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  • Articles  (62)
  • 2010-2014  (62)
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  • Articles  (62)
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  • 1
    Publication Date: 2014-12-31
    Description: In this paper, the mechanical and long-term durability behaviour of structural lightweight concrete (LWC) produced with natural lightweight scoria aggregate (LWSA) from the Azores is analysed. Among the properties studied are the compressive and tensile strength, modulus of elasticity, shrinkage, capillary absorption, carbonation and chloride resistance. To this end, a comprehensive experimental study was carried out on different concrete compositions involving common and high-performance structural concrete produced with coarse or coarse and fine volcanic scoria aggregate. Generally, all the mechanical properties worsened with the incorporation of LWSA. It is concluded that the most efficient LWC with LWSA can attain compressive strength up to about 35 MPa. On the other hand, the production of LWC of more than 50 MPa implies a marked reduction in compressive strength for a small reduction in density. The long-term shrinkage increased with the replacement of normal-weight aggregate by LWA and was higher when coarse and fine scoria were used. However, the early age shrinkage can be counterbalanced by the internal curing effect provided by LWSA. The capillary absorption, chloride and carbonation resistance were affected by the high porosity of LWSA. The absence of the outer shell in scoria aggregate and the surface effects during tests can partly explain the lower performance of the LWC. Nevertheless, it is shown that LWC with scoria can be durable, and even for low- to moderate-strength LWC the corrosion induced by carbonation may not be relevant.
    Print ISSN: 1319-8025
    Electronic ISSN: 2191-4281
    Topics: Natural Sciences in General , Technology
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  • 2
    Publication Date: 2014-12-31
    Description: A new CMOS current-mode four-quadrant analog multiplier is presented. The design is based on the square-difference approach using short-channel MOSFETs operating in saturation region and a rectifier. The squaring circuit used is free of error due to carrier mobility reduction and hence an accurate multiplier is achieved. Tanner T-spice is used to confirm the functionality of the proposed design using 0.18μm TCMS CMOS process technology. Simulation results shows that the relative error is 1.8% and –3dB frequency is 230MHz.
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    Electronic ISSN: 2191-4281
    Topics: Natural Sciences in General , Technology
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  • 3
    Publication Date: 2014-12-31
    Description: The purpose of this study was to conduct seismic hazard analysis for Al-Tajiat and Al-Zawraa stadiums using probabilistic and deterministic approaches. The stadiums of Al-Tajiat and Al-Zawraa are located at latitude of \({33^{\circ}}\) 25ʹ25.80ʺN, longitude of \({44^{\circ}}\) 17ʹ9.28ʺE and latitude of \({33^{\circ}}\) 20ʹ39ʺN, longitude of \({44^{\circ}}\) 22ʹ5.81ʺE, respectively. To assess the seismic hazard, the region was divided into five seismic sources, and the seismic parameters were calculated for each selected seismic source. According to the results obtained from probabilistic seismic hazard analysis, the maximum horizontal accelerations on bedrock for return periods of 75, 475, and 2,475 years are equal to 0.06, 0.12, and 0.21 g, respectively. The result of deterministic seismic hazard assessment proves that the maximum horizontal and vertical acceleration on bedrock are 0.31 and 0.16 g, respectively. In order to analyze the response of structures against calculated acceleration, studies related to response spectrum and design spectrum have been carried out based on statistical analysis of appropriate time histories. The computer program Equivalent linear Earthquake Response Analysis was used to process the selected records and to consider the effect of soil conditions. Design spectrum has also been presented based on the response spectrum of the selected time histories and then was compared with the corresponding ones in UBC 1997 and ISIRI 2800 codes. The comparison shows that the presented design spectrum is more conservative than the results provided by the above codes.
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  • 4
    Publication Date: 2014-12-30
    Description: This work developed an integrated scheduling model which incorporated both production scheduling and maintenance planning for a single machine problem and considered the multiple objectives of minimizing total weighted completion time and maximizing average timeliness level under a fuzzy environment. First, a fuzzy random variable for maintenance time windows was considered, and this model was then transformed using the expected value. Finally, a numerical example was used to demonstrate the value of this improved algorithm, the computational results from which proved its efficiency.
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  • 5
    Publication Date: 2014-12-30
    Description: The paper presents entropy analysis of MHD generalized Couette fluid flow inside a composite duct. The composite duct is composed of two parallel walls. The upper wall is uniformly moving impermeable plate, whereas the lower wall is porous strata of finite thickness with impermeable bottom. The upper plate and the bottom are asymmetrically convectively cooled. The solutions obtained for the velocity and the temperature are exploited to enumerate entropy generation. The effects of pertinent parameters on the quantities of interest are portrayed graphically and are discussed.
