Simple Obstacles Research Articles

Wave amplitude gain within wedge waveguides through scattering by simple obstacles.

Wave confinement, e.g., in waveguides, gives rise to a huge number of distinct phenomena. Among them, amplitude gain is a recurrent and relevant effect in undulatory processes. Using a general purpose protocol to solve wave equations, the boundary wall method, we demonstrate that for relatively simple geometries, namely, a few leaky or opaque obstacles inside a θ wedge waveguide (described by the Helmholtz equation), one can obtain a considerable wave amplification in certain spatially localized regions of the system. The approach relies on an expression for the wedge waveguide exact Green's function in the case of θ=π/M (M=1,2,...), derived through the method of images allied to group theory concepts. The formula is particularly amenable to numerical calculations, greatly facilitating simulations. As an interesting by-product of the present framework, we are able to obtain the eigenstates of certain closed shapes (billiards) placed within the waveguide, as demonstrated for triangular structures. Finally, we briefly discuss possible concrete realizations for our setups in the context of matter and electromagnetic (for some particular modes and conditions) waves.

Traveling Salesperson Problem with Simple Obstacles: The Role of Multidimensional Scaling and the Role of Clustering

The objective of the traveling salesperson problem (TSP) is to find a shortest tour through all nodes in a graph. Euclidean TSPs are traditionally represented with “cities” placed on a 2D plane. When straight line obstacles are added to the plane, a tour has to visit all cities while going around obstacles. The resulting problem with obstacles remains metric, but is not Euclidean because the shortest paths are no longer straight lines. We first revise a previous version of multiresolution graph pyramid by modifying the hierarchical clustering stage. Next, we describe two new experiments with human subjects. In the first experiment, the effect of the length of obstacles on the quality of tours produced by subjects was tested with three problem sizes. Long obstacles affect the tours to a greater degree than short obstacles, but long obstacles create obvious clusters and limit the ways in which the tours can be produced. In the second experiment we evaluated the degree to which Multidimensional Scaling (MDS) can compensate for the presence of obstacles. The results show that although MDS approximation can compensate to a large degree for the presence of obstacles, it cannot fully account for human performance. This fact suggests that mental representation of a TSP with obstacles is not Euclidean. Instead, it is likely to be based on hierarchical clustering in which pairwise distances represent the shortest paths around obstacles.

Runout and deflection of granular flow past an array of obstacles on a slope

The interaction between granular flow and an array of obstacles is an important research subject in geological disaster prevention. We used a numerical model for the depth-integrated equations formulated in a bed-fitted coordinate system to simulate the runout and deflection of granular flow past an array of simple obstacles. Numerical simulations of granular flow encountering arrays of square columns and tetrahedral pyramids gave a lot of interesting and fascinating deposits. We then proposed a new dimensionless index called deflection efficiency (DE), together with the runout efficiency (RE) to quantify the comprehensive effects of the obstacle system on the runout and deflection behavior. There are three physical effects, a dissipation effect through bow shock, a deflection effect by splitting the granular flow, and a retention effect via trapping some granular mass, the combinations of which determine the mobility and deposit of granular flow. We found that RE decreases as either the height or number of rows of obstacles increases because of the dissipation effect, while DE increases as height increases due to the deflection effect for both tetrahedral and square column obstacles. However, if the retention effect was as important as the deflection effect, the DE could be reduced when the row numbers increase further. These findings provide some constructive help for the design of obstacle arrays and complement the understanding of subsequent flow after interaction between granular flow and obstacle arrays.

Small Scale Experiments of PM10 Dispersion Around Obstacles

Particulate matter (PM) is the main determinant of air pollution caused by a variety of natural and human-caused sources. Because it can be suspended in the atmosphere for long periods of time and travel long distances, it can cause a major health crisis for humans and damage the environment as well. Studies are still required to understand how the PM moves around obstacles, especially in urban areas. In this study, small scale experiments were carried out to look into the effects of simple obstacles, heights and distance from the source on the PM10 concentration. Results show that when obstacle heights and distance from the source increase, the PM10 average concentration decrease. Also, turbulence created by the obstacles affects the PM10 concentration in both sensors before and after the obstacle, mainly in cases of high wind speed. In addition, the use of incense sticks as a source of PM pollution illustrated that moderate burning of incense sticks in indoor places could skyrocket the PM10 concentration to an unhealthy level.

