Experimental and Numerical study of piano key weir with upstream nose and semicircular section at the outlet

Document Type : Research Article


1 M.Sc, Faculty of Civil and Environmental Engineering, Tarbiat Modares University, Tehran, Iran

2 Tarbiat Modarres


The newest Type of nonlinear weir is piano key weir. Initial studies on this weir have shown that this weir significantly increases the discharge, has a relatively simple structure, and is an economic structure. Various studies have been to investigate the factors affecting the discharge and optimization of this Type of weir. The purpose of this study was to modify the Type A piano key weir to better perform this Type of weir in discharge and Ventilation at the outlet. In this research, by applying changes such as a triangular nose at the entrance and a semicircular crest in the outlet of the A-type piano key weir, its performance has been investigated numerically by Flow-3D software. In the numerical simulation, the RNG turbulence model used. Validate the numerical model by the results of an experimental study on a Type A piano key weir in a laboratory flume with a length of 10 meters, a width of 0.75 meters, and a height of 0.8 meters used. The laboratory channel wall made of glass and its floor is metal. In the numerical simulation of the four piano key weir models, the first type PKW A has the same physical model characteristics and the modified model concurrently with changes such as upstream nose and semicircular cross-section (PKW B +). The third Type was added by removing the input overhang and using a semicircular cross-section at the modeled outlet (PKW C), and the fourth Type was added by the nose only upstream of the Type A piano key weir (PKW D). In the numerical model, of the three meshes, one in the upstream weir with dimensions of 2×0/4×0/6, the second in the weir site with dimensions of 0/5×0/4×0/6 and the third in the downstream weir dimension 1×0/4×0/6 was used. At the inlet boundary of block 1, the volume flowrate boundary condition, in the outlet boundary of block 3, outflow boundary condition, at the upper boundaries of the field, left and right of the inlet flow, and between the blocks, the symmetry boundary condition, and at the bottom boundary, the wall boundary condition.
The simulation time is set to 15 seconds, according to the software, which states that the current is stable after about 13.5 seconds.
The performance of PKW C and PKW D weir in the discharge coefficient is better than PKW A weir. One of the most important causes of PKW C weir coefficient is the low total length compared to PKW A as well as no downstream Overhang. Decreasing the weir length due to the general equation of weirs and the constant of all its parameters increases the discharge coefficient. Also, the lack of Downstream overhang of the weir reduces downstream turbulence and better directs the outflow. As a result, we see an increase in the discharge coefficient in PKW C weir over PKW A weir. Also, the dewatering coefficient by creating a nose upstream of the weir (PKW D) has increased, mainly due to better conduction of the current to the weir input key. However, with the increase of the head, due to more interference of the flow layers, the effect of this separation is reduced and results in a decrease in the discharge coefficient compared to the lower heads.
Each of the changes in Type A piano key weir on the discharge coefficient individually compared to the simple piano key weir mode improved the discharge coefficient on average by 5 to 10%. Also, the simultaneous effect of these two changes, based on the results of the piano key weir, improved the discharge by about 12%. By tec-plot software, the flow lines in PKW A and PKW B + weirs are plotted and compared. Flow lines are drawn in the PKW A weir indicate a 90-degree deviation of the flow lines after collision weir and scrolling further to pass through the weir input keys. Also, the focusing of the flow lines at a part of the lateral length (Bb) interferes with the flow layers, thereby reducing the flow velocity within that range. But by creating a nose upstream of the weir, the flow lines are redirected to the weir inlet prior to the weir by the nose. Also, no overhang in the PKW B + weir reduced the formation of vortices downstream of the weir, which eventually reduced the turbulence. Another benefit of non-evacuation is the reduction of the volume of air trapped in the weir outlet key, which increases the weir discharge.

Piano key weir (PKW), Numerical, Coefficient Discharge, Aeration, Triangular nose


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