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SEMINAR PRESENTED BY : Roy Roshan Chandy S7 M.E

7442 GUIDE:ARUN KUMAR. V.

LECTURER M.E.

NATURE OF PRESSURE WAVES INDUCED BY A HIGH-SPEED

TRAIN TRAVELLING THROUGH A TUNNEL

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INTRODUCTION• Increase in personal mobility has led to new high speed railway

needing new lines and straighter tracks. To avoid obstacles and environmental barriers longer & numerous tunnel sections are created.

• The passage of a high-speed train in a tunnel causes aerodynamic problems which do not appear in open air:

• Compression and expansion waves are generated when the train enters the tunnel, when its velocity changes and wherever the tunnel crosssection is varied. These pressure waves can cause relevant aerodynamic loads on vehicle and tunnel structures.

• Aerodynamic noise, forces and moments acting on the train, and especially the aerodynamic drag, grow due to the confinement of the surrounding space; moreover, at the tunnel exit, micro-pressure waves and sonic boom can generate inconveniences to the nearby residents.

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The entity of aerodynamic drag depends on several parameters such as •the blockage ratio,•the tunnel network geometry and surface,•the number of pressure relief ducts,•the train type and its speed, the presence of other trains, etc.

If this drag is underestimated during design, either the required operating speed cannot be attained, or the air temperature resulting from the dissipated power can exceed safety limits. Such negative effects can be minimized by reducing the blockage ratio,or by connecting the tunnel to the atmosphere, or to a second parallel tunnel.

The blockage ratio is the ratio of vehicle to free tunnel cross-section area

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Nature of fluid flow in the tunnel

• Flow generated by trains in the tunnel is COMPRESSIBLE,TURBULENT AND THREE DIMENSIONAL IN NATURE.

• Pressure, density and velocity fields around the train are affected by the confining effects of the tunnel walls even at steady state;

• unsteady phenomena develop whenever the relative motion between train and tunnel imposes strongly unsteady boundary conditions to the flow.

• This is the case when the train ends cross the tunnel portals, when train passing occurs in the same tunnel and, in general, whenever the tunnel section encounters a change or a connection with a different tunnel or atmosphere.

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MODEL PROBLEM • Results were obtained on different underground high-speed railway

networks connecting two stations 220m long which are 60km apart.• These different tunnel networks include single-track tunnels either with

or without connections with other parallel tunnels or atmosphere.• The blockage ratio is always 0.52, except for the case of a single

open tunnel at atmospheric pressure, considered as a reference case, where 0.13.

• The initial air temperature is fixed at 300 K. • The initial pressure level in the closed tunnel is 10 000 Pa, while in

open tunnels it is equal to the atmospheric pressure level of 101 300 Pa.

• In all cases, the tunnel walls have been considered adiabatic, with a friction coefficient Cfg =0.003 corresponding to a mean wall roughness of 0.5 mm. Pressure relief ducts, when present, are 25m long & 3 m diameter.

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MODEL PROBLEM • Pressure relief ducts, when present, are 25m long, cylindrical with 3m

diameter, and connect perpendicularly two main tunnels with equal section. • Their friction coefficient is 0.005, higher than that of the main tunnel, to take

into account the fact that these short ducts are usually bored manually.• Each train running on the rail link is Lt = 200 m long, has 10m

long,sinusoidally shaped nose and tail, and a circular cross-section At of 3.6 m diameter. Its friction coefficient is Cft = 0.003. The aerodynamic design of the train geometry has been considered ideal: a shape coefficient Cs = 1 has therefore been introduced.

• On the train tail, the correction coefficient has been set to the experimentally determined value Cdt=0.99, which is valid for high-speed trains. The train accelerates at 2m/s^2 for 60s reaching its cruise speed of 120m/s, which is maintained constant until the distance from the arrival station reaches 3.86 km. At this point, a constant deceleration of 2m/s2 brings the train to rest in 60 s . The total time needed to cover the 60 200m distance is therefore 561.66 s.

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MODEL PROBLEM

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TUNNELS AT ATMOSPHERIC PRESSURE

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Pressure initially increases in front of the train in the 60-op-5 case owing to tunnel friction; it reaches its maximum at t ’ 266 s, when the compression wave generated by the train during its acceleration reaches the train nose after having been partially reflected at t = 173 s as an expansion wave at the tunnel open end. In the 60-op-10 tunnel, viscous and compressibility effects areinstead negligible due to the large tunnel section and low air velocity; the pressure level in front of the train nose remains in this case approximately constan as due to air outflow piston effect is reduced..

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NEED FOR TRAIN TUNNELS UNDER PARTIAL VACUUM

• if the tunnel diameter must be kept small and the required power for 200m long trains has to be maintained in the order of 10–15 MW at 120m/s –i.e., the required power for very high-speed trains or Maglevs in open air –the aerodynamic constraints impose a reduction either of the cruise velocity or of the pressure level inside the tunnel.

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aerodynamic drag is one-tenth of the one observed previously in the open configuration

while the flow around the train is still choked, i.e., sonic conditions occur at the end of the annular space. Later, differences appear due to the closed portals.

Actually, the tunnel wall at the end station reflects the compression wave generated during the train acceleration as a compression wave –unlike the open air portals which reflect it as an expansion wave.

When the reflected wave reaches the train nose (t =266 s), it leads to an increase of the pressure level in front of the train.