Experimental tests of inhomogeneous hydrogen deflagrations in the presence of obstacles

Explosion venting is a frequently used measure to mitigate the outcome of deflagration in closed environments. Despite the effort to improve engineering formulas and CFD tools used to predict the vent area needed to meet the desired enclosure protection, work has still to be done to reliably predict the outcome of a vented gas explosion. Blind-prediction exercises recently published by the HYSEA project show a large spread in the predictions by engineering models, including semi-empirical correlations and computational fluid dynamics (CFD) tools. University of Pisa performed experimental tests in a 25 m3 facility in inhomogeneous conditions and with the presence of simple obstacles constituted by plates bolted to HEB beams. The present paper is aimed to share the results of hydrogen dispersion and deflagration tests and discuss the comparison of maximum peak overpressure generated with different blockage ratio and repeated obstacles rows. Description of the experimental setup includes all the details deemed necessary to reproduce the phenomenon with a CFD tool.

Improved approximation bounds for the minimum constraint removal problem

In the minimum constraint removal problem, we are given a set of overlapping geometric objects as obstacles in the plane, and we want to find the minimum number of obstacles that must be removed to reach a target point t from the source point s by an obstacle-free path. The problem is known to be intractable and no sub-linear approximations are known even for simple obstacles such as rectangles and disks. The main result of our paper is an approximation framework that gives an O(nα(n))-approximation for polygonal obstacles, where α(n) denotes the inverse Ackermann's function. For pseudodisks and rectilinear polygons, the same technique achieves an O(n)-approximation. The technique also gives O(n)-approximation for the minimum color path problem in graphs. We also present some inapproximability results for the geometric constraint removal problem.

DESIGN AND CONTROL OF SPIDER ROBOT MODEL

Robots imitating spider’s moving have many advantages such as flexible movement, high stability, diversity in movements performed, especially in terrain crossing, in military reconnaissance, in surveying and collecting environmental data in dangerous areas,.... In this article with the main objective is to exploit multiple control methods to support applications of a spider robot with low-cost, a spider robot with 6 legs and 18 joints was designed. The ESPWROOM-32 module (ESP32-D0WDQ6 chip) and MIT App Inventor were used as the main tools for conducting this research. As a result, the robot is controlled via Bluetooth and Wifi to move, making some actions by self-written software running on the Android operating system. In addition, the robot has the capacity of self-propelled to avoid simple obstacles and send some environmental parameters to the software, including obstacles distance, humidity and temperature.

An Efficient 3-D FDTD-PE Hybrid Model for Radio Wave Propagation With Near-Source Obstacles

A novel 3-D hybrid model is presented for the prediction of propagation of radio waves in the presence of complex obstacles located on regular perfectly conducting terrain. The finite-difference time-domain (FDTD) method is implemented to model the source and obstacle regions, and the wave is then propagated by using the parabolic equation (PE) method-based algorithm. The source region contains all the complex obstacles and takes into consideration all the possible wave interactions in that region. The transition region between the FDTD domain and the PE domain is carefully treated in order to avoid numerical incompatibilities and to realize the time-/frequency-domain transition. The method is applied for different scenarios, starting with relatively simple obstacles like knife-edges for the verification and validation of the approach and then including more complex obstacles to the propagation medium. The results are compared to those obtained from full FDTD implementations of the same scenarios.

A Novel Feature Optimization for Wearable Human-Computer Interfaces Using Surface Electromyography Sensors.

The novel human-computer interface (HCI) using bioelectrical signals as input is a valuable tool to improve the lives of people with disabilities. In this paper, surface electromyography (sEMG) signals induced by four classes of wrist movements were acquired from four sites on the lower arm with our designed system. Forty-two features were extracted from the time, frequency and time-frequency domains. Optimal channels were determined from single-channel classification performance rank. The optimal-feature selection was according to a modified entropy criteria (EC) and Fisher discrimination (FD) criteria. The feature selection results were evaluated by four different classifiers, and compared with other conventional feature subsets. In online tests, the wearable system acquired real-time sEMG signals. The selected features and trained classifier model were used to control a telecar through four different paradigms in a designed environment with simple obstacles. Performance was evaluated based on travel time (TT) and recognition rate (RR). The results of hardware evaluation verified the feasibility of our acquisition systems, and ensured signal quality. Single-channel analysis results indicated that the channel located on the extensor carpi ulnaris (ECU) performed best with mean classification accuracy of 97.45% for all movement’s pairs. Channels placed on ECU and the extensor carpi radialis (ECR) were selected according to the accuracy rank. Experimental results showed that the proposed FD method was better than other feature selection methods and single-type features. The combination of FD and random forest (RF) performed best in offline analysis, with 96.77% multi-class RR. Online results illustrated that the state-machine paradigm with a 125 ms window had the highest maneuverability and was closest to real-life control. Subjects could accomplish online sessions by three sEMG-based paradigms, with average times of 46.02, 49.06 and 48.08 s, respectively. These experiments validate the feasibility of proposed real-time wearable HCI system and algorithms, providing a potential assistive device interface for persons with disabilities.

