چكيده لاتين :
Background and Objectives: The combination of weir and gate, resolves some the defects of using them separately, as allows passing deposits under the gate and suspended materials over the weir, simultaneously. The cylindrical weir-gate, as one of the weir-gate types, has various advantages like higher discharge coefficient and lower energy head loss. Several researches have been conducted on the hydraulic aspects of the structure, confirm that the discharge coefficient of the combined model of cylindrical weir-gate (discharge coefficient is the most important hydraulic parameter of weir-gate design), decreases in each parts of weir and gate, compared to their function separately. In addition to effective hydraulic parameters on discharge coefficient, extensively have been studied by previous researchers, hydrodynamic phenomena such as flow separation, vortex shedding and the convergence point of shear layer passing through the both sides of the structure, 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 behavior of separation points under and above the cylinder, 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 respect to the gate opening height, 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 hydrodynamic laboratory of mechanical engineering department of Çukurova University using the PIV technique on a cylinder with a diameter of 5 cm for five different relative gate opening height between 0.1-0.5 (ratio of the gate opening height to the cylinder diameter). The employed PIV system include dual Pulse Lasers (Model 120 mJNd: Yag,), a digital camera with a resolution of 1024×1600 pixels with 30 Hz frequency and particles with the density equal to the water, with a diameter of 10 micrometers made of S-HGS were used. For each experiment, using PIV 1000 images from flow field were recorded and then processed by the program, velocity and time-averaged features of the flow field were calculated. Besides, for all the tests, the upstream water depth and entrance velocity were constant: 15 (cm) and 0.58 (m/s), respectively.
Results: The results show that for all the studied ratios of the diameter to gate opening height, 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 were 40 and 3 degrees. As the gate opening increases from 0.1D to 0.4D, the horizontal distance between the convergence points of two separated boundary layer to structure would be less. As well, when the gate .
opening height increasing, a fluctuation can be observed in vertical direction of the convergence point, the convergence point (S), in the opening range of the present study, had the vertical deviation between 0.1 to 0.15D to horizontal axis crossing the cylinder center and locates under the axel in all cases. For all the runs, the velocity profiles, recorded from a distance equal to the diameter of the structure, away upstream of the cylinder, along both the X-axis (u/U0) and (v/U0) Y, for all the gate openings, are strongly affected by hydrodynamic factors. Above the weir-gate while the angel increase from 0 to 90 degrees, (toward the crest) the ratio of maximum velocity to the entrance velocity rises, so that, at the crest level an increasing about 27% is recorded, compared to the zero point could be observed. In the higher levels, data exhibits a fluctuation and this situation, in the gate opening equal to 0.5D, causes that the value of the ratio of maximum velocity to entrance velocity near the separation point, reaches to about 17%, in comparison with those obtained at the crest level.
Conclusion: The results indicate that by changing the gate opening height, flow separation point is displaced and reduce in the length of the wake tail in horizontal direction, as well as elevation fluctuations at the point of convergence of the shear layer can be observed. In the downstream, up to a distance equal to the diameter of the structure, velocity profile is greatly influenced by the presence of weir-gate and in the distance; the flow is transition, that it is very important in design to protect the bed.
