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Your search keyword '"Hamidi, Sajad Ahmad"' showing total 27 results
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27 results on '"Hamidi, Sajad Ahmad"'

2. On Sensitivity of Time Step for Dynamic Analysis of Bridges under Moving Loads

Author

Hamidi, Sajad Ahmad

Subjects
Mathematics - Numerical Analysis
Abstract

In the analysis of structures under dynamic loads, selection of a proper time step has a great influence to reach exact results. In this research determination of proper time step in dynamic analysis of railway bridges under high speed moving loads is considered. Dynamic responses of four simple span steel bridges with 10, 15, 20 and 25 meter length to moving trains with speed from 100 to 400 km/h and axle distances from 13 to 23 meter are calculated considering different time steps in analysis. The results indicate that by increase in moving speed of vehicle (increase in loading speed) the length of proper time step for dynamic analysis is reduced. In contrast by increase in span length (increase in bridge vibration period) longer time steps can be used in dynamic analysis. In this research by investigation of dynamic analysis results, an equation is suggested for determination of proper time step for dynamic analysis of bridges under moving loads.

Published
2015

3. Preliminary Investigations on Uncertainty Analysis of Wind-Wave Predictions in Lake Michigan

Author

Nekouee, Navid and Hamidi, Sajad Ahmad

Subjects
Physics - Atmospheric and Oceanic Physics
Abstract

With all the improvement in wave and hydrodynamics numerical models, the question rises in our mind that how the accuracy of the forcing functions and their input can affect the results. In this paper, a commonly used numerical third generation wave model, SWAN is applied to predict waves in Lake Michigan. Wind data were analyzed to determine wind variation frequency over Lake Michigan. Wave predictions uncertainty due to wind local effects were compared during a period where wind had a fairly constant speed and direction over the northern and southern basins. The study shows that despite model calibration in Lake Michigan area, the model deficiency arises from ignoring wind effects in small scales. Wave prediction also emphasizes that small scale turbulence in meteorological forces can increase error in predictions up to 35%. Wave frequency and coherence analysis showed that both models are able to reveal the time scale of the wave variation with same accuracy. Insufficient number of meteorological stations can result in neglecting local wind effects and discrepancies in current predictions. The uncertainty of wave numerical models due to input uncertainties and model principals should be taken into account for design risk factors., Comment: This paper has been withdrawn by the author due to a crucial sign error in equation 1

Published
2015

26. Multi-decadal and multi-centennial variability in Colorado River streamflow.

Author

Hamidi, Sajad Ahmad and Namdar Ghanbari, Reza

Subjects
*RIVERS, *SPECTRUM analysis, *CLIMATE change research, *ATMOSPHERIC composition, *ATMOSPHERIC circulation
Abstract

A 1240-year-long record of reconstructed annual flows of the Colorado River at Lees Ferry is analysed using singular spectrum analysis and multi-taper method of spectral analysis. Spectral analysis of the 100-year-long recent record of annual flows of Colorado River in the measured and reconstructed forms shows similar oscillations in high- and low-frequency bands. Therefore, the oscillatory components and trends extracted from the reconstructed data are a good representative of inter-annual, multi-decadal, and multi-centennial variability of natural flow in the river. In Colorado River the length of flow data is 1240 years and multi-centennial and multi-decadal oscillatory components can be extracted at a high confidence level. In this research we attempt to find whether the changes in streamflow in the twentieth century are due to an external cause such as climate change or whether they are part of the natural variability of flow observed in the past. The results suggest that there is only a part of the linear trend, caused by climate change or man-made effects, and an important part of that is due to climate variability which is believed to be totally natural. The same line of climate variability is still acting on our planet, and it may have gained new aspects due to the change in atmospheric composition and circulation as a result of anthropogenic effects. This may cause serious limitations to the water sustainability and water availability on the earth. [ABSTRACT FROM PUBLISHER]

Published
2016
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