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Slipstream and Flow Structures in the Near Wake of High-Speed Trains
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Train transportation is a vital part of the transportation system of today. Asthe speed of the trains increase, the aerodynamic effects become more impor-tant. One aerodynamic effect that is of vital importance for passengers’ andtrack workers’ safety is slipstream, i.e. the induced velocities by the train.Safety requirements for slipstream are regulated in the Technical Specificationsfor Interoperability (TSI). Earlier experimental studies have found that forhigh-speed passenger trains the largest slipstream velocities occur in the wake.Therefore, in order to study slipstream of high-speed trains, the work in thisthesis is devoted to wake flows. First a test case, a surface-mounted cube, issimulated 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) andthe computed flow field is decomposed into modes with Proper Orthogonal De-composition (POD) and Dynamic Mode Decomposition (DMD). The computedmodes on the surface-mounted cube compare well with prior studies, whichvalidates the use of DDES together with POD/DMD. To ensure that enoughsnapshots are used to compute the POD and DMD modes, a method to inves-tigate the convergence is proposed for each decomposition method. It is foundthat there is a separation bubble behind the CRH1 and two counter-rotatingvortices behind the ATM. Even though the two geometries have different flowtopologies, the dominant flow structure in the wake in terms of energy is thesame, namely vortex shedding. Vortex shedding is also found to be the mostimportant flow structure for slipstream, at the TSI position. In addition, threeconfigurations of the ATM with different number of cars are simulated, in orderto 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, theStrouhal number decreases as the momentum thickness increases. The velocityin ground fixed probes are extracted from the flow, in order to investigate theslipstream velocity defined by the TSI. A large scatter in peak position andvalue for the different probes are found. Investigating the mean velocity atdifferent distances from the train side wall, indicates that wider versions of thesame train will create larger slipstream velocities.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , xii, 64 p.
Series
TRITA-AVE, ISSN 1651-7660 ; 2012:28
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-94182ISBN: 978-91-7501-392-3 (print)OAI: oai:DiVA.org:kth-94182DiVA: diva2:528024
Public defence
2012-06-13, F3, Lindstedsv. 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
TrenOp, Transport Research Environment with Novel Perspectives
Note

QC 20120530

Available from: 2012-05-30 Created: 2012-05-09 Last updated: 2014-02-11Bibliographically approved
List of papers
1. Mode Decomposition on Surface-Mounted Cube
Open this publication in new window or tab >>Mode Decomposition on Surface-Mounted Cube
2012 (English)In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 88, no 3, 279-310 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, the flow around the surface-mounted cube is decomposed into modes using Proper Orthogonal Decomposition (POD) and Koopman mode decomposition, respectively. The objective of the paper is twofold. Firstly, a comparison of the two decomposition methods for a highly separated flow is performed. Secondly, an evaluation of Detached Eddy Simulation (DES) for simulating a time-accurate flow, to be used as input data for the two mode decomposition methods, is accomplished. The knowledge on the accuracy and usefulness of the modes computed with from DES flow fields can then be the foundation for other studies for applied geometries in vehicle aerodynamics. The flow is simulated using DES, which enables time-accurate simulations on flows around realistic vehicle geometries. Most of the first eight modes computed with DES in a reference domain can also be found among the first eight computed with LES in reference work. Since the POD modes computed with DES resemble those computed with LES, the conclusion is that DES is suitable to use for mode decomposition. When comparing the POD and Koopman modes, many similarities can be found in both the spatial and temporal modes. For this case, where the flow contains a broad band of frequencies, it is concluded that the advantage of using Koopman modes, decomposing by frequency, cannot be fully utilized, and Koopman modes are very similar to the POD modes.

