# High-Speed Vehicles in Low-Pressure Tunnels--Influence of Choked Flows
## 5 Alternative Tunnel Configuration  
In practical systems, there will commonly be two tunnels--one for each direction of travel. In this case, designers will need to decide whether these should be aerodynamically separate or should be connected in some manner. One obvious possibility is to link the tunnels at their ends, thereby causing them to act aerodynamically as a single continuous loop (Figure 14). In this case, all vehicles will tend to cause the air to circulate in a common direction. This has the twin advantages of (a) increasing the number of vehicles that provide mutual help and (b) removing the closed-end effect that is responsible for the strong pressurisation at the end of a single tube. Figure 13: Influence of vehicle speed (30 km tunnel), (**a**) vehicle pressures: 150 m/s, (**b**) vehicle temperatures: 150 m/s, (**c**) vehicle pressures: 200 m/s, (**d**) vehicle temperatures: 200 m/s, (**e**) vehicle pressures: 250 m/s, (**f**) vehicle temperatures: 250 m/s. (\(N_{Tant}\), \(T_{Tunt}\) = small distances ahead of the nose and behind the tail. \(N_{amt}\), \(T_{amt}\) = in the annulus, small distances from the nose and tail). Figure 15 shows pressure and air velocity histories for a twin tunnel system that is connected in this way. For simplicity, the connections are at the extreme ends of the tunnels, and they have the same cross-sectional area as the tunnels. The actual geometry in any real system will be less simple, partly because of the need to make provision for embarking and disembarking, but also for many other reasons. However, the aerodynamic consequences are unlikely to depend strongly on the particular geometry. Indeed, it will be a design requirement to prevent significant divergence from the ideal behaviour. In the case considered in Figure 15, there are three vehicles in each direction, and, in each tube, the specified journeys are identical to those considered in Figure 11 for a  
Figure 14: Aerodynamically linked tunnels for two-way operation. Figure 15: Influence of position in convoy on flow conditions (linked tunnels), (**a**) pressures along leading vehicles, (**b**) velocities along leading vehicles, (**c**) pressures along middle vehicles, (**d**) velocities along middle vehicles, (**e**) pressures along trailing vehicles, (**f**) velocities along trailing vehicles. (\(N_{tunn}\), \(T_{tunn}\) = small distances ahead of the nose and behind the tail. \(N_{tunn}\), \(T_{tunn}\) = in the annulus, small distances from the nose and tail). single tube. When the vehicles begin to move, they generate pressure disturbances in both directions. In the single-tube cases, the influence of waves propagating in the same direction as the vehicles is dominant. In the twin-tube system, however, waves propagating in the opposite direction can have a stronger influence on the outcomes. This is a consequence of the time taken for waves to travel. First, consider a wave propagating ahead of the first vehicle to begin its journey. This will reach the remote end of the tunnel after about 45 s and it will then begin to travel along the second tube. By that time, however, the vehicles in the second tunnel will already have moved several kilometres and will be accelerating rapidly. As a consequence, an additional period of more than 5 s will elapse before the wave reaches the tail of the trailing vehicle. Furthermore, these times relate only to the tip of the wavefront. Much greater times will elapse before the arrival of the strongest parts of the wavefront. By that time, the leading vehicles will already be decelerating. Now consider a wave travelling in the opposite direction to the vehicles. The leading tip of the wave initiated when the first vehicle begins to move will reach all three vehicles in the second tube before it reaches the remote end of that tube. Thus, it will begin to \"assist\" these vehicles before waves travelling in the same direction as the vehicles can do so.