An experimental investigation of the detention of airborne smoke in the wake bubble behind a disk

Abstract

Experiments have been performed in a low-speed wind tunnel to determine the detention time of airborne smoke particles that become trapped in the wake vortex (or bubble) region behind flat disks placed perpendicular to the flow. Using a laser transmissometer to detect the smoke, its detention time was obtained from the time-dependent decay of the smoke in the disk bubble during the time immediately following the removal of the source of smoke. The dimensionless groupH, the product of the detention time and the mainstream air velocity divided by the disk diameter, is seen to be a constant equal to 7.44 ± 0.52 for Reynolds numbers in the range 2000–40000. This result is compatible with a simple fluid-mechanical model which describes the transport of fluid-borne scalar entities across the bubble boundary by turbulent diffusion. The investigation suggests thatHshould be unique for the flow about a disk over a wide range of conditions, and further suggests the possibility that similar unique values forHcan exist for flow about other obstacles. The numberHhas potential applications in a number of physical and engineering research areas.