Мобильный ускоритель на базе безжелезного импульсного бетатрона для радиографирования динамических объектов

The paper concerns the mobile accelerator based on the ironless pulsed betaron. The accelerator is aimed to radiograph dynamic objects with a large optical thickness. It has a possibility to obtain up to three γ-pulses in one cycle of the acceleration. The accelerator operation description and results of its testing powering in a single-pulse mode are provided. The estimated boundary energy of an electron beam is equal to 60 MeV at the capacitance value of 1.8 mF of the storage of the betatron electromagnet pulsed power system. The thickness of the lead test object examined with γ-rays is 140 mm at 4 m from the tantalum target. The full width of the output γ-pulse at half maximum is equal to 120 ns. The dimension of the radiation source is 3×6 mm. The application of these accelerators within the radiographic complex will allow increasing the investigation efficiency due to the optimization of the hydrodynamic experiments geometry and the cost reduction.

Research on the influence of ground effect on the performance of robotic fish propelled by oscillating paired pectoral fins

Purpose This paper aims to carry out the research on the influence of ground effect on the performance of robotic fish propelled by oscillating paired pectoral fins. Design/methodology/approach The two-dimensional ground effect model of the oscillating pectoral fin without considering flexible deformation is established by introducing a two-dimensional fluid ground effect model. The parameters of the influence of ground effect on the oscillating pectoral fin are analyzed. Finally, the ground effect test platform is built, and a series of hydrodynamic experiments are carried out to study the influence of ground effect on the propulsion performance of the robotic fish propelled by oscillating paired pectoral fins under different motion parameters. Findings The thickness of the trailing edge and effective clearance are two important parameters that can change the influence of ground effect on the rigid pectoral fin. The experimental results are consistent with that obtained through theoretical analysis within a certain extent, which indicates that the developed two-dimensional ground effect model in this paper can be used to analyze the influence of ground effect on the propulsion performance of the oscillating pectoral fin. The experiment results show that the average thrust increases with the decreasing distance between the robot fish and the bottom. Meanwhile, with the increase of oscillation frequency and amplitude, the average thrust increases gradually. Originality/value The developed two-dimensional ground effect model provides the theoretical basis for the further research on the influence of ground effect on the propulsion performance of the oscillating pectoral fin. It can also be used in the design of the bionic pectoral fins.

A soft manipulator for efficient delicate grasping in shallow water: Modeling, control, and real-world experiments

Collecting in shallow water (water depth: ~30 m) is an emerging field that requires robotics for replacing human divers. Soft robots have several promising features (e.g., safe interaction with the environments, lightweight, etc.) for performing such tasks. In this article, we developed an underwater robotic system with a three-degree-of-freedom (3-DoF) soft manipulator for spatial delicate grasping in shallow water. First, we present the design and fabrication of the soft manipulator with an opposite-bending-and-stretching structure (OBSS). Then, we proposed a simple and efficient kinematics method for controlling the spatial location and trajectory of the soft manipulator’s end effector. The inverse kinematics of the OBSS manipulator can be solved efficiently (computation time: 8.2 ms). According to this inverse kinematics method, we demonstrated that the OBSS soft manipulator could track complex two-dimensional and three-dimensional trajectories, including star, helix, etc. Further, we performed real-time closed-loop pick-and-place experiments of the manipulator with binocular and on-hand cameras in a lab aquarium. Hydrodynamic experiments showed that the OBSS soft manipulator produced little force (less than 0.459 N) and torque (less than 0.228 N·m), which suggested its low-inertia feature during the underwater operation. Finally, we demonstrated that the underwater robotic system with the OBSS soft manipulator successfully collected seafood animals at the bottom of the natural oceanic environment. The robot successfully collected eight sea echini and one sea cucumber within 20 minutes at a water depth of around 10 m.

Scour-resilient bio-inspired geomorphic designs: The Male Japanese Puffer Fish Nest

<p>In 1995, divers noticed a strange circular pattern on the seabed off Japan. The geometric formations mysteriously appeared and dissolved, and no-one knew what made them. Finally, the creator of these remarkable formations was found: a new species of pufferfish from the genus Torquigener. The male puffer fish executes a design of mathematical perfection in the form of ornate circles. As he swims along the seabed, he laboriously flaps his fins and rearranges the sand, creating the geomorphic feature dubbed crop circle by the pioneers who first noticed them. The significance to understand the puffer fish design is magnified when we consider that the nest is able to maintain its morphological features for long periods even though it is built entirely of mobile particles in an area where the flow does not stop.</p><p>As a relatively new discovery, the exact reasons behind why the pufferfish spends such a long time constructing and cultivating the nest it still a question that is shrouded in a substantial amount of mystery. Male puffer fish spend many days caring for the eggs, the only puffer fish genus to be overserved doing so; suggesting that Torquigener place an unusually large emphasis on ensuring the survival of their eggs. It is hypothesised that the nest is created as a mating display, as female puffer fish will visit the site, presumably assessing various characteristics of the nest. It is not known exactly what parameters the females judge the nest on; whether it be size, symmetric properties or decorative choice. However, due to some basic hydrodynamic experiments performed by Hiroshi Kawase, there is some evidence to suggest that there may be more to building the nest than solely attracting a mate.</p><p>Several questions therefore arise regarding the nest. Is there an evolutionary reason that male puffer fish build these nests? Which characteristics of a nest make it attractive to female puffer fish? Are the eggs safer in a nest, perhaps from incoming currents? How exactly does fluid flow through nest, and can it be replicated and simulated? This project begins to tackle these questions through a numerical investigation (CFD) of the fluid flow through the nest in order to identify key fluid dynamic features, which may play a significant role in egg incubation and spawning using Star CCM+.</p>

