426 research outputs found
An elliptical cover problem in drone delivery network design and its solution algorithms
Given n demand points in a geographic area, the elliptical cover problem is to determine the location of p depots (anywhere in the area) so as to minimize the maximum distance of an economical delivery trip in which a delivery vehicle starts from the nearest depot to a demand point, visits the demand point and then returns to the second nearest depot to that demand point. We show that this problem is NP-hard, and adapt Cooper’s alternating locate-allocate heuristic to find locally optimal solutions for both the point-coverage and area-coverage scenarios. Experiments show that most locally optimal solutions perform similarly well, suggesting their sufficiency for practical use. The one-dimensional variant of the problem, in which the service area is reduced to a line segment, permits recursive algorithms that are more efficient than mathematical optimization approaches in practical cases. The solution also provides the best-known lower bound for the original problem at a negligible computational cost
Super-directivity formation and numerical analysis of acoustic array
The formation of super-directivity of an acoustic array is firstly analyzed to construct a general mathematical model of the array with super-directivity and maximum signal-to-noise ratio (SNR). Then the numerical simulation on the super-directivity of the array is carried out for the arrays with different shapes, element number and apertures. It shows that, circular array with regular shape and Archimedean spiral array with irregular shape have optimum directivity
Rethinking Adversarial Policies: A Generalized Attack Formulation and Provable Defense in Multi-Agent RL
Most existing works consider direct perturbations of victim's state/action or
the underlying transition dynamics to show vulnerability of reinforcement
learning agents under adversarial attacks. However, such direct manipulation
may not always be feasible in practice. In this paper, we consider another
common and realistic attack setup: in a multi-agent RL setting with
well-trained agents, during deployment time, the victim agent is
exploited by an attacker who controls another agent to act
adversarially against the victim using an \textit{adversarial policy}. Prior
attack models under such setup do not consider that the attacker can confront
resistance and thus can only take partial control of the agent , as
well as introducing perceivable ``abnormal'' behaviors that are easily
detectable. A provable defense against these adversarial policies is also
lacking. To resolve these issues, we introduce a more general attack
formulation that models to what extent the adversary is able to control the
agent to produce the adversarial policy. Based on such a generalized attack
framework, the attacker can also regulate the state distribution shift caused
by the attack through an attack budget, and thus produce stealthy adversarial
policies that can exploit the victim agent. Furthermore, we provide the first
provably robust defenses with convergence guarantee to the most robust victim
policy via adversarial training with timescale separation, in sharp contrast to
adversarial training in supervised learning which may only provide {\it
empirical} defenses
Simulation and analytical modeling of high-speed droplet impact onto a surface
The fluid dynamics of liquid droplet impact on surfaces hold significant
relevance to various industrial applications. However, high impact velocities
introduce compressible effects, leading to material erosion. A gap in
understanding and modeling these effects has motivated this study. We simulated
droplet impacts on surfaces and proposed a new analytical model for impact
pressure and droplet turning line, targeting at predictions for enhanced
cavitation. The highly compressed liquid behind the droplet expands sideways,
causing lateral jetting. As the droplet encounters a shock wave, it reflects as
a rarefaction wave, leading to low-pressure zones within the droplet. These
zones converge at the droplet's center, causing cavitation, which, upon
collapse, induces another shock wave, contributing to erosion. Using the
well-established model for the low-velocity impact shows a significant
discrepancy. Hence, an analytical model for the turning line radius is
introduced, incorporating the lateral jetting's characteristic length scale.
Comparing our model with existing ones, our new model exhibits superior
predictive accuracy
The Performances Optimization of Finger Seal Based on Fuzzy Game Theory
AbstractLeakage and abrasion are two key performances of finger seals (FS). They not only contradict each other in FS design but also relate to many design parameters. Moreover, in the multi-objective optimization progress, the problems of optimizing results decision and preference requirement for optimization objectives are still challenge to researcher. So far, they are still important influence factors for advanced FS design. Therefore, the current work presents a new multi-objective optimization method by introducing game theory and fuzzy comprehensive evaluation theory. The optimizing results are compared to that of the general optimization method and finite element method (FEM). The study show that the FS, which is obtained by presented optimization method, has good performances. Compared respectively with the general optimization method and FEM, the computational results indicate that the presented method can effectively reflect the different response requirements of optimization objectives. Furthermore, the decision-making difficulty for multi-objective optimization of FS performances is significantly reduced
Elements content in tree rings from Xi'an, China and environmental variations in the past 30 years
Using inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES), the characteristics of chemical elements were analyzed in white poplar (Populus bonatii Levl.) and ailanthus (Ailanthus altissima (Mill.) Swingle) from three sites in the town of Xi'an, China. The results indicated that the concentration variations of Pb and Cd in tree rings were consistent with that of the environment where the trees were growing. P and Zn were translocated within tree rings to a certain degree, which led to an inaccurate pollution reconstruction. We also found that white poplar had a stronger absorptive capacity of Cd and Zn than ailanthus, which could make white poplar better as a species in environmental remediation. From this research we can see the great potential of tree rings for studying the history of different element pollution in the environment, showing that dendrochemical methods could be used as a powerful component in environmental monitoring programmes, to reconstruct past pollution history at the time when monitoring systems were not yet installed. (c) 2017 Elsevier B.V. All rights reserved
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