Bambang Yulistiyanto
Civil and Environmental Engineering Department, Faculty of Engineering, Gadjah Mada University Jl. Grafika 2 Yogyakarta 55281

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Vorticity Fields on Flow with Vortex System Yulistiyanto, Bambang
Jurnal Teknik Sipil Vol 16, No 2 (2009)
Publisher : Institut Teknologi Bandung

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Abstract

Abstract. The flow around a circular cylinder mounted normal to the channel bed is essentially a three-dimensional one. The flow upstream undergoes a separation of the turbulent boundary layer and rolls up to form a flow system, known as horseshoe vortex, which is swept around the cylinder. This vortex system plays an important role, if the bed material is movable. The shear stress of the vortex system is responsible to the bed erosion. The shear stress depends on the velocity gradient, may be presented as the vorticity. This study is done to gain a better understanding of the vorticity around a cylinder, especially where the system of vortices exist. Flow measurements were used to study the vorticity fields on flow with a horse-shoe vortex system around a pile. The velocity vector plots show the presence of a primary vortex upstream of the cylinder. Based on these velocity vectors, the vorticity can be analyzed by using central finite difference approximations. Results of the vorticity calculation are presented as the vorticity contours. Results of study show the greatest value of the positive-vorticity on the horse-shoe vortex system is in the plane upstream of cylinder. This value decreases in the planes downstream, attaining the lowest value in the plane downstream, where the wake vortex established. The strength of the positive vorticity increases at larger Reynolds number. Underneath the field of positive vorticity, stretching around the cylinder, it appears a field of negative vorticity. This negative vorticity near the bed is high in front of the cylinder, proportional to the bed shear stress, which is responsible to the development of local scour at the movable bed. The line of zero vorticity is plotted originated at the bed, being as the separation point. It is also concluded that the maximum positive-vorticity is not necessary coincided with the center of the vortex.Abstrak. Pola aliran di sekitar silinder bulat yang dipasang tegak lurus dasar saluran merupakan aliran tiga dimensi Aliran di hulu silinder membentuk separasi pada lapis batas turbulen, membangkitkan terbentuknya pusaran tapal-kuda yang mengelilingi silinder. Sistem pusaran ini mempunyai peranan penting jika material dasar mudah bergerak. Tegangan geser pada sistem pusaran akan menyebabkan terbentuknya erosi lokal. Besarnya tegangan geser bergantung pada perbedaan kecepatan, yang dapat dipresentasikan dengan nilai vortisitasnya. Studi ini bertujuan untuk mendapatkan pemahaman yang lebih baik tentang vortisitas di sekitar silinder, terutama di lokasi dimana terbentuk system vortex. Data hasil pengukuran kecepatan aliran dipakai untuk mempelajari system vortisitas pada aliran dengan sistem pusaran di sekitar pilar. Gambar vektor kecepatan di sekitar pilar silinder memperlihatkan keberadaan pusaran utama di depan silinder. Berdasarkan vector kecepatan tersebut, dapat dianalisis vortisitasnya dengan menggunakan metode beda hingga. Hasil hitungan vortisitas digambarkan dalam bentuk kontur vortisitas. Hasil studi menunjukkan nilai maksimum dari vortisitas positif (searah jarum jam) dari sistem pusaran tapal kuda adalah pada bidang simetri di hulu silinder. Nilai vortisitas tersebut menurun, mencapai nilai minimum di hilir silinder. Besarnya vortisitas positif meningkat pada angka Reynold yang lebih tinggi. Di bawah bidang vortisitas positif, diperlihatkan terbentuknya system vortisitas negatif. Vortisitas negative di dekat dasar di depan silinder mempunyai nilai yang besar, proporsional dengan nilai tegangan geser di dasar, yang bertanggungjawab pada terbentuknya erosi lokal pada saluran dengan dasar bergerak. Garis yang memberikan nilai vortisitas sama dengan nol, merupakan garis separasi antara vortisitas positif dan negative. Dari hasil kajian juga disimpulkan bahwa posisi dari vortisitas positif maksimum tidak selalu berimpit dengan pusat pusaran tapal kuda.
ANALISIS HIDRAULIKA BANJIR TUKAD BADUNG Wiarta, I Nyoman; Yulistiyanto, Bambang; Nizam, Nizam
Civil Engineering Forum Teknik Sipil Vol 18, No 2 (2008): MEI 2008
Publisher : Civil Engineering Forum Teknik Sipil

