Rotation of Nonspherical Particles in Turbulent Channel Flow.

The effects of particle inertia, particle shape, and fluid shear on particle rotation are examined using direct numerical simulation of turbulent channel flow. Particles at the channel center (nearly isotropic turbulence) and near the wall (highly sheared flow) show different rotation patterns and surprisingly different effects of particle inertia. Oblate particles at the center tend to rotate orthogonally to their symmetry axes, whereas prolate particles rotate around their symmetry axes. This trend is weakened by increasing inertia so that highly inertial oblate spheroids rotate nearly isotropically about their principle axes at the channel center. Near the walls, inertia does not move the rotation of spheroids towards isotropy but, rather, reverses the trend, causing oblate spheroids to rotate strongly about their symmetry axes and prolate spheroids to rotate normal to their symmetry axes. The observed phenomena are mostly ascribed to preferential orientations of the spheroids.

Into turbulent air: size-dependent effects of von Kármán vortex streets on hummingbird flight kinematics and energetics.

Animal fliers frequently move through a variety of perturbed flows during their daily aerial routines. However, the extent to which these perturbations influence flight control and energetic expenditure is essentially unknown. Here, we evaluate the kinematic and metabolic consequences of flight within variably sized vortex shedding flows using five Anna's hummingbirds feeding from an artificial flower in steady control flow and within vortex wakes produced behind vertical cylinders. Tests were conducted at three horizontal airspeeds (3, 6 and 9 m s(-1)) and using three different wake-generating cylinders (with diameters equal to 38, 77 and 173% of birds' wing length). Only minimal effects on wing and body kinematics were demonstrated for flight behind the smallest cylinder, whereas flight behind the medium-sized cylinder resulted in significant increases in the variances of wingbeat frequency, and variances of body orientation, especially at higher airspeeds. Metabolic rate was, however, unchanged relative to that of unperturbed flight. Hummingbirds flying within the vortex street behind the largest cylinder exhibited highest increases in variances of wingbeat frequency, and of body roll, pitch and yaw amplitudes at all measured airspeeds. Impressively, metabolic rate under this last condition increased by up to 25% compared with control flights. Cylinder wakes sufficiently large to interact with both wings can thus strongly affect stability in flight, eliciting compensatory kinematic changes with a consequent increase in flight metabolic costs. Our findings suggest that vortical flows frequently encountered by aerial taxa in diverse environments may impose substantial energetic costs.

Networks, dynamics, and modularity.

The identification of general principles relating structure to dynamics has been a major goal in the study of complex networks. We propose that the special case of linear network dynamics provides a natural framework within which a number of interesting yet tractable problems can be defined. We report the emergence of modularity and hierarchical organization in evolved networks supporting asymptotically stable linear dynamics. Numerical experiments demonstrate that linear stability benefits from the presence of a hierarchy of modules and that this architecture improves the robustness of network stability to random perturbations in network structure. This work illustrates an approach to network science which is simultaneously structural and dynamical in nature.

Improving the density of jammed disordered packings using ellipsoids.

Packing problems, such as how densely objects can fill a volume, are among the most ancient and persistent problems in mathematics and science. For equal spheres, it has only recently been proved that the face-centered cubic lattice has the highest possible packing fraction phi=pi/18 approximately 0.74. It is also well known that certain random (amorphous) jammed packings have phi approximately 0.64. Here, we show experimentally and with a new simulation algorithm that ellipsoids can randomly pack more densely-up to phi= 0.68 to 0.71 for spheroids with an aspect ratio close to that of M&M's Candies-and even approach phi approximately 0.74 for ellipsoids with other aspect ratios. We suggest that the higher density is directly related to the higher number of degrees of freedom per particle and thus the larger number of particle contacts required to mechanically stabilize the packing. We measured the number of contacts per particle Z approximately 10 for our spheroids, as compared to Z approximately 6 for spheres. Our results have implications for a broad range of scientific disciplines, including the properties of granular media and ceramics, glass formation, and discrete geometry.