MEASURES to mitigate aerodynamic phenomena and noise became critical challenges in developing Japan's Shinkansen, when the maximum speed was almost doubled from 110km/h for conventional trains to over 200km/h when the Tokaido Shinkansen opened in 1964. This is because aerodynamic phenomena and noise associated with high-speed trains are closely related to running speed.
For example, a two-fold increase in speed causes a four-fold rise in air resistance - proportionate to the square of train speed - and an 18dB rise in the sound pressure level because aerodynamic noise is proportional to the sixth power of speed.
Reduction of air resistance, which directly influences the operating speed and the economics of train operation, has been a major subject of study since the beginning of Shinkansen development. Investigations such as wind-tunnel tests to support decisions on the shape of the nose at each end of the train have been performed extensively. The original 0-series Shinkansen train had a characteristically streamlined nose and a body surface smoother than that of conventional trains. However, there was still scope for further reduction of air resistance relating to elements such as underfloor equipment exposed to the train's exterior for ease of maintenance.
The contribution of the head and tail to overall air resistance in a long train with a streamlined nose, such as a Shinkansen, is only around 10%. To reduce overall resistance, it is therefore necessary to flatten the outer surfaces of centre cars in the train, such as roofs, underfloor equipment and gaps between cars.
In recent years, great progress has been made in smoothing the outer surfaces of Shinkansen vehicles as countermeasures to suppress noise and avoid snow accumulation. These developments have also significantly reduced air resistance.
In the development of the first Shinkansen trains, it was necessary to investigate a new phenomenon in which two trains running at more than 200km/h pass each other in close proximity. As a way of addressing the impact force created by the wind pressure caused from oncoming trains, theoretical analysis and model experiment were performed to study the scenario. The outcomes indicated that the resulting impact force would not interfere with safe running either inside or outside tunnels.
Shinkansen tunnels have a smaller cross-section than those on European double-track high-speed lines. However, Shinkansen vehicles are wider than their Europe counterparts to allow three-plus-two seating. Although these features have brought significant advantages in the form of lower construction costs and higher transport capacity, they have also led to aerodynamic phenomena having a greater influence on high-speed running in tunnels.
A typical problem is air pressure fluctuations which result in ear popping. This phenomenon is observed when a pressure wave generated by a train running into a tunnel infiltrates inside the train. Such phenomena had been foreseen theoretically, and a need for related countermeasures was identified in studies based on prototype train tunnel running tests on the Kamonomiya model test track of the Tokaido Shinkansen. As a result, the first-ever airtight structure on a mass-produced railway vehicle was adopted.
A further issue related to air pressure fluctuations is the micro-pressure wave phenomenon caused when a pressure wave generated by a train running into a tunnel radiates from the exit, causing adverse effects in nearby areas. Such radiating waves are called micro-pressure waves. Because they tend to have significant effects in long tunnels with slab track, micro-pressure waves have been handled as a new environmental problem since the widespread introduction of slab track on Shinkansen lines in about 1975.
Countermeasures include the installation of tunnel entrance hoods, snow shelters and intermediate shafts at ground-side approaches, in addition to vehicle-related measures such as a reduction of the cross-section, elongation of the nose and optimisation of cross-sectional area variation of the leading car.
Other aerodynamic phenomena include the effect of air pressure from passing trains on structures and equipment along the line, train-induced air flow on ballasted tracks or on passengers on platforms, the influence of cross winds on vehicles, and aerodynamic jolting associated with tunnel running.
Noise-related research was conducted during the development of the Tokaido Shinkansen but mainly addressed vehicle internal noise. However, following environmental noise problems arising along Shinkansen lines, installation of noise barriers at key locations began a few years later. Technology to deal with environmental noise has been treated as of critical importance on Shinkansen lines ever since.
Shinkansen noise sources are roughly categorised into those coming from the lower body, the upper body including the nose, pantographs and bridges. Lower-body noise is further classified as rolling noise caused by the vibration of rolling wheels and rails, noise from equipment such as running gear, and aerodynamic noise generated by air turbulence and vortices in bogie parts. Pantograph noise is further classified as aerodynamic noise, sparking noise generated by pantographs detaching from overhead wires and contact strip sliding noise from the vibration of pantograph heads and catenary.
It is important to clarify the locations of noise sources and to what degree each source affects wayside environmental noise. A directional microphone array developed to help clarify these various noise sources generated enormously useful insights.
The contribution levels of each noise source to environmental noise along Shinkansen lines have changed over time. In the early days of Shinkansen operation, when there were no noise barriers, rolling noise was dominant around lines. Later, rolling noise was reduced by the installation of noise barriers and the improvement of the wheel tread roughness condition due to the adoption of resin abrasive blocks to the wheel tread. Consequently, both rolling and sparking noise became dominant in around 1975.
Rolling noise was then significantly reduced thanks to the application of rail grinding. In addition, sparking noise was also virtually eliminated by connecting nultiple pantographs with extra-high voltage bus line wiring and thereby preventing contact breaks. It also made it possible to reduce the number of pantographs required.
As a result, the contribution of the remaining aerodynamic noise from vehicle upper bodies including pantographs became prominent in 1991. Since then measures to counter aerodynamic noise, which is highly dependent on train speed, have been indispensable. These include surface smoothing to reduce vehicle aerodynamic noise and the mitigation of such noise from current collectors, through the adoption of low-noise pantographs and insulator covers, which were introduced in 1999.
A large-scale low-noise wind tunnel constructed by RTRI is extensively used in research and development for these aerodynamic noise issues. At present, the contribution levels of aerodynamic noise from vehicle upper bodies including pantographs and that from the lower body comprising aerodynamic and rolling noise are almost comparable. Continuing speed increases are causing a shift in lower-body noise dominance from the rolling noise component to the aerodynamic noise component.
The Shinkansen has a proud record of safety and stable transport, but its main characteristic is high-speed operation. Measures relating to aerodynamics and noise will undoubtedly continue to play one of key technological roles as speeds increase in the future, just as they have done over the Shinkansen's 50-year history.