Section 9-25 S789

B. SCHULTE-WERNING, c. HEINE AND G. MATSCHKE

Slipstream Development and Wake Flow Characteristics of modern High-speed trains

The tendency to increased cruising velocity of high-speed trains is unbroken, although stabilising the level already reached is dominant [ 13. In parallel the weight of the end coaches in the future train sets is decreased because of replacing the power car concept by the distributed traction concept and because of axle load limitations in the trans-European interoperability rules. This allows both for a less overall weight of the train with its reduced energy consumption effect and for an end car equipped with passenger seats, thus increasing the transport capacity of the train.

As higher speed means higher aerodynamic forces which then act on vehicles with lower mass, the vehicle reaction onto unsteady air force excitation, e.g. of the end car in trailing position, have come into the view of the railway operators. In the BriteEuram-funded research project RAPIDE (Railway Aerodynamics of Passing and Interaction with Dynamic Effects) the railway undertakings Deutsche Bahn AG (DB), the French SNCF and the Italian FS, joined their forces to investigate, among other examinations, the boundary layer flow of a modem high-speed train and the wake flow characteristics by means of currently available off-the-shelf CFD- codes. Full scale measurements both with a multi-pressure probe device and a LDA-system were performed for validation purposes up to train velocities of 280 kmh ([2]).

When the thick turbulent boundary layer separates at the end of the train the points of separation on the train surface may shift periodically in time, thereby causing aerodynamic excitation of oscillations of the last coach around the longitudinal axis; a phenomenon first detected by the Japanese railways some years ago ([3], [4]). The oscillation frequency with its related amplitude may cause discomfort to passengers in the rear coach because of its ''nausea'' effect. Confirmation of this effect has been given by Japanese measurements on the STAR2 1 [3] experimental train with speeds up to 3 15 kmh, showing these lateral oscillations to be at several Hertz as a peak value. As the lateral oscillations in a tunnel were found to be 10 times higher than in the open air because of the asymmetric flow expansion at the tail, the Japanese RTRI presented an initial CFD study of the train wake [4] in which they suggested an improved tail shape blunter than the original head geometry, to reduce drastically the oscillations in tunnels. Although the suggested tail shape reduces the lateral oscillations in the tunnel to a value only 2 times higher compared to the value for open air conditions, a blunter head shape clearly contradicts the efforts concerning the reduction of the micro-pressure wave emission ("sonic boom" effect) at the tunnel exit.

To feed the numerical calculations of the flow around the trailing car with the correct inflow conditions, the structure of the turbulent boundary layer was measured under full scale conditions with Re=12* lo6 based on the equivalent diameter. Figure 1 shows the test arrangement and the data obtained in a log-log scaling. Approximated by a power law, for this highly turbulent flow the measured variation of the velocity with the distance to the train surface can be well represented by a power factor of 1/10, this factor is also supported by

TO come to a clear understanding of the train wake flow phenomena, the analysis methods of flow topology are used (see e.g. [6 ] ) . The surface flow field structure is represented due to its skeleton via the detection of the "singular points" in which in the wall plane the skin friction vector vanish identically. These singular points can be of the so-called nodal, saddle and focus type. Once having located these points on the train surface and the separating streamlines connecting them following the rules of flow topology, the visualisation of closed separation bubbles, separating and reattaching shear layers reaching into mid-air as well as vortical flow structures, leads to a clear portrait of the wake flow situation. Figure 2 exhibits a first comparison between a 1 :7 model scale test of the ICE trailing car and the related numerical simulation using the commercial Navier- Stokes code FLUENT with a standard k-E turbulence model. Concerning the overall flow separation on the trailer shoulders there is a quite resemblance between the experimental "oil flow visualisation" and the computational skin friction lines, although the details of this separation process were not resolved in the experiment.

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S 790 ZAMM Z. Angcw. Math. Mcch. 81 (2001) S3

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Figure 1 : Test arrangement and measured data in log-log scaling near the surface of an ICE trailing car with Re=l2* lo6

Roof

Underfloor

Figure 2: Wall flow pattern on the surface of the ICE trailing car (Top: wind tunnel test scale 1:7 with Re=2*106, Bottom: numerical simulation of 1 : 1 scale test with Re = 12* lo6, Right: topological structure at rear part with onflow from roof, side walls and underfloor)

The skin friction computation shows clearly the two vortices spiralling out of wall focus points on each of the nose shoulders and a vortex starting at the trailing "tip". All these vortices, on each side with the same sense of rotation, will combine in the wake to a complex overall left-right vortex system now subject to the classical rules of vortex dynamics. It can be expected that due to the induction law of vortex motion these vortices stay close together while slowly coming down to the ground, i.e. the track, where they then separate promptly for leaving the train's near wake to the left and right hand remaining close to the ground. This effect amplifies the slipstream velocity in the train wake basically created by the boundary layer thickening along the train and causes the massive train induced velocity increase in the slipstream after the complete train has passed, this is directly felt e.g. by passengers on the platform or maintenance workers near the tracks.

References 1 2 3

4

5 6

C. Taylor: Quantily not quality, as high-speed operators mark time; Railway Gazette Int., October 1999 RAPIDE Project Programme: Annex 1 to BriteEuRamIII-Contract BRPR CT97-0603 (1998) Kohama Y, Koshikawa T & Okude: Wake Characteristics of a High Speed Train in Relation to Tailcoach Oscillations, Vehicle Aerodynamics Conference, Loughbrough University, UK ( 1994) Suzuki M, Maeda T & Arai N: Numerical Simulation of Flow Around Train, IMACS-COST Conference on 3D Complex Flows, Lausanne, Switzerland (1 995) Wieghardt K: Theoretische Stromungslehre, LAMM 4, B. G.Teubner, Stuttgart (1974) P e k e D.J. and Toback M., Three Dimensional Interaction and Vortical Flows with Emphasis on High Speeds, AGARDograph No. 252, (1980)

Adresses: DR. B. SCHULTE-WERNING, DR. C. HEINE AND G. MATSCHKE Deutsche Bahn AG, Forschungs-und Technologie-Zentrum, Vblckerstr. 5, D-80939 Milnchen