低压天然气管道在有限空间中泄漏计算研究

在有限空间内,可燃气体泄漏扩散极其危险,甚至会引起火灾,因此对低压天然气管道在有限空间中泄漏进行计算研究意义非常重大。存在障碍物的有限空间中,气体泄漏后,在有限空间内形成的危险区域会受到风速和障碍物的影响。用流体力学软件模拟泄漏后气体形成的危险区域浓度分布情况,对模拟结果进行分析可知,危险气体扩散受到障碍物的阻碍,会在障碍物周围形成不规则的危险区域,风速不同,危险区域的大小也不同。泄漏口位置对危险区域的大小也有影响,离障碍物近的泄漏口附近,气体容易堆积,危险系数相对较高。     

Event-triggered path tracking control with obstacle avoidance for underactuated surface vessel compliant with COLREGs-constraints: Theory and experiments

This paper addresses the path tracking problem for an underactuated surface vessel with model parametric uncertainties, in the presence of unknown ocean currents under input saturation and limited transmission resources, while guaranteeing no collision can occur with a nearby target ship (TS) in compliance with the International Regulations for Preventing Collisions at Sea (COLREGs) rules. In view of COLREGs practice, this paper proposes a strategy to address the issue of collision avoidance in the presence of an encounter situation. Based on different stages of encounter situation, a local path re-planning-based repulsive potential function technique is proposed, such that the potential force generated by a decision-making algorithm, is directly acted on the original desired trajectory deviating thus the Own Ship (OS) to take direction in compliance with the COLREG constraints. By restoring to the path-tracking framework, this paper proposes a novel navigation control algorithm with ship-to-ship collision avoidance. At the navigation level, a finite-time prescribed performance control strategy with the deployment of novel asymmetrical envelopes is proposed to prescribe path-tracking errors within user-defined constraints while reducing transient response overshoots. Furthermore, in order to enhance the tracking performance under model uncertainties and external disturbances, a radial basis function neural network (RBFNN) technique is used. We first propose a second-order neural network disturbance observer to restrain the compound disturbances which are combined with external disturbances and neural network approximation errors. Next, based on the estimated compound disturbances, an adaptive NN controller is designed via an event-triggering mechanism along with a transverse function approach and the backstepping technique. It is shown that with the proposed technique, all signals in closed-loop systems are semi-globally uniformly bounded, while the output tracking errors converge to a prescribed arbitrarily small region within a finite time. In addition, we prove via the direct Lyapunov approach, the existence of minimal inter-event time and thus Zeno-behavior is avoided. Finally, experiments are conducted to validate the effectiveness and performances of the proposed control strategy.     

Developing and testing a novel pressure-controlled hydraulic profile for siphon-shaft spillways

The operating heads of the siphon-shaft spillways are limited by cavitational pressure due to low siphonic pressure inside the spillway shaft. Therefore, these low pressures inside the shaft must be eliminated to control the cavitation. An innovative idea is to enlarge the shaft cross-section upwards where siphonic pressure decreases. For this purpose, in this study, a new pressure-controlled siphon-shaft profile was developed analytically based on Bernoulli principle and cavitational pressure. The hydraulic performance of the profile is numerically tested by means of computational fluid dynamics (CFD). The numerical model used was validated with the results of a benchmark model, and the discretization errors were estimated based on a procedure recommended by American Society of Mechanical Engineers (ASME). The verified models used to analyze the novel hydraulic design together with the classical Wagner profile originally applied to shaft/morning glory spillways. The analysis results indicated that the pressure-controlled siphon-shaft profile is quite effective in terms of cavitation protection and hydraulics performance for siphon-shaft spillways.