Journal of Advanced Research in Fluid Mechanics and Thermal Sciences 88, Issue 2 (2021) 50-63 50 Journal of Advanced Research in Fluid Mechanics and Thermal Sciences Journal homepage: https://semarakilmu.com.my/journals/index.php/fluid_mechanics_thermal_sciences/index ISSN: 2289-7879 Design Intent of Future Tunnels Mohammed Asadullah 1,* , Sher Afghan Khan 2 , Parvathy Rajendran 3,4, , Ervin Sulaeman 2 1 Mechanical Engineering Department, Faculty of Engineering, Lords Institute of Engineering and Technology, Hyderabad, India 2 Mechanical Engineering Department, Faculty of Engineering, International Islamic University Malaysia, Kuala Lumpur, Malaysia 3 School of Aerospace Engineering, Universiti Sains Malaysia, Nibong Tebal, Penang, Malaysia 4 Faculty of Engineering & Computing, First City University College, Bandar Utama, 47800 Petaling Jaya, Selangor, Malaysia ARTICLE INFO ABSTRACT Article history: Received 14 August 2021 Received in revised form 26 September 2021 Accepted 28 September 2021 Available online 27 October 2021 The sound barrier for bullet trains remains a challenge due to the piston effect causing compression waves at the entry and exit of the tunnel. The air ahead of the train nose is compressed, and the wave propagates through the tunnel at the speed of sound and exits with the generation of micro pressure waves. It gives rise to a complex wave pattern comprising compression at the train nose & expansion at the train tail leading to the positive pressure around the nose and suction around the tail. This is intended to provide exhaustive input for the proper design of a futuristic tunnel. The cross- sectional shapes of the tunnel, whether square, rectangular, circular, or semi-circular, will experience pressure compression wave generated by high-speed train but will influence the flow pattern and hence the compression wave. This paper presents the pressure load on the walls of long and short tunnels for subsonic compressible and transonic flows. The experimental investigation is carried out only for length parameters to study short and long tunnels. Further, flow visualization is also provided after the formation of the sonic boom. The results of this investigation can be an essential data source for optimum design of high-speed tunnels so as to suppress or break the sound barriers, thus, resulting in a safer high-speed train network. Keywords: High-speed Train; High-speed Tunnel; Pressure Load; Mach Number 1. Introduction Many countries have switched over to underground tunnel network construction in order to counter the issue of ever-rising traffic on the road, railways, and air, but the design and safety of high-speed train tunnels remain a daunting challenge. At present, the operating speed of trains is above Mach 0.3, giving rise to the compressibility effect. As the train enters a high-speed tunnel, it gives rise to highly transient air velocity and pressure fluid inside the tunnel from the time of its entry until well after its departure [1]. Moreover, much research has gone into designing trains running through tunnels to meet the need to travel at high velocity, but tunnels remain an obstacle for these trains [2]. Early researchers have studied train and tunnel systems from an aerodynamic and * Corresponding author. E-mail address: shiblibhai@gmail.com  Corresponding author. E-mail address: aeparvathy@usm.my https://doi.org/10.37934/arfmts.88.2.5063