جستجوی مقالات مرتبط با کلیدواژه « Density current » در نشریات گروه « عمران »

Sedimentation in dam reservoirs is known as an effective factor of reducing dams’ life-time and reservoir capacity, disruption of dam operation, blockage and corrosion in intake structures, drain valves and other sediment venting structures and consequently increase costs of maintenance and other related damages. This proves the importance of understanding the hydraulic behavior of density current in dam reservoirs. In this study, the trend of density current moving in Lorestan Roudbar reservoir has been investigated by Mike 3 model. The aim of this study is to withdraw maximum gravity current concentration in flood time period from bottom outlets in order to decrease sedimentation in reservoirs active volume. Therefore, three-dimensional hydrodynamics simulation of density current in this reservoir has been done. The modeling scenarios have been designed to calculate how much mass can be vented from bottom outlet to save more capacity in reservoir when flood with various return time period occur. The results show that increasing the return period of the flood the sediment discharge efficiency, increases. Sediment venting efficiency is increasing with a two-year return period and reaches a maximum in floods with a 20-year return period when the maximum efficiency of sediment venting is roughly 17%.

Gravity currents, also known as density currents, or turbidity currents, are happened by the density difference between the flow and its ambient fluid. The density difference can be due to suspended particles, chemicals, soluble materials, and temperature differences. In dams reservoir ambient fluid, usually has a vertical stratification. When the gravity current arrived to ambient fluid, in the position that density of both gravity current and ambient fluid is equal the gravity current abandon the bed and flows in ambient fluid horizontally. Therefore the density current into this reservoir maybe intrude such as interflow density current.This study investigates the inter flow density current in a stratification ambient. The investigation of velocity profiles showed that the flow is self-similar and velocity fluctuations Continues maximum up to 2.5 times greater than current thickness in the lower layer. The front velocity of currents in stratified environments increases at first then sizeable decreases. It shows that stratified can limited the flow movement. In each three slope, increasing of discharge and concentration increase velocity head of density current in stratified environment. As the slope increases, the current velocity increases at the underflow stage, and in the interflow stage, the slope does not have much effect on the current velocity. Interflow Travel Time decrease in increasing of discharge and concentration. Density current in weaker stratified can travel more distance in the slope and separate latter from the bed.

Density currents are stratified flows caused by density differences between the inflow and the still water. The inflow could be moving under, through, or over the reservoir water depending on the density of each. The stratification is caused by differences in temperature, the presence of dissolved materials, or the presence of suspended solids. If the presence of suspended sediment in the inflowing water is the main or only cause of stratification, the density current is known as turbidity current. When turbidity currents enter a reservoir, they plungebeneath the fresh water traveling down the slope. If the turbidity current reaches the dam, it may be vented through the low-level outlets, thus preventing the deposition of sediment in the reservoir. There are many studies in the literature regarding the transport mechanism and characteristics of density currents

The Hydraulic Laboratory of Shiraz University, Iran, was used to conduct density current tests. The intended flume is related to hydraulic single-phase open channels, which is transformed into a density current flume by modifying and adding accessories. Flume length is 8m, width 35cm and height 60cm. A 1000 liter tank containing sedimentary turbidity current was used by a 2-inch outlet pump with a maximum passing discharge of 35 m3/hr. A flow meter was used to control the density current inlet. The flume was filled with water using a pipe connected to a 20000 liter water tank. During the experiment, the inlet valve of this tank is disconnected and a 500 liter regulating water tank connected to the pump is used to control the water table in the flume. The channel has sloping capability. The velocity measured by a 2D electromagnetic flowmeter, a product of UK’s Valeport Company with a precision of ±5 mm/sec. The flowmeter includes a data logger and sensors to transmute measured current velocity and discharge time series, etc. to the computer for further analysis. A number of siphons with a diameter of 5 mm suction tube were used to remove the sediment. The tubes connected to the siphons were located at 10 points along with the direction of current depth, through which the average current concentration was measured. An experiments were performed on 3 model samples. Model 1 is a simple, sloping flume without obstacle. In model 2, the flume has a decreased section and continuous constriction at 4.5 m, which is the passing section of the half of channel. In model 3, the flume has a local constriction in 4.5 m, and the passing current through the sides is 10 cm. All records were conducted at 5 m distance from the channel. The parameters of velocity, concentration, and thickness of density current were measured at the desired position.

