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Research Papers: Flows in Complex Systems

LES Study of the Influence of a Train-Nose Shape on the Flow Structures Under Cross-Wind Conditions

[+] Author and Article Information
Hassan Hemida

Division of Fluid Dynamics, Department of Applied Mechanics, Chalmers University of Technology, SE-41296 Gothenburg, Swedenhemida@chalmers.se

Siniša Krajnović

Division of Fluid Dynamics, Department of Applied Mechanics, Chalmers University of Technology, SE-41296 Gothenburg, Sweden

J. Fluids Eng 130(9), 091101 (Aug 12, 2008) (12 pages) doi:10.1115/1.2953228 History: Received December 20, 2006; Revised April 26, 2008; Published August 12, 2008

Cross-wind flows around two simplified high-speed trains with different nose shapes are studied using large-eddy simulation (LES) with the standard Smagorinsky model. The Reynolds number is 3×105 based on the height of the train and the freestream velocity. The cross section and the length of the two train models are identical while one model has a nose length twice that of the other. The three-dimensional effects of the nose on the flow structures in the wake and on the aerodynamic quantities such as lift and side force coefficients, flow patterns, local pressure coefficient, and wake frequencies are investigated. The short-nose train simulation shows highly unsteady and three-dimensional flow around the nose yielding more vortex structures in the wake. These structures result in a surface flow that differs from that in the long-nose train flow. They also influence the dominating frequencies that arise due to the shear-layer instabilities. Prediction of vortex shedding, flow patterns in the train surface, and time-averaged pressure distribution obtained from the long-nose train simulation are in good agreement with the available experimental data.

Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) The shape of the simplified train models. (b) Computational domain.

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Figure 2

Coarse mesh (a) cross section of the mesh shows the first and the second O-grids around the train model. (b) Mesh shape around the train nose in the symmetric x−y plane of the train.

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Figure 3

Time-averaged local surface pressure coefficient, Cp, at x∕D=6.5. (a) Long-nose model; (b) short-nose model. The experimental data of Chiu (3) shown in (a) and (b) have been collected on the long-nose model.

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Figure 4

Long-nose model flow: time-averaged trace lines projected on the train surface

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Figure 5

Short-nose model flow: time-averaged trace lines projected on the train surface

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Figure 6

Vortex cores in the wake flow showing the time-averaged flow structures. (a) Short-nose model; (b) long-nose model. Upper figures: view from the exit; lower figures: view from the roof.

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Figure 7

Long-nose model flow: time-averaged streamlines showing the circulation regions projected onto the y−z planes at (a) x∕D=1.5, (b) x∕D=2.5, (c) x∕D=3.5, (d) x∕D=4.5, (e) x∕D=6, and (f) x∕D=8.

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Figure 8

Short-nose model flow: time-averaged streamlines showing the circulation regions projected onto the y−z planes at (a) x∕D=1.5, (b) x∕D=2.5, (c) x∕D=3.5, (d) x∕D=4.5, (e) x∕D=6, and (f) x∕D=8.

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Figure 9

Time-averaged velocity vectors on the leeward side of the train close to the nose: (a) short-nose train; (b) long-nose train

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Figure 10

Vortex cores in the time-averaged wake flow close to the nose: (a) short-nose train; (b) long-nose train

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Figure 11

Schematic representation showing the time-averaged flow structures around the nose: (a) short-nose train; (b) long-nose train

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Figure 13

Short-nose model flow: time-averaged streamlines and velocity vectors showing the separation bubble on the roof-side face, projected onto the y−z planes at (a) x∕D=1, (b) x∕D=3, (c) x∕D=5, and (d) x∕D=8.

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Figure 14

Long-nose model flow: (a) time-averaged streamlines projected onto the y−z plane x∕D=4 showing the separation bubble on the roof-side face, (b) zoom of region C from Fig. 1 and (c) a zoom of region R in Fig. 4 showing vector plots on the separation region

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Figure 15

Comparison of the surface pressure distributions along the train length: (solid line) long-nose LES; (dashed) short-nose LES; (symbols) experiment from Chiu (3)

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Figure 16

Long-nose model flow: temporal evolution of the coherent structures in the region between x∕D=3 and x∕D=6. (a), (b), and (c) show the isosurface of the instantaneous static pressure, p=−0.19Pa, at three different times. (a′), (b′), and (c′) show the isosurface of the instantaneous second invariant of the velocity gradient Q=7000. The time difference between two successive pictures is t′=0.12.

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Figure 17

Long-nose model flow: time history of the force coefficients; (a) Side force coefficient; (b) lift force coefficient.

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Figure 18

Short-nose model flow: time history of the force coefficients. (a) Side force coefficient; (b) lift force coefficient.

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Figure 19

Long-nose model flow: autopower spectra E(St)=F(St)F¯(St) of the time-varying force coefficients, where F(St) is the Fourier transform of the time-varying signal and F¯(St) is the complex conjugate of F(St). (a) and (b) are the autopower spectra of Cs and Cl, respectively, drawn against Strouhal number St=fD∕U∞, where f is the force’s time-varying frequency.

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Figure 20

Short-nose model flow: autopower spectra of the time-varying force coefficients.

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Figure 12

Short-nose model flow: time-averaged streamlines and velocity vectors projected onto the x−z planes parallel to the train length: (a) y∕D=−0.5, (b) y∕D=−0.4, (c) y∕D=−0.3, (d) y∕D=−0.2, (e) y∕D=−0.1, (f) y∕D=0, (g) y∕D=0.1, and (h) y∕D=0.2. View is from the upper side of the train.

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