Numerical investigation of transmission of low frequency sound through a smooth air-water interface

It is the traditional belief that sound transmission from water to the air is very weak due to a large contrast between air and water impedances. Recently, the enhanced sound transmission and anomalous transparency of air-water interface have been introduced. Anomalous transparency of air-water interface states that the sound generated by a submerged shallow depth monopole point source localized at depths less than 1/10 sound wavelength, can be transmitted into the air with omni-directional pattern. The generated sound has 35 times higher power compared to the classical ray theory prediction. In this paper, sound transmission through air-water interface for a localized underwater shallow depth source is examined. To accomplish this, two-phase coupled Helmholtz wave equations in two-phase media of air-water are solved by the commercial finite element based COMSOL Multiphysics software. Ratios of pressure amplitudes of different sound sources in two different underwater and air coordinates are computed and analyzed against non-dimensional ratio of the source depth (D) to the sound wavelength (λ). The obtained results are compared with the experimental data and good agreement is displayed.     

Three-dimensional numerical investigation of the separation process in a vortex tube at different operating conditions

Air separators provide safe, clean, and appropriate air flow to engines and are widely used in vehicles with large engines such as ships and submarines. In this operational study, the separation process inside a Ranque–Hilsch vortex tube cleaning (cooling) system is investigated to analyze the impact of the operating gas type on the vortex tube performance; the operating gases used are air, nitrogen, oxygen, carbon dioxide and nitrogen dioxide. The computational fluid dynamic model used is equipped with a three-dimensional structure, and the steady-state condition is applied during computations. The standard k–ε turbulence model is employed to resolve nonlinear flow equations, and various key parameters, such as hot and cold exhaust thermal drops, and power separation rates, are described numerically. The results show that nitrogen dioxide creates the greatest separation power out of all gases tested, and the numerical results are validated by good agreement with available experimental data. In addition, a comparison is made between the use of two different boundary conditions, the pressure-far-field and the pressure-outlet, when analyzing complex turbulent flows inside the air separators. Results present a comprehensive and practical solution for use in future numerical studies.     

Modeling the flow regime near the source in underwater gas releases

Recent progress in calculating gas bubble sizes in a plume, based on phenomenological approaches using the release conditions is a significant improvement to make the gas plume models self-reliant. Such calculations require details of conditions Near the Source of Plume (NSP); (i.e. the plume/jet velocity and radius near the source), which inspired the present work. Determining NSP conditions for gas plumes are far more complex than that for oil plumes due to the substantial density difference between gas and water. To calculate NSP conditions, modeling the early stage of the plume is important. A novel method of modeling the early stage of an underwater gas release is presented here. Major impact of the present work is to define the correct NSP conditions for underwater gas releases, which is not possible with available methods as those techniques are not based on the physics of flow region near the source of the plume/jet. We introduce super Gaussian profiles to model the density and velocity variations of the early stages of plume, coupled with the laws of fluid mechanics to define profile parameters. This new approach, models the velocity profile variation from near uniform, across the section at the release point to Gaussian some distance away. The comparisons show that experimental data agrees well with the computations.     

峡谷河流的挂网式人行悬索桥设计

为了降低峡谷河流人行悬索桥的造价,本文提出一种峡谷河流的双曲抛物面空间缆索挂网式人行悬索桥,将双曲抛物面空间缆索网分散锚固于峡谷河流两侧的悬崖陡壁岩石之上,取消桥塔,取消吊索体系,管桁架式桥面加劲梁直接搁置在双曲抛物面空间缆索网之上,节约造价,装配式施工简便。结合某150m跨径峡谷河流人行景观悬索桥,建立Midas有限元分析模型,开展动力模态特性分析研究,验证了双曲抛物面空间缆索挂网式人行悬索桥结构具有良好的抗风稳定性。     

Workshop report - Benchmarking the outcome of competitive tendering

Workshop 1 was concerned to identify the real results of competitive tendering, and experience on how to make it work better, using evidence from the rail and bus sectors in a large number of countries spread around the world. It was found that competitive tendering had generally been successful in terms of quality and costs, but problems had occurred in a number of cases, so careful attention must be paid to the design of tendering exercises, details of the contract, risk-sharing arrangements and the approach to any re-negotiation found to be necessary. As a result, an important conclusion is that the tendering authority needs a high degree of expertise in these issues; any thought that competitive tendering relieves the public authority of the need for expertise in public transport is mistaken.