Visualization of the Airflow around a Life-Sized, Heated, Breathing Mannequin at Ultralow Windspeeds

Abstract

During the past two decades, there has been considerable progress in developing particle size-selective criteria for aerosol sampling and exposure assessment that relate more realistically to actual human exposures than previously. An important aspect has been the aspiration efficiency—the ‘inhalability’—with which particles enter through the nose and mouth of aerosol-exposed individuals during breathing. Most of the reported experiments to determine inhalability have been conducted in wind tunnels with life-sized, breathing mannequins, for windspeeds from 0.5 m s⁻¹ and above. A few experiments have been reported for calm air. However, nothing has been reported for the intermediate range from 0.5 m s⁻¹ downward, and it so happens—as we now know—that this corresponds to most industrial workplaces. The research described in this paper represents a first step toward filling this knowledge gap. It focuses on identifying the features of the airflow near the mannequin at such low windspeeds that might have important influences on the nature of particle transport, and hence on inhalability, and eventually the performances of personal aerosol samplers mounted in the breathing zone. We have carried out flow visualization experiments for the realistic range of windspeeds indicated, investigating specifically the effect of the air jet released into the freestream during expiration and the effect of the upward-moving boundary layer near the body associated with the buoyancy of air in that region as a result of heat received from the warm body. We set out to identify the combinations of conditions—external windspeed, breathing mode (nose versus mouth breathing), breathing rate and body temperature—where such factors need to be taken into account. We developed an experimental system that allowed the visualization of smoke traces, providing very good observation of how the flow was modified as conditions changed. From inspection of a large number of moving pictures, we developed a matrix of regimes—categorized by windspeed and breathing rate—where the effect of the expired air is sufficient to permanently and seriously destabilize the airflow approaching the mannequin. It was found that the effect of body temperature was minimal. Such results will be important in the interpretation of current and future inhalability experiments carried out at realistic low windspeeds.

INTRODUCTION

During the past two decades, there has been considerable progress in developing particle size-selective criteria for aerosol sampling and exposure assessment that relate more realistically to actual human exposures than previously. This progress has been matched by a considerable degree of international harmonization, as reflected in the criteria listed by the Comité Européen de Normalisation (CEN) (1992), the International Standards Organisation (ISO) (1992) and the American Conference of Governmental Industrial Hygienists (Vincent, 1999). In these, an important feature has been the aspiration efficiency with which particles enter through the nose and mouth of aerosol-exposed individuals during breathing—what has become widely referred to as ‘inhalability’.

The first experiments to measure the aerosol aspiration efficiency of the human head as a function of particle size and external windspeed involved life-sized human mannequins to represent the worker in purpose-built aerosol wind tunnels (Ogden and Birkett, 1977; Armbruster and Breuer, 1982; Vincent and Mark, 1982). These studies, supported by subsequent ones, formed the basis of the definition of the inhalable fraction (or inhalability) (Vincent and Armbruster, 1981), expressed as the efficiency with which particles of given size enter the nose and/or mouth during breathing. Notably, the windspeeds in the experiments leading to this definition were relatively high, in the range from 0.5 m s⁻¹ upward, driven in part by the initial intended application of such work to aerosol exposures in highly ventilated underground mining environments. In more recent years, however, it has been shown that windspeeds in most workplaces are generally much lower, more typically in the range from ∼0.05 to 0.25 m s⁻¹ (e.g. Berry and Froude, 1989; Baldwin and Maynard, 1998; and others). Although some experiments have been reported for very low windspeeds, approximating to calm air (Ogden et al., 1977; Aitken et al., 1999; Hsu and Swift, 1999), there has been nothing reported for the important range of realistic windspeed conditions identified as pertaining to actual workplaces. So, there remains a significant knowledge gap that needs to be filled in order to enable standard-setting bodies to better define the inhalable fraction. With this in mind, a new body of work has been undertaken.

Little is known about the true nature of the air movement near a warm, breathing human being. An early flow visualization study carried out with a breathing mannequin in a wind tunnel by Wood and Birkett (1979) did show that there were significant disturbances to the approaching freestream resulting from the air jet associated with the expired air, hinting that such motions might in turn have a strong influence on particle motion near the human head and hence on the efficiency with which particles might be inhaled. But no information was provided about under what ranges of conditions such effects might be more or less important. And the role of body heat has never been studied in this regard. In general, in the large number of wind tunnel studies that have been reported to investigate inhalability, it has been tacitly assumed that the external windspeed was high enough that the local air movements induced near the mannequin due to the breathing actions and body temperature were negligible.

