Tipstreaming of a drop in simple shear flow in the presence of surfactant

We have developed a multi-phase SPH method to simulate arbitrary interfaces containing surface active agents (surfactants) that locally change the properties of the interface, such the surface tension coefficient. Our method incorporates the effects of surface diffusion, transport of surfactant from/to the bulk phase to/from the interface and diffusion in the bulk phase. Neglecting transport mechanisms, we use this method to study the impact of insoluble surfactants on drop deformation and breakup in simple shear flow and present the results in a fluid dynamics video.Comment: Two videos are included for the Gallery of Fluid Motion of the APS DFD Meeting 201

Optimum reentry trajectories of a lifting vehicle

Research results are presented of an investigation of the optimum maneuvers of advanced shuttle type spacecraft during reentry. The equations are formulated by means of modified Chapman variables resulting in a general set of equations for flight analysis which are exact for reentry and for flight in a vacuum. Four planar flight typical optimum manuevers are investigated. For three-dimensional flight the optimum trajectory for maximum cross range is discussed in detail. Techniques for calculating reentry footprints are presented

Anomalous scaling of conductivity in integrable fermion systems

We analyze the high-temperature conductivity in one-dimensional integrable models of interacting fermions: the t-V model (anisotropic Heisenberg spin chain) and the Hubbard model, at half-filling in the regime corresponding to insulating ground state. A microcanonical Lanczos method study for finite size systems reveals anomalously large finite-size effects at low frequencies while a frequency-moment analysis indicates a finite d.c. conductivity. This phenomenon also appears in a prototype integrable quantum system of impenetrable particles, representing a strong-coupling limit of both models. In the thermodynamic limit, the two results could converge to a finite d.c. conductivity rather than an ideal conductor or insulator scenario.Comment: 6 pages, 3 figures. Submitted to PR

Adaptive pumping for spectral control of random lasers

A laser is not necessarily a sophisticated device: Pumping energy into an amplifying medium randomly filled with scatterers, a powder for instance, makes a perfect "random laser." In such a laser, the absence of mirrors greatly simplifies laser design, but control over emission directionality or frequency tunability is lost, seriously hindering prospects for this otherwise simple laser. Lately, we proposed a novel approach to harness random lasers, inspired by spatial shaping methods recently employed for coherent light control in complex media. Here, we experimentally implement this method in an optofluidic random laser where scattering is weak and modes extend spatially and strongly overlap, making individual selection a priori impossible. We show that control over laser emission can indeed be regained even in this extreme case by actively shaping the spatial profile of the optical pump. This unique degree of freedom, which has never been exploited, allows selection of any desired wavelength and shaping of lasing modes, without prior knowledge of their spatial distribution. Mode selection is achieved with spectral selectivity down to 0.06nm and more than 10dB side-lobe rejection. This experimental method paves the way towards fully tunable and controlled random lasers and can be transferred to other class of lasers.Comment: 23 pages, 7 figure

Fighting coal — Effectiveness of coal-replacement programs for residential heating in China: Empirical findings from a household survey

Household fuel substitution has been a crucial step for controlling air pollution in China, but the performance evaluation of household fuel substitution policies is overlooked. This study capitalized on the opportunity to use data collected during the household coal-replacement program in North China to evaluate the effect of a mandatory policy on fuel substitution at the micro-level. The results indicate that there is a significant effect of the coal-replacement program on fuel substitution, as we expected. The coal-to-electricity policy is effective in achieving the goal of a clean winter but not a warm winter due to the decline of delivered energy, while the high-quality coal replacement policy results in better performance in delivered energy but no improvement in indoor air quality. It is recommended to prioritize supporting measures on both the supply and demand sides before implementation, along with undertaking differential measures during the implementation phase to better address energy inequality