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Mode Decomposition on Surface-Mounted Cube
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. 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: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 88, no 3, p. 279-310Article 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.

Place, publisher, year, edition, pages
2012. Vol. 88, no 3, p. 279-310
Keywords [en]
Detached Eddy Simulation, Koopman mode decomposition, Proper orthogonal decomposition, Surface-mounted cube
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-12884DOI: 10.1007/s10494-011-9355-yISI: 000303203500001Scopus ID: 2-s2.0-84861458133OAI: oai:DiVA.org:kth-12884DiVA, id: diva2:319517
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
In thesis
1. Analysis of Flow Structures in Wake Flows for Train Aerodynamics
Open this publication in new window or tab >>Analysis of Flow Structures in Wake Flows for Train Aerodynamics
2010 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Train transportation is a vital part of the transportation system of today anddue to its safe and environmental friendly concept it will be even more impor-tant in the future. The speeds of trains have increased continuously and withhigher speeds the aerodynamic effects become even more important. One aero-dynamic effect that is of vital importance for passengers’ and track workers’safety is slipstream, i.e. the flow that is dragged by the train. Earlier ex-perimental studies have found that for high-speed passenger trains the largestslipstream velocities occur in the wake. Therefore the work in this thesis isdevoted to wake flows. First a test case, a surface-mounted cube, is simulatedto test the analysis methodology that is later applied to a train geometry, theAerodynamic Train Model (ATM). Results on both geometries are comparedwith other studies, which are either numerical or experimental. The comparisonfor the cube between simulated results and other studies is satisfactory, whiledue to a trip wire in the experiment the results for the ATM do not match.The computed flow fields are used to compute the POD and Koopman modes.For the cube this is done in two regions of the flow, one to compare with a priorpublished study Manhart & Wengle (1993) and another covering more of theflow and especially the wake of the cube. For the ATM, a region containing theimportant flow structures is identified in the wake, by looking at instantaneousand fluctuating velocities. To ensure converged POD modes two methods toinvestigate the convergence are proposed, tested and applied. Analysis of themodes enables the identification of the important flow structures. The flowtopologies of the two geometries are very different and the flow structures arealso different, but the same methodology can be applied in both cases. For thesurface-mounted cube, three groups of flow structures are found. First groupis the mean flow and then two kinds of perturbations around the mean flow.The first perturbation is at the edge of the wake, relating to the shear layerbetween the free stream and the disturbed flow. The second perturbation isinside the wake and is the convection of vortices. These groups would then betypical of the separation bubble that exists in the wake of the cube. For theATM the main flow topology consists of two counter rotating vortices. Thiscan be seen in the decomposed modes, which, except for the mean flow, almostonly contain flow structures relating to these vortices.

Publisher
p. 136
Series
Trita-MEK, ISSN 0348-467X ; 2010:04
Keywords
Train Aerodynamics, Slipstream, Wake Flow, Detached-EddySimulation, Proper Orthogonal Decomposition, Koopman Mode Decomposi-tion, Surface-mounted Cube, Aerodynamic Train Model
National Category
Fluid Mechanics and Acoustics Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-12746 (URN)978-91-7415-651-5 (ISBN)
Presentation
2010-05-28, MWL74, Teknikringen 8, KTH, 13:15 (English)
Opponent
Supervisors
Projects
Gröna Tåget
Note
QC 20100518Available from: 2010-05-18 Created: 2010-05-07 Last updated: 2012-03-21Bibliographically approved
2. 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. p. xii, 64
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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