Reduction in Motorcyclist Pollutant Exposure Intensity via the Aid of a Porous

Scooters are the most common form of transport for commuters in Taiwan, and their emissions account for the majority of the airborne pollutants present in metropolitan areas. These pollutants have a serious adverse effect on the health of motorcyclists and their passengers, and thus effective methods are required for minimizing the pollutant exposure intensity. Accordingly, the present study performs numerical simulations to investigate the pollutant dispersion efficiency of four different front fender and vent designs, namely (1) no fender spoiler and vents on two sides of scooter frame; (2) a fender spoiler and vents on two sides of scooter frame; (3) no fender spoiler and vents beneath scooter frame; and (4) a fender spoiler and vents beneath scooter frame. For each design configuration, the simulations consider two different fender porosity ratios, namely 50% and 70%. In addition, the effectiveness of a rear-mounted exhaust suppressor and various saddle designs in reducing the exhaust exposure concentration is also briefly examined. Overall, the simulation results show that, compared to a conventional fender/vent design (i.e., zero fender porosity, no fender spoiler, and no vents), the use of a fender with a porosity ratio of 70% and two vents beneath the scooter frame reduces the exhaust exposure concentration by 36% given no fender spoiler and 43% given the use of a fender spoiler. Furthermore, it is shown that the addition of an exhaust suppressor to the rear of the saddle or the use of a wide saddle width also achieves a moderate improvement in the pollutant dispersion efficiency.


INTRODUCTION
Scooters are the most common form of commuter transportation in Taiwan; particularly in metropolitan areas.It has been estimated that there are around 15 million scooters in Taiwan, of which around 30% are located in Taipei, Taichung or Kaohsiung.It has been reported that mobile sources account for almost 98% of the carbon monoxide (CO) emissions in Taipei (Liu et al., 1994).Many studies have shown that long-term exposure to high air pollution concentration levels causes serious adverse health effects (Tsai et al., 2010;Su et al., 2011).Thus, the effects on human health of airborne pollutants such as CO, nitrogen oxide (NO), particulate matter (PM), volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), dioxins, and so forth, has attracted significant attention in Taiwan (Lee et al., 1995;Mi et al., 2000;Tsai et al., 2004;Lin et al., 2005;Lin et al., 2007;Lin et al., 2008;Wang et al., 2008;Chuang et al., 2010;Hsieh et al., 2011;Tsai et al., 2011;Cheng et al., 2012;Hung et al., 2012;Wang et al., 2012).Many vehicular emission concentration distribution and personal exposure level studies have been conducted in Taiwan (Chan et al., 1993;Chan et al., 1995).In general, the results have shown that commuters in Taipei experience VOC concentration levels around three to eight times higher than those experienced by commuters in Los Angeles (Chan et al., 1993).Various researchers have investigated the exhaust exposure levels experienced in different commuter receptor modes (e.g., public bus, private car, taxi, scooter or motorcycle) (Zhao et al., 2004;Chan et al., 2010).Many studies have shown that commuting by motorcycle or bicycle during heavy traffic periods leads to significantly increased levels of exposure to CO, respirable suspended particulates, and VOCs (Bevan et al., 1991;Chan et al., 1993;Liu et al., 1994;Vellopoulou and Ashmore, 1998;Liu et al., 2011).
With growing awareness regarding the adverse health effects of CO and other VOCs, the government in Taiwan has put in place various mechanisms to measure and report the CO level in metropolitan areas.One of the most common forms of determining the CO concentration is that of fixed-site monitors (Kåre and Finn., 1999).However, the monitoring data obtained by fixed-site monitors cannot accurately access the exposure dose received in the various commuter mode microenvironments.For example, it has been reported that the average PM 2.5 concentration measured at a fixed monitoring station may be as much as three times lower than the average level actually experienced by pedestrian commuters and up to eight times lower than the maximum exposure.Similarly, the measured CO concentration level may be as much as three times lower than the actual value.Furthermore, it is both difficult and time consuming to precisely quantify the pollutant exposure level utilizing on-site measurements (Zhao et al., 2004;Kuar et al., 2005).
Accordingly, the use of numerical simulation techniques to evaluate the pollutant dispersion process and the commuter exposure level has received increasing attention in recent years (Sharma et al., 2001;Vardoulakis et al., 2003;Kim et al., 2004;Liu et al., 2011;Zhao et al., 2011).In general, the numerical results confirm the finding of experimental studies that motorcyclists are exposed to a far higher air pollutant exposure level than pedestrians or those riding in a private car or public bus (Liu et al., 1994;Kaur et al., 2005).Various researchers have used numerical models to examine the downwind dispersion process of the CO emissions in a vehicle exhaust plume (Gosse et al., 2006;Wang et al., 2006;Chan et al., 2008).It was reported by Wang et al. (2006) that the CO dispersion process is less influenced by the ambient wind velocity than by the vehicular exhaust velocity.In addition, Kaur et al. (2005) showed that the pollutant exposure level experienced by motorcyclists and their passengers in the commute mode is particularly high due to the characteristic short distance between the exhaust tailpipe and the receptor. .
The present study performs full-scale computational fluid dynamics (CFD) simulations to investigate the motorcyclist pollutant exposure intensity in the scooter commute mode.The simulations consider the CO dispersion efficiency of four different front fender and vent designs, namely (1) no fender spoiler and vents on two sides of scooter frame; (2) fender spoiler and vents on two sides of scooter frame; (3) no fender spoiler and vents beneath scooter frame; and (4) fender spoiler and vents beneath scooter frame.For each design configuration, the simulations consider two different fender porosity ratios, namely 50% and 70%.In addition, the effects on the CO dispersion efficiency of a rear-mounted exhaust suppressor and various saddle designs are also briefly explored.

