|Title:||Vehicle queue effect on the characteristics of air flow, and exhaust scalar dispersion and distribution fields in the vehicle wake|
Motor vehicles -- Aerodynamics.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Mechanical Engineering|
|Pages:||xxv, 237 leaves : ill. (some col.) ; 30 cm.|
|Abstract:||The characteristics of air flow, and vehicular exhaust scalar (i.e., pollutant) dispersion and distribution fields in the near-wake region of a scaled-down vehicle were experimentally investigated using the particle image velocimetry (PIV) and cold-and hot-wire anemometers for selected urban vehicle speeds. The studied vehicle was placed alone or behind preceding vehicle(s) inside a wind tunnel facility. The front and body of all studied vehicles are of the same shape except for different rear slant angles (α =25° and 60°) in order to generate three-dimensional and quasi two-dimensional wake flows behind the vehicle, respectively. The structures of wake flow are almost independent of the studied vehicle speeds but the presence of the preceding vehicle(s) can change the wake structures especially in the recirculation zone. The incoming flow toward the following vehicle can be blocked by the preceding vehicle(s). This blockage effect is more prominent with shorter vehicle spacing. For two-dimensional wake flow, the flow velocity behind the vehicle can be reduced by about 40% for 3.1H vehicle spacing, 20% for 9.3H and 10% for 15.5H, respectively. The second preceding vehicle can further reduce no more than 10% of the flow velocity. Compared with the two-dimensional wake flow, the three-dimensional wake flow can double the blockage effect of preceding vehicle.|
The vehicular exhaust scalar (i.e., pollutant) distribution pattern is found to mainly conform to the main flow structure behind the vehicle, whereas the flow turbulence velocity field can slightly expand the scalar dispersion and distribution region. For the studied vehicle (α = 60°), the cross-section of exhaust scalar dispersion and distribution region behind the vehicle looks like an "n-shape" profile whose size is initially almost identical to vehicle rear. It will grow to about 2 times of the vehicle width and 1.5 times of the vehicle height near the end of near-wake region where the highest scalar region stays at about half of the vehicle height (i.e., close to the height level of human inhalation zone). With the presence of the preceding vehicle(s), the size and shape of the scalar profile will not change too much, but the highest scalar contour region will be lowered from half of the vehicle height toward ground surface. On the other hand, for the studied vehicle (α = 25°), the cross-section of exhaust scalar distribution region behind the vehicle looks like an "m-shaped" profile. It is initially about 0.5 times of the vehicle height and 1.5 times of the vehicle width, and will grow to about 0.8 times of the vehicle height and 3 times of the vehicle width near the end of near-wake region. One side of the "m-shaped" profile which is right behind the tailpipe exit will share a larger portion of scalar jet exit, thus this side has a much higher scalar concentration, and its size is 50% taller than the other side of the "m-shaped" profile but not much wider. In addition, each side of the "m-shaped" profile has its highest concentration zone located very close to ground surface. With the presence of the preceding vehicle(s), the "m-shaped" profile of scalar distribution will not change too much, but the unbalance of scalar distribution between the two sides of "m-shaped" profile would be enhanced.
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