Due to the piston effect, this increase is accentuated when the train approaches the arrival station, as shown in Fig. the supersonic expansion on the tail becomes more intense. A stronger shock wave appears on the train tail, and the Mach number reaches the value of 1.7 before the deceleration phase.

Aerodynamic power reaches instead its maximum value at the end of the cruise phase the closure of the end portals causes a 130% increase of the maximum aerodynamic power Wmax(tot) , and a 50% increase of the average power (Wtot).

A reduction of the sharp increase of aerodynamic drag can be obtained by a tunnel prolongation behind both stations.

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NEED FOR TUNNEL WITH PRESSURE RELIEF DUCTS

reduction in the tunnel diameter by a factor 2, although leading to a decrease in building costs approximately proportional to the tunnel diameter, also implies an increase of energy costs by a factor 7. This fact plays an important role in the choice of the tunnel diameter for high-speed lines.

Another relevant environmental issue raised by high-speed train aerodynamics in tunnels is the high velocity of air flow at the tunnel exit. At the higher blockage ratio tested, wind in the arrival station reaches nearly 50 m/s, while at the lower one it does not exceed 5m/s.

These values should be compared with the current requirements for passenger comfort in underground stations, which prescribe an air velocity not higher than 5m/s

The addition of 13 pressure relief ducts of 3m diameter, one every 5 km (configuration 60-op-5-13), causes an average power decrease of 17MW; when ducts are placed every kilometer this decrease reaches 32MW.

However, the required power, needed to overcome aerodynamic forces at these blockage ratios and train velocities, remains too high for a realistic underground transportation system, due to energy consumption and thermal evolution.

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TUNNEL NETWORK WITH CONNECTIONS

•60-cl-5-13 refers to twin single track parallel tunnels, each 60km long & interconnected with each other by 13 pressure relief ducts each 25m long & 3m diameter.

•The presence of pressure relief ducts allows the generation of an air flow in the second tunnel from the high-pressure regions in front of the train nose to the low pressure regions behind the tail. •This alleviates the piston effect, thus reducing the pressure drag and the avg power by factor of 1.9,peak power by factor of 2.6.•Air flow also remains subsonic & does not exceed 0.9 at the end of the annular space.

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DRAWBACKS OF TUNNELS WITH CONNECTIONS:

There is a strong drag increment at each shaft crossing and the drag may even rise to the value of 10kn.

Cross flows are generated by pressure difference between the tunnels, reaching upto 60 m/s. thus resulting in a strong lateral force of newtons acting on train at each shaft crossing.

These generate high structural load and pose problems for train control.

Trains travelling in opposite tunnels can reciprocally interact and cause high unsteady aerodynamic loads.

MODIFICATIONS MADE TO REDUCE THE DRAWBACKS:

Design of shafts so as to avoid direct impingement of the cross flow on the train.

Locate the airshafts where the train speed and the upstream pressure level are not high enough to increase the piston effect and generate high velocity air flow in the ducts such as in the 60-cl-5-2x2 configuration of the tunnel shown thus reducing drag by 20 to 40%.

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AEROTHERMAL LOADS UNDER PARTIAL VACUUMEFFECTS ON TRAINS:The compression due to the piston effect generated by the train leads to an increase in the air temperature upstream of the train nose: the increment with respect to the initial air temperature ranges from 30K (60-cl-5-2 2) to 80K (60-cl-5).The rapid expansion along the train brings to a strong reduction of the airtemperature which reaches a minimum after the supersonic expansion. At t=500s, in single tunnel configurations, the air temperature can fall 60K below the initial reference temperature. Such a reduction in air temperature requires additional care in the structural and thermal design of the vehicle.

EFFECT ON DEPARTURE STATION:

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EFFECT ON ARRIVAL STATIONS:

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CONCLUSIONS:

The passage of a high-speed train in a tunnel causes aerodynamic problems whichdo not appear in open air: compression and expansion waves are generated when the train enters the tunnel, when its velocity changes and wherever the tunnel cross section is varied. These pressure waves can cause relevant aerodynamic loads on vehicle and tunnel structures.

The reduction of the diameter of the tunnels is desirable in order tolimit construction costs. This reduction increases blockage ratios forgiven train geometries, thus leading to an unwanted rise of propulsion costs.

The construction of underground connections under partial vacuum seems to be the most viable solution to the problem. However, the effects of multiple reflected compression waves on tunnel closed ends reduces strongly the advantages of partial vacuum in single closed tunnels.

In this framework, the best configuration for this long-range, high-blockage ratiotunnel network seems to consist in two coupled tunnels connected by a number ofpressure relief ducts.

The power required for train motion is, in this case, more than 50% lower than in the single tunnel connection and more than 90% lower than in a single tunnel connection at low blockage ratio and atmospheric conditions.

Side effects of these connections are not always desirable: sudden increasesin aerodynamic drag and strong lateral wind loads on the train can be generated.A solution to this problem can be found by placing pressure relief ducts onlyin proximity of the stations, where the high-speed train is in its accelerating=decelerating phase.

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References:[1] A.Baron ,The alleviation of

the aerodynamic drag and wave effects of high speed trains in very long tunnels,Italy,Sept 2000 pp 365-401

[2] P.Ricco ,Nature of pressure waves induced by high speed train travelling through a tunnel,Belgium,July 2005 pp 781-808

[3] Yahya,Fundamentals of compressible flow.