Aeroheating and aerodynamic performance of a transonic hyperloop pod with radial gap and axial channel: A contrastive study

https://doi.org/10.1016/j.jweia.2021.104591Get rights and content

Highlights

  • Effects of the radial gap on the flow field differ between different stages

  • Flow fields around the pods were affected by the channels, especially the radial gap

  • Use of channels has a small effect on pressure drag except during deceleration

  • High temperatures from shockwaves can be reduced using radial and axial channels

Abstract

The aerodynamic characteristics of a hyperloop vehicle with or without channels operating at subsonic speeds in a low-pressure tube were simulated using a scale-adaptive simulation (SAS) method based on the shear-stress transport (SST) κ-ω turbulence model. The aerodynamic behavior of the hyperloop pod, fluid flow, and heat transfer phenomena in the tube were analyzed and evaluated. The grid resolution was analyzed, and the numerical algorithm was validated using a wind tunnel test. The results show that the effects of using channels on the pressure drag of the pod are mainly noticeable at the end of the deceleration stage, and using channels has a great influence on the friction drag relative to the pressure drag, especially during deceleration. The friction drag of such a pod increased with the inclusion of an axial channel in both the acceleration and deceleration stages, whereas it was decreased using a radial gap inside the pod in conjunction with an axial channel during deceleration. The flow field around the pod tail is greatly affected when channels are used relative to other areas around the pod. Using both radial gap and axial channel can effectively reduce the abnormally high temperature in the wake of the pod when the pod surpasses the critical speed. The effects of the radial gap on the flow field in the radial gap are different during different stages of operation, especially in terms of the pressure field. The inclusion of a radial gap is not conducive to improving the surrounding flow field, especially during deceleration. It was also found that the aerodynamic profile design of the channel exit is very important, and the pod speed must be considered.

Introduction

The pursuit of higher vehicle speeds has never ceased. However, in dense atmospheres, the aerodynamic drag of a vehicle has a quadratic relationship with its speed. When the running speed reaches 300 ​km/h, the aerodynamic drag can account for more than 80% of the total drag (Brockie and Baker, 1990; Schetz, 2001; Raghunathan et al., 2002; Tian, 2019; Baker, 2014a, 2014b). By extracting a certain amount of air within a tube to create a low vacuum environment of 10–1000 ​Pa (wherein the continuum assumption still holds), a maglev vehicle may operate with low aerodynamic drag at high speeds under all weather conditions. In addition, such a vehicle is not disturbed by the external environment, as shown in Fig. 1. Therefore, evacuated tube maglev transportation is an important concept for creating green, energy-saving, and high-speed rail transit technology in the future (Janić, 2019).

However, running at high speed causes the gas in front of the vehicle in the tube to be strongly compressed, and the high-speed relative motion between the tube and the vehicle causes intense friction as the gas is in an enclosed space, which generates a large amount of heat (i.e., aerodynamic heating) (Von Driest, 1956; Nonweiler, 1959). This causes the temperature within the tube to increase with a low vacuum pressure (Baron et al., 2001; Kim et al., 2011; Niu et al., 2019) and deteriorates the thermal environment inside the tube. When the vehicle runs at or above the speed of sound, not only are the original aerodynamic effects experienced in the tube aggravated, but new aerodynamic phenomena, such as shock waves, will occur. These phenomena further deteriorate the complicated flow and heat transfer processes within the tube (Zhou et al., 2019; Yang et al., 2017). Fatigue damage to the tube structure may be caused by shock waves and thermal stresses.

At present, many countries worldwide have begun to conduct research on vacuum tube maglev transportation. In 2011, Switzerland announced their intent to build a pipeline automobile transportation system with corresponding plans and was the first country in the world to begin construction on a pipeline transportation system (CASSAT and ESPANET, 2004). In 2013, SpaceX and Tesla, Inc. of the United States proposed a hypothetical scheme for the California Hyperloop between Los Angeles and San Francisco in California. (SpaceXTeslamotors, 2013). In 2016, the US Hyperloop One company conducted a comprehensive test of its maglev hyperloop system in a vacuum environment for the first time, reaching a speed of 70 mph (https://www.wired.com/sto). The Korea Railway Research Institute has also started to fund the vacuum pipeline project, and Hyper Tube Express (HTX), similar to Hyperloop, is also being developed in Korea (Oh et al., 2019; Lee et al., 2019). In China, several institutions have begun to study vacuum pipeline maglev transportation systems. In 2014, a “Super-Maglev” test line was established in Chengdu, with a maximum current speed of 1500 ​km/h. In addition, in 2018, China presented plans to build a hyperloop test line in Tongren, Guizhou, which will be the first hyperloop in China (Zhou et al., 2020a; Rob et al., 2019). The Netherlands officially launched the first full-size hyperloop test facility in Europe in 2019 and plans to build the European Hyperloop Center in 2022, with the test line following in 2023 (http://us.cnn.com/travel/). At present, research on the hyperloop has reached the transition stage between theoretical conception and the establishment of test lines, but it has not been put into practical application. The next key is to establish relevant test lines for field tests, laying a foundation for future practical operations.

