The answer is simple: DLR has gained a great deal of experience with aerodynamics since it was founded over 100 years ago, and has developed a number of sophisticated techniques to measure the behaviour of moving objects in a variety of atmospheric conditions. This expertise can be put to good use for high-speed trains, as their aerodynamic characteristics are crucial to safety and performance.
The DLR's Next Generation Train (NGT) project, designed to create a double-deck very-high-speed train, involves researchers from nine DLR institutes, including the Institute of Aerodynamics and Flow Technology in Göttingen, where model trains have been undergoing tests in specially-built wind tunnels since October 2010. The aim is to increase the maximum commerical speed of high-speed trains to 400km/h, while at the same time halving energy consumption, increasing passenger comfort, cutting aerodynamic noise, improving safety standards and reducing wear and life-cycle costs.
The new €3m research facility for high-speed trains in Göttingen has two specially-designed tunnels, which DLR says are unique, to investigate various factors affecting train behaviour. The tunnel simulation facility (TSG) is also unique - a 60m track equipped with a plexiglass tunnel which can accommodate model trains of any scale from 1:20 to 1:100.
The really surprising feature is that the model trains are accelerated by one of the oldest methods known to mankind: a catapult. The idea surfaced when archaeologists found a large number of catapult missiles on an old Roman site nearby. As Professor Andreas Dillmann, director of the Institute of Aerodynamics and Flow Technology, comments with a smile: "We thought if the Romans could do it, so can we." And it does the job perfectly: models can be catapulted at up to 100m/s to a maximum speed of 400km/h.
One of the biggest problems facing high-speed trains is the impact of cross winds on stability. "Trains are becoming lighter, which means that at high speed the leading vehicle can leave the rails," says Dillmann. "At speeds of 300km/h the front section of a double-deck train can start to lift, making it prone to tilting in strong side winds, despite the fact that the train may weigh several hundred tonnes. There have already been 30 to 40 incidents in Germany alone where this is likely to have been a factor. One of the most critical times is when a train comes out of a wind shadow - a tunnel for example - and is suddenly subject to a cross wind." The same applies if a train is crossing a bridge or if there is an oncoming train.
To measure this effect, the TSG is equipped with a 5m wind channel module as well as the plexiglass tunnel. Here the model can be subjected to cross winds directed at various angles, and the flow structures can be observed with the use of smoke and laser beams. The effect of cross winds can also be measured in the cross-wind tunnel (SWG), a closed system with a model train inside attached to a fixed plate. The flow field is obtained by means of particle image velocimetry (PIV), which gives a picture of the forces at work. In addition, aerodynamic loads are measured with a strain-gauge balance in the rear car.
The TSG is also being used to investigate the propagation of pressure waves in tunnels, another major challenge. The faster a train travels and the narrower the tunnel, the higher the pressure; in addition when pressure waves reach the tunnel portal they are partially reflected back into the tunnel which puts increased aerodynamic forces not only on the train but also on the tunnel structure - not to mention the acoustic shock for the passengers. This becomes even more pronounced when trains pass in double-track single-bore tunnels.
One solution is to install vertical ventilation slots at the tunnel entrance so that the pressure builds up more slowly and smoothly. Small sensors placed in fixed positions along the plexiglass tunnel measure the pressure during a test, and the model train is equipped with a pressure sensor, an acceleration sensor and a light sensor. Data is recorded by computer to form the basis for calculations to find the optimum configuration.
A German Rail (DB) ICE3 is almost completely airtight and has extra thick walls to counter the effect of these pressure waves. If the proposed double-deck train has even stronger seals it will be heavier and need more energy, which is why another solution has to be found. Part of the NGT project is to develop lightweight trains, with for example sandwich structures comprising a honeycomb core and glass-fibre reinforced skin. This coupled with energy saving techniques during acceleration and braking would reduce energy consumption considerably and help to cut emissions.
Flying ballast is a growing problem. Formerly associated with the accumulation of ice and snow beneath a train, the phenomenon has now become more prevalent at other times of the year, since the aerodynamic forces generated by the current generation of ICE trains is enough to lift ballast off the track. Dillmann refers to an avalanche effect: one stone is spun into the underside of the train, dislodges two or three more as it falls, and so on. "This can do enormous damage to the underside of a train," he says. To find a solution, PIV measurements from a water tank wagon on a model train are being collected to provide information on the flow field generated under high-speed trains.
It is too early to say how the aerodynamics of a train could be modified to reduce the effects of cross winds, pressure waves and the slipstream from a passing train, which, as Dillmann points out, can be strong enough to pluck people off a station platform. The new facility is still being optimised to develop the calculation tools needed to produce solid results.
As Mr Sigfried Loose, the DLR's specialist in vehicle aerodynamics, says the technical limit for commercial high-speed trains is 400-440km/h, because a disproportionate amount of energy is needed to achieve only a small increase in speed beyond 440km/h. Even so, to reach 440km/h safely and at an affordable operating cost means building light trains and modifying their shape. For this, Loose says information on the aerodynamic flows round the underside of a train in particular is vital to design the train of the future, and none of the test facilities elsewhere can give a realistic picture of this due to inadequate simulation techniques. This is one of the reasons why the Göttingen facility was built.
"Generally our main goal at the moment is to develop measurement techniques for aerodynamic drag, slipstream, flying ballast and aerodynamic characteristics as a whole," Loose says, explaining that data is still being collected for comparison with results from standard wind tunnels and observations of trains in service. Loose predicts that many of the findings from the new facility will be incorporated in the Technical Specifications for Interoperability (TSI) when they are revised in five years time. In the meantime, the current phase of the NGT project is due for completion at the end of this year.
Aside from the aerodynamic issues associated with the NGT project, many other aspects are being covered by this inter-disciplinary programme. Other objectives include improving passenger comfort by reducing vibration, climate control, noise, and cabin pressure, as well as finding ways to speed up boarding and alighting, and developing an efficient baggage handling system.
It is also hoped that the cost of producing new high-speed trains can be considerably reduced by using modular designs and system integration, as well as finding ways to reduce life-cycle costs. In addition the DLR is looking at related areas such as track design and automatic train control, and it also sees significant potential for improving development and approval procedures.
It will be interesting to see how long it will take for a next-generation high-speed train to come into operation, and where the limits really lie.