PubMed · 7391663
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J H Neame, W C Wright. 1980. Wind noise in hearing aids.. https://doi.org/10.1017/s0022215100088988
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The well-mixed room model is traditionally used to predict the concentration of contaminants in indoor environments. To account for imperfect air mixing, the room supply/exhaust air rate Q is frequently multiplied by a mixing factor m, where 0 < m < or = 1, and an effective ventilation rate QE = m . Q is used in place of Q in the well-mixed room equations. However, this procedure is inappropriate because a well-mixed room model, albeit with an adjusted ventilation rate, is still used to describe an imperfectly mixed room. To illustrate the errors that may result, a two-zone model is described in which a room is conceptually divided into an upper zone and lower zone, where the latter is the zone of occupancy. Air is supplied to and exhausted from the upper zone at rate Q, and air exchanges between the two zones at rate beta. The lower zone's true ventilation rate is termed its purging flow rate QL, where QL = [beta/(beta + Q)]Q. Expressions for the steady-state contaminant levels in the two zones and for decay from the steady-state levels are presented. In a two-zone room, if one ignores imperfect air mixing and attempts to estimate QE from a decay curve, QE will usually be greater than QL. Given that contaminant is emitted in the lower zone, subsequent use of QE rather than QL to predict steady-state exposure intensity in the room will cause an underestimation error. For a room with an upper- to lower-zone volume ratio of 2:3, the underestimation error can reach 40%. If a room has a single or dominant point source of contaminant, it is recommended that the purging flow rate near the release point be determined, which permits a more accurate prediction of a worker's exposure intensity near the source. Alternative methods for determining the local purging flow rate are described. It is also shown that age-of-air analysis techniques do not provide information directly relevant to estimating exposure intensity.
Short-track speed skating involves pack-style racing where five to seven skaters may be on the ice at once. Since average speed for a 3000-m event may exceed 35 km.h-1, drafting may be beneficial. However, the short (111 m) oval track could limit effective drafting space, and high forces required in cornering may compromise potential benefits. We evaluated heart rate (HR)-lactate (LA) responses and post-drafting 3-lap sprint performance using 18 National Team and developmental skaters. Two 4-min trials, one drafting and one leading at 8.8 m.s-1, were performed. In addition, six skaters performed three 3-lap sprints, rested, immediately after a 4-min drafting trial at 9.2 m.s-1, and immediately after an unaided 4-min trial at 9.2 m.s-1. Results demonstrated lower HR and LA responses during drafting (174.0 +/- 9.0 and 5.56 +/- 2.18 vs 180.4 +/- 8.7 and 7.75, P < 0.05) at 8.8 m.s-1. After 4-min trials at 9.2 m.s-1, HR deltas were 6 bpm, lactate values were 9.00 +/- 1.84 and 5.22 +/- 1.18 for unaided and drafting, respectively. Sprint performance was better following drafting (33.46 +/- 1.19 vs 34.03 s, P < 0.05). HR and LA deltas during the 8.8 m.s-1 trials ranged from 0.8 to 12.4 and -0.18 to 5.37, respectively, indicating that some skaters were more effective drafters than others. These results suggest that drafting could be an important strategy in short-track speed skating, and drafting technique should be emphasized in training.
The velocity field downstream of a worker is approximated with a discrete vortex algorithm. This information is used to calculate trajectories of massless tracer 'particles' released from a point-source of contaminant. Concentrations in the plane of this source are estimated by averaging over a number of such trajectories. Approximations include: (1) representing the worker by a two-dimensional elliptical cylinder; and (2) representing tracer gas contaminant by massless particles generated without momentum. These particles are transported by both vortex shedding and turbulent diffusion. Computer-predicted mean concentrations in the near-wake region downstream of the worker compare well with results from wind-tunnel tracer gas experiments employing a mannequin. Subsequently, the concept of a computational breathing zone is introduced, and predictions of worker exposure are made. These simulations of time-integrated breathing zone concentration also compare well with measured values.