In the new body of work referred to above, the first part consisted of new flow visualization experiments, performed specifically at the realistic range of windspeeds that have now emerged as being of particular interest. This paper reports results on the nature of the airflow near the human body in air at ultralow windspeeds, specifically the effect of the air jet released into the freestream during expiration and the effect of the upward-moving boundary layer near the body associated with the buoyancy of air in that region as a result of heat received from the warm body. In particular, we set out to identify the combinations of conditions—external windspeed, breathing mode (nose versus mouth breathing), breathing rate and body temperature—where such factors need to be taken into account in considerations of aerosol movement as it relates to inhalation. The study that is reported was largely qualitative, based on motion picture records of smoke traces for conditions in the ranges indicated.

EXPERIMENTAL RATIONALE AND SETUP

The nature of aerosol aspiration, including during inhalation by humans and practical sampling as it is carried out by occupational and environmental hygienists, is strongly dependent on the nature of the airflow that is bringing the particles into proximity to the entry (as reviewed by Vincent, 2007). Therefore, it is important to know the extent to which the actions of inspiration and—especially—expiration may disturb or otherwise influence the approaching airflow. Although the role of body temperature has not been specifically studied in this regard, it has frequently been mentioned as an influential factor and so needs to be clarified.

Ultralow speed wind tunnel

For the work under way in our laboratory, including studies of air movement, aerosol transport, inhalability and aerosol sampler performance at ultralow windspeeds, a special new wind tunnel was built (with Engineering Laboratory Design, Lake City, MN, USA). In brief, it was designed to (i) contain a life-sized human mannequin, including the full torso above the waist, (ii) provide uniform, smooth airflow at velocities continuously variable between 0.05 and 0.50 m s⁻¹ and (iii) enable the injection of spatially uniform test aerosols with well-defined particle size distributions for particle aerodynamic diameter in the range up to ∼100 μm. Full details, in particular those that pertain to the performance of the new facility for the aerosol-related parts of the research, will be provided in a subsequent publication. Here, we will concentrate on the features relevant to the flow visualization investigation, and these are shown schematically in Fig. 1. Notably, the working section measured 1.22 × 1.22 m in cross section and 3.05 m in overall length. The air entered through a pre-filter into the upstream mixing chamber, after which it passes through a honeycomb screen that serves to straighten the airflow as it enters into the working section of the wind tunnel. The airflow was developed by four downstream fans, which are regulated by a frequency inverter that allows easy manipulation of the windspeed. At such low windspeeds, conventional anemometry—hot-wire or pitot static—could not be used. So, we developed a time-of-flight method in which the smoke trace (see below) was given a small mechanical disturbance to produce a ‘blip’ that could be observed visually and its passage through the wind tunnel timed over a predetermined distance by means of a stop watch. This ‘old-fashioned’ method was considered accurate enough for the purpose in hand. In this way, the actual velocity in the wind tunnel was calibrated against the pressure drop across the large high efficiency particulate air (HEPA) filters at the inlet to the tunnel as measured using a micromanometer. It was this pressure drop that was used on a day-to-day basis for determination of windspeed. Its calibration was checked periodically throughout the duration of the experimental program to take account of any changes in the pressure drop characteristics of the filter media. In addition, the air velocity was shown to be uniform to within a few percentage points throughout the entire cross section of the wind tunnel.

Fig. 1.

Schematic drawing of new ultralow speed wind tunnel, showing the location of the primary components, notably the smoke generation setup and the illumination for flow visualization.

The overhead mixing chamber, shown above the working section, was designed primarily for the purpose of test aerosol injection in the proposed future experiments (which will be described in future publications arising from this body of work), but here served to contain the illumination system for the flow visualizations. This system took the form of a series of five 250 W floodlight bulbs inverted on top of the honeycomb floor of the upper section such that the light entering the working section was collimated by the honeycomb elements. These elements also served to provide sufficient separation between the upper section and the working section such that heating of the air flowing in the wind tunnel was negligible in terms of any influence on the flow itself. By such means, a sheet of light illuminated the portion of the working section immediately upstream of the mannequin that, as shown in Fig. 1, was positioned in the center of the working section.