Rational adaptive blends among obstacles in 3D by contour method

In this paper we will continue in investigating ‘contour method’ and its using for the computation of rational parameterizations of canal surfaces without a need of sum of squares (SOS) decomposition. Further approaches for constructing flexible smooth transitions between canal surfaces will be presented. Mainly, we focus on one particular application of recently introduced rational envelope curves, newly constructed over an arbitrary planar rational curve in space. Using this type of curves significantly simplifies the previous methods discussed in Bizzarri (2015), and mainly new situations, which could not have been handled with the previous setup, are successfully solved, now. Especially a method for constructing rational adaptive blends which bypass a given obstacle (or more given obstacles when needed) is thoroughly discussed and its functionality is demonstrated on a number of examples. The designed approach works not only for simple obstacles represented by one-dimensional medial axis transforms but also for more general obstacles described by two-dimensional medial surface transforms.

Collective strategy for obstacle navigation during cooperative transport by ants.

Group cohesion and consensus have primarily been studied in the context of discrete decisions, but some group tasks require making serial decisions that build on one another. We examine such collective problem solving by studying obstacle navigation during cooperative transport in ants. In cooperative transport, ants work together to move a large object back to their nest. We blocked cooperative transport groups of Paratrechina longicornis with obstacles of varying complexity, analyzing groups' trajectories to infer what kind of strategy the ants employed. Simple strategies require little information, but more challenging, robust strategies succeed with a wider range of obstacles. We found that transport groups use a stochastic strategy that leads to efficient navigation around simple obstacles, and still succeeds at difficult obstacles. While groups navigating obstacles preferentially move directly toward the nest, they change their behavior over time; the longer the ants are obstructed, the more likely they are to move away from the nest. This increases the chance of finding a path around the obstacle. Groups rapidly changed directions and rarely stalled during navigation, indicating that these ants maintain consensus even when the nest direction is blocked. Although some decisions were aided by the arrival of new ants, at many key points, direction changes were initiated within the group, with no apparent external cause. This ant species is highly effective at navigating complex environments, and implements a flexible strategy that works for both simple and more complex obstacles.

Dynamics of glide avalanches and snow gliding

AbstractIn recent years, due to warmer snow cover, there has been a significant increase in the number of cases of damage caused by gliding snowpacks and glide avalanches. On most occasions, these have been full‐depth, wet‐snow avalanches, and this led some people to express their surprise: how could low‐speed masses of wet snow exert sufficiently high levels of pressure to severely damage engineered structures designed to carry heavy loads? This paper reviews the current state of knowledge about the formation of glide avalanches and the forces exerted on simple structures by a gliding mass of snow. One particular difficulty in reviewing the existing literature on gliding snow and on force calculations is that much of the theoretical and phenomenological analyses were presented in technical reports that date back to the earliest developments of avalanche science in the 1930s. Returning to these primary sources and attempting to put them into a contemporary perspective are vital. A detailed, modern analysis of them shows that the order of magnitude of the forces exerted by gliding snow can indeed be estimated correctly. The precise physical mechanisms remain elusive, however. We comment on the existing approaches in light of the most recent findings about related topics, including the physics of granular and plastic flows, and from field surveys of snow and avalanches (as well as glaciers and debris flows). Methods of calculating the forces exerted by glide avalanches are compared quantitatively on the basis of two case studies. This paper shows that if snow depth and density are known, then certain approaches can indeed predict the forces exerted on simple obstacles in the event of glide avalanches or gliding snow cover.

Journeys Together: Horses and Humans in Partnership

Many living with companion animals hope for “good relationships” based on trust, mutuality, and cooperation. Relationships develop from mutual actions, yet research often overlooks nonhumans as mindful actors within relationships. This is a study of horse/human dyads, using multimethod approaches intended to include horses as participants. We ask: can “good relationships” be observed, especially when the pair know each other well? We studied familiar/unfamiliar pairs, negotiating simple obstacles, to explore qualities of cooperation between pairs. Interviews with human participants elicited perceptions of horses’ “personalities” and reactions. We analyzed video recordings of interactions and also showed them to external observers. We identified differences in attention, tension and coordination: familiar pairs were more coordinated, mutually attentive, and less tense, and they showed less resistance. That is, some relationships displayed discernible qualities of “working together.” We cannot know nonhuman animals’ experiences, but knowing how they behave says something about their agency within interspecies relationships.