Keyword
Detached Eddy Simulation, Koopman mode decomposition, Proper orthogonal decomposition, Surface-mounted cube
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-12884 (URN)10.1007/s10494-011-9355-y (DOI)000303203500001 ()2-s2.0-84861458133 (Scopus ID)
Funder
TrenOp, Transport Research Environment with Novel PerspectivesSwedish e‐Science Research Center
Note

QC 20120511

Available from: 2010-05-18 Created: 2010-05-18 Last updated: 2017-12-12Bibliographically approved
2. Analysis of flow structures in the wake of a high-speed train
Open this publication in new window or tab >>Analysis of flow structures in the wake of a high-speed train
Show others...
2016 (English)In: Proceedings aerodynamics of heavy vehicles III, buses, trucks and trains, Springer, 2016, Vol. 79Conference paper, Published paper (Refereed)
Abstract [en]

Slipstream is the flow that a train pulls along due to the viscosity of the fluid. In real life applications, the effect of the slipstream flow is a safety concern for people on platform, tracksideworkers and objects on platforms such as baggage carts and pushchairs. The most important region for slipstream of high-speed passanger trains is the near wake, in which the flow is fully turbulent with a broad range of length and time scales. In this work, the flow around the Aerodynamic Train Model (ATM) is simulated using Detached Eddy Simulation (DES) to model the turbulence. Different grids are used in order to prove grid converged results. In order to compare with the results of experimental work performed at DLR on the ATM, where a trip wire was attached to the model, it turned out to be necessary to model this wire to have comparable results. An attempt to model the effect of the trip wire via volume forces improved the results but we were not successful at reproducing the full velocity profiles. The flow is analyzed by computing the POD and Koopman modes. The structures in the floware found to be associated with two counter rotating vortices. A strong connection between pairs of modes is found, which is related to the propagation of flow structures for the POD modes. Koopman modes and POD modes are similar in the spatial structure and similarities in frequencies of the time evolution of the structures are also found.

Place, publisher, year, edition, pages
Springer, 2016
Series
Lecture Notes in Applied and Computational Mechanics, ISSN 1613-7736
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-65755 (URN)10.1007/978-3-319-20122-1_1 (DOI)2-s2.0-84951325441 (Scopus ID)978-331920121-4 (ISBN)
External cooperation:
Conference
Aerodynamics of heavy vehicles III: Trucks, buses and trains, September 12-17, Potsdam, Germany, 2010
Note

QC 20160202

Available from: 2012-01-25 Created: 2012-01-25 Last updated: 2016-09-05Bibliographically approved
3. Flow structures around a high-speed train extracted using Proper Orthogonal Decomposition and Dynamic Mode Decomposition
Open this publication in new window or tab >>Flow structures around a high-speed train extracted using Proper Orthogonal Decomposition and Dynamic Mode Decomposition
2012 (English)In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 57, 87-97 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are used to extract the most dominant flow structures of a simulated flow in the wake of a high-speed train model, the Aerodynamic Train Model (ATM). The use of decomposition methods to successfully identify dominant flow structures for an engineering geometry is achieved by using a flow field simulated with the Detached Eddy Simulation model (DES), which is a turbulence model enabling time accurate solutions of the flows around engineering geometries. This paper also examines the convergence of the POD and DMD modes for this case. It is found that the most dominant DMD mode needs a longer sample time to converge than the most dominant POD mode. A comparison between the modes from the two different decomposition methods shows that the second and third POD modes correspond to the same flow structure as the second DMD mode. This is confirmed both by investigating the spectral content of the POD mode coefficients, and by comparing the spatial modes. The flow structure associated with these modes is identified as being vortex shedding. The identification is performed by reconstructing the flow field using the mean flow and the second DMD mode. A second flow structure, a bending of the counter-rotating vortices, is also identified. Identifying this flow structure is achieved by reconstructing the flow field with the mean flow and the fourth and fifth POD modes.

