https://dx.doi.org/10.20961/mekanika.v22i2.75079 Revised 17 August 2023; received in revised version 27 August 2023; Accepted 18 September 2023 Available Online 30 Septmber 2023 2579-3144 © 2023 Mekanika: Majalah Ilmiah Mekanika. All right reserved 76 Mekanika: Majalah Ilmiah Mekanika Volume 22 (2) 2023 Page 76 - 87 Predicting the Drag Coefficient Characteristics of Ocean Bottom Unit (OBU) Float Array Model for Early Warning Tsunami Systems Using Computational Fluid Dynamics (CFD) Method Yudiawan Fajar Kusuma 1* , Ilham Hariz 2 , Hanni Defianti 3 , Buddin Al Hakim 1 , Arfis Maydino F. Putra 4 1 Research Center for Hydrodynamics Technology, National Research and Innovation Agency, BRIN, Surabaya, Indonesia 2 Directorate of Laboratory Management, Research Facilities, and Science and Technology Park, South Tangerang, Indonesia 3 Research Center for Aeronautics Technology, National Research and Innovation Agency, Bogor, Indonesia 4 Department of Naval Architecture and Ocean Engineering, Marine Hydro-Science and Engineering Laboratory, Osaka University, Osaka, Japan *Corresponding Author’s email address: yudi018@brin.go.id Keywords: Floater OBU Computational fluid dynamics Drag coefficient INA-TEWS Abstract As a country along the Pacific Ring of Fire, Indonesia faces various natural disaster threats, including tsunamis. Therefore, an early tsunami warning system is crucial for detecting potential tsunami waves. The early tsunami warning system encompasses several complex components, one of which is the Ocean Bottom Unit (OBU) floater. This paper discusses the performance of various types of floater arrays for tsunami early warning systems using Computational Fluid Dynamics (CFD) simulations. The study focuses on coefficients, especially the drag coefficient, and the influence of the number of float arrangements on the flow pattern around the buoy or Ocean Bottom Unit (OBU) array. Among the five numerical simulation models, the six-couple floater has the highest drag and lowest lift coefficients, while the single floater has the lowest drag coefficient. The percentage of difference in drag coefficient between single floater and couple series floater is quite significant, reaching up to 50%. The moment coefficient is also affected by the number of floaters, with a series of five couple floaters having the highest moment coefficient at a Reynolds (RE) number of 2 × 10 6 . The advantage of using the CFD method is that it can visualize current velocity, which is crucial for understanding the flow pattern around the float system. The results indicate that the flow pattern becomes more complex as the number of floater arrays increases, which leads to more vortices between the floater, resulting in increased turbulence and drag coefficient. 1 Introduction Indonesia, as an archipelagic country located in the Ring of Fire, is vulnerable to natural disasters such as earthquakes, volcanic eruptions, tsunamis, and sea-level rise [1-3]. Indonesia is geologically situated at the convergence of four major active tectonic plates and several minor plates. The four major active plates are the Eurasian, Indo-Australian, Pacific, and Philippine Sea Plates [4][5]. The speed and direction of active movement between these plates vary [5-7], leading to earthquakes and tsunamis. In Indonesia, there are at least 13 megathrust zones [7][8], which have a high potential for significant