Limpahan sifon adalah sejenis alur limpah saluran tertutup yang dibina di dalam empangan kecil dan rangkaian pengairan untuk menyalurkan limpahan dengan cepat dari takungan. Aliran keluar dari limpahan sifon memasuki kolam-kolam empang di hilirnya supaya tenaga dapat diserakkan. Kolam empang yang bertempat di hilir alur limpah akan menenggelamkan salur keluar sifon lalu memberikan kesan yang negatif terhadap operasi sifon. Kajian ini bertumpu pada kesan penenggelaman ini terhadap operasi sifon dan cara menambahkan serakan tenaga dengan menggantikan kolam empang dengan pelongsor bertangga. Pelongsor bertangga adalah sejenis binaan penyerak tenaga yang bercirikan rintangan aliran dan penyerakan tenaga yang nyata dengan penggunaan tangga. Dalam kajian ini, beberapa susunan pelongsor bertangga yang sederhana cerun digunakan pada salur keluar limpahan sifon. Aliran air diukur bagi setiap susunan dan hasilnya dibentangkan dalam bentuk aturan aliran, ketinggian permukaan air, halaju huluan dan salur keluar, serakan tenaga, dan rintangan aliran. Simulasi berangka, termasuklah pembinaan model ciri-ciri aliran di dalam, di hulu dan di hilir alur limpah, aturan aliran pada pelongsor bertangga, dan ketinggian permukaan air di hilir pelongsor bertangga, dilakukan untuk menentukan kecerunan, lebar pelongsor, dan bilangan tangga yang optimum. Berdasarkan hasil ujian eksperimen dan model berangka ini, pelongsor bertangga yang berkecerunan 14°, mempunyai 4 anak tangga, dan berukuran 0.14 m lebar didapati mencapai kadar penyerakan tenaga yang tertinggi, iaitu sehingga 92%, tanpa kesan yang negatif terhadap operasi limpahan sifon. Berdasarkan ujian pengesahan pada model-model berangka, model golakan k-ε didapati lebih memuaskan berbanding dengan model RNG apabila mensimulasikan operasi sifon.
_______________________________________________________________________________________________________
A siphon spillway is a type of spillways with a closed conduit that is constructed in small dams and irrigation networks for rapid evacuation of overflow in a reservoir. The outflow from siphon spillways enters to pool sills at downstream for energy dissipation. Using pool sills at downstream of spillways will cause submergence of siphon outlet, and this has a negative effect on siphon operations. The present study focuses on the effect of submergence on the siphon operation and increases energy dissipation by replacement of the pool sill with the stepped chutes. The stepped chute is a type of energy dissipation structure, which is characterized by significant flow resistance and energy dissipation via the steps. In this study, several stepped chute configurations with moderate slopes were applied to the siphon spillway outlet. Water flow measurements were carried out for each configuration and results were presented in terms of flow regimes, water surface elevation, upstream and outlet velocity, energy dissipation, and flow resistance. Numerical simulations, including the modeling of the flow characteristics inside, upstream and downstream of the spillway, the flow regimes on the stepped chutes, and water surface elevation at downstream of stepped chutes were performed to determine the optimum slope, width of stepped chute, and number of steps. Based on the results of experimental tests and numerical models, the stepped chute with a slope of 14°, 4 step numbers, and width of 0.14 m achieves the highest energy dissipation up to 92%, and had no negative effect on the operation of the siphon spillway. Based on the verification tests in numerical models, the k-ε turbulence model is more satisfactory than the RNG model to simulate siphon operation.
Effects of stepped chute in siphon spillway outlet on the performance of spillway / Amin Ghafourian_A9