In this paper hydraulics and flow structure over the rectangular piano key weirs with different heights have been studied experimentally and numerically and effects of different fillet shapes on hydraulic performance are investigated. Experiments are conducted in a one meter width flume. Models of the tested weirs are made from PVC Plates in 3 different heights and with the same L/W ratios equal to 5. Discharge coefficient curve for wide range of heads over height ratio for 3 different weir heights are determined and effect of triangular, half round and oval fillets on increasing of discharge coefficient are investigated. In this research, the chosen range for ratio of head over height is in good agreement with ratio which has been used to design of prototype weirs. Large triangular and oval shaped fillets have significant effects on improvement of performance of the piano key weirs. Main effect of fillets is more uniformly distribution of the flow streamlines over the downstream part of the side crest. In second part of this paper, one and half-key piano key weirs with oval noses and without any nose (net) have been numerically modeled using Flow-3D model. Discharge coefficients of one of the numerically simulated rectangular models is compared with derived discharge coefficient curve from physical model. Convergence of inflow and outflow of numerical model has been controlled. Satisfactory correspondence presents between the experimental and numerical studies. Discharge distribution over the crest of the normal PKW and the weir equipped with oval fillet are compared. Result showed that due to uniformly distribution of the streamlines, the fillet notably increases flow rate at the downstream part of the side crest. Results of numerical simulations are exported to Tecplot software, in order to visualize the flow streamlines at different parts of the studied weirs. The weir with oval fillet affects the flow streamlines in three positions: the near bead streamlines of the weir with oval fillet, have less lateral diversion when they reach beneath the overhang of the outlet keys. In other word, streamlines pass this region more smoothly rather than normal PKW. As a result, the fillet decreases the local head loss, when the flow enters the inlet keys. Streamlines of the mid depth level show less contraction at the entrance and middle of the inlet keys. Consequently, lower velocity of the flow along the inlet keys, helps to more evacuation of the flow from side crests. Finally, streamlines release more uniformly from downstream part of the side crest. These phenomena results low submergence level at the middle of the outlet keys. The outlet keys are the brake of nonlinear weirs. By decreasing the submergence level of the outlet keys, flow from the side crests discharge more freely from inlet to the outlet keys.
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