CFD MODELING
The simulations were performed using commercial Fluent software (Version 6.3) based on the Reynolds Averaged Navier-Stokes equations and the RNG k-ε turbulence model (Choudhury (1993)) The computational domain was discretized non-uniformly using Gambit software.The flow fields produced by each fender/vent design configuration were determined by solving the 3D averaged Navier-Stokes equations subject to the following governing equations: where ρ is the density of the fluid and u i denotes the fluid velocity in the i-th direction.

Momentum Equation
where p is the static pressure and is the Reynolds stress caused by turbulence.
The CO concentration distribution was determined in accordance with 2 ( ) where C is the concentration, u i is the fluid velocity, ρ is the density, μ is the viscosity, D is the binary diffusion coefficient, and p is the pressure.Among the various turbulence models available, the present study deliberately adopted the RNG k-ε model for its ease of use.The complete formulation of the RNG k-ε turbulence model is expressed in the Einstein summation convention as follows: where k is the turbulence kinetic energy, ε is the kinetic energy dissipation rate, σ k is the Prandtl number of the turbulence kinetic energy, and σ ε is the Prandtl number of the turbulence kinetic energy dissipation rate.The remaining variables in Eqs. ( 4) and ( 5) are defined as follows: where S ij is the shearing-rate tensor and g i is the body force term in the x i direction.
In performing the simulations, the computational domain was meshed using a hexahedral grid system in approximately 600,000 cells, and the coefficients of the model were assigned the default values proposed by Fluent (ANSYS, ( 2006)), i.e., C u = 0.085, σ ε = 0.719, σ k = 0.719, C 1ε = 1.42,C 2ε = 1.68, β = 0.012, and η 0 = 4.38. it is seen that the wake flow velocity structures rotate in the downward direction above the seat but in the upward direction under the seat.As a result, the exhaust pollutants are confined to the region between the saddle and the ground (see Fig. 1(c)).In practice, the formation of a large exhaust contaminant region behind the scooter is due to the wake flow effect.Specifically, when the exhaust emissions enter the tailing vortices region, they become uniformly mixed within the rotating wake flow structure and are therefore not easily dispersed to the surrounding environment.

