Control of wake-induced exposure using an interrupted oscillating jet.

A problem may arise in ventilation design when the contaminant source is located in the worker's wake, where turbulence and vortex formation can carry the contaminant into the breathing zone even though the source is downwind. It was found previously that forced directional variations in the flow can reduce or eliminate the vortex formation that causes these local reversals. Reported here is a simple realization of this concept, in which an oscillating jet of air was directed at a mannequin in an otherwise steady flow of air. A 50th percentile male mannequin was placed in a nearly uniform flow of approximately 0.18 m/sec (36 ft/min). A low-velocity tracer gas source (isobutylene) was held in the standing mannequin's hands with the upper arms vertical and the elbows at 90 degrees. Four ventilation scenarios were compared by concentration measurements in the breathing zone, using photoionization detectors: (A) uniform flow; (B) addition of a steady jet with initial velocity 5.1 m/sec (1.0 x 10(3) ft/min) directed at the mannequin's back, parallel to the main flow; (C) making the jet oscillate to 45 degrees on either side of the centerline with a period of 13 sec; and (D) introducing a blockage at the centerline so the oscillating jet never blew directly at the worker. At the 97.5% confidence level the interrupted oscillating jet (case D) achieved at least 99% exposure reduction compared with the uniform flow by itself (case A), at least 93% compared with the steady jet (case B), and at least 45% exposure reduction compared with the unblocked oscillating jet (case C).