عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Weirs are used in different ways to control water levels and flow measurement. One of the most effective and economical way to increase the effective length of weir is using labyrinth spillways. The direction of flow of the labyrinth spillways is not perpendicular to the edge of the weir and it is oblique. On the upstream and downstream of the weir, the flow direction of the symmetry axis of labyrinth weirs is parallel. This phenomenon is more common in the downstream overflow because of flow stacking and, thus efficiency of weir is reduced (Hay the Taylor, 1970). Since, study about the using of upstream guide vanes in the direct channel has not been yet published, in this paper, flow guide vanes were used in the upstream of the triangular labyrinth spillway with vertex angles of 45° or 90°. The main purpose is guiding flow on weir wall perpendicularly and study of discharge coefficient.
The experiments were conducted in a canal with a length of 7 m, width of 0.32 m and height of 0.36m. The two triangular labyrinth spillways with vertex angle of 45° and 90° were used. The models were made of galvanized sheet with 1 mm thickness and smooth overflow threshold edge. According to previous researchers (w/P?2.5, in this case P is weir height and w is canal width), the height of the weir was considered equal to 12 cm. A direct spillway with the same width was used to calculate the equivalent discharge. Guide vanes were made from galvanized steel plates. The height of guide vanes was 12 cm and their widths were 2, 3, 4 and 5 cm. The width of guide vanes was a coefficient of the length of one side spillway. Spillways were installed at a distance of 4.5 meters from the beginning of the canal. The reason of choosing this distance is that the flow completely developed from 3.5 to 4.5 meters from the beginning of the canal. Height and velocity of upstream water flow were taken at 5 discharges. The upstream depth of water, above the weir and longitudinal profile of the water surface was measured by a depth meter. Acoustic Doppler Velocimeter was used to measure the flow velocity. The tests were conducted in 3 groups: Group I at vertex angle of 45 or 90°, the width of guide vanes were 2, 3, 4 or 5 cm at a direct distance of 8 cm from the weir with 45° wall angle. Group II at vertex angle of 45 or 90°, wall angle guide vanes with weir of 35, 45, 65 or 90 degrees at direct distance of 8 cm from the weir. Group III at vertex angle of 90°, direct distance of 8, 22 or 33 cm from the center.
In this study, Group I on the weir at vertex angle of 90 degrees and direct distance of 8 cm from the upstream weir does not positive effect on overflow efficiency, because efficiency in all tests were less than simple triangular labyrinth spillway with vertex angle of 90 degrees. The comparison shows that the highest efficiency occurred on weir with vertex angle of 45 degrees with guide vane width of 2 cm and on weir with vertex angle of 90 degrees without guide vane. In group II at the weir with vertex angle of 90 degrees, existence of guide vanes at direct distance of 8 cm from the weir with all angles of 35, 45, 65 and 90 degrees is reduced efficiency of flow. It is determined by comparing the curve of models, the maximum of efficiency occurred with vertex angle of 45 degrees and wall angle of 35 degrees, and on weir with vertex angle of 90 degrees and wall angle of 35 degrees. In group III, efficiency in all tests were less than simple triangular labyrinth spillway with vertex angle of 90 degrees and adistance of 8, 22 and 33 cm. In triangular labyrinth spillway with small vertex angle, using guide vanes could affect on vertical velocity component and this component could be increased. So at the same time, the flow is passed more quickly from weir crest, discharge is increased then for the same height water, the discharge coefficient increased. It was found that the triangular labyrinth weir with less vertex angle had very disturbance flow with increasing of discharge. These vanes could be guided flow perpendicularly on wall weir and then efficiency was improved.