Introduction:
Piano key weirs have rectangular, triangular, and trapezoidal shapes in terms of plan and are presented in four types A, B, C, and D. Piano key weirs are the evolution of nonlinear weirs with high efficiency; therefore, it is essential to study the flow energy loss and the methods to increase it.  In the present study the first time, various numbers of flow splitters were used in the trapezoidal piano key weir of type B to increase the flow energy loss. Also, two weirs with heights of 0.20 and 0.18 m were used. The flow splitters were cylindrical and are six, four, and two in each cycle.
Methods:
The experiments were conducted in a channel with 10 m length, 0.8 m width, and 1 m height. The slope of the channel was zero. Water temperature varied between 8 and 13°C. The flow was recharged into the tank by a pump, a 10,000 m³ underground tank, and a monitor. The pump had 0.01% error. Because the velocity profiles coincided at 3.5, 0.4, and 4.5 m from the beginning of the channel coincide, a weir was installed at a distance of 5.5 m from the start of the channel. The flow rates varied between 0.02 and 0.05 m³/s. Three ultrasonic sensors were used to measure the flow depth. The first sensor was installed at a distance of 2P upstream of the weir relative to its center, and the second sensor was positioned 8P downstream of the weir to record the flow depth. Two type B trapezoidal piano key weirs were used with constant geometry but different heights of 0.20 and 0.18 m. The width of the weir inlet keys (W_i) and outlet keys (W_o) was 0.215 m and 0.075 m respectively. The length of the weir side walls (B*) and the length of the upstream overhanging edges of the weirs (B_i) was 0.40 m and 0.15 m, respectively. The length of the weir crest (L) was 3.27 m, and the thickness of the weir (Ts) was 0.01 m.
Results:
The flow splitters act as a barrier and divert the upstream flow; increasing the water level upstream of the weir. The splitters also divide the water flow and enhance aeration at the inlet keys. By the results of this research, as the number of blades increased, the water flow became more distributed and more aeration occurred at the keys. In the last row of blades (downstream of the weir), the degree of flow separation was more significant. In the weir with a greater height, the energy loss was higher and mixing of the flow near the weir toe was more intense; therefore, the flow was transferred downstream with a slower velocity. For weirs without flow splitters, the discharge coefficient in the 0.20 m high weir was obtained about 5.60% lower than that of the 0.18 m high weir. Also, the discharge coefficient in the weir with six, four, and two flow splitters per cycle was about 6.80, 1.97, and 0.50% lower, respectively, compared with the 0.20 m height weir without blades. In the weir with a height of 0.20 m compared to the weir with a height of 0.18 m, both without flow splitters, the flow energy loss was reduced by about 8.52%. Also, in weirs with a height of 0.20 meters with six, four, and two flow splitters, the flow energy loss was about 5.06, 3.07, and 1.45 percent higher, respectively, comparing the weirs with the same height and without flow splitters.
 Conclusion:
Investigation of flow energy loss in piano key weirs is of interest and importance due to their high efficiency in the flow passage. In the present study, flow energy loss was researched in a trapezoidal piano key weir type B, with and without varying numbers of flow splitters. Dimensional analysis was also used to extend the results to other weirs and different types of piano key weirs in channels. The key findings of the current study are as follows: (1) Increasing the number of flow splitters (thereby reducing the effective length of the weir crest) led to a decrease in the discharge coefficient and increase in the flow energy loss. (2) Increasing the height of the weir resulted in a higher discharge coefficient, and reduced the flow energy loss. (3) The presence of splitters shifted the flow farther from the weir toe and reduced the specific energy downstream. (4) Increasing values of Hu/P and Le/P, the flow energy loss decreased. (5) A relationship was developed to calculate the flow energy loss with a correlation coefficient of 99.91%