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Flow structures around a high-speed train extracted using Proper Orthogonal Decomposition and Dynamic Mode Decomposition
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aeroacoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aeroacoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-9061-4174
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0001-7864-3071
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.

Place, publisher, year, edition, pages
2012. Vol. 57, 87-97 p.
Keyword [en]
Detached Eddy Simulation, Aerodynamic Train Model, Proper Orthogonal Decomposition, Dynamic Mode Decomposition, Slipstream, Train aerodynamics
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-65731DOI: 10.1016/j.compfluid.2011.12.012ISI: 000301683300007Scopus ID: 2-s2.0-84857034877OAI: oai:DiVA.org:kth-65731DiVA: diva2:483591
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
In thesis
1. Slipstream and Flow Structures in the Near Wake of High-Speed Trains
Open this publication in new window or tab >>Slipstream and Flow Structures in the Near Wake of High-Speed Trains
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:nbn:se:kth:diva-94182 (URN)978-91-7501-392-3 (ISBN)
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

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Efraimsson, GunillaHenningson, Dan S

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