## 1 Introduction  
Hydrogen has been widely utilized as a clean energy resource, and the hydrogen fuel cell bus is one of the key applications of hydrogen energy in transportation [1, 2, 3]. However, safety issues become more important due to the rapid flame speed of hydrogen and its high diffusion speed in air. Since hydrogen can burn in the air when the volume fraction of hydrogen reaches 4%, factors such as ventilation, obstacles, green walls, and so on may significantly influence utilizations of hydrogen [4, 5, 6]. In reference to the ventilation method, the hydrogen fuel bus is generally simplified as a cuboid space with two vents at the top and the side wall, when investigating the concentration distribution of hydrogen with different ventilations [7, 8]. Since the leakage might occur at any position in the bus, a series of experimental studies were conducted to investigate the influence of the leakage position on the concentration distribution of hydrogen in spaces with fixed vents [9, 10]. [PERSON] et al. [11] tested the hydrogen leakage in the vertical direction taking place at the center of the floor, and found the mole fraction of hydrogen above 2.6 m in the space reached about 29.3% (i.e., reaching the flammable concentration limit), which is higher than that at other altitudes 3500 s after the release. In another study with the same leakage position, [PERSON] et al. [12] found stratification happened in the space with a relatively uniform combustible layer formed at the ceiling 20 min after the release. Helium was also used for experiments instead of hydrogen, due to safety concerns [13]. Compared to the leakage taking place on the floor, the average helium concentration was lower when the leakage height was 0.725 m in a cuboid space with two vents [14], and the stratification was more evident when the helium gas was injected from the top [15]. A number of numerical studies have been conducted to analyze the general process of hydrogen dispersion [16; 17]. The behavior of a leakage from the bottom of a car was separated into two phases: the hydrogen firstly extending to the bottom of the car; and then rising to the ceiling of the parking garage [18]. It was also found that higher wind speed could accelerate the mixing of hydrogen and air, with the hydrogen composition decreasing faster [19]. [PERSON] et al. [20] found that hydrogen spread faster in spaces without any obstacles, than those with obstacles. [PERSON] et al. [21] found the flammable region expanded when increasing the height of the leakage position for different leakage directions. Therefore, the effect of leakage direction on hydrogen diffusion needs to be further studied. The concentration distribution of hydrogen for leakage in the vertical direction has also been investigated [22; 23; 24; 25]. [PERSON] et al. [8] found the velocity vectors varied when changing the positions of vents, with the leakage taking place in the vertical direction from the floor in a cuboid space. The mole fraction of the hydrogen was found to be higher than 4% in the area above 0.6 m, and less than 2% in the area below 0.2 m from the floor after the stratification. The flow velocity also varied with different locations of both the leak and the vent when the leakage took place at the center of the floor in a prismatic cavity. The hydrogen concentration with the vent at the top of the space was lower than that when the vent was located in the lower half of the space [26]. [PERSON] et al. [27] investigated a leakage taking place from the floor in a cuboid space and found a flow recirculation area existing between the leakage position and the vent in the wall. [PERSON] et al. [28] found the mole fraction increased as the height of the crossbeam increased for a leakage in the vertical direction in a cuboid space. The flammable region for the downward leakage disappeared much more rapidly than the upward leakage in the enclosure [21]. A leakage in the horizontal direction with leakage positions on a side wall in a cuboid space was also investigated.