RESULTS AND DISCUSSION
As described above, the high concentration of pollutants within the exhaust wake flow poses serious health hazards; particularly for the rider and passenger of any following scooter.According to the Environmental Protection Administration in Taiwan, the CO content of the exhaust flow emitted by a four-stroke scooter should not exceed Two different alignments of the following scooter were considered, namely in-line (i.e., the centerline of the following scooter was aligned with the centerline of the leading scooter) and off-set (i.e., the centerline of the following scooter was aligned with the tailpipe of the leading scooter).Fig. 3 shows the exhaust concentration distributions for the four different tailpipe angle and alignment scenarios.Note that the cruise velocity is assumed to be 30 km/hr in every case.Observing the four figures, it is seen that an in-line alignment of the two scooters reduces the contaminant exposure of the following scooter for both angles of the exhaust tailpipe.In the case of the off-set scooter alignment mode, the trailing motorcyclist's exposure concentration is 399 ppm following a 45° slant tailpipe but 179 ppm following a horizontal tailpipe.Clearly, the 45° slant tailpipe results in an exposure concentration more than twice of that a horizontal tailpipe would have yielded.On the other hand, in the in-line scooter alignment mode, the trailing motorcyclist's exposure concentration is 356 ppm following a 45° slant tailpipe but 141 ppm following a horizontal tailpipe.However, for both alignment modes, the 45° slant tailpipe results in a higher contaminant exposure intensity than the horizontal tailpipe since the exhaust flow is guided directly toward the following scooter.As shown in Fig. 3, a high-concentration contaminant region is formed between the front fender and the saddle of the following scooter due to recirculation effects.This scenario is extremely common in the commute mode and is a serious concern in Taiwan since a third passenger (typically a young child) is commonly carried in this particular region of the scooter.The results presented in Fig. 3 show that motorcyclists and their passengers experience a high level of exhaust contaminant exposure under typical commuting conditions.Thus, effective methods are required to disperse the exhaust pollutants in an efficient manner so as to minimize their impact on motorcyclist health.Accordingly, in the present study, numerical simulations were performed to examine the pollutant dispersion efficiency of four different front fender and vent designs, namely (1) no fender spoiler and vents on two sides of scooter frame; (2) fender spoiler and vents on two sides of scooter frame; (3) no fender spoiler and vents beneath scooter frame; and (4) fender spoiler and vents beneath scooter frame.For each design configuration, the simulations considered two different fender porosity ratios, namely 50% and 70%.In addition, the effectiveness of a rear-mounted suppressor and various saddle designs in reducing the exhaust exposure concentration was also briefly examined.
Fig. 4 illustrates the various fender/vent design configurations.As shown in Fig. 4(a), the effect of the fender porosity on the pollutant dispersion performance was examined by dividing the front fender into three sections (louver panels); where a porosity of 50% or 70% was applied to the upper two panels and the lower panel remained solid.Fig. 4(b) illustrates the addition of a spoiler with a width of 5 cm and a thickness of 3 mm to the top of the fender.Figs.4(c) and 4(d) show the addition of symmetrical vents to either side of the scooter frame and beneath the scooter frame, respectively.Finally, Fig. 4(e) shows the addition of an exhaust suppressor with dimensions of 16 cm × 6 cm × 6 cm behind the saddle.
Figs. 5(a)-5(d) show the pressure distribution contours, velocity distribution contours, velocity vector distribution, and in-fender flow streamlines, respectively, given a fender porosity ratio of 70%.As shown in Fig. 5(a), the fender and rider experience a high pressure since they impact the air directly as the scooter moves in the forward direction.By contrast, the regions behind the fender and the rider experience a low pressure since they are shielded from the on-coming air.exhaust fumes which enter the fender exit the vents and are dissipated at a height corresponding to the upper portion of the rear wheel.For both vent designs, a high velocity region is formed beneath the scooter frame due to pressure difference effects.Thus, as shown in Fig. 6(b), given the use of vents beneath the scooter frame, the exhaust fumes which enter the fender are rapidly dissipated at a height corresponding to the lower portion of the rear wheel.In other words, the use of under-frame vents results in a high mass flow rate of the exhaust contaminants and is therefore beneficial in reducing the contaminant concentration intensity.Table 1 summarizes the exhaust mass flow rates for the various front fender and vent designs.For all four design configurations, results are presented for both a 50% fender porosity ratio and a 70% fender porosity ratio.It is seen that given the use of side vents but no fender spoiler, the mass flow rate is equal to 0.101 kg/s for a porosity ratio of 50% and 0.115 kg/s for a porosity ratio of 70%.In other words, a porosity ratio of 70% improves the mass flow rate by around 15% compared to that achieved using a porosity ratio of 50%.Given the addition of a top spoiler to the front fender, the mass flow rate increases to 0.105 kg/s for a porosity