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Slipstream and Flow Structures in the Near Wake of High-Speed Trains

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Slipstream and Flow Structures in the Near Wake of High-Speed Trains Slipstream and Flow Structures in the Near Wake of High-Speed Trains by Tomas W. Muld June 2012 Technical Reports Royal Institute of Technology Department of Aeronautical and Vehicle Engineering SE-100 44 Stockholm, Sweden Akademisk avhandling som med tillst˚and av Kungliga Tekniska H¨ogskolan i Stockholm framl¨agges till offentlig granskning f¨or avl¨aggande av teknologie doktorsexamen onsdag den 13 juni 2012 kl 10.00 i sal F3, Kungliga Tekniska H¨ogskolan, Lindstedsv. 26, Stockholm. c Tomas W. Muld 2012 ISBN 978-91-7501-392-3 TRITA AVE 2012:28 ISSN 1651-7660 ISRN KTH/AVE/DA-12/28-SE Universitetsservice US–AB, Stockholm 2012 Till Mamma ♥ 1944-2009 iii iv Aerodynamics are for people who can’t build engines Enzo Ferrari 1898-1988 v Slipstream and Flow Structures in the Near Wake of High- Speed Trains Tomas W. Muld Linn´eFlow Centre, KTH Aeronautical and Vehicle Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Abstract Train transportation is a vital part of the transportation system of today. As the speed of the trains increase, the aerodynamic effects become more impor- tant. One aerodynamic effect that is of vital importance for passengers’ and track workers’ safety is slipstream, i.e. the induced velocities by the train. Safety requirements for slipstream are regulated in the Technical Specifications for Interoperability (TSI). Earlier experimental studies have found that for high-speed passenger trains the largest slipstream velocities occur in the wake. Therefore, in order to study slipstream of high-speed trains, the work in this thesis is devoted to wake flows. First a test case, a surface-mounted cube, is simulated to test the analysis methodology that is later applied to two differ- ent train geometries, the Aerodynamic Train Model (ATM) and the CRH1. The flow is simulated with Delayed-Detached Eddy Simulation (DDES) and the computed flow field is decomposed into modes with Proper Orthogonal De- composition (POD) and Dynamic Mode Decomposition (DMD). The computed modes on the surface-mounted cube compare well with prior studies, which validates the use of DDES together with POD/DMD. To ensure that enough snapshots are used to compute the POD and DMD modes, a method to inves- tigate the convergence is proposed for each decomposition method. It is found that there is a separation bubble behind the CRH1 and two counter-rotating vortices behind the ATM. Even though the two geometries have different flow topologies, the dominant flow structure in the wake in terms of energy is the same, namely vortex shedding. Vortex shedding is also found to be the most important flow structure for slipstream, at the TSI position. In addition, three configurations of the ATM with different number of cars are simulated, in order to investigate the effect of the size of the boundary layer on the flow structures. The most dominant structure is the same for all configurations, however, the Strouhal number decreases as the momentum thickness increases. The velocity in ground fixed probes are extracted from the flow, in order to investigate the slipstream velocity defined by the TSI. A large scatter in peak position and value for the different probes are found. Investigating the mean velocity at different distances from the train side wall, indicates that wider versions of the same train will create larger slipstream velocities. Descriptors: Train Aerodynamics, Slipstream, Wake Flow, Detached-Eddy Simulation, Proper Orthogonal Decomposition, Dynamic Mode Decomposition, Surface-mounted Cube, Aerodynamic Train Model, CRH1 vi Preface This doctoral thesis is written in the topic of engineering mechanics, and ad- dresses train aerodynamics. It contains original research by the candidate, except where explicit references are given. This thesis consists of two parts. The first part gives a background into the field and presents the numerical simulation as well as the analysis techniques used in thesis. The first part also gives an overview of the results in the appended articles. The second part is a collection of the following articles: Paper 1. T.W. Muld, G. Efraimsson and D.S. Henningson, Mode Decomposition on Surface-Mounted Cube Flow, Turbulence and Combustion 88(3) 279-310 (2012) Paper 2. T.W. Muld, G. Efraimsson, D.S. Henningson, A.H. Herbst and A. Orellano, Analysis of Flow Structures in the Wake of a High-Speed Train Proceedings of Aerodynamics of Heavy Vehicles III, Truck, Buses and Trains (2010) Paper 3. T.W. Muld, G. Efraimsson and D.S. Henningson, Flow structures around a high-speed train extracted using Proper Orthogonal Decomposition and Dynamic Mode Decomposition Computers & Fluids 57, 87-97 (2012) Paper 4. T.W. Muld, G. Efraimsson and D.S. Henningson, Mode Decomposition and Slipstream Velocities in the Wake of Two High-Speed Trains Submitted to The International Journal of Railway Technology (2012) Paper 5. T.W. Muld, G. Efraimsson and D.S. Henningson, Wake Characteristics of High-Speed Trains with Different