176 research outputs found
Experimental Study on the Small Strain Shear Modulus of Unsaturated Soils
University of Technology Sydney. Faculty of Engineering and Information Technology.The small strain shear modulus (Gâââ) is a key parameter in analysing and predicting the dynamic response of soils. Numerous experimental studies have been conducted to investigate Gâââ of unsaturated soils during drying and wetting processes and loading and unloading stages of net stress, however, limitations still exist, requiring more research studies in this field. Specifically, the impact of hydraulic hysteresis, an important characteristic of unsaturated soil experiencing a number of drying-wetting and loading-unloading cycles of net stress, has not been captured successfully. Another issue in existing research studies is that the variation of Gâââ during an air-drying process was measured without considering the effect of matric suction equalisation, resulting in misrepresentation of the response of the material. In addition, the effects of drying-wetting and loading-unloading cycles on Gâââ of cemented soils have received little attention, and thus need to be investigated.
In this study, experimental studies were conducted to address the above-mentioned issues. To investigate the impact of hydraulic hysteresis on Gâââ of an unsaturated reconstituted soil, the specimen was subjected to a multi-stage test during several drying-wetting cycles and a loading-unloading cycle of net stress using a modified unsaturated Rowe cell apparatus. The results revealed four key factors that directly influence the magnitude of Gâââ : the void ratio, the net stress, matric suction and degree of saturation. While variations of the void ratio, net stress, and matric suction cause persistent responses of Gâââ (i.e. if all other factors remain unchanged, Gâââ would then be reversely proportional to the void ratio and directly proportional to the net stress and matric suction), variations in the degree of saturation result in different responses. A decrease in the degree of saturation may induce a reduction or growth of Gâââ since on the one hand, it reduces the effect of matric suction, while on the other hand, it increases the total effect of van der Waals attractions and electric double layer repulsions. An analysis of the results showed that hydraulic hysteresis occurred in all the stress loops, and it directly influenced the response of Gâââ. The effect of hydraulic hysteresis can only be captured if the van der Waals attractions and electric double layer repulsions are considered. A model to estimate Gâââ while incorporating the van der Waals attractions and electric double layer repulsions has been proposed and it provided a good agreement with the experimental measurements. For practical issues, this model allows the determination of Gâââ of unsaturated soils based on the stress state, void ratio, and degree of saturation regardless of the stress and drying-wetting history, thus improving the accuracy of capturing response of Gâââ in complex loading-unloading and drying-wetting cycles.
To investigate the impact of matric suction equalisation on the measurement of Gâââ during an air-drying process, a weight-controlled bender element test was developed allowing the evolution of Gâââ at each test stage until matric suction equalisation is reached. Test results indicated that excluding matric suction equalisation causes underestimation of Gâââ measurement, especially in the middle range of the degree of saturation. This underestimation could be due to the non-uniform distribution of the water content and the corresponding matric suction across the cross section of the soil sample. The impact of matric suction equalisation on the measurement of Gâââ was rather small in the early stages of the air-drying process but accelerated when degree of saturation approached the threshold corresponding to the shrinkage limit. It was believed that water discontinuity, developed at water content smaller than the shrinkage limit, prevented the hydraulic flow from diminishing the non-uniform distribution of the water content, and thus, caused the underestimation of Gâââ. It was also found that effect of the unsaturated coefficient of permeability on the time required for matric suction equalisation is significant only at high degrees of saturation when the water phase is still continuous, while at lower degrees of saturation, when the water phase loses its continuity, effect of the unsaturated coefficient of permeability is gradually overcome by the effect of water evaporation. Two empirical equations were proposed to determine the time required for matric suction equalisation in experiments studying behaviours of unsaturated soil after air drying and to predict Gâââ of compacted soil layers near the ground surface which can experience significant evaporation when exposed to the open environment.