ratio of 50% and 0.118 kg/s for a porosity ratio of 70%; corresponding to an increase of around 3% in both cases.For the case of vents beneath the frame and no fender spoiler, the mass flow rates given fender porosity ratios of 50% and 70% are found to be 0.136 kg/s and 0.151 kg/s, respectively.In other words, as in the side-vent  design configuration, the mass flow rate increases with an increasing fender porosity ratio.Notably, for both fender porosity ratios, the mass flow rate in the under-vent design is around 35% higher than that in the side-vent design.Thus, the superior dispersion performance of the under-vent design implied in Fig. 6 is confirmed.As shown in the lower row of Table 1, given the use of vents beneath the frame and the addition of a spoiler to the fender, the mass flow rate is equal to 0.146 kg/s for a porosity ratio of 50% and 0.159 kg/s for a porosity ratio of 70%.Thus, it is evident that the under-vent with fender spoiler design achieves the best pollutant dispersion performance of the four designs.
Table 2 shows the average motorcyclist exhaust exposure concentration given the use of vents beneath the scooter frame and various fender designs.(Note that a horizontal exhaust tailpipe and an in-line alignment is assumed.)It is seen that for a conventional solid fender design, the average pollutant concentration is equal to 141 ppm.Given a fender porosity of 70%, the average concentration is equal to 90 ppm; corresponding to a reduction of around 36% compared to the original design.Finally, given a porosity ratio of 70% and the addition of a spoiler to the fender, the average concentration reduces to 81 ppm; corresponding to a reduction of 43% compared to the original design.In other words, the addition of the spoiler to the fender reduces the average concentration intensity by around 7%.The streamline fender design was mostly adopted by the manufacturers for reducing the form drag.However, the high pressure on the fender guides the exhaust to the motorcyclist's normal direction by the geometry, and the rider suffers high exposure concentration exhaust.Therefore, the fender geometry design reveals an excellent solution for motorcyclist exposure concentration reducing in the cruise condition.
The results presented above have shown that a significant improvement in the exhaust diffusion performance can be obtained by modifying the fender design and adding vents beneath the frame of the scooter.However, this inevitably implies an increased manufacturing cost.Accordingly, this study also investigated the feasibility of reducing the exhaust exposure intensity by adding an exhaust suppressor to the rear of the saddle (see Fig. 4(e)) or modifying the saddle design.Fig. 7 shows the various suppressor positions considered in the present study, namely the default position (Fig. 7(a), a height of 25 cm above the exhaust tailpipe), a position 5 cm below the default position (Fig. 7(b)), and a position 5 cm above the default position (Fig. 7(c)).The simulation results obtained for the motorcyclist exhaust exposure concentration given the default suppressor position and an off-set alignment of the leading and following scooters is shown in Fig. 8(a).In addition, the relative effectiveness of the three suppressor configurations in reducing the exhaust exposure concentration is summarized in Table 3.It is seen that the default suppressor design reduces the average exhaust exposure concentration from 141 ppm (no suppressor) to 101 ppm; corresponding to a reduction of 28%.In addition, it is observed that moving the suppressor 5 cm in either the upward or the downward direction also reduces the average exhaust concentration, but to a lesser extent, i.e., 20% (upward direction) and19% (downward direction).Overall, the results presented in Table 3 show that while the addition of a rear-mounted exhaust suppressor yields a less significant improvement in the exhaust diffusion performance than that obtained by modifying the fender and adding vents, the suppressor  nevertheless represents a potential solution for reducing the adverse effects of exhaust pollutants on motorcyclist health.Fig. 8(b) presents the simulation results for the motorcyclist exhaust exposure concentration given the use of a hollow saddle design and no suppressor.The average exposure concentration is found to be 128 ppm.This value is less than that in the original design (i.e., 141 ppm, see Table 3), but is higher than that obtained in the various solid seat with exhaust suppressor designs.This result implies that the hollow saddle fails to generate a wake flow and is therefore less able to confine the exhaust emissions to the region beneath the exhaust tailpipe and the ground.
As shown in Fig. 9, the present simulations also considered the effects of various saddle widths (i.e., normal, 20 cm; narrow, 10 cm; and wide, 40 cm) on the exhaust pollutant diffusion efficiency.Figs.10(a) and 10(b) present the simulation results for the exhaust exposure concentration given a narrow saddle and a wide saddle, respectively.It is seen that the narrow saddle results in a higher exhaust concentration behind the rider, while the wide saddle results in a higher exhaust concentration in front of the rider.This finding suggests that the wider seat tends to direct the exhaust fumes in the lower height and therefore increases the horizontal distance required for the fumes to disperse.As a result, the wider saddle design provides a more effective solution than the narrow saddle design for minimizing the adverse effects of exhaust pollutants on motorcyclist health.