Cost of Transport as a Discriminator of Conditioning in Horses Submitted to Incremental Jumping Tests

IntroductionHeart rate (HR) and blood lactate (LAC) are biomarkers whose responses to exercise are related to physical fitness. This study evaluated the cost of transport (CT) by assessing the cardiovascular and metabolic efficiency of jumping horses using an incremental jumping test.MethodsSixteen Brazilian Sport horses of two fitness levels were used: high‐level jumpers (HL, n = 8) and low‐level jumpers (LL, n = 8). Tests were performed in a sandy jumping arena at a mean velocity of 18 km/h; the tests were distributed into three steps of 4 laps of 74 m, each with 10 obstacles. Each stage consisted of simple obstacles that were 0.40, 0.60 and 0.80 m high. The costs of transport were determined as (HRstep‐HRrest/speed) and (LACstep‐LACrest/speed). During intervals between each stage, blood samples were drawn to determine LAC (mmol/L). Student's t‐test was used for the unpaired samples (P<0.05).ResultsThere was no difference in the stage 0.40 m for HR. There were differences in the HL and LL groups, respectively, in the HR related to the effort in the obstacle series 0.60 m [1.7 ± 0.2 bpm/(km/h) and 2.0 ± 0.2 bpm/(km/h) (P = 0.02)]; 0.80 m [1.5 ± 0.2 bpm/(km/h) and 1.7 ± 0.2 bpm/(km/h); (P = 0.04)], and in the LAC for the 0.40 m obstacle [0.05 ± 0.04 mmol/L/(km/h) and 0.10 ± 0.03 mmol/L/(km/h); P = 0.04]; 0.60 m [0.07 ± 0.06 mmol/L/(km/h) and 0.14 ± 0.03 mmol/L/(km/h); (P = 0.006)] and 0.80 m [0.11 ± 0.04 mmol/L/(km/h) and 0.19 ± 0.05 mmol/L/(km/h); P = 0.009].ConclusionsThe CT differentiated horses of high and low level jumping fitness. The HR method (rather than LAC) is particularly useful because it is simple, inexpensive and accessible to trainers and veterinarians.Ethical Animal ResearchAll procedures were approved by the institutional animal care and use committee of the university (CEUA, Process: 019281/13). Sources of funding: FAPESP ‐ Fundação de Amparo à Pesquisa do Estado de São Paulo (Process 2011/11080‐0). Competing interests: none.

The Nonnormative Celebrity Body and the Meritocracy of the Star System: Constructing Peter Dinklage in Entertainment Journalism

Examining the coverage of Peter Dinklage in entertainment journalism, this article interrogates the possibilities and limitations of a celebrity with a nonnormative body to actually challenge Hollywood’s stereotyping of little people and the cultural tendency to use little people as a source of humor or voyeuristic pleasure. Although the rise of Dinklage to celebrity status created an opportunity for entertainment journalism to question Hollywood casting politics and draw attention to the stigma faced by little people, the discourses surrounding Dinklage have largely created a meritocratic narrative in which the struggles faced by little people in Hollywood are acknowledged but depoliticized. Such a narrative affirms the inherent fairness of the celebrity system, casting the structural and cultural barriers that create inequality as simple obstacles that the talented and hard working will overcome.

Feasibility of shock attenuation by simple obstacles in mitigating severe accident explosive loads

Fukushima severe accident showed that explosions are no longer exclusive in nuclear power plants and creative safety features would be beneficial to preserve the integrity of vital components against such explosions. Based on this motivation, simple obstacles are suggested as a means to attenuate shock waves that may arise from severe accident explosions and the effectiveness is investigated by computations employing fully compressible viscous Navier–Stokes equations of the FLUENT code. Two- and three-dimensional computations for a shock tube filled with air are performed and the computed peak pressure and arrival time are validated against existing experimental data and analyses. Effects of structural shapes and rectangular rigid obstacles on shock attenuation in simple geometries are preliminarily analyzed and the results show that the end wall peak pressure is reduced by about 18–30%. As a demonstration in a plant scale, a high pressure source of 0.2m radius semicircle is positioned at the bottom center of the two-dimensional hypothetical reactor cavity model (5.4m by 9.4m) and multiple rigid obstacles are positioned at the half-way between a reactor vessel and the pressure source. Incident circular shock wave is dispersed by the obstacles and the peak pressures at the reactor vessel bottom head center and the 45° regions are reduced by about 35% and 50%, respectively. This study implies the feasibility of simple obstacles for attenuating shock waves from the explosions expected under nuclear power plant severe accidents.

