Document Type : Original Article
Authors
1 Former master's student, water and hydraulic engineering, Tarbiat Modares University-Tehran
2 Professor of Hydraulics, Faculty of Civil and Environmental Engineering, Tarbiat Modares University, Tehran
Abstract
Extended Abstract
Introduction
Piano key weirs (PKWs) are a kind of non-linear weirs, initially introduced by Hydrocoop in France (Blanc and Lempérière, 2001). PKWs include the inlet and outlet keys as well as the inclined bottom. Due to their high compatibility with the site and their economic and hydraulic performance, PKWs have been used in different countries including North America, Europe, Asia, and Australia (e.g., Malarce Dam, France, Lake Peachtree Dam, GA, USA, Dakmi 2 and Van Phong Barrage, Vietnam). There is a wide range of studies addressing the discharge coefficient of PKWs, but the energy dissipation of rectangular and trapezoidal PKWs has not been compared so far. Hence, in this paper the energy dissipation of these weirs are evaluated and compared.
Methodology
Tests were conducted in the hydraulic laboratory of Tarbiat Modares University, Tehran to assess the energy dissipation and flow properties downstream of rectangular and trapezoidal PKWs. Tests were performed using a flume with 10 m length, 0.75 m width, and 0.9 m height (Fig. 1). The water was provided by an underground sump. PK weir was installed and sealed at 4 m away from the flume inlet, so the minimum flow turbulence was achieved. The discharge was adjusted by changing the speed of the pumps using a control panel. The upstream and downstream flow depths were measured at (Crookston, 2010) and (Eslinger and Crookston, 2010) away from the weir upstream and downstream, respectively, using digital point gage with an accuracy of ±0.1 mm. The weir specifications are listed in Table 1. Experiments were conducted for various discharges and approach flow depths.
Results
The flow field upstream of PKWs was almost uniform and no turbulence was observed on the water surface. The flow deviated near the PKW with streamlines along the inlet and outlet keys and over the weir walls. Flow deviation led to an increased unit discharge in the outlet keys. Meanwhile, the local velocity was increased, leading to a positive acceleration. The observations showed that this was accompanied by water level decline, and increased downstream turbulence.
The flow jet passing over the crest was accompanied by the interaction of three colliding jets. This interaction was the result of the collision between the flow nappies from lateral crests and the approach flow in the outlet keys (Fig. 4). The resulting interactions led to significant turbulence, and expansion of flow at the downstream. As the head increased, the outlet key discharge was increased, and energy dissipation was decreased. Figure 5 shows variations of the energy dissipation for rectangular and trapezoidal PKWs. The relative energy dissipation by trapezoidal PKW was more than that of rectangular PKW, by average of abbot 3%. Figure 6 shows the residual energy of the PKW versus discharge for rectangular and trapezoidal PKWs. It is clear that as the discharge increased, increased. Furthermore, the ascending rate of was higher for lower discharges. This is due to the local submersion upstream of the weir at higher discharge.
Conclusion
Despite the length of the used crest of trapezoidal PKW being less than rectangular PKW, energy dissipation of trapezoidal PKW is higher than that of rectangular PKW.
The average discharge coefficient for trapezoidal PKWs is higher than that for rectangular PKW.
The flow characteristics is different for rectangular and trapezoidal PKWs. For , no aeration is occurred. For , a week hydraulic jump is formed at the end of the weir outlet keys.
New equation was obtained for estimation of energy dissipation.
Keywords
Main Subjects
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