Existing studies show that when the vehicle is traveling at or above the speed of sound, the original aerodynamic effect inside the tube will not only intensify but will also produce shock waves and other new aerodynamic phenomena. The shock cluster has bow shock, positive shock, reflected shock, rhombic shock, and other structures. The shock wave distributions differ greatly between different parts of the pod. The length of the high-pressure area in front of the train in the pipeline decreases linearly with the increase in running speed and increases linearly with the increase in running time. The shock wave near the head area is strongly curved, and the shock wave layer is very thin. At the same time, the tail shock length increases with increasing running speed. The suspension gap causes the shock waves at the rear of the train to appear asymmetrical. The stronger the shock wave, the greater the entropy increase caused by the fluid passing through the shock wave (Zhang et al., 2019; Deng et al., 2017; Zhou et al., 2020b; Li et al., 2013). Currently, no real hyperloop exists. However, because airplanes fly in high-speed and low-pressure environments and the aerodynamic characteristics of aircraft intake pipes are similar to those of pods, the shock wave characteristics of aircraft intake pipes are used as an important reference object for the study of hyperloop shock waves. The shock wave/boundary layer interference phenomena in aircraft inlets mainly include normal shock wave/boundary layer interference, oblique shock wave/boundary layer interference, and three-dimensional shock wave/boundary layer interference, which have significant coupling interference characteristics and obvious three-dimensional characteristics. The arrangement of baffles in the internal contraction section can significantly inhibit large-scale separation induced by strong shock wave/boundary layer interference in the inlet and improve the inlet flow field structure (Niu et al., 2019; Oh et al., 2019). For an inlet with a side plate, the larger the side plate area, the more obvious the shock wave. The high temperature rise of the pipe wall in the inlet enhances the interference between the shock waves and boundary layer, reduces the performance of the inlet, and even prevents the inlet flow from starting (Zhou et al., 2019). When an aircraft flies at supersonic speed, the compression of the bulge at the entrance of the large S-curved inlet with a bulge generates a conical wave which is continuously compressed to generate secondary flow. Within a certain range, appropriately reducing the height of the bulge or reducing the sweep angle of the lip margin is conducive to reducing the shock wave (Zhou and Zhang, 2020a). The shock wave distribution law in the aircraft inlet pipe can also be used to design a binary curved inlet, which can control the deformation of the compression surface by controlling the aerodynamic difference on the lower surface of the elastic compression surface, changing the position and shape of the curved shock wave, and improving the performance of the inlet (Zhang et al., 2018).

At present, research on evacuated tube maglev transportation is mainly focused on reducing the aerodynamic drag of the vehicle, as well as the relationship between the aerodynamic drag and other influential parameters (Kim et al., 2011; Zhang et al., 2014; Chen et al., 2015; Sheng et al., 2018; Kim. and Felder., 2011). Few studies have been conducted on the formation and space-time distribution characteristics of the flow and heat transfer within tubes that possess a low-vacuum environment. In this study, the flow and heat transfer characteristics in a tube resulting from a hyperloop vehicle running at a transonic speed were simulated. The aerodynamic behaviors within the tube resulting from a hyperloop vehicle with or without channels were compared and analyzed.

The remainder of this paper is organized as follows. Section 2 establishes the relevant numerical model and introduces the research methods adopted. Section 3 conducts mesh resolution analysis and comparison with wind tunnel experiments, and Section 4 describes the study results of aerodynamic drag, flow, and aerodynamic heating analysis. Finally, Section 5 concludes the study. The conclusions of this study may be helpful in guiding the design and operational parameters of transonic hyperloop vehicles.

Section snippets

Models

Three different configurations of channel models—C1 (without channels), C2 (with an axial channel), and C3 (with radial gap and axial channel)—were selected for installation inside the hyperloop pod, as shown in Fig. 2(a). The reason for using the channel and gap is that the front pressure is larger and the rear pressure is smaller, which results in a large pressure difference between the front and rear of the pod. The establishment of the channel and gap can transport part of the pod front gas

Grid resolution analysis

Grid independence analysis employed the case of a full-scale pod running in a tube at a uniform speed of 555 ​m/s. As shown in Table 1, the difference in the pressure and friction Cd between the Coarse and Fine grids is less than 1.08%, and the difference in mean Cd between the Medium and Fine grids is even smaller.

As shown in Fig. 6, the main difference in the mean values of Cp and CT on the train surface among the three grid densities was mainly concentrated at the pod tail, especially for CT

Aerodynamic drag analysis

Aerodynamic drag is composed of pressure and friction drag components (Brockie and Baker, 1990; Tian, 2019; Achenbach, 1968), which are used to analyze the effects of the channels on the aerodynamic drag of a pod operating under different conditions. As shown in the black box at the bottom of Fig. 15(a), the aerodynamic pressure drag (Fd) of the pod with both the radial gap and axial channel (C3) is the largest at the beginning of the acceleration stage, and the Fd of the pod with an axial

Conclusions

In this study, the aerodynamic characteristics of a tube with a low pressure resulting from a hyperloop pod with or without channels operating at supersonic speeds were investigated. The following conclusions were drawn:

  • 1.

    The use of channels has a small effect on the pressure drag of the pod, except at the end of the deceleration stage. The use of an axial channel delays the rate of pressure drag reduction, followed by a sudden decrease in the pressure drag. Considering the friction drag, the

CRediT authorship contribution statement

Kangyi Zhou: Writing – original draft, Data curation, Methodology. Guofu Ding: Supervision, Software, Resources, Project administration. Yueming Wang: Investigation, Methodology, Validation. Jiqiang Niu: Conceptualization, Visualization, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51805453), the China Postdoctoral Science Foundation (2019M663551), the Fundamental Research Funds for the Central Universities (2682018CX14), and the Sichuan Science and Technology Program (2020JDTD0012).

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