When two food sources are presented to the slime mold Physarum in the dark, a thick tube for absorbing nutrients is formed that connects the food sources through the shortest route. When the lightavoiding organism is partially illuminated, however, the tube connecting the food sources follows a different route. Defining risk as the experimentally measurable rate of light-avoiding movement, the minimum-risk path is exhibited by the organism, determined by integrating along the path. A model for an adaptive-tube network is presented that is in good agreement with the experimental observations. Introduction.-The plasmodium of Physarum polycephalum is an amoebalike organism with a body made up of a tubular network through which nutrients, signals, and body mass are transported. Studies of this organism have shown that it is able to determine the shortest path through a maze as well as ''solve'' other geometric puzzles [1][2][3]. In a maze, a starved organism forms a tube that connects food sources (FS) placed at the two exits of the maze via the shortest path, while nearly the entire protoplasm of the amoeba gathers over the two FS. The organism meets its physiological requirements in adopting this shape by absorbing nutrients from the FS as rapidly as possible while maintaining sufficient connectivity to permit intracellular communication. Such behavior in a primitive organism of this kind may offer insights into the evolutionary origins of biological information processing.Here we give the plasmodium a new type of task involving optimization behavior. Two separate FS are presented to the organism, which is illuminated by an inhomogeneous light field. Because the plasmodium is photophobic, tubes connecting the FS do not follow the simple shortest paths but form according to the illumination inhomogeneity. We report on the behavior of the organism under these conditions and discuss its physiological significance. We also propose a mathematical model for the cell dynamics and present a computational algorithm for its problem solving.Organism and methods.-The plasmodium of Physarum polycephalum, which regenerated from the sclerotia in ca. one-half day in the dark (25 C), was used in the experiments. A plastic film was placed onto a 1% agar gel, leaving a rectangular area (1 2 cm 2 ) of the gel uncovered. A few pieces (0:5 1 cm 2 ) of the regenerated plasmodium were placed in the rectangular area, and the preparation was placed in the dark for a few hours. The

We study the surface shape of water in an open cylinder driven by constant rotation of the bottom. Around the critical Reynolds number for the laminar-turbulent transition, the surface deformation, which is of the order of the container size, shows an aperiodic switching phenomenon between an axisymmetric shape and a nonaxisymmetric shape. The axisymmetric shape is observed as a steady state when the Reynolds number is smaller than that in the switching region, while the nonaxisymmetric shape is observed as a ͑quasi-͒ periodic state in which the surface rotates at almost constant angular velocity when the Reynolds number is larger than that in the switching region. A detailed analysis for the surface shape suggests that the flow with the nonaxisymmetric shape is turbulent.

Three-dimensional flow structure and aerodynamic loading on a revolving wing Phys. Fluids 25, 034101 (2013) On the interaction of shock waves and sound waves in transonic buffet flow Phys. Fluids 25, 026101 (2013) Thermochemical nonequilibrium modeling of a low-power argon arcjet wind tunnel J. Appl. Phys. 113, 053304 (2013) Biomimetic flow control based on morphological features of living creatures Phys. Fluids 24, 121302 (2012) Study of mechanisms and factors that influence the formation of vortical wake of a heaving airfoil Forward flights of a bilaterally symmetrically flapping butterfly modeled as a fourlink rigid-body system consisting of a thorax, an abdomen, and left and right wings are numerically simulated. The joint motions of the butterflies are adopted from experimental observations. Three kinds of the simulations, distinguished by ways to determine the position and attitude of the thorax, are carried out: a tethered simulation, a prescribed simulation, and free-flight simulations. The upward and streamwise forces as well as the wake structures in the tethered simulation, where the thorax of the butterfly is fixed, reasonably agree with those in the corresponding tethered experiment. In the prescribed simulation, where the thoracic trajectories as well as the joint angles are given by those observed in a free-flight experiment, it is confirmed that the butterfly can produce enough forces to achieve the flapping flights. Moreover, coherent vortical structures in the wake and those on the wings are identified. The generation of the aerodynamic forces due to the vortical structures are also clarified. In the free-flight simulation, where only the joint angles are given as periodic functions of time, it is found that the free flight is longitudinally unstable because the butterfly cannot maintain the attitude in a proper range. Focusing on the abdominal mass, which largely varies owing to feeding and metabolizing, we have shown that the abdominal motion plays an important role in periodic flights. The necessity of control of the thoracic attitude for periodic flights and maneuverability is also discussed. C 2013 American Institute of Physics. [http://dx.

We study 'surface switching' quantitatively in flows driven by the constant rotation of the endwall of an open cylindrical vessel reported by Suzuki, Iima & Hayase (Phys. Fluids, vol. 18, 2006, p. 101701): the deformed free surface switches between axisymmetric and non-axisymmetric shapes accompanied by irregular vertical oscillation. Detailed simultaneous measurements showed that the magnitude of the velocity fluctuations (turbulent intensity) temporally varies greatly and are strongly correlated with the surface height, suggesting that dynamic switching between laminar and turbulent states is accompanied by vessel-scale surface shape changes. The study also identified clear hysteresis in the turbulent intensity arising from changes in the Reynolds number; the bifurcation diagram consists of two overlapping branches representing a high-intensity (turbulent) state and a low-intensity (laminar) state. Based on the results, a switching mechanism is suggested.

We study spontaneous pattern formation and its asymptotic behaviour in binary fluid flow driven by a temperature gradient. When the conductive state is unstable and the size of the domain is large enough, finitely many spatially localized time-periodic travelling pulses (PTPs), each containing a certain number of convection cells, are generated spontaneously in the conductive state and are finally arranged at non-uniform intervals while moving in the same direction. We found that the role of PTP solutions and their strong interactions (collision) are important in characterizing the asymptotic state. Detailed investigations of pulse–pulse interactions showed the differences in asymptotic behaviour between that in a finite but large domain and in an infinite domain.