pioneering research in the 1950s where many of the experimental techniques and methods of analysis were developed, which we now take for granted. A brief review of shock-wave physics is given, which illustrates important hydrodynamic and thermodynamic concepts. Recent studies on the EOS of Ti are presented with emphasis on the-to- phase transition. VISAR wave profiles for polycrystalline Ni and singlecrystal Ni are presented to determine the strengths of these materials under pressure. Low-density polystyrene foam Hugoniot experiments are described and results analyzed.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1990.     

Transport phenomena in a plane shock wave

With the use of the nonpolynomial closure 1/ _z in the Mott-Smith approximation of the solution of the Boltzmann equation, we obtain a value of the density gradient in the limit of a very weak shock wave that is close to the correct value. For the determination of the transverse temperature gradient we calculated the _x ² / _z moment of the Mott-Smith collision integral. The effective values of viscosity and thermal conductivity in the limit of a very weak shock wave were calculated for inverse-power potentials and found to agree almost exactly with the Chapman-Enskog values. Such a comparison can serve as a criterion for the evaluation of different bimodal theories. Various bimodal theories give different values of viscosity and thermal conductivity, but all of them give 33 % too high a value of the Eucken ratio.     

Shock cooling of liquid nitrogen and superfluid helium

Abstract

Superfluid Helium (11) and liquid nitrogen are the only liquids which have been shown experimentally, to exhibit shock cooling. In the present paper we use the Landau model to demonstrate theoretically that the roton gap, which decreases with increasing density, plays the same role leading to shock cooling in liquid helium as the dissociation energy does in liquid nitrogen.     

Shock wave research for engineering physics

Abstract

In this paper, several recent activities on shock wave research for engineering physics performed in China will be presented as the supplement of author's previous report of Ref. 1.     

Spin-polarized current and tunnel magnetoresistance in heterogeneous single-barrier magnetic tunnel junctions

Current in heterogeneous tunnel junctions is studied in the framework of the parabolic conduction-band model. The developed model of the electron tunneling takes explicitly into account the difference of effective masses between ferromagnetic and insulating layers and between conduction subbands. Calculations for Fe/MgO/Fe-like structures have shown the essential impact of effective mass differences in regions (constituents) of the structure on the tunnel magnetoresistance of the junction.     

Ignition delay times of methane and hydrogen highly diluted in carbon dioxide at high pressures up to 300 atm

The need for more efficient power cycles has attracted interest in super-critical CO₂ (sCO₂) cycles. However, the effects of high CO₂ dilution on auto-ignition at extremely high pressures has not been studied in depth. As part of the effort to understand oxy-fuel combustion with massive CO₂ dilution, we have measured shock tube ignition delay times (IDT) for methane/O₂/CO₂ mixtures and hydrogen/O₂/CO₂ mixtures using sidewall pressure and OH* emission near 306?nm. Ignition delay time was measured in two different facilities behind reflected shock waves over a range of temperatures, 1045–1578?K, in different pressures and mixture regimes, i.e., CH₄/O₂/CO₂ mixtures at 27–286 atm and H₂/O₂/CO₂ mixtures at 37–311 atm. The measured data were compared with the predictions of two recent kinetics models. Fair agreement was found between model and experiment over most of the operating conditions studied. For those conditions where kinetic models fail, the current ignition delay time measurements provide useful target data for development and validation of the mechanisms.