One of the main challenges for structural damage detection using monitoring data is to acquire features that are sensitive to damages but insensitive to noise (e.g. sensor measurement noise) as well as environmental and operational effects (e.g. temperature effect). Inspired by the capabilities of deep learning methods in representation learning, various deep neural networks have been developed to obtain effective damage features from raw vibration data. However, most of the available deep neural networks are supervised, resulting in practical difficulties owing to the lack of damage labels. This article proposes a damage detection strategy based on an unsupervised deep neural network, referred to as deep convolutional denoising autoencoder, which accepts multi-dimensional cross-correlation functions as input. The strategy aims to extract damage features from field measurements of undamaged structures under the influence of noise and temperature uncertainties. In the proposed strategy, cross-correlation functions of vibration data are first calculated as basic features; then deep convolutional denoising autoencoder is developed to reconstruct cross-correlation functions from their noise-corrupted versions to extract desired features; exponentially weighted moving average control charts are finally established for these features to identify minor structural damages. The strategy is evaluated through a numerical simply supported beam model and an experimental continuous beam model. The mechanism of deep convolutional denoising autoencoder to extract damage features is interpreted by visualizing feature maps of convolutional layers in the encoder. It is found that these layers perform rough estimations of modal properties and preserve the damage information as the general trend of these properties in multiple extra frequency bands. The results show that the proposed strategy is competent for structural damage detection under the exposed environment and worth further exploring its capabilities in applications of real bridges.
The swirling flow in the submerged entry nozzle (SEN) is found to significantly improve the flow pattern in the mold, as well as the quality of the steel products. However, the devices generating swirling flow designed so far are mostly limited to be applied in the industries due to the cost, lifetime, external facility, and so on. Herein, a novel swirling flow generator (SFG) is intended to be installed in the conventional tundish to generate a swirling flow in the SEN driven by the gravitational potential energy of the molten steel. The computational fluid dynamics (CFD) simulations are performed to investigate the effects of the SFG by comparing the flow pattern in the conventional tundish. The result shows that a swirling flow is produced in the SEN without bringing a serious disturbance to the flow distribution in the tundish. The flow in the SEN becomes much even by adopting the SFG and the swirling velocity is found to decrease with the increasing distance from the bottom of the tundish. A parameter called swirling intensity is proposed to quantitatively evaluate the intensity of the swirling in the SEN.
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