Flow visualization

Several methods for flow visualization were considered. The ‘helium bubble’ technique has been widely used by aerodynamicists (Kerho and Bragg, 1994; Mueller, 1996). It involves the generation of helium-filled soap bubbles of a size such that the bubbles are neutrally buoyant and so should theoretically follow the airflow inside the wind tunnel. This method was attempted, but it was found to be prohibitively difficult to inject bubbles sufficient in quantity and stability and of appropriate size to adequately characterize the flow across the entire cross section of the tunnel with minimum external interference to the freestream airflow. Ultimately, therefore, this technique was abandoned. We also considered the heated, oil-covered wire approach that had been used in earlier studies in a smaller wind tunnel (e.g. Sreenath et al., 1997), but this too was found to be difficult for flow visualization purposes in our new larger wind tunnel, not least because the heating of the wire was sufficient to disturb the freestream at the low velocities of interest here.

The flow visualization option that proved most effective, and which was ultimately adopted, was much simpler, involving smoke derived from the burning of incense sticks. In order to eliminate any effect of the heat generated by the actual burning of the incense stick, the smoke was generated in a chamber located outside the wind tunnel. The chamber was constructed of particleboard with a plexiglas top and steel brackets holding all the sides together, measuring ∼0.6 m on each side. Eight incense sticks (Florasense Incense, Blyth HomeScents International, Des Plaines, IL, USA) were lit, placed inside this smoke chamber, and allowed to burn for at least 5 min before turning on the wind tunnel. This allowed the smoke to fill the smoke chamber and to cool sufficiently to minimize potential thermal effects once the smoke was delivered to the wind tunnel. In order to introduce the smoke into the wind tunnel, a length of flexible plastic tubing was attached from the smoke chamber through the floor of the upstream mixing chamber. A long, rigid plastic tube of diameter ∼5 cm and length ∼1.2 m was connected to the end of the flexible tubing and mounted vertically in the central plane of the wind tunnel just upstream of the honeycomb at the entrance to the working section. Twelve small holes were drilled in a line along the surface of the tube such that, due to the negative pressure inside the wind tunnel, the smoke was drawn from the smoke chamber into the tube and out into the wind tunnel through the 12 holes. Remarkably stable and continuous smoke traces were generated in this way, as will be seen.

High-quality images of the flow patterns around the mannequin were obtained using a digital camera (Panasonic, DMC-FZ50 Lumix) capable of producing high-quality video clips, from which additional still photographs could also be extracted. Although the most important information—leading to the primary conclusions of the work—was gained from viewings of the large number of video clips produced in this way, only selected still photographs were feasible for the purpose of this publication. In the figures presented here, still pictures were selected relating to the peaks of the inspiration and expiration parts of the breathing cycle.

Finally, it is important to note that all the streamlines visualized in the manner indicated were undisturbed and horizontal in the absence of the mannequin.

Mannequin

An important factor in aerosol inhalability experiments is a physical model of an adult human subject that adequately represents the important external physiological features of the head and body as well as the respiratory system (nose and mouth), including the ability to inspire and expire realistically. With this in mind, a new life-sized mannequin torso and head system was custom designed and built to provide continuously variable breathing capabilities in order to simulate a range of normal human inspiration and expiration, along with a heating mechanism to simulate actual body temperature (with Measurement Technology Northwest, Seattle, WA, USA). Again, greater detail as it relates to the wider body of aerosol research under way will be presented in a later publication.

Figure 2a and b shows photographs of the mannequin and its internal breathing structure. External to the body of the mannequin was a mechanical piston-type breathing machine for which breathing parameters (including minute volume, breath rate and flow pattern for both inspiration and expiration) were controlled through a computer. The design of the duct connections inside the head were such that any combination of breathing could be simulated, including in and out through the nose (nose only), in and out through the mouth (mouth only) and in through the nose and out through the mouth (nose–mouth). In addition, the option was available to breathe in through the mouth and out through the nose, but was not used during the present experiments since it was considered the least likely mode of breathing in the real world. Adjustments to the mode of breathing could be made simply by manually changing the connections inside the head indicated in Fig. 2b.

