Numerical comparisons of the aerodynamic performances of wind-tunnel train models with different inter-carriage gap spacings under crosswind

Highlights

• Time-averaged quantities of train vehicle models with different gap spacings were compared.

• Flow patterns and structures around different gap spacings were investigated.

• PSD of aerodynamic loads on train vehicles with different gap spacings was analyzed.

• Reasonable gap spacings between adjacent train vehicle models were recommended.

Abstract

Reasonable gap spacings between adjacent train vehicle models are of great significance for the data accuracy during a wind tunnel test. In this research, crosswind flows around 1/8th-scale high-speed train models with inter-carriage gap spacings of 0, 5, 8, 10, and 20 mm are studied using improved delayed detached eddy simulation (IDDES) with the SST (shear-stress transport) k-omega model. The numerical methodology is first validated against the wind tunnel experiment data. Then, the effects of gap spacings on aerodynamic loads and the flow field around the train are investigated. The results indicate that the side force of the tail car significantly increases with the gap spacing, in which the maximum difference occurs at 20 mm compared with the zero gap spacing, whereas the difference in the drag decreases. When the gap spacing is greater than 10 mm, the time-averaged pressure distribution, flow pattern and transient flow structures around the inter-carriage gap regions observe a significant difference from other gap spacings. Considering the overall aerodynamic effects induced by gap spacings and the practical handleability during a wind tunnel test, a gap spacing between 5 mm and 10 mm is therefore recommended for adjacent train vehicles with a 1/8th scale.

Introduction

HSTs (high-speed trains) operating under crosswind are surrounded by transient flow structures, which are random, chaotic, and full of turbulent eddies with different scales. A train immersed in such a turbulent flow is subjected to transient aerodynamic forces that determine the stability, safety, and efficiency of the HSTs (Dorigatti et al., 2015⁠; Versteeg and Malalasekera, 2007). Therefore, numerous studies have been conducted on HSTs cruising under crosswind based on the reduced-scale wind tunnel tests and numerical simulations (Baker et al., 2004⁠; Dorigatti et al., 2015⁠; Niu et al., 2016⁠; Schober et al., 2010⁠; Sicot et al., 2018⁠; Wang et al., 2018, Wang et al., 2018⁠; Zhang et al., 2018b). Owing to the high authenticity and easily controlled experimental conditions, the reduced-scale wind tunnel experiment has almost become one of the most widely used experimental methods for the studies of HST aerodynamic performances (Bocciolone et al., 2008⁠; Chen et al., 2018⁠; Dorigatti et al., 2015⁠; Li et al., 2018b⁠; Niu et al., 2016⁠; Zhang et al., 2018b).

When conducting wind tunnel tests on the multi-car marshaling HST models, such as the typical three-car marshaling HST model consisting of the head, middle, and tail cars, in order to separately test the detailed aerodynamic quantities of each train vehicle of the three-car train models, the head, middle, and tail car models cannot be connected through the internal and external windshields like the actual marshaling method of the operating HSTs. Thus, these three train vehicle models should be separately installed on the wind tunnel floor with a proper gap spacing between adjacent vehicle models to avoid the mechanical contact that causes inaccurate testing of the aerodynamic quantities of the high-speed train model. Zhang et al. (2018a, b) emphasized that in wind tunnel tests, an inter-carriage gap with a certain distance between adjacent vehicles should be adopted to determine the aerodynamic forces acting on individual vehicles and to avoid the mechanical force interference between adjacent vehicles. However, the appropriate gap spacing has not been given. Zhang et al. (2016) highlighted the internal and external windshields between the adjacent vehicle models were separated, not only to make independent and accurate measurements of the aerodynamic quantities for the HST model but also to guarantee the geometric similarity of the inter-carriage gap regions between the model and prototype trains. The gap spacing of 8 mm was adopted for the 1/8th-scale train model, however, without reasons for the gap spacing.

It should be noted that, for real HSTs, adjacent vehicles are connected by internal and external windshields with no gap spacing, while different gap spacings may be adopted in different wind tunnel tests. However, the theoretical basis for wind tunnel tests is based on the similarity criterion, and geometric similarity is essential to achieving complete similarity between the train model and prototype flow fields (Munson et al., 2009⁠; CEN European Standard 14,067-6, 2010). The existing experiences have shown that relatively minor simplifications and distortions of the train model geometry will alter the airflow structures, resulting in substantial variation in the aerodynamic performances. (Munson et al., 2009⁠; CEN European Standard 14,067-6, 2010). Thus, the differences between the gap spacings not only cause discrepancies between wind tunnel test results and an actual train, but also divergences between wind tunnel tests that adopt different gap spacings.

Under no crosswind, 10 different gap spacings were investigated by Li et al. (2019a). The results showed that the gap spacing should be smaller than 10 mm to obtain more realistic results for HSTs in the wind tunnel tests, however, in the study, different gap spacings were achieved by changing the spatial position of a certain vehicle longitudinally, which not only caused the variation of the gap spacing, but also that of the relative position of vehicles and the total length of the scale train models (the distance between the nose tips of the head and tail cars). These variations would lead to additional effects on the study results, in addition to the effects only caused by gap spacings. Thus, to avoid the additional effects caused by the variation of the relative position of vehicles and to focus on the pure effect of gap spacings, six different gap spacings obtained by installing windshields with different thicknesses on the vehicle end walls were systematically researched by Xia et al. (2020). The results showed that the airflow pattern and the aerodynamic loads acting on the vehicle parts surrounding the inter-carriage gap region significantly changed as the gap spacing increased, thus, the gap spacing should not exceed 8 mm to minimize the effect of gap spacings. In general, the aerodynamic performances of HSTs under crosswind are important content for wind tunnel experiments for reduced-scale train models. However, the studies mentioned above about the effects of gap spacings on the flow structures and aerodynamic performances of HST models did not consider the crosswind conditions, which gives impetus to systematically research the influence of gap spacings on HST models under crosswind.

In this study, the method used to achieve different gap spacings is the same as that adopted by Xia et al. (2020), which can be referred to for the details. The numerical simulations are conducted on the 1/8th-scale train models with five different gap spacings of 0, 5, 8, 10, and 20 mm under crosswind to investigate the aerodynamic effects of gap spacings on the HST models under crosswind. The train models and numerical simulation settings are introduced in section 2. Section 3 demonstrates the validation of the numerical methodology and analyses of the time-averaged and transient aerodynamic quantities and flow structures. Finally, the conclusions obtained are given in section 4.