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  • 6
    Publication Date: 2014-12-28
    Description: In this paper, a novel nine-level inverter topology for open-end winding induction motor is proposed. This topology requires less number of components as compared to conventional topologies. The proposed inverter uses two three-phase three-level cascade inverters to generate nine-level in the phase voltage of open-end winding induction motor. An interesting feature of this topology is that in case of failure of any three-level inverter, it can be bypassed and the proposed topology can operate in three-level mode hence reliability of the circuit is improved. Out of two three-level inverters, the inverter which operates at higher DC link voltage switches less as compared to the inverter which operates at a lower DC link voltage, thus switching losses are reduced. A 1.5-kW induction motor is run with the proposed inverter at different modulation indices, and the performance of the drive is evaluated under steady and transient states.
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  • 7
    Publication Date: 2014-12-28
    Description: In this paper, studies have been made to detect the changes in key hydroclimatic variables including precipitation, surface-air temperatures and stream flow with their trends, constructed projection scenarios, and simulated projected climate parameters for Naran watershed. Extreme indices have been determined by least squares and weighted average regression analysis. Temperature extremes have indicated signs of warming trends and increase in variability. Precipitation extremes have depicted decrease in occurrences and magnitudes. The analysis has provided evidence of high confidence about these changes. Trends of from hydroclimatic parameters and stream flows were also determined using Mann–Kendall test for the period 1962–2011. Inter-annual trends have been detected to determine the sensitivity with a base period (1962–1991) by using equal and unequal overlapped moving periods. It has been found that there is a clear trend reversibility during 1996–1997 for temperatures and precipitation pattern. However, the analysis has revealed that no long-term trends are persistent in inter-annual surface-air temperatures and precipitation. Intra-annual trends and variations on the monthly scale from data sets of 1962–2011 have also been detected with their significant values. There seems evidence that variation of parameters in monthly scale is occurring that is affecting the ice melt schedule and evapotranspiration demand.
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  • 8
    Publication Date: 2014-12-28
    Description: Estimation of runoff volume in Wadis of intermittent flow is very important in order to manage the scarce water resources and mobilize Wadi flows efficiently. In Yemen, the availability of accurate runoff information is rare, which poses a serious challenge for hydrologists. This study used the soil conservation services curve number (SCS-CN) method with conventional database and the geographical information system to estimate the runoff volume of Wadi Zaher, Harad and Al Ghayel. The Wadi is located in the southwest of Sana’a City, the capital of Yemen. The area of the Wadi is 350.24 km 2 , and the average annual rainfall is 284 mm. The study area is classified into four hydrological soil groups. The average curve number for normal condition is 77.6, where the averages of the dry and wet conditions are 59.5 and 88.8, respectively. The results, obtained by the SCS-CN method, show that the average annual runoff depth for Wadi Zaher Al Ghayel watershed is 212 mm, and the total runoff volume is estimated to be 75.80 mm 3 , which is 76 % of the total annual rainfall. This approach can be applied in other Wadis in Yemen for the purpose of planning and development.
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  • 9
    Publication Date: 2014-12-28
    Description: Cloud computing is currently one of the fastest growing segments of IT. To date, and according to a recent survey conducted by the International Data Corporation, security is the biggest challenge to cloud computing. A cloud introduces resource-rich computing platforms, where adopters are charged based on the usage of the cloud’s resources, known as “pay-as-you-use” or utility computing. However, a conventional Distributed Denial-of-Service (DDoS) attack on server and network resources compromises cloud computing services by charging cloud adopters more cost due to the attack activities that consume cloud’s resources. In such case, the main goal of such attack is to make the cloud computing unsustainable by targeting the cloud adopter’s economic resources. Thus, it constitutes a new breed of DDoS attacks, namely Economic Denial of Sustainability (EDoS) attack. In this paper, we study the impact of EDoS attacks on the cloud computing services, considering only a single class of service. We developed an analytical model verified by a simulation model to study such impact of EDoS attacks on the cloud computing. The analytical model relies on the queuing model that captures the cloud services and considers a number of performance and cost metrics including end-to-end response time, utilization of computing resources, throughput, and the incurred cost resulting from the attack.
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  • 10
    Publication Date: 2014-12-28
    Description: Floating-point (FP) multipliers are the main energy consumers in many FP applications. Recently several FP multipliers with multiple- precision modes have been designed to trade energy consumption with output accuracy of FP multiplication operation (MOP). To effectively apply these multi-mode multipliers to FP applications, this paper presents a fast instruction precision assignment method for reducing energy consumption under accuracy and performance constraints. To easily set and check the accuracy constraint, we first build an affine arithmetic based error model to evaluate the overall output accuracy loss caused by inaccurate FP MOPs. Moreover, a simplified instruction scheduling method is also developed to quickly check the performance constraint. Based on these two check functions and the characteristics of proposed multi-mode multiplier, a fast Tabu search (TS) algorithm is then proposed to assign the precision mode of each FP MOP under the accuracy and performance constraints imposed on the given application. Experimental results show that the proposed fast TS algorithm can find the precision assignment with more energy saving and less searching time when compared to previous methods.
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