To investigate effects of drying-wetting and loading-unloading cycles on Gâââ of cemented soils, a cemented sample cured under a constant stress was subjected to a multi-stage test during several drying-wetting and loading-unloading cycles using the modified unsaturated Rowe cell apparatus. The test results revealed that drying and wetting caused degradation of cementation of lightly cemented soil. The cementation degradation reduced the contribution of cementation and increased the contribution of the stress state to Gâââ. It was observed that, under constant matric suction, drying and wetting also occurred during the loading and unloading stages, respectively, due to the changes of the pore water pressure with loading-induced contraction and unloading-induce swelling. Consequently, degradation of cementation increased with increasing number of drying-wetting cycles as well as loading-unloading cycles. With an increase in the number of drying-wetting and loading-unloading cycles, Gâââ at the high stresses would intensify due to an increase in the contribution of the stress state, while Gâââ at the low stresses would decrease due to the reduction in the contribution of the cementation. In general, small strain shear modulus of lightly cemented soil was influenced by void ratio, cementation, degree of cementation degradation, and stress level. The effects of these influencing factors varied during drying-wetting cycles as well as loading-unloading cycles of net stress, thus, stress-strain history plays an important role in predicting the Gâââ of lightly cemented soils on site
Monitoring Heart Rate Variability Based on Self-powered ECG Sensor Tag
This paper proposes a batteryless sensing and computational device to collect and process electrocardiography (ECG) signals for monitoring heart rate variability (HRV). The proposed system comprises of a passive UHF radio frequency identification (RFID) tag, an extreme low power microcontroller, a low-power ECG circuit, and a radio frequency (RF) energy harvester. The microcontroller and ECG circuits consume less power of only ~30 ”A and ~3 mA, respectively. Therefore, the proposed RF harvester operating at frequency band of 902 MHz ~ 928 MHz can sufficiently collect available energy from the RFID reader to supply power to the system within a maximum distance of ~2 m. To extract R-peak of the ECG signal, a robust algorithm that consumes less time processing is also developed. The information of R-peaks is stored into an Electronic Product Code (EPC) Class 1st Generation 1st compliant ID of the tag and read by the reader. This reader is functioned to collected the R-peak data with sampling rate of 100ms; therefore, the user application can monitor fully range of HRV. The performance of the proposed system shows that this study can provide a good solution in paving the way to new classes of healthcare applications
SDP-Based Quality Adaptation and Performance Prediction in Adaptive Streaming of VBR Videos
Recently, various adaptation methods have been proposed to cope with throughput fluctuations in HTTP adaptive streaming (HAS). However, these methods have mostly focused on constant bitrate (CBR) videos. Moreover, most of them are qualitative in the sense that performance metrics could only be obtained after a streaming session. In this paper, we propose a new adaptation method for streaming variable bitrate (VBR) videos using stochastic dynamic programming (SDP). With this approach, the system should have a probabilistic characterization along with the definition of a cost function that is minimized by a control strategy. Our solution is based on a new statistical model where the future streaming performance is directly related to the past bandwidth statistics. We develop mathematical models to predict and develop simulation models to measure the average performance of the adaptation policy. The experimental results show that the prediction models can provide accurate performance prediction which is useful in planning adaptation policy and that our proposed adaptation method outperforms the existing ones in terms of average quality and average quality switch
Availability of Free-Space Laser Communication Link with the Presence of Clouds in Tropical Regions
Free-space laser communication (lasercom), a great application of using free-space optics (FSO) for satellite communication, has been gaining significant attraction. However, despite of great potential of lasercom, its performance is limited by the adverse effects of atmospheric turbulence and cloud attenuation, which directly affect the quality and availability of lasercom links. The paper, therefore, concentrates on evaluating the cloud attenuation in the FSO downlinks between satellite and ground stations in tropical regions. The meteorological ERA-Interim database provided by the European Center for Medium-Range Weather Forecast (ECMWF) from 2015 to 2020 is used to get the cloud database in several areas in tropical regions. This study proposed a novel probability density function of cloud attenuation, which is validated by using a well-known curve-fitting method. Moreover, we derive a closed-form of satellite-based FSO link availability by applying the site diversity technique to improve the system performance. Numerical results, which demonstrate the urgency of the paper, reveal that the impact of clouds on tropical regions is more severe than in temperate regions
Standard-Compliant Content Adaptation in IPTV Systems
IPTV, which is based on the universal IP infrastructure, has the inherent nature of heterogeneity in terms of content, networks, terminals, and users. An important solution to cope with such heterogeneity is content adaptation. This paper reviews the standardization issues related to content adaptation in IPTV standards. We first describe the basic architecture of content adaptation and its integration into the ITU-T IPTV architecture. Then typical methods of content adaptation in practical IPTV systems are discussed in detail. Especially, we highlight the standard metadata tools that are potential to support adaptation methods within ITU-T IPTV architecture. Some recent developments in other standard bodies are also discussed
Research on aerodynamics of a wing-in-surface-effect ship by theory and experiment
The paper presents the results of theoretical and experimental studies of the aerodynamic characteristics of a wing-in-surface-effect ship (WISES), which was made under coordination of the Institute of Mechanics, Vietnamese Academy of Science and Technology. The theoretical researches are carried out by the half analytical method and together by the linear and nonlinear vortex lattice methods. The experiment was conducted by the model on wind tunnel OTl of The Technical Institute of Air Force - Air Defense
Cumulative Quality Modeling for HTTP Adaptive Streaming
Thanks to the abundance of Web platforms and broadband connections, HTTP
Adaptive Streaming has become the de facto choice for multimedia delivery
nowadays. However, the visual quality of adaptive video streaming may fluctuate
strongly during a session due to bandwidth fluctuations. So, it is important to
evaluate the quality of a streaming session over time. In this paper, we
propose a model to estimate the cumulative quality for HTTP Adaptive Streaming.
In the model, a sliding window of video segments is employed as the basic
building block. Through statistical analysis using a subjective dataset, we
identify three important components of the cumulative quality model, namely the
minimum window quality, the last window quality, and the average window
quality. Experiment results show that the proposed model achieves high
prediction performance and outperforms related quality models. In addition,
another advantage of the proposed model is its simplicity and effectiveness for
deployment in real-time estimation. The source code of the proposed model has
been made available to the public at https://github.com/TranHuyen1191/CQM
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