The effect of gate opening differences on separation point, wake size and velocity distribution of submerged cylindrical weir-gate

Background and objectives:
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.
Materials and methods:
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 conclusion:
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.