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991.
Sílvia C. Hirata 《Physics letters. A》2010,374(26):2661-4613
By using the mathematical formalism of absolute and convective instabilities we study the nature of unstable three-dimensional disturbances of viscoelastic flow convection in a porous medium with horizontal through-flow and vertical temperature gradient. Temporal stability analysis reveals that among three-dimensional (3D) modes the pure down-stream transverse rolls are favored for the onset of convection. In addition, by considering a spatiotemporal stability approach we found that all unstable 3D modes are convectively unstable except the transverse rolls which may experience a transition to absolute instability. The combined influence of through-flow and elastic parameters on the absolute instability threshold, wave number and frequency is then determined, and results are compared to those of a Newtonian fluid. 相似文献
992.
993.
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995.
亚纳秒光脉冲抽运光子晶体光纤产生的瓦级超连续谱 总被引:1,自引:1,他引:0
研究了亚纳秒脉冲抽运光子晶体光纤产生高功率超连续谱的机理.采用掺镱锁模光纤激光器产生的脉宽570ps光脉冲,抽运1.8m光子晶体光纤,得到了平均功率为1.15W、光谱覆盖范围为750nm的超连续谱.通过实验和模拟结果的对比和分析,证实了亚纳秒脉冲抽运1.8m PCF产生超连续谱时,调制不稳定性效应起了重要作用.在研究了不同抽运功率下输出的超连续谱变化后,发现随着抽运功率的提高,输出功率也更高且超连续谱覆盖波段也更宽,在瓦级输出功率下依然未达到饱和展宽状态,还有进一步提高功率和展宽光谱的空间. 相似文献
996.
Modulation instability (MI) and evolution of pattern formation inside a passive non-linear photorefractive ring resonator having cavity-length longer than the coherence length of the light circulating in the resonating cavity has been theoretically analyzed. Dynamics of the system below the threshold conditions have also been analyzed, numerically using linear perturbation theory for the Kerr-type non-linearity. The process of modulation instability in a temporally incoherent cavity below the cavity threshold and the influence of various parameters such as the cavity-length, intensity feedback of the perturbed wave field, amplitude of the incoming beam and spatial frequency on the gain factor and the spatial spectral density have also been studied. For the perturbed wave field, it has been found that the growth rate of MI increases with increase in the intensity feedback as well as the spatial frequency. It has also been found that increasing the intensity feedback of the perturbed wave field, for a given value of the spatial frequency, the spatial spectral density of the perturbation at the output of the non-linear medium increases with the shifting peak positions to the higher spatial frequency side. 相似文献
997.
An interfacial approximation of the streamer stage in the evolution of sparks and lightning can be formulated as a Laplacian growth model regularized by a ‘kinetic undercooling’ boundary condition. Using this model we study both the linearized and the full nonlinear evolution of small perturbations of a uniformly translating circle. Within the linear approximation analytical and numerical results show that perturbations are advected to the back of the circle, where they decay. An initially analytic interface stays analytic for all finite times, but singularities from outside the physical region approach the interface for t→∞, which results in some anomalous relaxation at the back of the circle. For the nonlinear evolution numerical results indicate that the circle is the asymptotic attractor for small perturbations, but larger perturbations may lead to branching. We also present results for more general initial shapes, which demonstrate that regularization by kinetic undercooling cannot guarantee smooth interfaces globally in time. 相似文献
998.
将单层Kidder自相似解推广到双层,使得两层壳体的交界面两侧存在密度跳跃,使得轻流体向重流体加速产生Rayleigh-Taylor不稳定性;通过采用Lagrange坐标下的Godunov方法进行一维直接数值模拟,将模拟解与双层Kidder自相似基本解进行比较,验证了双层Kidder自相似解的可靠性;最后,通过编制球形内爆的三维扰动的线性稳定性分析程序,对双层Kidder自相似解的Rayleigh-Taylor不稳定性进行了分析计算.计算结果表明:初始扰动越集中于交界面,会造成后期扰动增长得越快,越不稳定;扰动波数越大,扰动增长得越快,越不稳定;从扰动在空间上的发展来看,可压缩性研究表明内外壳体的可压缩性对扰动增长起着相反的作用,外层壳体的可压缩性对Rayleigh-Taylor不稳定起失稳作用,而内层壳体的可压缩性对Rayleigh-Taylor不稳定起致稳作用. 相似文献
999.
Andrea Piccolroaz John R. Willis 《Journal of the mechanics and physics of solids》2006,54(11):2391-2417
Flutter instability in an infinite medium is a form of material instability corresponding to the occurrence of complex conjugate squares of the acceleration wave velocities. Although its occurrence is known to be possible in elastoplastic materials with nonassociative flow law and to correspond to some dynamically growing disturbance, its mechanical meaning has to date still eluded a precise interpretation. This is provided here by constructing the infinite-body, time-harmonic Green's function for the loading branch of an elastoplastic material in flutter conditions. Used as a perturbation, it reveals that flutter corresponds to a spatially blowing-up disturbance, exhibiting well-defined directional properties, determined by the wave directions for which the eigenvalues become complex conjugate. Flutter is shown to be connected to the formation of localized deformations, a dynamical phenomenon sharing geometrical similarities with the well-known mechanism of shear banding occurring under quasi-static loading. Flutter may occur much earlier than shear banding in a process of continued plastic deformation. 相似文献
1000.