Narrow Results

Dynamic trajectories of relativistic electrons injected into tightly focused ultra-intense laser field have been investigated. In addition to the previously-reported CAS (Capture and Acceleration Scenario) and IS (Inelastic Scattering) trajectories, a new kind of nonlinear electron trajectory is found when the beam waist radius w₀ is small enough (kw₀≤30, k is the wave number) and incident angle is small. We shall call it PARM (Penetrate into Axial Region and Move). The basic feature of PARM trajectory shows the strong diffraction effect of a tightly-focused laser field. Part of the incident electrons that experience the strong transversal force from the diffraction edge field as they travel toward the beam waist will follow the PARM trajectory. This force can push the electrons toward the beam center. Thus unlike the CAS and IS electrons, the PARM electrons will move along the region near the beam axis. We also found some of the PARM electrons can gain energy from the field. The conditions for PARM electrons to appear were examined and are presented here. The implication of the presence of PARM to the planned experimental test of the CAS scheme is addressed.

Blast wall is considered to be an effective passive measure for blast protection since it can effectively reduce the blast loads and protect the building structures and people behind it. However, the current practice in blast wall design mainly depends on the structural strength and ductility to resist blast loads. These designs often lead to huge solid walls which are not only expensive, but also unsuitable for construction in urban areas, as they are not aesthetically appealing. Moreover, failure of solid blast wall may generate a significant amount of debris, which imposes great threats to people and structures behind the wall. In this paper, a new fence type blast wall, instead of the solid wall, is proposed to resist the blast loads based on the concept of wave interference. The proposed fence wall uses structural columns placed at strategic locations as wave stoppers to generate wave reflection, diffraction and interaction between the reflected and diffracted waves from different columns to result in self-cancellation of wave energy, thus leads to substantial reduction in blast wall size in design. Numerical simulations are carried out to investigate the effectiveness of the fence wall layout with different column geometries, column spacing, column dimensions, and fence layers on blast loads reduction. Based on the results, an effective design of the fence type blast wall is proposed, which can reduce the pressure and impulse of the blast loads behind the wall upto 70%.

To protect structures from external explosions, solid protective barriers have been demonstrated by experimental and numerical studies to be able to effectively mitigate blast loads on structures behind them. However, to protect against blast loads, barriers normally need to be designed to have high structural resistance and ductility. This often requires bulky and heavy protective barriers which are not only highly costly but also often not appropriate for application in downtown areas as they are not friendly to city planning or appearance. Fence type blast wall consisting of structural columns was recently proposed and its effectiveness in mitigating blast loads was investigated through numerical simulations. It was found that the wave–fence interaction and interference of waves significantly reduced the wave energy when the blast wave passed through the fence blast wall. To further investigate the effectiveness and applicability of fence type blast wall as a highly potential technology for structural protection in an urban area, field tests have been conducted and results are reported in this paper. Columns with circular and triangular cross-sections were adopted to build fence blast walls. In addition, a masonry wall was also constructed as solid barrier for comparison. Hemispherical TNT explosive weighing 1.0 kg with different stand-off distances was detonated on the ground to generate the blast load. Blast overpressures in free air, behind the fence blast wall and behind the masonry wall were recorded by pressure sensors. The effectiveness of the fence blast wall in reducing blast wave and protecting structures was demonstrated by the test data.

A horizontal two-dimensional finite element model is developed in order to estimate the process of scour around a large-scale cylinder due to waves. The present model differs from previous models in the sense that the wave model is based on an elliptic mild slope equation and the sediment transport induced by the steady streaming is considered. The current induced by the gradient of radiation stress is considered and calculated using a depth integrated shallow water equation. The contributions of the Lagrangian drift velocity to the scour is also considered in this model. The model is validated against a few cases where experimental data are available. The comparison of the calculation results with the experimental data indicates that the present numerical model predicts the scour around a large cylinder reasonably well. The effects of Keulegan–Capenter (KC) number, the grain size of sediments and the model scale on scour around a large cylinder are also investigated.