Influence of turbulence and in-stream structures on the transport and survival of grass carp eggs and larvae at various developmental stages

Understanding the response of grass carp to flow and turbulence regimes during early life stages is fundamental to monitoring and controlling their spread. A comprehensive set of hydrodynamic experiments was conducted with live grass carp eggs and larvae, to better understand their drifting and swimming patterns with 3 different in-stream obstructions: (1) a gravel bump, (2) a single cylinder, and (3) submerged vegetation. The hydrodynamic behavior of eggs and larvae with each obstruction was continuously monitored for about 85 consecutive hours. Transient spatial distributions of the locations of eggs and larvae throughout the water column were generated for each flow scenario. Results show that the active swimming capabilities of larvae allow them to seek areas of low turbulence and low shear stresses, and that eggs are susceptible to damage by high levels of turbulence, which was further corroborated with tests in an oscillating grid-stirred turbulence tank. Our study seeks to better inform field collection of grass carp during early life stages, and to guide the design of alternative approaches to control the dispersal of this invasive species in North America.

Deciphering the Tsunami Wave Impact and Associated Connection Forces in Open-Girder Coastal Bridges

In view of the widespread damage to coastal bridges during recent tsunamis (2004 Indian Ocean and 2011 in Japan) large-scale hydrodynamic experiments of tsunami wave impact on a bridge with open girders were conducted in the Large Wave Flume at Oregon State University. The main objective was to decipher the tsunami overtopping process and associated demand on the bridge and its structural components. As described in this paper, a comprehensive analysis of the experimental data revealed that: (a) tsunami bores introduce significant slamming forces, both horizontal (Fh) and uplift (Fv), during impact on the offshore girder and overhang; these can govern the uplift demand in connections; (b) maxFh and maxFv do not always occur at the same time and contrary to recommended practice the simultaneous application of maxFh and maxFv at the center of gravity of the deck does not yield conservative estimates of the uplift demand in individual connections; (c) the offshore connections have to withstand the largest percentage of the total induced deck uplift among all connections; this can reach 91% and 124% of maxFv for bearings and columns respectively, a finding that could explain the damage sustained by these connections and one that has not been recognized to date; (e) the generation of a significant overturning moment (OTM) at the initial impact when the slamming forces are maximized, which is the main reason for the increased uplift in the offshore connections; and (f) neither maxFv nor maxOTM coincide always with the maximum demand in each connection, suggesting the need to consider multiple combinations of forces with corresponding moments or with corresponding locations of application in order to identify the governing scenario for each structural component. In addition the paper presents “tsunami demand diagrams”, which are 2D envelopes of (Fh, Fv) and (OTM, Fv) and 3D envelopes of (Fh, Fv, OTM), as visual representations of the complex variation of the tsunami loading. Furthermore, the paper reveals the existence of a complex bridge inundation mechanism that consists of three uplift phases and one downward phase, with each phase maximizing the demand in different structural components. It then develops a new physics-based methodology consisting of three load cases, which can be used by practicing engineers for the tsunami design of bridge connections, steel bearings and columns. The findings in this paper suggest the need for a paradigm shift in the assessment of tsunami risk to coastal bridges to include not just the estimation of total tsunami load on a bridge but also the distribution of this load to individual structural components that are necessary for the survival of the bridge.

Numerical simulation of the von Kármán sodium dynamo experiment

We present hydrodynamic and magnetohydrodynamic (MHD) simulations of liquid sodium flows in the von Kármán sodium (VKS) set-up. The counter-rotating impellers made of soft iron that were used in the successful 2006 experiment are represented by means of a pseudo-penalty method. Hydrodynamic simulations are performed at high kinetic Reynolds numbers using a large eddy simulation technique. The results compare well with the experimental data: the flow is laminar and steady or slightly fluctuating at small angular frequencies; small scales fill the bulk and a Kolmogorov-like spectrum is obtained at large angular frequencies. Near the tips of the blades the flow is expelled and takes the form of intense helical vortices. The equatorial shear layer acquires a wavy shape due to three coherent co-rotating radial vortices as observed in hydrodynamic experiments. MHD computations are performed: at fixed kinetic Reynolds number, increasing the magnetic permeability of the impellers reduces the critical magnetic Reynolds number for dynamo action; at fixed magnetic permeability, increasing the kinetic Reynolds number also decreases the dynamo threshold. Our results support the conjecture that the critical magnetic Reynolds number tends to a constant as the kinetic Reynolds number tends to infinity. The resulting dynamo is a mostly axisymmetric axial dipole with an azimuthal component concentrated near the impellers as observed in the VKS experiment. A speculative mechanism for dynamo action in the VKS experiment is proposed.