Keyword
Detached Eddy Simulation, Aerodynamic Train Model, Proper Orthogonal Decomposition, Dynamic Mode Decomposition, Slipstream, Train aerodynamics
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-65731 (URN)10.1016/j.compfluid.2011.12.012 (DOI)000301683300007 ()2-s2.0-84857034877 (Scopus ID)
Funder
Swedish e‐Science Research CenterTrenOp, Transport Research Environment with Novel Perspectives
Note

QC 20120411

Available from: 2012-01-25 Created: 2012-01-25 Last updated: 2017-12-08Bibliographically approved
4. Mode Decomposition and Slipstream Velocities in the Wake of Two High-Speed Trains
Open this publication in new window or tab >>Mode Decomposition and Slipstream Velocities in the Wake of Two High-Speed Trains
2012 (English)In: The international Journal of railway technology, ISSN 2049-5358, E-ISSN 2053-602X, The International Journal of Railway TechnologyArticle in journal (Other academic) Submitted
Abstract [en]

Two different train geometries, the Aerodynamic Train Model (ATM) and the CRH1, are studied in order to compare the flow fields around the trains. This paper focuses on the flow structures and flow topologies in the wake. The flow is simulated with Detached Eddy Simulation and decomposed into modes with Proper Orthogonal Decomposition and Dynamic Mode Decomposition, respectively. The topology of the flow is found to be different for the two train geometries, where the flow behind the ATM separates with two counter-rotating vortices, while the flow behind the CRH1 separates with a separation bubble. The difference in flow topology is seen, for instance,  in the mean pressure at the tail, the mean flow in the wake and streamlines of the flow. Despite the different flow topology, there are also similar flow structures in the wake behind the ATM and the CRH1, such as vortex shedding. In order to measure the slipstream effect of the two vehicles, the velocity in a ground fixed point has to be extracted from the train fixed flow field. The resulting velocity is averaged with an equivalent of 1s time average at full scale. The contribution of the DMD modes to slipstream has been analyzed and it is found that the same flow structure that is dominant in energy is also important for slipstream.

Keyword
Detached Eddy Simulation, Aerodynamic Train Model, CRH1, Proper Orthogonal Decomposition, Slipstream, Train aerodynamics
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-94098 (URN)
Projects
Gröna Tåget: Front shape and slipstream for wide body trains at higher speeds
Note

QS 2012

Available from: 2012-05-07 Created: 2012-05-07 Last updated: 2017-12-07Bibliographically approved
5. Wake characteristics of high-speed trains with different lengths
Open this publication in new window or tab >>Wake characteristics of high-speed trains with different lengths
2014 (English)In: Proceedings of the Institution of mechanical engineers. Part F, journal of rail and rapid transit, ISSN 0954-4097, E-ISSN 2041-3017, Vol. 228, no 4, 333-342 p.Article in journal (Refereed) Published
Abstract [en]

Three different train configurations with different numbers of cars are analysed in order to investigate the effect of the train length on wake structures. The train geometry considered is the aerodynamic train model and the different versions have two, three and four cars. Due to the different lengths of the trains, the boundary-layer thickness will be different at the tail of each configuration. The flow is simulated using detached eddy simulation, and coherent flow structures are extracted via proper orthogonal decomposition and dynamic mode decomposition. As a result of reconstruction of the flow field using coupling of the mean flow and the first fluctuating proper orthogonal decomposition mode, it is found that the dominant flow structure in the wake is the same for all three cases. However, this structure has different frequencies and wavelengths depending on the boundary-layer thickness in front of the separation. It is shown that the frequency decreases as the boundary-layer thickness increases for these train configurations.

Keyword
Detached eddy simulation, aerodynamic train model, proper orthogonal decomposition, slipstream, train aerodynamics
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-94099 (URN)10.1177/0954409712473922 (DOI)000335657000001 ()2-s2.0-84899802755 (Scopus ID)
Projects
Gröna Tåget: Front shape and slipstream for wide body trains at higher speeds
Note

QC 20140625. Updated from submitted to published.

Available from: 2012-05-07 Created: 2012-05-07 Last updated: 2017-12-07Bibliographically approved

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Output format
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