Euclidean TSP on two polygons

We give an O ( n 2 m + n m 2 + m 2 log m ) time and O ( n 2 + m 2 ) space algorithm for finding the shortest traveling salesman tour through the vertices of two simple polygonal obstacles in the Euclidean plane, where n and m are the number of vertices of the two polygons. By obstacle, we mean that the tour may not cross between the interior and exterior of either polygon. We also consider the problem’s extension to higher dimensions, proving that, if P ≠ NP , constructing a shortest TSP tour on the vertices of two non-intersecting polytopes is NP-hard if the polytopes are similarly viewed as obstacles.

Lamb mode conversion at edges. A hybrid boundary element–finite-element solution

Two general and flexible numerical techniques based on the finite-element and boundary element methods developed by the authors in a previous paper are applied to study Lamb wave propagation in multilayered plates and Lamb mode conversion at free edges for frequencies beyond the first cutoff frequency. Both techniques are supported by a meshing criterion which guarantees the accuracy of the results when a condition is fulfilled. A finite-element formulation is directly applicable to study Lamb wave propagation and reflection by simple obstacles such as a flat edge. In order to tackle Lamb wave diffraction problems by defects with more complex geometries, a hybrid boundary element-finite-element formulation is used. This technique provides a major improvement with respect to the only previous boundary element application on Lamb waves: the connecting boundary might be placed as close to the reflector as desired, reducing greatly the requirement on mesh size. Two main application problems on practical metallic plates are studied and compared with reported numerical, theoretical, and experimental results: (1) Lamb wave propagation in degraded titanium diffusion bonds, and (2) Lamb mode conversion at inclined or perpendicular free edges of steel plates for frequencies beyond the first cutoff frequency.

Explosive Volume Formation Form The Release Of Vapors Or Liquids In Enclosures

Evaluation of the explosion hazard from accidental releases in enclosures involves estimating the flammable volume produced by mixing of the released material with air. Two specific situations are considered in the paper. In the first case dealing with gaseous jet releases, recent results are discussed on the effects of simple obstacles on the flammable volume. These results illustrate how the flammable volume can increase or decrease, depending on the values of dimensionless parameters that define the release and the obstacle. The second case considered is that of the flammable layer produced by the evaporation of a liquid pool. This particular release scenario has been studied using a computational approach based on a one-dimensional model. The paper discusses the rationale behind this approximation and the adjustments that have been found to be necessary to include multi-dimensional effects into the approximate treatment. The common thread among the correlations or calculation methods discussed in the paper is the desire to identify tools that can be of use in engineering applications.

Distributed Algorithms for Multi-Robot Observation of Multiple Moving Targets

An important issue that arises in the automation of many security, surveillance, and reconnaissance tasks is that of observing the movements of targets navigating in a bounded area of interest. A key research issue in these problems is that of sensor placement—determining where sensors should be located to maintain the targets in view. In complex applications involving limited-range sensors, the use of multiple sensors dynamically moving over time is required. In this paper, we investigate the use of a cooperative team of autonomous sensor-based robots for the observation of multiple moving targets. In other research, analytical techniques have been developed for solving this problem in complex geometrical environments. However, these previous approaches are very computationally expensive—at least exponential in the number of robots—and cannot be implemented on robots operating in real-time. Thus, this paper reports on our studies of a simpler problem involving uncluttered environments—those with either no obstacles or with randomly distributed simple convex obstacles. We focus primarily on developing the on-line distributed control strategies that allow the robot team to attempt to minimize the total time in which targets escape observation by some robot team member in the area of interest. This paper first formalizes the problem (which we term CMOMMT for i>Cooperative Multi-Robot Observation of Multiple Moving Targets) and discusses related work. We then present a distributed heuristic approach (which we call A-CMOMMT) for solving the CMOMMT problem that uses weighted local force vector control. We analyze the effectiveness of the resulting weighted force vector approach by comparing it to three other approaches. We present the results of our experiments in both simulation and on physical robots that demonstrate the superiority of the A-CMOMMT approach for situations in which the ratio of targets to robots is greater than 1/2. Finally, we conclude by proposing that the CMOMMT problem makes an excellent domain for studying multi-robot learning in inherently cooperative tasks. This approach is the first of its kind for solving the on-line cooperative observation problem and implementing it on a physical robot team.