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Analysis of Flow Structures in Wake Flows for Train Aerodynamics
KTH, School of Engineering Sciences (SCI), Mechanics.
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.

Place, publisher, year, edition, pages
2010. , 136 p.
Series
Trita-MEK, ISSN 0348-467X ; 2010:04
Keyword [en]
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: urn:nbn:se:kth:diva-12746ISBN: 978-91-7415-651-5 (print)OAI: oai:DiVA.org:kth-12746DiVA: diva2:318538
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
List of papers
1. Mode Decomposition of the Flow Behind the Aerodynamic Train Model Simulated by Detached Eddy Simulation
Open this publication in new window or tab >>Mode Decomposition of the Flow Behind the Aerodynamic Train Model Simulated by Detached Eddy Simulation
2010 (English)Report (Other (popular science, discussion, etc.))
Series
Trita-AVE, ISSN 1651-7660 ; 2010:28
Identifiers
urn:nbn:se:kth:diva-12886 (URN)
Note
QC 20100518Available from: 2010-05-18 Created: 2010-05-18 Last updated: 2012-03-21Bibliographically approved
2. Detached Eddy Simulation and Validation on the Aerodynamic Train Model
Open this publication in new window or tab >>Detached Eddy Simulation and Validation on the Aerodynamic Train Model
Show others...
2009 (English)In: EUROMECH COLLOQUIUM 509: Vehicle Aerodynamics, 2009Conference paper, Published paper (Other academic)
Abstract [en]

We present CFD-simulations of the flow around the aerodynamic train model(ATM). The turbulence modelling technique is detached eddy-simulation(DES), where the DES model is based on the k-ω SST RANS model. TheReynolds number for the simulation is 60.000 based on the hydraulic diame-ter (3m in full scale), free-stream velocity and kinematic viscosity of air. Themodel used is in 1:50 scale. The numerical results are compared to water tunnelexperimental data on the ATM available from the German Aerospace Center(DLR). The velocity field is measured using particle image velocimetry (PIV).The numerical setup is made to match the experimental setup as close as possi-ble. Focus of the analysis is on the flow in the wake of the train. Comparisonsof the averaged velocity and the velocity fluctuations in the wake shows that theoverall levels and trends are captured by the numerical simulations. However,the peak value of the velocity magnitude in the wake seems to be overestimatedby the DES technique used.

Identifiers
urn:nbn:se:kth:diva-12745 (URN)
Projects
Gröna Tåget
Note
QC 20100518Available from: 2010-05-18 Created: 2010-05-07 Last updated: 2012-03-21Bibliographically approved
3. 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

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