David Soper

A model-scale study to assess the influence of ground geometries on aerodynamic flow development around a train

The need for interoperability for rail operators across Europe has resulted in the development of the technical specifications for interoperability: requirements and regulations which include safety limits for train aerodynamics. Safety limits are calculated within guidelines, including environmental conditions, train speeds and ballast shoulder height. However, there are many cases on the European rail network which fall outside ballast shoulder height limits, raising questions about the suitability of the technical specifications for interoperability limits, where European homologation is a requirement. Ballast is a layer of crushed stone onto which the railway track is laid; a ballast shoulder is defined from the top of the ballast layer to the base of the track foundation or ground. This paper describes the detailed model-scale experiments carried out at the University of Birmingham’s moving model TRAIN rig facility to assess the influence of ground geometries on aerodynamic flow development around a train. The technical specifications for interoperability methodology was questioned in relation to whether modest changes to include a wider cross-section of ballast shoulder heights, more appropriate to actual operating conditions, would affect limit values in relation to safety. The influence of ballast shoulder height was investigated for three typical train types. The results showed a similar static pressure development for all the ballast shoulder heights tested. Passenger train results indicated that shallow ballast shoulders confine the aerodynamic flow within a smaller area, increasing the magnitude of slipstream velocities in respect to larger ballast shoulders. The largest slipstream velocities were found for the ground configuration with no ballast shoulder modelled. Measurements within the technical specifications for interoperability-specified range of ballast shoulder heights exhibited little difference in flow development. Analysis of maximum 1 s gusts, calculated using the current technical specifications for interoperability methodology, found values lie close to, but do not break, the existing limits. Increasing ballast shoulder height was shown to decrease values away from technical specifications for interoperability limits.

A comparative study of methods to simulate aerodynamic flow beneath a high-speed train:

The introduction of dedicated high-speed railway lines around the world has led to issues associated with running trains at very high speeds. Aerodynamic effects proportionally increase with train speed squared; consequently, at higher speeds aerodynamic effects will be significantly greater than those of trains travelling at lower speeds. On ballasted track beds, the phenomenon in which ballast particles become airborne during the passage of a high-speed train has led to the need for understanding the processes involved in train and track interaction (both aerodynamical and geotechnical). The difficulty in making full-scale aerodynamic measurements beneath a high-speed train has created the requirement to be able to accurately simulate these complex aerodynamic flows at the model scale. In this study, the results of moving-model tests and numerical simulations were analysed to determine the performance of each method for simulating the aerodynamic flow underneath a high-speed train. Validation was provided for both cases by juxtaposing the results against those from full-scale measurements. The moving-model tests and numerical simulations were performed at the 1/25th scale. Horizontal velocities from the moving-model tests and computational fluid dynamics simulations were mostly comparable except those obtained close to the ballast. In this region, multi-hole aerodynamic probes were unable to accurately measure velocities. The numerical simulations were able to resolve the flow to much smaller turbulent scales than could be measured in the experiments and showed an overshoot in peak velocity magnitudes. Pressure and velocity magnitudes were found to be greater in the numerical simulations than in the experimental tests. This is thought to be due to the influence of ballast stones in the experimental studies allowing the flow to diffuse through them, whereas in the computational fluid dynamics simulations, the flow stagnated on a smooth non-porous surface. Additional validation of standard deviations and turbulence intensities found good agreement between the experimental data but an overshoot in the numerical simulations. Both moving model and computational fluid dynamics techniques were shown to be able to replicate the flow development beneath a high-speed train. These techniques could therefore be used as a method to model underbody flow with a view to train homologation.

Introduction and Literature Review

This thesis presents an experimental investigation carried out to analyse the aerodynamics of freight trains, undertaken as part of the author’s Ph.D. studies. The details and results of these experiments form the main body of this thesis. Preliminary versions of the results in this thesis have been published and presented in a series of conference papers, included in Appendix E.

Slipstream Data Processing Methodology

This chapter introduces the methodology of processing full and model scale experimental data in preparation for analysing results in Chap. 4. Through developing a series of computer scripts using Matlab, the data is converted from raw voltage data to meaningful physical data (Sects. 3.1.1 and 3.1.2). Section 3.2 discusses the process of normalising data to a non-dimensionalised coefficient to enable ease of comparison when analysing results. In Sect. 3.3 the method and choice for aligning data with respect to the train nose is discussed. Section 3.4 calculates associated result uncertainties with respect to instrumentation and methods of collection. Finally Sect. 3.5 presents an introduction to the development of wavelet analysis from Fourier transforms, an analysis technique utilised in Chap. 4.

