Active tail flexion in concert with passive hydrodynamic forces improves swimming speed and efficiency

Fish typically swim by periodic bending of their bodies. Bending seems to follow a universal rule; it occurs at about one-third from the posterior end of the fish body with a maximum bending angle of about $30^{\circ }$ . However, the hydrodynamic mechanisms that shaped this convergent design and its potential benefit to fish in terms of swimming speed and efficiency are not well understood. It is also unclear to what extent this bending is active or follows passively from the interaction of a flexible posterior with the fluid environment. Here, we use a self-propelled two-link model, with fluid–structure interactions described in the context of the vortex sheet method, to analyse the effects of both active and passive body bending on the swimming performance. We find that passive bending is more efficient but could reduce swimming speed compared with rigid flapping, but the addition of active bending could enhance both speed and efficiency. Importantly, we find that the phase difference between the posterior and anterior sections of the body is an important kinematic factor that influences performance, and that active antiphase flexion, consistent with the passive flexion phase, can simultaneously enhance speed and efficiency in a region of the design space that overlaps with biological observations. Our results are consistent with the hypothesis that fish that actively bend their bodies in a fashion that exploits passive hydrodynamics can at once improve speed and efficiency.

Mapping and determining the center of mass of a rotating object using a moving observer

For certain applications, such as on-orbit inspection of orbital debris, defunct satellites, and natural objects, it is necessary to obtain a map of a rotating object from a moving observer, as well to estimate the object’s center of mass. This paper addresses these tasks using an observer that measures its own orientation, angular rate, and acceleration, and is equipped with a dense 3D visual sensor, such as a stereo camera or a light detection and ranging (LiDAR) sensor. The observer’s trajectory is estimated independently of the target object’s rotational motion. Pose-graph mapping is performed using visual odometry to estimate the observer’s trajectory in an arbitrary target-fixed frame. In addition to applying pose constraint factors between successive frames, loop closure is performed between temporally non-adjacent frames. A kinematic constraint on the target-fixed frame, resulting from the rigidity of the target object, is exploited to create a novel rotation kinematic factor. This factor connects a trajectory estimation factor graph with the mapping pose graph, and facilitates estimation of the target’s center of mass. Map creation is performed by transforming detected feature points into the target-fixed frame, centered at the estimated center of mass. Analysis of the algorithm’s computational performance reveals that its computational cost is negligible compared with that of the requisite image processing.

Evaluation and Improvement of Productivity in High-Speed NC Machining

The productivity of machining centers is influenced inherently by the quality of NC programs. To evaluate productivity, first an effective feedrate factor and a productivity evaluation factor are proposed. It has been found that in high-speed machining, these two factors depend on a kinematic factor which is a function of (1) command feedrate, (2) average per-block travel of the tool, (3) moving vectorial variation of the tool, and (4) ac/deceleration or time constants. Then an NC program simulator has been developed to evaluate productivity. With the simulator, the machining time can be calculated accurately and the cutting conditions can be extracted. Finally, three NC programs were implemented on high-speed machining centers and analyzed by the simulator. It was found that in mold and die machining, the productivity can be improved by increasing the acceleration and average travel and reducing the vectorial variation of the tool rather than the command feedrate. [S1087-1357(00)01303-4]

(e, 2 e)-Spectroscopy and the Electronic Structure of Molecules. A Short Review with Selected Examples

The simple plane wave target Hartree-Fock impulse approximation for the (e, 2e) reaction is developed. One result of the approximation is the separation of the expression for the (e, 2e) cross-section into a kinematic factor and a structure factor that contains all of the information about the target. When the target is a molecule, the structure factor can be further separated into atomic terms and a geometric term. This is illustrated for a simple one-electron homonuclear diatomic molecule. Three examples of the application of (e, 2e) spectroscopy to systems of chemical interest are given. They are borazine (inorganic benzene), the methyl siloxanes and the inorganic complex trimethylamine boron trifluoride.