جستجوی مقالات مرتبط با کلیدواژه « separation point » در نشریات گروه « آب و خاک »

One of the most important problems arising after the construction of a dam is sedimentation in the dam’s reservoir. In turn, one of the phenomenon that may affect the sedimentation is density currents. A density current is, indeed, the movement of a fluid through another one that has a different density. If there was saline stratification in the vertically downward direction in the reservoir of the static fluid, density current occurs as interflow. Kao (1977) derived the flow diffusion velocity along a common surface between two homogeneous fluids based on Bernoulli's theory. Likewsie, Ungarish (2012) and Sahuri et al. (2015) conducted studies on the interflow density current.

The experiments were conducted in a flume of 8 meters long, 34 centimeters wide and 70 centimeters high. To that end, a solution of water and salt was used to form the ambient fluid with saline stratification and a mixture of water and sediment particles as the dense fluid. 48 experiments were, then, carried out with 4 discharges (1, 1.5, 2, and 2.5 l/s), 3 slopes (2.5, 3.25, and 4 percent) and 4 concentrations (5, 10, 15, and 20 g/l). The motion of the interflow density current in a layered fluid is like that the flow entering the reservoir first expands as an underflow current and after moving a distance, it enters through saline layers of the fluid with saline stratified layers. The current, then, separates itself from the bed at a location in the reservoir where the density of the dense flow equals to that of the layered one. This way, the current finds the suitable density in its environment and moves in a horizontal layer (Fig.1).

Underflow density currents traveling through density-stratified fluids begin to separate from the bed as they reach areas of similar densities, after which they continue their path as interflows through the surrounding ambient fluid. This point, which is referred to as “separation point” of density currents, acts as the boundary between the underflow and interflow density currents. A density current within a stratified environment is known as intrusive gravity currents (IGC), which travel horizontally at a roughly constant velocity U within the stratified layer after propagation (Nokeset al., 2008). Regarding underflow density currents, efforts have been made by many researchers e.g. Singh, and Shah (1971), Lee, and Yu (1997) and Farrell, and Stefan (1986) in the past decades to determine the location of the plunge point of underflow density currents (which is similar to the separation point in interflow currents) using experimental and theoretical methods based on the Densimetric Froude Number at the plunge point Fp. Although studies have been conducted on interflow density currents, including Kao (1977), Wells, and Nadarajah (2008), An, and Julien (2014) and Zhang et al. (2015). Not any research has been found in the literatures about the height of density current at the point of separation from the bed. Hence, in addition to examining the height variations in density currents at the separation point with respect to changes in hydraulic parameters such as flow rate, density of the flow at the inlet, as well as the bed slope, the present study attempted to derive a relation for prediction of density current height at the point of separation from the bed under different conditions.

The combination of weir and gate creates a new structure, resolve some the defects of using them separately, such as the simultaneous flow pass of deposits under gate and suspended material over the weir, also benefits the ability of vertically movement and making different gate openings according to the changes of the discharge, control the water level more accurate and adjust constant head of water for the side channel. Likewise, the cylindrical weir-gate, as one the weir-gate’s type, has various advantages like higher discharge coefficient and lower energy loss. Researchers have been conducted on the hydraulic aspects of this structure state that the discharge coefficient of combined model of cylindrical weir-gate (discharge coefficient is the most important hydraulic parameter of designing the weir-gates), decreases in each parts of weir and gate, regarding to their separated function. On the other hand, in addition to effective hydraulic parameters on discharge coefficient, extensively have been studied by previous researchers, hydrodynamic phenomena such as flow separation, vortex shedding, the convergence point of shear layer passing through the both sides of the structure and etc., resulting from the assumption of hydrostatic pressure on the structure body, play a crucial role in discharge coefficient, discharge rate and other flow characteristics. Therefore, in order to investigate the changes of separation points under and above the structure, the convergence point of boundary layer of the both sides, velocity distribution above and at the downstream of the structure, as well as the size of the wake with gate opening, a series of runs using the technique of particle imaging velocimeter (PIV) were carried out, and the results were analyzed.
The experiments were conducted at the hydrodynamics laboratory of mechanical engineering department of Çukurova University using the PIV technique on a cylinder with a diameter of 50 mm for five different relative gate opening between 0.1-0.5 (ratios of the gate opening to the cylinder diameter). Besides, for all the tests, the upstream water depth and entrance velocity were constant:150 (mm) and 0.58 (m/s) respectively.
Results and The results show for all ratios of the diameter to gate opening, the separation point of flow from the body of the structure was different in function as gate and weir parts and the separation occurs earlier as gate, so that, at the test range, the maximum and minimum of the separation angle’s deviation of the weir and gate were40 and 3 degrees. As the gate opening increases from 0.1D to 0.4D, The horizontal distance between the convergence point of two separated boundary layer to structure would be less. As well, the gate opening increasing cause fluctuations in vertical direction of the convergence point, the convergence point (S), in the opening range of the current study, had the vertical deviation between 0.1 to 0.15 D to horizontal axis crossing the cylinder center and locates under the axel in all cases. The velocity profile along the X-axis (u/Uo) and (v/U0) Y for all the gate openings to a distance equal to the diameter of the structure behind of it, is strongly affected by hydrodynamic factors. Above the weir-gate while the angel increase from 0 to 90 degrees, the ratio of maximum velocity to the entrance velocity rises, so that, at the crest level and increasing about 27 percent compared to the zero point could be observed, but in the higher levels, exhibits fluctuation and this fluctuations in the gate opening equal to 0.5D causes the dimensionless value of maximum velocity to entrance velocity near the separation point is about 17 percent compared to values obtained the crest level.