In the inner region below the maximum velocity, drag on the lower boundary is the main controller parameter, and the proper expression for the velocity profile is a power law distribution as follows in stratified currents. For the whole experiment, the variation of the coefficient n is almost large and varies from 3.027 to 5.33 in different experiments. High variation of this coefficient can be due to the effect of the shear in bed on the velocity profiles, and the coefficient α ranges from 0.13 to 0.84. The value of β coefficient also ranges from 1.185 to 1.758 in all experiments. From the general comparison of all the conditions for the dimensionless velocity profiles in wall region, the best coefficient n is equal to 3.86, which shows a high correlation of 0.878, and for the jet region, the best coefficient α and β are 0.412 and 1.343, with a correlation coefficient of 0.92.Subsequent density current velocity increases and its concentration decreases by creating a constriction and this is due to the accumulation of current behind the obstacles. The constriction also causes an average increase of current velocity by 2.26 times, and the concentration of accumulated current behind the obstacles increases by 1.45 times. The Richardson criterion and the sediment trap efficiency were used in order to correlate accumulation of sediment with the hydraulic parameters of density current, the Richardson number is reduced with increasing trap efficiency, and the correlation coefficient is well established by 0.83 value in all the test modes. On average, about 29.8% from sediments of density current is accumulated behind obstacles for two models with all different states.

The dimensionless velocity profiles in the upper edge of current show greater dispersion due to the non-permanent behavior of the current in this region. The maximum current velocity in the wall region is greater than the jet region. The best coefficient n is 3.86 for dimensionless profile of velocity in wall region, which shows a high correlation of 0.878; the best coefficients α and β are 0.412 and 1.343, respectively, with a correlation coefficient of 0.92 for the jet region; the best coefficient α is 0.1 for the dimensionless profile of concentration in wall region; and the best coefficients β and γ are 1.15 and 0.8, respectively, with a correlation coefficient of 0.94 for the jet region. Also, the effect of local and continuous constriction showed that the constriction increased the velocity of density current by 2.26 times, and also increased the concentration of current sediments behind obstacles by 1.45 times, and trap efficiency rate of sediments was 29.8%.

Flood flow in rivers is often of density current type. Hence, recognizing and exploring these currents can solve some problems of sedimentation. In this study, the effect of porosity and the angle of permeable obstacles on the control and trapping of density current have been investigated in the laboratory. For this purpose, an expanded polystyrene (EPS) polymer was used with 1.135 g/L density and average diameter of 1.15 mm. The experiments were carried out with two concentrations (10 and 20%) and 5 porosity and 4 angles. The obstacles were made of palsy glass plates and two types of groove and cavity with 8.2 mm width of the groove and the diameter of the cavity. The results showed that, with an increase in porosity ratio, the amount of trapping to optimum porosity decreases and then increases. The optimal porosity of the cavity and groove is 22 % and 19%, respectively. In experiments, the cavity trapping was observed more than the groove, in the concentrations of 10.20% it was 0.13 and 0.14%, respectively. In addition, with the increase of the angle, the amount of trapping has reduced and its value was observed in the groove more than the cavity. The correlation coefficient in the grooves and cavities was 0.996 and 0.937, respectively. The major effect of obstacles, reducing velocity and slowing flow were identified as the average velocity in the cavity was 62.3% higher than the groove. Accordingly, in the same conditions, the cavity obstacles have better performance than the groove obstacles.

The present paper investigated the simultaneous effect of the density and pattern of bed roughness with obstacle on the control of a dense current head. The tests were run in a flume (length 720 cm, width 35 cm, and height 70 cm) using two roughness patterns (parallel and zigzag), and with four roughness densities (0%, 10%, 30%, and 50%). In all tests, the dense fluid was assumed to be of the salt type with a density of 20 g/lit. Two series of tests were performed using rough surfaces with and without obstacle. In total, 56 tests were conducted. The results showed that in all of the tested sloped, for the zigzag pattern, the highest control for the head of the dense current occurred at a density of 30%; while this density value was 10% for the parallel pattern. Moreover, using an obstacle in addition to roughness increased the control of the density current head. The mean control percentage in the tested slopes was 97% for the zigzag pattern at a density of 30% compared to the mode without roughness and with a zero percent slope; however this mean control percentage increased to 163% by implementing an obstacle. Further, in the parallel pattern with a density of 10%, the mean control percentage for the modes without and with an obstacle were respectively 44% and 74%.

Gravity currents, also called density currents or buoyancy currents, are flows driven by the density difference between the flow and its ambient environments. When a gravity current reaches the level of neutral buoyancy in a stratified water body, it can separate from the bed as an intrusion. Laboratory experiments are performed to investigate dense flow behavior in stratified ambient in this study. Experiments were carried out on 2.5% bed slope by 4 discharges, 1, 1.5, 2 and 2.5 l/s, and 4 concentrations 5, 10, 15 and 20 mg/l, that created current with a density of 1003.2, 1006.3, 1009.4 and 1012.5 respectively. Stratification was made by mixture of water and salt with vertical gradient. For creating density flow, silica particles with 8 diameters in average and density of 2.673 g/cm3 was used. To investigate the concentration profile, density and concentration were measured in three sections along the flume with three centimeters interval in depth using siphon mechanism. Experimental observations showed that the currents would separate from the bed and then horizontally intrude into the ambience and the density of ambient fluids below the separation point decreases. The measurements in this study indicate that the maximum concentration depth and thickness of body current decreases with increasing concentration in interflow and body current thickness stabilize with increasing discharge.