كليدواژه :
كنترل بدنه جريان غليظ , مانع هاي متوالي , پروفيل سرعت و پروفيل غلظت , مشخصات و كنترل بدنه جريان غليظ
چكيده لاتين :
1. Introduction Density current is the most important factor of sediment transport in the dam reservoirs. Density current is the current which is caused by density difference between two fluids on the acceleration of gravity. The effective acceleration of gravity acting on the flow, which is known as driving force for density flow is expressed as follows:
(1)
𝑔′=𝑔(𝜌𝑡−𝜌𝑎)𝜌𝑎 Where, 𝑔 is the acceleration of gravity as (𝑚𝑠2⁄), 𝑔′is the reduced acceleration of gravity as (𝑚𝑠2⁄), 𝜌𝑎 and 𝜌𝑡 are the density of ambient fluid and density fluid respectively as (𝑘𝑔𝑚3⁄). Fig. 1 shows the schematic of density current. Fig. 1. A schematic of density current Installing a single obstacle to the density current, Long (1970) concluded that when the obstacle height is approximately twice the height of the body of density current, it is fully controlled. Asghari Pari et al. (2016) studied the effect of single obstacle height on controlling the density current for two types of subcritical and supercritical density currents. Ohey et al. (2010) investigated the impact of inclined plane on the characteristics of the density current. As mentioned before, Long’s studies (1970) showed that the density current will be fully controlled if we install an obstacle twice the height of the body of the density current. However, it should be noted that in addition to economic issues for building an obstacle to the density current, which is twice the height of it, it may then lead to the formation of stagnant flow behind it and increases the risk of entraining of large volume of sediments to dam if the obstacle breaks. Therefore, this study employs higher number of obstacles with less height. This study aims to examine the effect of using consecutive obstacles on the general
Seyed Zaniyar Nikkhah et al. / J. Civ. Env. Eng. 48 (2018)
form of velocity profiles as well as the concentration of the density current body and the impact of these obstacles on the controlling the body of density current. 2. Methodology The experiments were carried out in a flume with a length of 780 cm and a height of 70 cm with adjustable bed slope of -1 to 3.8%. The flume had a mixing tank with the capacity of 1000 litres and a head tank to maintain a head flow constant (Fig. 2.). Fig. 2. Schematic of the flume used in the laboratory This study investigated four different modes for the installation of 3 consecutive obstacles in order to control the density current. Three height proportions of obstacles were determined according to the average body height of density current of the control samples, namely ℎ𝑟=0.5,0.75,1 ℎ𝑟=ℎ𝑚ℎ where ℎm is the obstacle height and h is the average body height of density current of the control sample). The average value of the density current body height of the control samples was determined to be 5 cm. Therefore, the values of obstacle heights used were determined to be 2.5, 3.75 and 5 cm. The heights were considered equal in the 3 modes of installation, and in the fourth mode, the three obstacles were installed in ascending order of the values mentioned earlier. The obstacles were placed at the constant distance of 50 cm. During the experiments, the density current discharge was considered constant and equal to 1𝑙𝑠⁄. Moreover, the saline density current used in the study has a constant density value of 20 𝑔𝑟𝑙𝑖𝑡⁄ (𝜌𝑡=1013.5 𝑘𝑔𝑚3⁄). The experiments were carried out using 3 bed slopes of 0%, and 1.5%, 2.5%. Moreover, 3 control experiments were carried out without considering the obstacles under the aforementioned conditions. A total of 15 series of experiments were carried out. 3. Results and discussion Fig. 3 provides, as an example, the velocity profiles before and after the consecutive obstacles installed using the bed slope of 0%. 𝑢𝑚𝑎𝑥𝑢, ℎ𝑡ℎ and ℎ𝑚𝑎𝑥ℎ are three parameters to indicate the general form of velocity profile. The results showed that with an increase in the heights of obstacles from ℎ𝑟=0.5 to ℎ𝑟=1, the parameter 𝑢𝑚𝑎𝑥𝑢 experiences the minimum rate of change which is about 4%, which is decreasing, and the parameter ℎ𝑚𝑎𝑥ℎ experiences the maximum rates of change which 225%, which is increasing. Fig. 4 provides, as an example, the concentration profiles before and after the consecutive obstacles installed using the bed slope of 0%. 𝑐𝑏𝐶𝑠 and 𝑐𝑏𝐶𝑚𝑎𝑥 are two parameters to indicate the general form of concentration profile. The rate of change for the parameter 𝑐𝑏𝐶𝑠 is about 142%, which is increasing, and for the parameter 𝑐𝑏𝐶𝑚𝑎𝑥 is about 229%, which is also increasing. The maximum percentage value of the discharge control of density current body is associated with hr=1using a bed slope of 0%, and the minimum percentage value is associated with hr=0.5 using a bed slope of 2.5% which are respectively 63% and 22%. With an increase in the heights of consecutive obstacles from hr=0.5 to hr=1, the percentage values of the discharge control of density current body increased by 75%, 76%, and 105% respectively using the bed slopes of 0%, and 1.5%, 2.5%. In case of using ascending order of obstacles, the percentage values of the discharge control of density current body was obtained 50, 44 and 36% respectively using the bed slopes of 0%, and1.5%, 2.5%.
Seyed Zaniyar Nikkhah et al. / J. Civ. Env. Eng. 48 (2018)
4. Conclusions - Installation of consecutive obstacles has significant effect on the velocity and concentration profiles. - Using consecutive obstacles in the mode of ℎ𝑟 =1 has the highest impact on the parameter of ℎ𝑚𝑎𝑥ℎ
of the velocity profile by the rate of +221% compared with the control mode. - Using consecutive obstacles in the mode of ℎ𝑟 =1 has a significant impact on the parameters of 𝑐𝑏𝐶𝑠
and 𝑐𝑏𝐶𝑚𝑎𝑥
of concentration profile by the rate of +150 and +250 percent compared with the control mode. - The impact of consecutive obstacles on controlling of density current body is reduced by increasing the bed slope.
