A wind-tunnel study on exhaust-gas dispersion from road vehicles—Part II: Effect of vehicle queues

By a reduced-scale wind-tunnel experiment, we investigate the dispersion behavior of exhaust gas from automobiles. Based on the results of single-vehicle cases in Part I of our work, we consider vehicle queues consisting of passenger cars (P)(P) and small-size trucks (T)(T). The roles of the vehicles before and after the gas-emitting vehicle are as follows. By their turbulent wake, the preceding vehicles make the concentration field approximately symmetric in the spanwise direction despite the significant lateral offset of the exhaust pipe. The vehicles behind the gas-emitter expand the exhaust plume considerably in the spanwise direction; TT expands the plume also in the vertical direction in its roll-up wake, while PP scoops up the oncoming plume only around the centerline. For vehicle queues of mixing ratio P:T=2:1P:T=2:1, the concentration fields when one of the component vehicles emit the gas are measured. It turns out that the plume shape is determined mostly by the type of the gas-emitting vehicle whereas the type of the following vehicles has minor effect. We also present an analytical procedure to approximate the overall contribution from all the queue-forming vehicles by a Gaussian line-source plume formula.