Fig. 2.

Two views of the mannequin used in this research: (a) assembled and (b) with breathing manifold open to reveal the assembly for controlling the flow paths for inspiration and expiration.

Changes in body surface temperature were made possible by electrical heating coils located just below the surfaces of the head, torso and arm structures. Body surface temperature was monitored by means of strategically located thermocouples and controlled by means of the computer.

In the experiments described in this paper, the mannequin was located ∼1.5 m downstream of the honeycomb screen at the entrance to the working section.

EXPERIMENTAL METHODS

The wind tunnel and mannequin system were set up to enable visualization of the airflow around the human body, for ranges of conditions of interest, using the smoke tracers described earlier. The nature of the airflow was visualized for a relevant range of windspeed, breathing rate, breathing mode (nose and/or mouth breathing), mannequin body temperature, orientation and clothing. We chose (i) three different windspeeds: 0.10, 0.24 and 0.42 m s⁻¹; (ii) two different breathing rates: minute volume rates of 6 and 20 l min⁻¹ (to simulate breathing ‘at rest’ and for ‘moderate work’), for either 12 or 20 breaths per minute with either 0.5 or 1.0 L tidal volume, respectively, and (iii) three modes of breathing: nose only, mouth only and nose–mouth. As mentioned earlier, the other possible combination of inspiration through the mouth with expiration through the nose was not included because it was not considered to be a likely breathing mode. Also, for the higher breathing rate, nose-only breathing was not examined because this too was not considered to be a likely breathing mode (Saibene et al., 1978). In addition to the flow conditions summarized above, the mannequin body temperature was set at either room temperature (i.e. no heating) or at typical skin temperature (33°C). All these conditions of interest are summarized in Table 1. Here, it is seen that a total of 30 combinations of experimental conditions were studied.

Table 1.

Values of experimental conditions studied, applied to full set of experiments covering all 30 possible combinations

Windspeed (m s−1)	Breathing minute volume (l min−1)	Mode of breathing	Body temperature
0.10		In through nose/out through mouth
0.24	6	In through mouth/out through mouth	Unheated
0.42	20	In through nose/out through nose	Heated (33°C)

Most of the experiments that will be described were carried out for the case where the mannequin was facing forward into the wind, this being the orientation where the influences under consideration were likely to be greatest. However, a small number of additional experiments were performed for the unheated mannequin at other orientations, including 90° from the wind and facing directly downstream from the wind (180°). Furthermore, some experiments were also carried out to examine the effects of clothing, including lab coat, safety glasses and a hard hat.

As already mentioned, the research was largely qualitative, based on close evaluation of the detailed smoke tracer records that were accumulated for the various sets of conditions studied. For each set of conditions, in viewing the moving pictures, we asked the following questions:

• (i) Is the airflow (which would be carrying the aerosol to which the mannequin would be exposed) approaching the mannequin from upstream significantly modified by the breathing action of the mannequin? That is, does the jet of expired air disturb the airflow? If so, does it persist long enough to influence the air motion—and hence aerosol motion—during the subsequent inspiration?

• (ii) Is the flow approaching the mannequin from upstream significantly modified by any buoyancy-driven air movements associated with differences between the temperature of the body and that of the surrounding air?

If the answer to either of these were to be ‘yes’, then it could be concluded that the alteration of the airflow would indeed have been sufficient to significantly influence the transport of aerosol during aspiration and hence influence aspiration efficiency (in a way not assumed to be present for the higher windspeeds of nearly all previously reported inhalability studies).

RESULTS

Answers to questions like those posed above were obtained by inspection of the movie clips of the flow patterns over a number of breathing cycles. Of course, it is not possible to present such moving pictures in a paper like this. Therefore, the illustrations described below are given in terms of sequences of representative still frames taken from the more detailed movie clips.

