The resistance force of a vehicle is usually made up of the rolling resistance and the aerodynamic resistance. The aerodynamic resistance part is usually a constant C times velocity squared, where C is the aerodynamic constant for open road and equal to 0.5*air density*drag coefficient*vehicle cross sectional area. For a tunnel the constant C is modified, which is expected to be higher by a factor, depending on the blockage ratio of vehicle area to tunnel area. The question is how much higher? Has anyone any done any related research? Note some environmentalists are using this to argue against building of tunnels, so it is good to quantify this.
Beginning by assuming a tunnel for traffic going in both directions, where the two directions are separated from each other by a wall.
If traffic through tunnels acts as a piston, pushing air through from entrance to exit, then it seems to me that some benefit could be derived if the pressurized air toward the outlet of the tunnel could be passively diverted to the inlet of the tunnel for traffic heading in the opposite direction. This would reduce the pressure differential from entrance to exit.
If the two directions are not separated by a wall, so air is allowed to flow freely from one side of the tunnel to the other, then one should notice less of a pressure differential.
I've observed this effect in a metro system, where the tunnels are not separated in each direction.
If one stands at the entrance to the train station, a train arriving into the station will create a significant air draft heading out of the station. If a train is leaving the station, the air draft goes from outside the station into the tunnel. This is as one would expect, because the train acts as a piston, and the tunnel is the cylinder.
The interesting point is that if a train is arriving into the station while another is leaving from the station, there will be virtually no draft through the station entrance, from or to the exterior.
So, seems to me that any energy-wasting issues with tunnels can perhaps be alleviated, not just with vents to the exterior, but also with vents that shift higher pressure air from one traffic direction to the lower pressure areas in the opposite traffic direction. The shorter these vents are, the better.
So this rambling post is to suggest that how much C is increased will depend on tunnel design. Air shafts to the exterior, at least, are commonplace in tunnels, just for this reason.
Dear Vitalii,
Many thanks for the papers. My colleagues are translating; hopeful for some results.
M
The first question is about the energy consumed. In a twin tube tunnel, the power required for driving in a tunnel should be less, as the vehicle is driving in air flow direction (same as wind coming from the back). I would expect this effect to be more important than the variation of the drag coefficient - at least for passenger vehicles.
On the drag coefficient, I read comments on increased drag coefficients due to the enclosure. But I don't recall having seen any data.
