Magnetic Fluctuations for Small Magnetic Prandtl Numbers

Numerous studies of magnetic fluctuations with a zero mean-field for small magnetic Prandtl numbers (Pr _m 1) show that magnetic fluctuations cannot be generated by turbulent fluid flow with the Kolmogorov energy spectrum. In addition, the generation of magnetic fluctuations with a zero mean-field for Pr _m 1 were not observed in numerical simulations. However, in astrophysical plasmas the magnetic Prandtl numbers are small and magnetic fluctuations are observed. Thus a mechanism of generation of magnetic fluctuations for Pr _m 1 still remains poorly understood. On the other hand, in astrophysical applications (e.g., solar and stellar convection zones, galaxies, accretion disks) the turbulent velocity field cannot be considered as a divergence-free. The generation of magnetic fluctuations by turbulent flow of conducting fluid with a zero mean magnetic field for Pr _m 1 is studied by means of linear and nonlinear analysis. The turbulent fluid velocity field is assumed to be homogeneous and isotropic with a power law energy spectrum ( k ^–p ) and with a very short scale-dependent correlation time. It is found that magnetic fluctuations can be generated when the exponent p > 3/2. It is shown also that the growth rates of the higher moments of the magnetic field are larger than those of the lower moments, i.e., the spatial distribution of the magnetic field is intermittent. In addition, the effect of compressibility (i.e., u 0) of the low-Mach-number turbulent fluid flow u is studied. It is demonstrated that the threshold for the generation of magnetic fluctuations by turbulent fluid flow with u 0 is higher than that for incompressible fluid. This implies that the compressibility impairs the generation of magnetic fluctuations. Nonlinear effects result in saturation of growth of the magnetic fluctuations. Asymptotic properties of the steady state solution for the second moment of the magnetic field in the case of the Hall nonlinearity for the low-Mach-number compressible flow are studied.     

锚泊辅助动力定位系统艏向控制模式研究

锚泊辅助动力定位系统是一种将锚泊定位与动力定位结合起来的新型定位方式。对该系统的艏向控制模式进行了研究。在该模式下,控制系统将只对平台的艏向运动施加约束。同时对平台在该模式下的定位进行了时域模拟,并与锚泊定位和水平面三自由度均施加约束的全定位模式进行了对比。结果表明,在一般海况下,相比锚泊定位,该定位模式能显著提高平台的艏摇控制精度;相比全定位模式,该定位模式消耗的功率更小、更经济。艏向控制模式是锚泊辅助动力定位系统的一种工作模式,在相应海况条件下采取该定位模式,能够实现精确和经济的定位。