Simulation and experiment of density current on venting efficiency and movement mechanism
Date Issued
2013
Date
2013
Author(s)
Lee, Fong-Zuo
Abstract
When turbidity current flows into a reservoir, the main cause of sedimentation problem is the sediment transport ability. The sediment transport ability decays by means of velocity when turbidity current flows into a reservoir. After that, the inflow sediment may deposit on the bottom of the reservoir or develops to the density current. When density current develops and moves to the downstream, its flow mechanism is affected by dam. Therefore, this study focus on the behavior and character of density current during density current is affected by different dam slopes. The experimental results present the density current climbing velocity and the maximum climbing height are directly proportional to the dam slopes. But, the maximum density current climbing height and its thickness are inverse proportional to the inflow concentration. On the other hand, the positive surge is mainly affected by Richardson number, the relative distance traveled by positive surge and dam slopes. Based on this research, the experimental formulas can utilize to compute positive surge velocity under variant slopes of the dam. It can be helpfully to estimate the timing of reservoir turning into turbidity.
A laboratory study was conducted using a flume which combined CFX, fluid simulation software with the experimental results to simulate the density urrent movement. Algebraic Slip Model (ASM) scheme of CFX was adopted to simulate the hydraulic properties of density current including the head velocity, concentration and velocity distribution of body movement, climbing velocity and climbing height along the dam slope, thickness of density current and positive surge velocity. Since particle fall velocity was the major physical parameter in ASM scheme. It was taken into account for sediment transport between sediment and clear water. The kaolin and saline water were used as density current fluid materials. The results of the experiment were employed for numerical calibration and verification. The CFX was then employed to simulate the movement of density current movement with downstream shape of 900 and 450. The calibration and verification results showed that relative deviation (5%) in concentration and velocity distribution was satisfied. The simulation results indicate that relative deviation in climbing velocity and climbing height along the slope were 6.98% and 5.94%, respectively. Other hydraulic properties mentioned above also had less than 6% relative deviations. These experimental values agree well with the results of numerical simulation. In the result, this numerical tool is suitable for simulation the movement of the density current.
This study based on empirical formula, theoretical derivation and field observation to investigate plunge point variation in Shihmen reservoir. A successful estimation of plunge area was been used to analysis the possibility position of plunge point. Based on the investigation results, the plunge point is approximated at . In addition, the plunge area have 2 section difference due to the inflow sediment concentration is from regression formula and field measurement. According to the estimation of , there are 4 times difference and 1.5 times difference before peak discharge and recession duration, respectively. Therefore, the is suggested to estimate plunge point when inflow sediment concentration is used from regression formula.
The research results of Tsengwen reservoir indicate that proposed equation is relatively accuracy to estimate venting efficiency of power plant intake and permanent river outlet. The simulated hydrograph of venting efficiency using modified equation is much better than others. And, the venting coefficient K of modified equation is calibrated to 0.2 that was using Typhoon event in TsengWen reservoir. Besides, although lower structures can drainage higher sediment concentration and reach higher venting efficiency, but, large sediment would be carried to near the dam to increase deposition elevation. Over a period of time, deposition elevation would higher than intake elevation and affect its operation. Therefore, if bypass tunnel and desiltation passage can be established at upstream the deposition delta can be controlled and deposition elevation of dam site also could be maintained. Moreover, if drainage facility could be set more upstream of reservoir, coarse material could be flushing out by bypass tunnel and fine sediment could be vented by desiltation passage. In addition, such planed concept can prevent reservoir sedimentation and keep more water resources.
Subjects
異重流
理查遜數
壩體坡度
CFX
ASM
正湧波
潛入點
密度福祿數
排砂效率
減淤通道
Type
thesis
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