Comparative analysis of the slipstream of different nose lengths on two trains passing each other

Highlights

• Temporal and spatial evolution of the flow field was presented.

• The slipstreams for different nose length trains were evaluated.

• Aerodynamic forces for different nose length trains were described.

Abstract

The scenario of high-speed trains (HSTs) passing each other is usually encountered, which causes aerodynamic effects such as high slipstream velocity and strong aerodynamic loads, leading to trackside worker injuries, equipment fatigue damage, and snake-like train motions. This study used a three-dimensional, compressible, improved delayed detached eddy simulation (IDDES) method based on an SST k-ω turbulence model to research the turbulent flow around the train bodies generated by two trains passing each other in the open air. An overset grid method was utilised to tackle the moving boundary problem. The numerical results were firstly verified by comparison with the results of a moving model test. Then, temporal and spatial evolution of the flow field was analysed in detail based on a 4 ​m nose length model. The results show that the process of intersecting has a pronounced effect on the vortices as well as the boundary layer at the bottom of train side. The peaks of three slipstream velocity component profiles decrease as the distances from the centre of the track (COT) and the top of the rail (TOR) increase. Finally, considering three different nose lengths (4 ​m, 7 ​m, and 9 ​m), the differences in the instantaneous flow structures, slipstream profiles, and aerodynamic coefficients were elucidated. The longer nose lengths were found to reduce the scale and strength of the counter-rotating vortices, thereby lowering the maximum slipstream peaks in the wake. The slipstream resultant velocity at trackside height for the 7 ​m nose length case is 22.2% lower and 9.2% higher than the corresponding values for the 4 ​m and 9 ​m nose length cases, respectively. The side force coefficient is also influenced by the nose length, with the stronger effect being exerted on the head car and tail car than on the middle car.

Introduction

Slipstream, the air flow generated by a moving train continues to be an important factor affecting aerodynamic performance and safe operation. Such flows increase the risk to trackside workers and passengers on station platforms and cause reductions in the serviceable life of trackside furniture (Flynn et al., 2014). Notably, the peak slipstream flow is proportional to train speed (Meng et al., 2019). With the continual increase in the operating speeds of trains over the past few decades, issues related to slipstream safety are attracting more attention around the world, particularly in countries with operational speeds for HST of more than 300 ​km/h, such as China, Japan, and France. Thus, the slipstream needs to be given full attention and this issue has been the focus of multiple research projects.

The studies on slipstream can be divided into two categories based on the effect on slipstream: external conditions and the conditions of the train itself. The first includes investigations of the effect of the external facilities or environment on the slipstream, such as the ground conditions (Xia et al., 2017; Wang et al., 2018a), the rails (Wang et al., 2020), and cases with and without a crosswind (Hemida and Krajnović , 2010; Flynn et al., 2016). Moreover, the geometric structures of the trains have also proven to be a significant factor affecting slipstreams, including the length of the streamlined head (Chen et al., 2019; Baker et al., 2014; Hemida and Krajnović , 2010), the marshalling arrangement (namely, a single train or a double-unit train (Guo et al., 2018)), whether or not the train has an obstacle deflector (Niu et al., 2018a), whether or not the train has bogies (Wang et al., 2018b), and the complexity of the bogie (Dong et al., 2019). The above studies have explored the development mechanism and the effect factors of slipstreams induced by a single train; however, with the rapid development of high-speed rail, there are nearly 2400 high-speed trains (HSTs) in operation and they account for 63% of all passenger trains according to statistics from the National Railway Administration of China (Jiang et al., 2019). Therefore, the scenario of trains passing each other becomes more frequent. The scenario of trains passing each other causes more severe dramatic aerodynamic effects such as high slipstream velocity, strong aerodynamic loads, leading to trackside worker injuries, equipment fatigue damage, and snake-like train motions (Hwang et al., 2001). Niu et al. (2017) investigated the effect of the coupling region on train aerodynamic performance, and how the coupling region affects aerodynamic performance of coupled multiple-unit trains when they pass each other in open air. The results show that a positive pressure pulse was introduced in the alternating pressure produced by trains passing by each other in the open air, and the amplitude of the alternating pressure was decreased by the coupling region. Chen et al. (2018) conducted a numerical simulation analysis on the effects of train speeds on the pressure waves and aerodynamic forces (force moments) of two trains meeting on parallel tracks in the open air based on a $k-\epsilon$ turbulence model. They concluded that the wave pressure amplitudes of both head car and tail car were in direct proportion to the square of the train speed. Huang et al. (2019) studied the transient flow field induced by two maglev trains passing each other at a speed of 430 ​km/h in open air. It was reported that the peak values of pressure coefficient for two trains passing each other are about twice as large as that of a single train passing due to the blocking effect and interference by the motion of the other train in the opposite direction. They also showed that the maximum slipstream velocity occurs at the place where the trains meet.

In view of the above studies, although the temporal and spatial pressure variation, aerodynamic forces and flow structures of two trains passing each other in open air have been investigated in detail, few studies on trains with different nose lengths regarding the slipstreams induced by trains passing each other in the open air have been published. A comprehensive understanding of the flow mechanism, such as the development of the vortex structure and boundary layer, and how it alters the slipstream is yet to be revealed and this has motivated the present study. Therefore, the main aim of this research is to describe how changing the nose lengths of HST affects the slipstream and flow structures that are induced by trains passing each other in open air using the IDDES method. Moreover, considering the side force is an important parameter when evaluating the safety of the train at intersection: this will be addressed to achieve a fuller understanding of the effect of nose lengths on trains passing each other.