Showing 5 results for Afzalimehr
H. Afzalimehr,
Volume 5, Issue 3 (fall 2001)
Abstract
Over the past decades, numerous regime formulations, describing channel width, average flow depth and channel slope, have been proposed for gravel-bed rivers. Using the downstream hydraulic geometry measurements from 280 field channels, nine gravel-bed predictive formulations were compared. They are attributed to Bray, Chang, Hey and Thorne, Kellerhals, Neil, Parker and Simons, and Albertson. The comparison showed that the dominant discharge and the median grain size are not sufficient hydraulic variables to predict the channel geometry (width, depth, and especially channel slope). Seeking more complex formulations with the measured parameters, it was not possible to improve the existing formulations significantly without spurious influences of common variables.
A second database is thus gathered from 19 laboratory-based boundary-layer measurements published in Kironoto and Graf (13) and Song et al. (19). In fact, the boundary-layer theory will respond to a fundamental question: are the existing available variables in literature not sufficient or the methods of their estimations are not appropriate? Based on the present study, the methods of their estimations should be changed. The second database allows proving the usefulness of the Shields parameter for the prediction of average flow depth and channel slope. Hence, using boundary-layer theory for prediction of Shields parameter and stable channel parameters is recommended.
M. Heidarpour, H. Afzalimehr, E. Khorami,
Volume 6, Issue 3 (fall 2002)
Abstract
Of the many hydraulic structures developed by man, the weir is perhaps the oldest. Weirs are used for the measurement of discharge and regulation of water flow. The most common types of weirs are broad-crested, sharp-crested, circular-crested and cylindrical, and ogee crest weirs. Advantage of the circular-crested and cylindrical weir compared to the other weirs include simplicity of design, stable overflow pattern, larger coefficient of discharge and the associated lower costs. In the present study, potential flow around a circular cylinder are adapted to determine the velocity distribution at the crest section and to develop a model for coefficient of discharge (Cd) for circular-crested weirs. These results were evaluated using present test data for three types of weir models, namely, cylindrical, semicylindrical and semicylindrical with different heights and also Dressler theory. The results of the study showed that the experimental velocity profile agree very well with the theoretical profiles for the range of the study. Also, the prediction of the velocity distribution over the weir crest using Dressler theory is always less than the proposed model and measured data. The predicted values of coefficient of discharge (Cd) based on the proposed model agree well with Cd determined from direct discharge measurements. For the cylindrical model, the coefficient of discharge can be predicted from the proposed model within an error of –7% and for the semicylindrical and semicylindrical with different heights within ± 5%.
H. Afzalimehr, M. Heidarpour, S. H. Farshi,
Volume 7, Issue 1 (spring 2003)
Abstract
In this study, two data ranges of uniform flow (bulk parameters) and non-uniform flow (local parameters) are employed to investigate resistance to flow and the factors affecting it using velocity distribution of boundary layer theory.
The results indicate that the cross-section form factor or adjustment roughness coefficient of logarithmic law can not improve the prediction of flow resistance. On the other hand, it is possible to ameliorate the prediction of resistance to flow by application of the Froude number and the Shields parameter along with the integration constant of the logarithmic velocity distribution. Also, there is no improvement in flow resistance prediction by taking into account power equations. On the other hand, the application of boundary-layer characteristics such as displacement thickness and momentum thickness in velocity profile can remove the risk of spurious correlation. Based on the measured velocity profiles for non-uniform flow in Gamasiab river, a new flow resistance equation is suggested in which the following bulk parameters are considered: flow depth, maximal velocity at the water surface, and friction slope.
M. Heidarpour, H. Afzalimehr, M. Naderi Bani,
Volume 7, Issue 3 (fall 2003)
Abstract
The use of slot through a pier is a new method proposed to control local scour at bridge piers. In this study, control of local scour at bridge pier is studied using 20 pier models under clear water conditions. The models consist of one circular pier without slot, three round-nosed piers without slot, and 16 piers with slot. Two slot lengths were chosen (yl=b and yl=2b, where y1 is the length of slot and b is the width (diameter) of the pier). Furthermore, they were located at two positions (near the bed and near the water surface).
The results showed that for a circular pier, the slots with the lengths of b and 2b, located near the water surface, had no influence on the equilibrium scour depth. It was also found that for all piers, maximum reduction in scour depth occurred for piers with a slot length yl=2b and close to the bed. Also, the efficacy of a slot for scour protection in the case of a round-nosed pier was more than a circular pier (with a diameter equal to the width of a round-nosed pier).
H. Afzalimehr, M. Heidarpour, S. H. Farshi,
Volume 10, Issue 1 (spring 2006)
Abstract
Suitable stable channel design and optimization of river geometry can reduce cost of projects. The regime theory provides the possibility of empirical and semi-empirical investigations of stable channel design in which erosion and sediment transport are in equilibrium. The objective of this research is an investigation and a comparison of the influence of uniform and non-uniform flows on the prediction of stable channel characteristics. The following empirical and semi-empirical (extremal hypothesis) equations were selected to study the effect of uniform flow: Lacey, Chital, Kondap and Garde, and Chang. Using 24 regime channels in USA, the statistical and graphical approaches were applied to compare and to evaluate the power of prediction of the selected equations. In order to investigate the effect of non-uniform flow structure on the stable channel characteristics, 21 measured velocity profiles in Gamasiab River were applied. Using the boundary-layer theory, shear velocity was computed for each profile. Accordingly, the estimated Shields parameter using the boundary-layer approach is the most effective parameter on the regime channel prediction. Simultaneous application of the non-uniform flow effect and the boundary-layer theory not only remove the risk of spurious correlation but also improve the estimation of stable channel characteristics.