CONCLUSIONS
The exhaust emissions of scooters have a serious adverse effect on motorcyclist health.Accordingly, the present study has performed a series of numerical simulations to investigate the effectiveness of various scooter fender and vent designs in improving the exhaust pollutant dispersion  efficiency.In general, the results have shown that the addition of a spoiler to the top of the fender and two vents beneath the scooter frame provides an effective reduction in the exhaust concentration experienced by the rider on the following scooter.It has been shown that the pollutant dispersion efficiency can be further improved by increasing the porosity of the fender.Specifically, given a fender porosity of 70%, a fender spoiler and two under-frame vents, the average pollutant concentration can be reduced from 141 ppm (conventional solid fender design with no vents) to 81 ppm; corresponding to a reduction of 43%.In addition, it has been shown that adding an exhaust suppressor to the rear of the saddle or reducing the saddle width provides a low cost solution for obtaining a moderate improvement in the pollutant dispersion efficiency.The results presented in this study are of significant benefit to scooter manufacturers in modifying the fender design in such a way as to improve motorcyclist health.In addition, the results are also of interest to commuters in understanding the regions of the scooter prone to particularly high pollutant concentration levels and in modifying their passengercarrying habits accordingly.

Fig. 1
Fig. 1 illustrates the wake flow velocity contours and exhaust concentration distribution in the tailing vortices region of a typical scooter.As shown in Fig. 1(a), a large

Fig. 1 .
Fig. 1.(a) Photograph of exhaust distribution in rear region of scooter, (b) wake flow velocity contours in rear region of scooter, and (c) exhaust concentration distribution in rear region of scooter.

Fig. 6 .
Fig. 6.Exhaust flow patterns given (a) vents on both sides of scooter frame, and (b) vents beneath scooter frame.

Table 1 .
Exhaust mass flow rates given different fender and vent design configurations.

Table 2 .
Exhaust exposure concentration levels given different fender designs and vents beneath frame.

Table 3 .
Effect of rear-mounted suppressor on exhaust exposure concentration level.