Superseismic Loading and Shock Polars: An Example of Fluid-Solid Coupling

We propose a two-dimensional problem involving fluid-solid coupling where a solution is given in closed form. The upper half of the domain is modeled as an isotropic solid; the lower part as a compressible gas. The loading of the solid at the fluid-solid boundary is called superseismic when its speed is larger than the speed of propagation of disturbances in the bulk of the material. The loading is modeled by a shock coupled to the deformation of the boundary. The problem is relevant to high explosive applications, since it is very similar to the interaction between an explosive and the casing in a cylinder test experiment. Within this framework, we show the existence of self-similar solutions in the reference frame moving with the shock wave

Rehabilitation interventions for improving balance following stroke: an overview of systematic reviews

Background The aim of this study was to synthesize evidence from systematic reviews, to summarise the effects of rehabilitation interventions for improving balance in stroke survivors. Methods We conducted an overview of systematic reviews (SRs). We included Cochrane Systematic Reviews and non-Cochrane Systematic Reviews of randomized-controlled clinical trials and not-randomized clinical trials, in all types of stroke, comparing the effects of interventions, control interventions and no interventions on balance-related outcomes. We conducted a comprehensive search of electronic databases, from inception to December 2017. Data extracted included: number and type of participants, type of intervention, control intervention, method of assessing risk of bias of primary studies, balance outcome measures and results of statistical meta-analyses. Methodological quality of included reviews was assessed using AMSTAR 2. A narrative description of the characteristics of the SRs was provided and results of meta-analyses summarised with reference to their methodological quality. Results 51 SRs (248 primary studies and 10,638 participants) met the inclusion criteria and were included in the overview. All participants were adults with stroke. A wide variety of different balance and postural control outcomes were included. 61% of SRs focussed on the effectiveness of physical therapy, 20% virtual reality, 6% electromechanical devices, 4% Tai-Chi, whole body vibration and circuit training intervention, and 2% cognitive rehabilitation. The methodology of 54% of SRs were judged to be of a \u201clow or critically low\u201d quality, 23% \u201cmoderate\u201d quality and 22% \u201chigh\u201d quality. Conclusions There are 51 SRs of evidence relating to the effectiveness of interventions to improve balance in people with stroke, but the majority of these are of poor methodological quality, limiting our ability to draw clear implications. Only 22% of these SRs were judged to be of high quality, highlighting the need to address important methodological issues within rehabilitation research

A Level Set Approach to Eulerian-Lagrangian Coupling

We present a numerical method for coupling an Eulerian compressible flow solver with a Lagrangian solver for fast transient problems involving fluid-solid interactions. Such coupling needs arise when either specific solution methods or accuracy considerations necessitate that different and disjoint subdomains be treated with different (Eulerian or Lagrangian)schemes. The algorithm we propose employs standard integration of the Eulerian solution over a Cartesian mesh. To treat the irregular boundary cells that are generated by an arbitrary boundary on a structured grid, the Eulerian computational domain is augmented by a thin layer of Cartesian ghost cells. Boundary conditions at these cells are established by enforcing conservation of mass and continuity of the stress tensor in the direction normal to the boundary. The description and the kinematic constraints of the Eulerian boundary rely on the unstructured Lagrangian mesh. The Lagrangian mesh evolves concurrently, driven by the traction boundary conditions imposed by the Eulerian counterpart. Several numerical tests designed to measure the rate of convergence and accuracy of the coupling algorithm are presented as well. General problems in one and two dimensions are considered, including a test consisting of an isotropic elastic solid and a compressible fluid in a fully coupled setting where the exact solution is available

A numerical study of detonation diffraction

An investigation of detonation diffraction through an abrupt area change has been carried out via a set of two-dimensional numerical simulations parameterized by the activation energy of the reactant. Our analysis is specialized to a reactive mixture with a perfect gas equation of state and a single-step reaction in the Arrhenius form. Lagrangian particles are injected into the flow as a diagnostic tool for identifying the dominant terms in the equation that describes the temperature rate of change of a fluid element, expressed in the shock-based reference system. When simplified, this equation provides insight into the competition between the energy release rate and the expansion rate behind the diffracting front. The mechanism of spontaneous generation of transverse waves along the diffracting front is carefully analysed and related to the sensitivity of the reaction rate to temperature. We study in detail three highly resolved cases of detonation diffraction that illustrate different types of behaviour, super-, sub- and near-critical diffraction

A soft unmanned underwater vehicle with augmented thrust capability

The components which could make Soft Unmanned Underwater Vehicles a winning technology for a range of marine operations are addressed: these include vortex-enhanced thrust, added mass recovery and high degree of compliance of the vehicle. Based on these design criteria and recent advancement in soft-bodied, pulsed-jet thrusters, a new underwater vehicle is developed and tested

PoseiDRONE: design of a soft-bodied ROV with crawling, swimming and manipulation ability

The design concept and development of a multi-purpose, underwater robot is presented. The final robot consists of a continuum composed for 80% of its volume of rubber-like materials and it combines locomotion (i.e. crawling and swimming) and manipulation capabilities. A first prototype of the robot is illustrated based on the integration of existing prototypes

A general method for the design and fabrication of shape memory alloy active spring actuators

Shape memory alloys have been widely proposed as actuators, in fields such as robotics, biomimetics and microsystems: in particular spring actuators are the most widely used, due to their simplicity of fabrication. The aim of this paper is to provide a general model and the techniques for fabricating SMA spring actuators. All the steps of the design process are described: a mechanical model to optimize the mechanical characteristic for a given requirement of force and available space, and a thermal model for the estimation of the electrical power needed for activation. The parameters of both models are obtained by experimental measurements, which are described in the paper. The models are then validated on springs manufactured manually, showing also the fabrication process. The design method is valid for the dimensioning of SMA springs, independently from the external ambient conditions. The influence on the actuator bandwidth was investigated for different working environments, providing numerical indications for the utilization in underwater applications. The spring characteristics can be calculated by the mechanical model with an accuracy of 5%. The thermal model allows one to calculate the current needed for activation under different ambient conditions, in order to guarantee activation in the specific loading conditions. Moreover, two solutions were found to reduce the power consumption by more than 40% without a dramatic reduction of bandwidth

Shock and detonation modeling with the Mie-Grüneisen equation of state

We consider the numerical simulation of inviscid reactive flows with application to high density explosive detonation. The numerical model is based on the Euler equations and the Mie-Grüneisen equation of state extended to treat chemical energy release and expanded states. The equations are computed with a Roe-Glaister solver on a Cartesian mesh. We present results for two substances, a binder and an explosive. Our solution method is verified against the exact solution of the shock tube problem for solid materials. We show under what conditions a "physical" expansion shock can appear in this example. We then address the problem of modeling expanded states, and show results for a two-dimensional shock distraction around a sharp corner. In the last part of the paper, we introduce a detonation model that extends the Mie-Grüneisen equation of state to enable high explosive simulations without the complexity of mixture equations of state. We conclude with two examples of corner-turning computations carried out with a pressure-dependent reaction rate law