NUMERICAL SIMULATION AND EXPERIMENTAL STUDY ABOUT MAJOR AND MINOR LOSSES IN STRATIFIED PIPE

ANI HAIRANI, Ir. Adam Pamudji Rahardjo, M.Sc, Ph.D

2015 | Tesis | S2 Teknik Sipil

Jakarta merupakan inukota Indonesia yang banyak mengalami masalah banjir. Permasalahan banjir sudah menjadi bencana rutin yang harus dihadapi oleh warga kota Jakarta dan sekitarnya. Penurunan kapasitas drainase, buruknya daerah tangkapan di hulu hingga isu pemanasan menjadikan problem ini semakin bertambah parah. Di Malaysia, sebuah modifikasi drainasi bawah tanah yang disebut SMART (Stormwater Management and Road Tunnel) telah diimplementasikan untuk mengatasi dua permasalahan utama yatiu banjir, dan kemacetan. Ide memperkenalkan sistem penanganan banjir ini di sungai Ciliwung dirasa cukup menarik untuk mengatasi keterbatasan lahan yang ada. Meskipun begitu, perilaku aliran dan parameter hidraulik yang terkait dengan terowongan ini perlu untuk diteliti lebih lanjut. Penelitian ini bertujuan untuk mengetahui perilaku aliran, penurunan tinggi energi potensial, serta koefisien kehilangan akibat perubahan tampang pipa yang mendadak dengan menggunakan pemodelan turbulensi dengan numeris dan eksperimen. Percobaan dilakukan dengan menggunakan pipa akrilik sepanjang 3 m dan diameter pipe besar 7.4 cm. Tipe transisi pipa mengikuti desain SMART dimana rasio luasan pipa A1/A2= 0.50. Penelitian divariasikan terhadap angka Reynolds pada pipa kecil dimana Re = 8568, 10710, and 14994. Simulasi dilakukan dengan pendekatan 3D menggunakan FLUENT software. Standard k- turbulence dan equilibrium wall function diaplikasikan pada pipa lurus, sedangkan pipa kontraksi dan ekspansi menggunakan model turbulensi k- modified dan non equilibrium wall function. Imput parameter aliran yang disimulasikan mengikuti parameter pada eksperimen dimana Re=8568, 10710, and 14994, selain itu digunakan variasi luasan pipa dimana A1/A2 = 0.20 (Y = 0.25D2), 0.50 (Y = 0.50D2), dan 0.80 (H=0.75D2). Hasil eksperimen menunjukkan bahwa parameter hidraulik dan rasio luasan pipa berpengaruh terhadap penurunan tinggi potensial energi. Pengukuran tinggi energy potensial pada percobaan tidak cukup baik dimana jika dibandingkan dengan persamaan Darcy Weisbach. Namun, FLUENT cukup memberikan hasil yang mendekati teori Darcy-Wesibach. Koefisien kehilangan akibat kontraksi (KC) dan ekspansi (KL) berdasarkan eksperimen (rasio luasan A1/A2=0.50) adalah sebesar 0.40 dan 0.59, sedangkan hasil simulasi memprediksi KC = 0.60, 0.35, 0.15 dan and KL= 0.71, 0.30, 0.10 berturut-turut dimana A1/A2 = 0.20 (H = 0.25D2) , 0.50 (H = 0.50D2), dan 0.80 (H=0.75D2). Hasil tersebut menandakan bahwa koefisien kehilangan energi sekunder berkurang seiring dengan meningkatnya rasio luasan pipa. Nilai koefisien kontrakta pada model kontraksi sama besar untuk Re=8568, 10710, and 14994. Peningkatan tinggi energi ini dipengaruhi oleh separasi aliran di sekitar daerah transisi yang cukup besar yang dibuktikan dengan perbandingan nilai reattachment length berdasarkan hasil simulasi FLUENT.

Jakarta, capital city of Indonesia is one of city in Indonesia which has flood inundation problem. Flood has become an annual disaster that has to be faced by people there. Inadequate capacity of existing drainage system, lack of catchment areas in the upstream watersheds make the flood problem becomes more severe. In Malaysia, a modified of underground tunnel known as Stormwater Management and Road Tunnel (SMART) is implemented to aid flood prevention and traffic control system. An idea of introducing a floodway of Ciliwung River inspired by SMART system is considered attractive to minimize land utilization. However, it is necessary to know hydraulic performance of a modified ground pipe. This research was aimed to know the flow behaviour due to sudden transitions, pressure losses and loss coefficients along the pipe flow using numerical modeling and experimental validation. The experiment was carried out in smooth wall condition made of acrylic pipe with 3 m in length, and 7.4 cm in diameter. The type of transition followed the geometrical design of SMART where the area ratio A1/A2= 0.50 was specified. The experiment and simulation was varied in Reynolds number of smaller pipe which were 8568, 10710, and 14994. The simulation of numerical modeling was analyzed by using 3D approach of FLUENT Software. Standard k- turbulence and equilibrium wall function were used in straight pipe model, while contraction and enlargement pipe model used k- modified turbulence model and non equilibrium wall function. Simulation models followed the experiment with varied area ratio which A1/A2=0.20(H= 0.25D2), 0.50 (H=0.50D2), and 0.80 (H=0.75D2). Experiment and simulation shows that hydraulic parameters and area ratio of sudden transitions pipe give significant effect toward pressure head loss along pipe. The experiment did not give good accuracy in measuring pressure drop since the experiment results were somewhat big discrepancy with Darcy-Weisbach equation. However, FLUENT simulation result gave a good agreement with Darcy-Weisbach formula. Contraction loss coefficient (KC) and enlargement loss coefficient (KL) based on experiment (in which area ratio A1/A2 is 0.50) were 0.40 and 0.59, while simulations predicted KC = 0.60, 0.35, 0.15 and KL= 0.71, 0.30, 0.10 which A1/A2 = 0.20 (H=0.25D2), 0.50 (H=0.50D2), and 0.80 (H=0.75D2), consecutively. Those results indicate that loss coefficient decreases with the increase in pipe area ratio. Contraction coefficient (Cc) generated at contraction model simulation was equal for Re=8568, 10710, and 14994. The increase in pressure head loss was incurred by severe separated regions in the vicinity of pipe transition which was proved by typical reattachment length from the FLUENT simulation.

Kata Kunci : tinggi energi, koefisien kehilangan, FLUENT, model turbulensi, separasi