Experimental study of saline density control by using cylindrical obstacles

In general, the Density current is motion of a fluid with a density into relatively stationary fluid with different density. The difference in density may be caused by the suspended materials, dissolved materials, temperature, or a combination of them. In fact, since these currents occur because difference in specific weight or the effects of gravity on density, these may called gravity current too. When the density of entry fluid is more than the density of stationary fluid, then the dense fluid will move beneath the stationary fluid and called underflow density current. In this paper, the effect of cylindrical obstacles on the controlling of density current is experimentally studied. Laboratory equipment consists of two main parts: the flume and mixing system. Experiments were performed in a flume with the length of 7.8 m, 0.34m in width and 0.7m in height and the bed slope was variable. The flume was divided into two parts by a sluice gate. The water level in the flume was fixed by a 66 cm spillway installed at the end of the flume. The flume was filled with clear water before starting the experiments. The level of dense fluid and clear water in both sides of the gate are balanced, the gate was opened 5cm suddenly. The total number of 27 experiments with the bed slope of 0.5, 1.25 and 2 percent and with concentrations of 10, 15 and 20 g/L were performed. Discharge of entry dense fluid was controlled almost 1L/s by using an electromagnetic flow meter during the experiments. Cylindrical obstacles were installed on 4 meter of the flume- 1.5m from the sluice gate to 5.5m from the sluice gate. The obstacles were made from P.V.C material with a diameter equal to 1 cm and the height was 30 cm. The arrangement of the obstacles was staggering. The obstacle height was more than the density current head in all experiments. At first, nine experiments, the distance between two obstacles within a row (perpendicular to current direction) and the distance between two consecutive rows of obstacles were 8 and 8 cm, respectively. In second nine experiments, these distances were 4 and 8 cm, respectively. At last nine experiments, these distances were 4 and 4 cm, respectively. Inside the obstacles, four sections named 1, 2, 3 and 4 at spacing of 0.75, 1.5, 2.25 and 3 meters from the first row of obstacle are selected to measure the hydraulic parameters. The distribution of concentration in the head of density current was measured directly by using four siphons in sections 1, 2, 3 and 4. Also, the velocity of the head was measured in mentioned sections. Wake space and vortex caused around obstacles can effect on hydraulic of density currents. Impact rate and the flow lines encounter will increase at the space between two obstacles and lead to increase friction and resistive force and also increasing mixing rate. The best longitudinal distance to attach next obstacles is wake space length. In fact, by attaching of the next obstacle at end of wake space the possibility of parallel and uniform flow lines is reduced very much and flow lines is encounter together during the experiment. Impact and encounter of flow lines are important to increase the rate of mixing. In this research, for increasing the obstacle effect, they are attached to the bed as stagger form.The results showed the head velocity and height decreases in the beds with obstacles in comparison with the bed without obstacles. When the number of obstacles are increased, the reduction rate of head height and velocity goes up. The discharge of the head is reduced along the flume in the beds with obstacles. This reduction rate is more intensified as the number of obstacles increases. Mixing rate of head increase in the beds with obstacles in comparison with the bed without obstacles. Reduction of height and velocity in the bed with arrangement 4×4 of obstacles is more than bed with arrangement 4×8. So in the bed with arrangement 4×8 of obstacles is more than a bed with arrangement 8×8. Reduction of discharge of head in bed with arrangement 4×4 of obstacles is more than them. Reduction rate of height, velocity, discharge and mixing at all arrangements decrease by increasing slope of bed and density.