## 1 Introduction  
Several railways have been constructed in the deserts of Northwest China as part of the Belt and Road initiative [1]. Deserts and Gobi landforms are common in this region and are characterized by extensive wind-blown sand accumulation, threatening the safety of railway operations and facilities. Therefore, international researchers have focused on train operation safety and railway track maintenance in sandy environments [2; 3; 4; 5]. Numerous studies were conducted on the prevention and control of railway wind-blown sand disasters, and many practical ideas and measures for sand prevention were put forward. Based on the concept of CWE, the prevention and control of wind-blown sand along the railway were deeply analyzed [7; 8; 9]. Then, the sand bed collision, sand transport and environmental characteristics in Gobi area were discussed from different angles by means of numerical simulation and wind tunnel experiment [10; 11; 12]. Meanwhile, the characteristics of wind-blown sand flow field, optimization design and protection effect of sand barriers were studied [13; 14; 15; 16]. And starting from the characteristics of subgrade sand accumulation, the problem of sand disasters behind railway windbreak wall was investigated [17]. Overall, this kind of research focuses in this field is mainly on the protection of sand disaster along the railway, and the serious consequences of sand accumulation around the railway culvert are ignored.  
Culverts are used to provide a passage for people, livestock, and vehicles to cross the railway, but they are adversely affected by sand accumulation in the culvert. Sand sedimentation hinders or prevents traffic through the culvert [18; 19], as shown in Figure 1. Therefore, in-depth research is required on the sand sedimentation mechanism and formation reasons around railway and in culverts. Some studies have found that wind speed and direction [20], diversion dikes, and wing walls can change the airflow and minimize or prevent sand sedimentation at the culvert inlet. [PERSON] et al. studied the sand sedimentation mechanism around a slab culvert of the Jitong Railway in China using field measurements and pointed out that diversion dikes reduced the wind speed at the culvert inlet, minimizing sand [21]. [PERSON] et al. used computational fluid dynamics simulation to analyze the wind-blown sand flow around the culvert and found erosion occurred near the wing walls at the culvert inlet, and the airflow fluctuation index decreased, increased, and decreased [22]. [PERSON] et al. researched the wind-blown sand flow field around the culvert of the Qinghai-Tibet Railway and observed that the attenuation of the airflow caused sand sedimentation at the culvert inlet [23].  
Above studies on the wind-blown sand flow near railway culverts have focused on the sedimentation and the reasons for sand deposition in front of the culvert inlet. However, few studies investigated the sand distribution inside the railway culvert. Most importantly, there few studies have focused on the optimization of the culvert structure to prevent sand sedimentation inside the culvert.  
This study investigates the wind-blown sand flow based on the two-phase flow theory and establishes a CFD simulation model of the sand movement around the railway culvert. The sand sedimentation mechanism is analyzed according to the transport characteristics of wind-blown sand around the culvert. Two culvert designs are proposed, and their ability to minimize sand accumulation is evaluated. The results provide insights into designing railway culverts that prevent wind-blown sand accumulation.