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Abstract

There is densely populated residence along Tukad Badung reach resulting in the misuse of stream for waste disposal outlet. This condition causes the stream becoming the disposal place of waste, household sewage, and industrial waste. Also, there is erosion and sliding of river bank in the upstream. This condition also causes river reaches to shallow due to sedimentation resulting in the decrease of stream capacity to carry off the flood discharge. During recent years, Tukad Badung has been degraded in its function due to various factors occurring from the upstream to downstream resulting in the stream incapable to carry off the flood discharge optimally. The change of land use occurred in the watershed causes the change in stream flow pattern that can be seen from the high discharge fluctuation during rainy and dry seasons. The objective of this study is to observe 50 year return period flood water surface profile and to determine the flood control alternatives. Hydrology and hydraulic analyses are carried out in the study. Hydraulic modeling of the river system is accomplished using the Hydrologic Engineering Center-River Analysis System (HECRAS) software of version 3.1.2. The stream is assumed to be a single reach using the upstream boundary at AWLR Wangaya and downstream boundary at Nusa Dua estuary dam. The geometric input is conducted for the existing, normalization, and dikes conditions, whereas the flow hydrograph and rating curve are using for upstream boundary and the downstream boundary respectively. Based on the result of hydraulic analysis, the overtopping is occurred at 27 River Stations at the left bank and 23 River Stations at the right bank. The flood controlling by normalization is capable of decreasing flood water surface with the non existence of overtopping both at the left and right banks. The flood controlling by dike is also capable of retaining the flood discharge without the overtopping at the left and right banks. That can be concluded, by normalization or by dike the Tukad Badung is capable of carrying off the 50 year return period flood discharge. Calculation results shows the flood controlling by dike requires cost of Rp 1.118.376.638,35,- and by normalization requires cost of Rp. 1.316.571.000,00,- .
KARAKTERISTIK KECEPATAN ALIRAN DI DEKAT DASAR PINTU PELUAPAN BAWAH (SLUICE GATE) Laksitaningtyas, Agatha Padma; Legono, Djoko; Yulistiyanto, Bambang
Jurnal Teknik Pengairan Vol 11, No 1 (2020)
Publisher : Jurusan Teknik Pengairan, Fakultas Teknik, Universitas Brawijaya

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Abstract

Sluice Gates are the most important and often studied in hydraulic structures that are widely used and found, located in nature or artificial open channels. Sluice gates can be opened and closed at the bottom vertically. The sluice gate functions as a regulator of water flow which functions to control the discharge, control the water level and for monitoring the discharge. Water flowing through the sluice gate can be in a free flow condition or in a submerged flow condition that depends on depth of tailwater. The characteristics of the flow that flows through the sluice are based on the equation of energy and momentum conservation law, which depends on the pressure, velocity and depth of water that is upstream or downstream of the sluice. Hydraulics of sluice especially in free-flow conditions is very dynamic, including the profile of water level, the force that occurs on the sluice gate, or the velocity distribution of the bottom sluice gate, which is largely ignored. There are several basic development formulas of flows through the floodgates, especially in the development of vena contracta. Vena contracta will produce the values of the coefficient of discharge (Cd) and the coefficient of contraction (Cc) obtained from the calculation ratio of existing methods. Calculation of events theoretically and through experiments in the laboratory. However, the equation of discharge through the sluice gate is not very effective when used in a large opening condition and in a fully submerged condition.