Lengths Submitted to Journal of Rail and Rapid Transit (2012) vii Other publications by the candidate: Paper A1. T.W. Muld, G. Efraimsson, D.S. Henningson, A.H. Herbst and A. Orellano, Detached Eddy Simulation and Validation on the Aerodynamic Train Model Proceedings of EUROMECH COLLOQUIUM 509: Vehicle Aerodynamics (2009) Paper A2. T.W. Muld, G. Efraimsson, D.S. Henningson, Proper Orthogonal Decomposition of Flow Structures around a Surface-Mounted Cube Computed with Detached-Eddy Simulation SAE-2009-01-0331 (2009), SAE World Congress 2009 Paper A3. T.W. Muld, G. Efraimsson, D.S. Henningson, Mode Decomposition of Flow Structures in the Wake of Two High-Speed Trains in ”Proceedings of the First International Conference on Railway Technology: Research, Development and Maintenance”, J. Pombo, (Editor), Civil-Comp Press, Stirlingshire, UK, Paper 156, (2012) doi:10.4203/ccp.98.156 viii Division of work between authors The work is done within the research program Gr¨ona T˚aget, which is financed by Trafikverket (the Swedish Transport Administration). The title of the project is Front shape and slipstream for wide body trains at higher speeds and is a collaboration between KTH and Bombardier Transportation. The work has been supervised by Docent Gunilla Efraimsson(GE) and Prof. Dan S. Henningson(DH) at KTH. Leading the research project is Dr. Astrid H. Herbst (AH) from Bombardier Transportation, also involved in the project is Dr. Alexander Orellano(AO), also from Bombardier Transportation. Paper 1 TM performed the simulations, the post-processing, the mode decomposition, the analysis and the writing of the paper together with GE and DH. Paper 2 TM performed the simulations, the post-processing, the mode decomposition, the analysis and the writing of the paper together with GE and DH. Original mesh, geometry and experimental data were provided by AH and AO. Paper 3 TM performed the simulations, the post-processing, the mode decomposition, the analysis and the writing of the paper together with GE and DH. Paper 4 TM performed the simulations, the post-processing, the mode decomposition, the analysis and the writing of the paper together with GE and DH. Paper 5 TM performed the simulations, the post-processing, the mode decomposition, the analysis and the writing of the paper together with GE and DH. ix Nomenclature ATM Aerodynamic Train Model ATM2C, ATM3C, ATM4C ATM with 2,3,4 cars BL Boundary Layer CFD Computational Fluid Dynamics Cp Pressure Coefficient DDES Delayed Detached Eddy Simulation DES Detached Eddy Simulation d Wall Distance d˜ Modified Wall Distance dh Hydraulic Diameter DMD Dynamic Mode Decomposition DNS Direct Numerical Simulation ∆t DES Time step ∆tP OD Time step between POD snapshots EFD Experimental Fluid Mechanics G Giga (109) H LengthoftheCube Kph Kilometers per hour LES Large Eddy Simulation LT Turbulent Length Scale M Mega(106) MARS Monotone Advection and Reconstruction Scheme ν Kinematic Viscosity νt Turbulent Kinematic Viscosity Ω POD Domain RANS Reynolds Average Navier-Stokes Re Reynolds Number POD ProperOrthogonalDecomposition SA Spalart-Allmaras St Strouhal Number TA Sampling Time TI Turbulent Intensity TOR Top of Rail TSI Technical Specification for Interoperability TVD Total Variation Diminishing θ Momentum Thickness U,V,W Mean Velocity Components U∞ Free Stream Velocity urms Root-mean-square Velocity x Abstract vi Preface vii Nomenclature x Chapter 1. Introduction 1 1.1. Train Aerodynamics 3 1.2. Wake Flows behind Bluff Bodies 10 1.3. Wake Flows in Train Aerodynamics 14 Chapter 2. Geometries 15 2.1. Surface-Mounted Cube 15 2.2. High-Speed Trains 16 Chapter 3. Numerical Simulations 20 3.1. Detached-Eddy Simulations 20 3.2. Discretization Methods and Numerical Grid 23 3.3. Boundary Conditions 24 3.4. Computational Time 25 Chapter 4. Decomposition Methods 27 4.1. ProperOrthogonalDecomposition 28 4.2. Dynamic Mode Decomposition 30 4.3. Similarities and Differences between POD and DMD 33 4.4. Decomposition Domains 33 Chapter 5. Results and Discussion 35 5.1. Verification and Validation of Methods to identify Flow Structures 35 5.2. Slipstream Velocities at TSI-measurement point 41 5.3. Flow Structures in the Wake of Bluff Bodies 45 5.4. Comparing Flow Structures from POD and DMD 48 5.5. Size of the Boundary Layer and Wake Flow Patterns 51 Chapter 6. Conclusion 55 Chapter 7. Outlook 56 Chapter 8. Summary of Papers 58 Acknowledgements 61 Bibliography 62 xi Paper 1. Mode Decomposition on Surface-Mounted Cube 71 Paper 2. Analysis of Flow Structures in the Wake of a High- Speed Train 109 Paper 3. Flow structures around a high-speed train extracted using Proper Orthogonal Decomposition and Dynamic Mode Decomposition 129 Paper 4. Mode Decomposition and Slipstream Velocities in the Wake of Two High-Speed Trains 161 Paper 5. Wake characteristics of High-Speed trains with different lengths 203 xii Part I Introduction CHAPTER 1 Introduction Trains have moved people and goods around countries and continents ever since Stephenson’s Rocket won the Rainhill Trials in 1829; a competition where the winning design would traffic the newly built railway between Liverpool and Manchester.
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