Effect of windspeed

Figure 3 shows examples of pictures of the airflow pattern at different windspeeds, indicating the situation for no breathing and for the peaks of the expiration and inspiration phases of the breathing cycle, respectively. The results shown are for the unheated mannequin and mouth-only breathing at a minute volume of 6 l min⁻¹ (corresponding to at rest). They reveal clear differences as a function of windspeed. During expiration in the lowest windspeed (0.10 m s⁻¹), there was a pronounced disturbance of the airflow pattern directly in front of the mannequin, as observed by comparison to the case where there was no breathing. For the inspiration phase, which is the most important in terms of possible effects on aerosol inhalation, the disturbance introduced during the preceding expiration phase had clearly persisted. That is, the flow pattern did not have time to recover between the expiration and inspiration phases of the breathing cycle. By contrast, at the higher windspeeds (0.24 and 0.42 m s⁻¹), the initial disturbance during the expiration phase was less evident, and the smooth flow—similar to that for no breathing—had recovered by the time of the subsequent inspiration phase.

Fig. 3.

Typical flow visualizations to demonstrate the effect of windspeed, for no breathing, peak expiration and peak inspiration, respectively. Results are shown for the unheated mannequin for mouth-only breathing at 6 l min⁻¹, for (a) 0.10 m s⁻¹, (b) 0.24 m s⁻¹ and (c) 0.42 m s⁻¹.

Similar experiments (photographs not shown) were carried out for other flow and breathing combinations, including (variously) nose-only and nose–mouth breathing, and the higher breathing minute volume of 20 l min⁻¹ (corresponding to moderate work). In general, for a given windspeed, the impact of the expired air on the approaching freestream was less for the lower breathing flow rate and when expiration took place through the nose.

Effect of breathing parameters

Mode of breathing.

Figure 4 shows examples of pictures of the airflow pattern when different modes of breathing were used, for the peaks of the expiration and inspiration phases of the breathing cycle, respectively. Results are shown for the unheated mannequin and for the single windspeed of 0.24 m s⁻¹. These show that the amount and nature of the disturbance to the approaching airflow was strongly related to the orifice—nose or mouth—from which expiration took place. In general, expiration through the mouth created a jet that projected into the freestream directly in front of the mannequin head. But when expiration took place through the nose, the jet was directed downward and projected less far forward into the approaching airstream, thus creating a disturbance that was closer to the torso of the mannequin. The result for the condition where inspiration took place through the nose but expiration took place through the mouth was similar to that for mouth-only breathing. Similar results were obtained for the other windspeeds of interest, and the general trend was that the impact of the expiration on the approaching freestream, as it relates to the different modes of breathing, was greater the lower the windspeed.

Fig. 4.

Typical flow visualizations to demonstrate the effect of breathing mode, for peak expiration and peak inspiration, respectively. Results are shown for the unheated mannequin breathing at 6 l min⁻¹ in a windspeed of 0.24 m s⁻¹ for (a) nose-only breathing, (b) mouth-only breathing and (c) nose–mouth breathing (in through nose/out through mouth).

Breathing flow rate.

Figure 5 shows a typical set of pictures for the flow pattern for different breathing rates (corresponding to at rest and moderate work, respectively), for the peaks of the expiration and inspiration parts of the breathing cycle, respectively, at a single windspeed of 0.24 m s⁻¹. Not surprisingly, the effect of the expired air and the persistence of the disturbance into the subsequent inspiration cycle were much more evident for the higher breathing flow rate, as a greater volume of air was projected into the approaching airstream by the jet. That is, the initial disturbance at the higher flow rate was much greater and so too was the persistence into the inspiration phase of the breathing cycle. Again, in general, the trend from all the results obtained was that the effect of the expiration on the approaching freestream was greatest for the highest breathing flow rate.

Fig. 5.

Typical flow visualizations to demonstrate the effect of breathing flow rate, for peak expiration and peak inspiration, respectively. Results are shown for the unheated mannequin in a windspeed of 0.24 m s⁻¹ and mouth breathing only for (a) 6 l min⁻¹ (at rest) and (b) 20 l min⁻¹ (moderate work).

Effect of body temperature

Figure 6 shows a typical set of pictures of the flow pattern for the non-breathing mannequin, with the mannequin both heated and not heated, and for the two windspeeds of 0.10 and 0.24 m s⁻¹. The rationale for this part of the experimental inquiry was that, in most indoor working environments, the human body would be warmer than the surrounding air and therefore there may be thermal influences on air movement around the body that might in turn influence the inhalation of airborne particles. In general, however, the pictures show no evidence that the airflow pattern in the immediate breathing zone of the mannequin was influenced in any way by the heat emanating from the heated mannequin.