Aerodynamic Load Experiment Processing Methodology

This chapter introduces the methodology of processing container surface pressure data collected in the open air and crosswind experiment in preparation for analysing results in Chap. 7. Data processing methods developed by Sanquer et al. (Journal of Wind Engineering and Industrial Aerodynamics, 92(7):535–545, 2004), Quinn et al. (Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 221(11): 1367–1379, 2007) and Dorigatti, F., Rail vehicles in crosswinds: analysis of steady and unsteady aerodynamic effects through static and moving model tests, Dorigatti (2013) are adapted for container surface pressure measurements on the freight train (Sect. 6.1). As in Chap. 3, a series of computer scripts are developed in Matlab to convert raw voltage data to meaningful physical data (Sect. 6.2). Throughout this study data is presented in a non-dimensionalised form; Sects. 6.3.1 and 6.3.2 discuss the process of normalising data with respect to the crosswind and open air test sections, respectively. Section 6.4 introduces the method developed by Sanquer et al. (Journal of Wind Engineering and Industrial Aerodynamics, 92(7):535–545, 2004), Quinn et al. (Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 221(11): 1367–1379, 2007) of discretising a pressure tapped vehicle into smaller areas to create coefficients for forces and rolling moments. Finally Sect. 6.5 calculates associated result uncertainties with respect to instrumentation and methods of collection.

Aerodynamic Load Experiment Setup

This chapter introduces a series of model scale experiments undertaken at the TRAIN rig facility to assess aerodynamic forces subjected on an ISO container loaded onto a Class 66 hauled moving model container freight train in an open air and crosswind simulation. The TRAIN rig facility and crosswind generator, previously introduced in Sect. 2.1.1, are discussed in further detail in Sect. 5.1. Section 5.2 discusses adaptions to the moving model to include an on-board pressure monitoring system and data logger. Section 5.3 defines the coordinate system adopted for the aerodynamic load experiments and details of measuring positions. Trackside instrumentation, including measuring probes, position finders and ambient condition monitors are described in Sect. 5.3.2. The experiment methodology for the open air and crosswind sections, bringing together the experiment setup and instrumentation, is discussed in Sect. 5.4.

Slipstream Experiment Setup

This chapter describes the experiment setup and methodology adopted for the slipstream development research. Section 2.1 introduces the model scale slipstream experiment design and methodology undertaken at the TRAIN rig facility; outlining the facility features in Sect. 2.1.1 and the motivation and development of the model design in Sect. 2.1.2. Section 2.1.3.1 defines the coordinate system used for both model and full scale experiments. The model scale trackside instrumentation, including measuring probes, position finders and ambient condition monitors are described in Sect. 2.1.3.2 and the experiment methodology is discussed in Sect. 2.1.4. Similarly Sect. 2.2 discusses the full scale experiment design and methodology undertaken at Uffington. The Uffington test site is introduced in Sect. 2.2.1, and previous Uffington research carried out is discussed in relation to site suitability for slipstream experiments. Sections 2.2.2 and 2.2.3 describe the full scale experiment setup, including trackside instrumentation, and the experiment methodology respectively. Section 2.2.4 gives an overview of all the trains which passed the test site during the experiment and how specific trains have been chosen for this study.

Conclusions and Recommendations for Further Work

The main aim of this research was to characterise the aerodynamic flow around a container freight train and investigate how changing the container loading configuration affects the magnitude of aerodynamic forces measured on a container. In this section a series of conclusions are drawn in terms of the research aim and objectives stated in Chap. 1. Each objective is presented in the following section with a discussion of the work carried out to meet the objective and the conclusions that can be drawn. Recommendations for further work on freight train aerodynamics are considered in Sect. 8.2.

Slipstream Data Analysis, Results and Discussion

In this section the analysis and results from the freight slipstream experiments will be discussed. Throughout the analysis model scale results are presented in terms of a series of container loading configurations defined as consists 1–5, in Fig. 2.8, measured at a series of probe positions defined in Table 2.1. Results will be presented in terms of the equivalent full scale distances in various flow regions at train side and above train roof (Sects. 4.2 and 4.3). Firstly a comparison of flow development for different loading efficiencies is undertaken. By expanding this comparison and focusing on key flow regions (Sect. 4.5), conducting a series of analyses including displacement thickness, turbulence intensity and autocorrelation calculations, a thorough view of slipstream development for a Class 66 hauled container freight train and the influencing factors on slipstream development is possible. Section 4.6 discusses the influence of train speed on slipstream development at model scale. Finally comparisons are made between full and model scale data with conclusions drawn on the suitability of using model scale experiments to understand freight slipstream development (Sect. 4.7).

Aerodynamic Load Analysis, Results and Discussion

In this chapter the analysis and results from the freight aerodynamic load experiments are discussed. Results are presented in terms of the open air (Sect. 7.1) and crosswind (Sect. 7.2) test sections. Firstly a series of individual pressure coefficient time histories are analysed to assess the stability of the on-board data logger system in relation to each test section (Sects. 7.1.1 and 7.2.1). Mean ensemble pressure coefficients are calculated for each pressure tap. The analysis is split into a series of individual container surfaces, presented in Sects. 7.1.2 and 7.2.2. Aerodynamic load coefficients are calculated by discrete integration of mean pressure coefficients across the container surface. The results are analysed to assess the influence of container loading efficiency on aerodynamic load magnitudes and compared with previous freight studies in Sects. 7.1.3 and 7.2.3. A discussion of Reynolds number effects and the influencing factors on uncertainty are presented in Sect. 7.3. Finally general conclusions are drawn in Sect. 7.3.3.