عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Stepped spillways are technically and economically regarded as one of the most efficient options for energy dissipation of spillway flows. A remarkable amount of the flow energy is dissipated due to the hydraulic resistance of rough elements or those steps. Therefore, the energy dissipation obviates the need to build an energy dissipation system at the end of the downstream and reduces its size to a large extent. Considering the mentioned point and the required technology to use RCC in building such spillways, it is indispensable to thoroughly examine the hydraulic processes and the flows passing through them. The flows passing through the stepped spillways are divided into three types of nappe, transition, and skimming. Most studies concerned with stepped spillways have focused on nappe and skimming flows, while transition flows have not been fully addressed. Moreover, most of the stepped spillways’ designs have been based on nappe and skimming flows types. Therefore, the present study employed laboratory methods to examine the flow formation in transition mode in order to determine the upper and lower limit of the transition flow regime, and finally, provides more space for studying its characteristics. The flow regimes’ type depends on the step discharge and geometrical shape. Due to variations in the depth and velocity of flow, air concentration and energy dissipation in the three different regimes, it is essential to evaluate the flow in the regimes. Thus, the flow regimes’ onset has been the subject of many laboratory studies. The previous experiments and studies have shown that the nappe flow onset is a function of the height and length of step and also, the critical depth. This research was conducted in Water Research Center of Power Administration in order to determine the range of transition flow regime and hydraulic parameters of various flow regimes, including dynamic pressure at the steps’ bottom and the depth of flow passing over the steps as a flow profile. To do so, spillways’ models of Siahbisheh dams were used, which were constructed with the scale of 1:15 and 1:20.
The flow will be fully developed, after passing through the spillway crest and the aeration inception point. The upper and lower limits of the transition flow regime were determined by letting different discharges pass over the spillways. To detect the nappe flows’ entrance into the transition flows, i.e. the lower limit of transition flow regime, the inception of water spray can be used. The water spray level will be higher in sloped spillways. Moreover, the flow jet has more fluctuations in the transition flow regimes of pool; in other words, it will be in immersion mode. In addition, some flow rotation works can be noticed. The criterion for determining the threshold for the onset of skimming flow, i.e. the upper limit of transition flow, is the flow slipping from the edge of one step onto the next. It is worth mentioning that existence or lack of air holes does not play any role in proving this. This is in contrast to Chanson’s theory; however, it is in line with Chamani and Rajaratnam’s view. The onset of flow rotation in the pool below the step was also taken into account; these rotations are not still complete enough to enter the skimming flow section. After determining the flow regimes, the dynamic pressure and profile of passing flow were measured in transition regimes. In order to measure the pressure exerted on the steps’ bottom, 3 piezometers at (y/l), which are equal to 0.14, 0.45, and 0.71, were installed on four spillways. Transducer system adapters, computer systems and an information processing software were used in order to measure the dynamic pressure moment-by-moment. The pressure sensors used in the experiment were 0.150 kg/?Cm?^2 (150 mbar). The pressure fluctuations in each piezometer were recorded during 30 seconds with the frequency of 100 hertz. To measure the flow profile, without accounting for water spray, a point gauge was used.
The results of the study revealed that the increase in the spillway slope reduces the area for occurrence of transition flow regime. Moreover, as the ratio of h/l increase the ratio of y_c/h decreases, which shows faster formation of a skimming flow. In the nappe flow regime, it is noticed that pressure increases from the step bottom to the edge, where it moves with a slight slope to the middle of the step and then, continues with a steep slope. In the nappe flow regime, increasing the spillway slope, increases the pressure from the step bottom toward the edge. Moreover, the maximum and minimum pressures occur near the step edge. With respect to the transition flow regime, the pressure remains almost constant from the step bottom up to y/l=0.4 and then continues with a steep slope toward the step edge. In addition, in the transition flow regime, the maximum and minimum pressures occur on the step edge. In the skimming flow regime, the mean pressure increases from the step bottom toward theedge. This increasing trend has a direct influence on the spillway slope. The maximum pressure occurs in the middle of the step and then decreases with a slight slope. Similarly, the minimum pressure occurs in the middle of the step and then, increases toward the step edge. Finally, in the skimming flow regime, increasing the spillway slope is followed by a decrease in the maximum pressure.