EXPERIMENTAL INVESTIGATION ON TUNNEL SONIC-BOOM

Upon the entrance of a high-speed train into a relatively long train t unnel, compression waves are generated in front of the train. These co mpression waves subsequently coalesce into a weak shock wave so that a unpleasant sonic boom is emitted from the tunnel exit. In order to in vestigate the generation of the weak shock wave in train tunnels and t he emission of the resulting sonic boom from the train tunnel exit and to search for methods for the reduction of these sonic booms, a1 : 30 0 scaled train tunnel simulator was constructed and simulation experim ents were carried out using this facility. In the train tunnel simulat or, an 18 mm dia. and 200 mm long plastic piston moves along a 40 mm d ia. and 25 m long test section with speed ranging from 60 to 100 m/s. The tunnel simulator was tilted 8 degrees to the floor so that the att enuation of the piston speed was not more than 10% of its entrance spe ed. Pressure measurements along the tunnel simulator and holographic i nterferometric optical flow visualization of weak shock waves in the t unnel simulator clearly showed that compression waves, with propagatio n, coalesced into a weak shock wave. Although, for reduction of the so nic boom in prototype train tunnels, the installation of a hood at the entrance of the tunnels was known to be useful for their suppression, this effect was confirmed in the present experiment and found to be e ffective particularly for low piston speeds. The installation of a par tially perforated wall at the exit of the tunnel simulator was found t o smear pressure gradients at the shock. This effect is significant fo r higher piston speeds. Throughout the series of train tunnel simulato r experiments, the combination of both the entrance hood and the perfo rated wall significantly reduces shock overpressures for piston speeds of u(p) ranging from 60 to 100 m/s. These experimental findings were then applied to a real train tunnel and good agreement was obtained be tween the tunnel simulator result and the real tunnel measurements.