Fig. 6.

Typical flow visualizations to demonstrate the effect of body temperature for the heated and unheated mannequin, respectively. Results are shown for the non-breathing mannequin in windspeeds of (a) 0.10 m s⁻¹ and (b) 0.24 m s⁻¹.

There was one interesting observation that does not necessarily bear directly on the subject of our research, but should be noted nonetheless. That is, for the lower windspeed, there appear to have been a marked influence of the body heat on the shape of the approaching airflow not seen at higher windspeeds. This is believed to be an artifact of our experimental system, where the plume of buoyant air derived from the heating of the mannequin body rose to the upper part of the confined space of the wind tunnel, inducing a secondary, recirculating flow in the upper portion of the tunnel. This resulted in the observed depression of the flow lines in the approaching airflow, as shown most notably in the picture for the heated mannequin at the lowest windspeed.

Effect of orientation

All the experiments described so far were performed with the mannequin facing directly into the wind. However, for the quantitative inhalability experiments that will be studied in our future work, and as in all the previous related research, the mannequin will be continuously rotated through a full 360° to enable orientation-averaged results. With this in mind, we examined the effect of orientation in the present study. Our impressions were based on inspection of the flow patterns both upstream and downstream of the mannequin, which was useful in forming a general picture of the effect of different orientations. However, no pictures are available for this part of the study because of the difficulty in obtaining clear, unambiguous photographs of the disturbances introduced into the airflow for orientations other than forward facing.

To summarize briefly what was observed, turning the mannequin to a 90° angle, facing toward the wind tunnel sidewall, appeared to affect the smoke pattern only slightly. It was observed that eddies formed on the downstream side of the head independent of the breathing action. These appeared to be close enough to the nose and mouth that aerosol inhalation could potentially be affected, in the way that has previously been discussed in the literature (again, as reviewed by Vincent, 2007). However, the breathing action itself did not appear to affect the airflow significantly for any of the freestream and mannequin breathing parameters studied.

Similar results were found for the mannequin at the 180° orientation. That is, there was the expected eddy formation in the downstream wake region quite independent of any breathing action, but there appeared to be no significant influence associated with the breathing action.

Effect of clothing

We also studied the influence of garments (including lab coat, glasses and hard hat) for the mannequin, both heated and unheated but not breathing. Although there were some minor changes in the shape of the airflow very close to the mannequin associated with these features, these appeared to be second order in nature and not significant in influencing the general nature of the airflow approaching the mannequin. Hence, we believe, these would be unlikely to significantly impact on aerosol inhalability.

DISCUSSION

We now proceed to draw together the results that have been described in order to make some useful generalizations. To this end, Fig. 7 shows a matrix of regimes—categorized by windspeed and breathing rate and indicated by the hatched regions—where the effect of the expired air is sufficient to permanently and seriously destabilize the airflow approaching the mannequin. That is, it indicates the regimes where the disturbance induced by the expiration persists strongly into the subsequent inspiration phase of the breathing cycle. Results are shown for both mouth and nose expiration, respectively.

Fig. 7.

Summary of flow visualization results for (a) expiration through the mouth and (b) expiration through the nose, describing the relative influence of expired air on the approaching freestream for ranges of windspeed and breathing flow rate.

It is in the regimes in which the airflow is permanently destabilized where it is expected that aerosol inhalability might be significantly affected, although we are not yet able to predict in what ways. The same is likely to apply to the performance of personal samplers placed on the torso, perhaps differently depending on where the samplers are actually located. Importantly, information like that summarized in Fig. 7 will provide useful input to the discussion of the results of the planned future inhalability experiments, as well as to the ongoing discussion of the whole subject of inhalability at ultralow windspeeds. In addition to the information summarized in Fig. 7, it is useful to add that no significant effects were found associated with the elevated temperature of the body of the mannequin.

As standard-setting bodies like ISO and CEN, among others, move toward expanding the inhalability criteria to take into account more realistic conditions, a more complete understanding of the nature of the air movement near a heated, breathing individual (as represented here by the mannequin) has been provided by this work.

FUNDING

United States National Institute for Occupational Safety and Health (5-RO1-OH002987-09).