Document Type : Original Article

Authors

• Mohammad Masoud Vaseti ¹
• Ali Akbar Akhtari ²
• Reza Tarinejad ³

¹ PhD student, Department of Civil Engineering, Faculty of Engineering and Technology, Razi University, Kermanshah, Iran.

² Associate Professor, Department of Civil Engineering, Faculty of Engineering, Razi University, Kermanshah, Iran.

³ Professor Faculty of Civil Engineering, University of Tabriz, Tabriz, Iran.

Abstract

Nowadays, the use of numerical models has become highly prevalent due to the substantial increase in computer memory capacity and processing speed. The growth in computational power, paralleled by remarkable advancements in numerical models—particularly owing to their cost-effectiveness compared to laboratory models—indicates that numerical modeling and computational simulations will see even wider application in the future. However, commercial numerical models are highly sensitive to boundary and initial conditions; without a proper understanding of the governing physics of these conditions, this invaluable tool can lead to inaccurate and misleading results. This study presents an analytical and numerical comparison between pressure-inlet and velocity-inlet boundary conditions in computational fluid dynamics (CFD) simulations, with a focus on the significant numerical and physical consequences arising from a common yet incorrect choice of inlet boundary condition. For this purpose, a 1/15-scale model of the bottom outlet gate of the Diyaraba Dam (located in Sri Lanka) was employed. Simulations were conducted in ANSYS Fluent software using the standard k-ε turbulence model, corresponding to operating conditions with 10% gate opening. The computational domain mesh size was determined through sensitivity analysis and grid independence studies. To enhance the accuracy of simulating real flow behavior, a two-phase water–air model was utilized. The results reveal that applying a velocity-inlet boundary condition leads to excessively high velocity and pressure fields within the computational domain, resulting in order-of-magnitude discrepancies compared to the physically realistic case. Examination of the mass flow rate in the domain, compared with measurements from the laboratory model, demonstrates that the pressure-inlet boundary condition provides a more physically appropriate representation of the flow. In contrast, the velocity-inlet condition, in this specific model, yields unrealistic and engineering-wise inaccurate results.

Keywords

• Velocity boundary condition
• Pressure boundary condition
• Bottom outlet gate
• Computational fluid dynamics
• Fluent

Main Subjects

• Water and hydraulic structures