In this work, the vortex-induced vibration of a rectangular prism with a side ratio of 4 is numerically investigated using a two-dimensional spectral-element method for several structural damping. The prism is constrained to vibrate only in the cross-flow direction. The vibration is governed by an impinging leading-edge vortex instability, while the trailing-edge vortex between two leading-edge vortices also contributes to the amplitude. The results show that increasing the damping reduces not only the displacement amplitude but also the range of lock-in. It is found that the phase difference between the lift force and motion plays an important role that is related to the added mass. After suppressing the trailing-edge vortex using a splitter plate, the vibration amplitude varies almost linearly with the phase difference. In addition, the essence of the lock-in reduction is that there is a critical phase difference above which lock-in disappears for a certain reduced velocity and the critical phase difference changes linearly with the reduced velocity. A new semi-empirical model based on the phase difference is proposed to predict the vibration amplitude and lock-in regime under different damping values. Rectangular prism experiments with two different side ratios are used to verify the effectiveness of the model.

A compact dual band antenna operating simultaneously at 5.4 GHz and 15 GHz is presented in this paper. It is a planar array composed of a grid of 6×8 (48) patch antennas working at 5.4 GHz and 7×9 (63) patch antennas working at 15 GHz. The whole antenna array occupies an area of 150 mm × 180 mm which fits the ordinary size requirement of wireless access point. All the antenna patch antennas have a reflection coefficient lower than -6 dB across 320 and 2000 MHz bandwidth at 5 and 15 GHz. The electromagnetic coupling between any two antennas is lower than - 10 dB within the specified bandwidth. Measurement results show good agreement with the simulation of input matching, mutual coupling and radiation pattern. Compared to the ordinary antenna array design with half-wavelength spacing between two neighboring antennas, our proposed dual band array design accommodates 1.6 times more antennas at 5.4 GHz and extra 63 antennas at 15 GHz, while maintaining sufficient radiation efficiency, matching bandwidth and decoupling.

Fatigue damage can lead to failures of structural systems. To reduce the failure risk and enhance the reliability of structural systems, inspection and maintenance interventions are required, and it is important to develop an efficient inspection strategy. This study, for the first time, develops a system-level reliability growth model to establish efficient inspection planning. System-level reliability growth is defined as an increase in the percentage of the system reliability index with and without inspection. The probabilistic S-N approach is used to obtain the reliability index without inspection. Moreover, advanced risk analysis and Bayesian inference techniques are used to obtain the reliability index with inspection. The optimal inspection planning is obtained by maximizing system-level reliability growth. This model is applied to an offshore wind farm. The results show that inspection efficiency can be improved by increasing the number of repair objects in response to a ‘detection’ inspection outcome, changing the inspection object for each inspection, and increasing the inspection quality. The maximum system-level reliability growth gained from one additional inspection decreases as the number of inspections increases. This study quantifies the inspection efficiency of offshore wind farms by explicit system-level reliability growth computation, offering valuable insights for promoting sustainable energy solutions.

Non-destructive testing (NDT) methods have been widely used for damage examination and structural maintenance, e.g., detecting and repairing fatigue cracks. In-service inspections help increase fatigue reliability by providing new information for updating structural failure probability and making repair decisions. However, these benefits are often compromised by uncertainties associated with inspection methods. Sometimes, existing cracks may not be identified, and positive inspection indication may actually not exist. It is of great interest to consider the influence of inspection uncertainty in maintenance optimization, because the benefits and costs of maintenance are affected by inspection decisions (e.g., inspection times and methods), which are subjected to inspection uncertainty. The influence of inspection uncertainty on maintenance optimization has not been explicitly and adequately covered in the literature. In this paper, the problem has been investigated by probabilistic modelling of the qualities of inspection methods via probability of detection (PoD) functions. A new PoD function is proposed to characterize the inspection quality when inspection uncertainty is neglected. Optimum inspection decisions are derived by the objective of maximizing lifetime reliability index under two scenarios (considering and not considering inspection uncertainty). The effectiveness index of a planned inspection is defined based on the max reliability indexes under the two scenarios. It is shown that the max lifetime reliability index generally deceases when inspection uncertainty is considered. Inspection uncertainty may have little influence on the lifetime reliability index, depending on the planned inspection time. The effectiveness index of a planned inspection increases with the decrease in the mean detectable crack size. However, inspection uncertainty can result in significant increases in expected life cycle costs and maintenance costs.