Aerodynamic characteristics of a tube train

Abstract

Recently, full-scale research about a passenger tube train system is being progressed as a next-generation transportation system in Korea in light of global green technology. The Korea Railroad Research Institute (KRRI) has commenced official research on the construction of a tube train system. In this paper, we studied various parameters of the tube train system such as the internal tube pressure, blockage ratio, and operating speed through computational analysis with a symmetric and elongated vehicle. This study was about the aerodynamic characteristics of a tube train that operated under standard atmospheric pressure (open field system, viz., ground) and in various internal tube environments (varying internal tube pressure, blockage ratio, and operating speed) with the same shape and operating speed. Under these conditions, the internal tube pressure was calculated when the energy efficiency had the same value as that of the open field train depending on various combinations of the operating speed and blockage ratio (the P–D relation). In addition, the dependence of the relation between the internal tube pressure and the blockage ratio (the P–β relation) was shown. Besides, the dependence of the relation between the total drag and the operating speed depending on various combinations of the blockage ratio and internal tube pressure (the D–V relation) was shown. Also, we compared the total (aerodynamic) drag of a train in the open field with the total drag of a train inside a tube. Then, we calculated the limit speed of the tube train, i.e., the maximum speed, for various internal tube pressures (the V–P relation) and the critical speed that leads to shock waves under various blockage ratios, which is related to the efficiency of the tube train (the critical V–β relation). Those results provide guidelines for the initial design and construction of a tube train system.

Introduction

A tube train system is a high-speed ground vehicle through a sealed tube or tunnel. Regarding travel in a sealed and sub-vacuum tube, aerodynamic noise that propagates to the ambient environment is shielded and energy efficiency is increased through a decrease in the total or aerodynamic drag (Kim, 2008, Retrieved 23.05.07.,, Retrieved 23.04.07.,). The tube system originated from a Pneumatic Capsule Pipeline (PCP) system. After the PCP system was officially proposed by the English businessman, George Medhurst, in 1810, a variety of studies and actual construction were undertaken. After the first proposition, the constructed PCP system for which the driving force was the pressure difference in the tube has been used for small-cargo haulage in the US, the UK, and France. PCP transport for personnel also has been studied, but not been built as yet (Retrieved 03.05.09.,, Retrieved 03.05.09.,). This system rose to prominence in the late 1960s and early 1970s during the first oil shock. In recent times, the PCP system has attracted renewed interest in the light of energy and environmental concerns. Hence, studies of small vehicles for personnel and cargoes in the tube train or capsule system have been performed in the US, Germany, Italy, Switzerland, and so on (Retrieved 29.05.07.,, Retrieved 06.12.07.,, Retrieved 05.12.07.,, Retrieved 24.04.07.,). In those studies, Swissmetro proposed and studied a tube train system based on a magnetic levitation (maglev) train for the large-scale transportation of personnel (Retrieved 23.05.07.). The Korea Railroad Research Institute (KRRI) also has commenced official research on a tube train system based on a maglev train traveling at 700–1000 km/h for the construction of a passenger tube train system. However, systematic research about the aerodynamic characteristics of such a high-speed tube train system is rather scant (Kwon et al., 2008, Lee et al., 2008, Kwon, 2008).

The most intensive study about the aerodynamic analysis and energy efficiency of a tube system was conducted from September 1966 through October 1969 in a project undertaken by the US Transportation Bureau; the experimental results were published in 1970 (Trzaskoma, 1970). According to this study, the drag coefficient (C_D) of a vehicle is about 0.015–0.11 when the vehicle has a semicircular nose and rear under the atmospheric internal tube pressure condition. The Reynolds number (Re) is 10⁵ and the blockage ratio is one of 0.125, 0.22, 0.38, and 0.5. Harman and Davidson (1977) compared C_D under the wind-tunnel condition with C_D under the moving-vehicle condition. An ogive-shaped vehicle was used and Re was 10⁵. The blockage ratio was varied from 0.6 to 0.9 under the internal tube condition of about one atm (14.5 psia) and 74 ° (296.5 K) (Harman and Davidson, 1977). However, those experiments had limitations and showed only a trend in C_D because the tube train model had a simple geometry; a semicircular nose and an ogive rear, and had a relatively short length (the ratio of the length to diameter was 8.85). Furthermore, those experimental conditions and data were not suitable for the recently required tube train systems that operate at a Mach number of 0.6 and internal tube pressure of 0.01 atm, because the maximum operating speed of the experimental vehicle model was a Mach number of 0.4 or less and the internal tube pressure was the atmospheric condition of about one atm. In a doctoral dissertation at Ecole polytechnique fédérale de Lausanne (EPFL) in 1999, a relationship between the pressure waves and aerodynamic drag in a tunnel was studied through the Swissmetro model (Bourquin and Alexis Monkewitz, 1999). However, this study needs to be complemented for accuracy because the numerical models used were a 1-D unsteady pressure-wave model and a 1-D laminar unsteady friction model with heat transfer. It said that these models had not enough accuracy itself for prediction of drag with an unsteady flow pattern in a long tunnel and it is necessary to combine 3-D numerical computation to obtain the overall drag and its components on a high-performance train.

Thus, in this paper, we studied the aerodynamic drag of a tube train by changing important initial design variables such as the blockage ratio, internal tube pressure, and operating speed. A full-scale investigation of the tube train system was conducted for the initial design state. Since experimental analysis has limitations with regard to geometry, temperature, pressure, and operating speed, we performed computational analysis based on unsteady compressible Navier–Stokes equations. These study results, which include various driving aerodynamic drag data and the trend of C_D with respect to each blockage ratio, internal tube pressure, and operating speed, will provide guidelines for the construction of a tube train system and related research.