Results
Effect of Trailer Height on Aerodynamic Drag
Mean data for the effect of trailer height on the drag force. Click for a larger version.
The graphed data shows that an increase in the height of the trailer has a substantial impact on the amount of aerodynamic drag force the model is subjected to. The data also illustrates the effectiveness of the sloped front of the tractor in improving air flow. Figure 1 shows the predicted path of air over the slope and the flow meeting the heightened trailer.
Fig 1: The predicted path of air flow over the slope of the tractor.
As a result of the slope, any increases in trailer height only marginally increased drag forces until the height was out of the range of the slope’s angle. At this point, the drag forces saw much larger growth.
The amount of drag force growth caused by height increases at the high level seem to, based on the graphed data, have relatively little impact on the drag force. While this is possible, it is more likely that this is due to an oversight on the part of the experimenter. Even though the flow conditions in the tunnel were turbulent, there was a small boundary layer of slightly decreased flow velocity around the edge of the tube. The highest trailer heights pushed the trailer into this boundary layer. The top of the trailer would thus be exposed to a flow of smaller velocity than the rest of the model, resulting in a diminished increase in drag force. Under normal conditions, it is probable that a tractor-trailer would have encounter much greater increases in drag force for the same relative height augmentations.
Ultimately, the results show that the height of the trailer has a substantial impact on the drag forces experienced by a moving tractor trailer. Furthermore, it is especially noticeable once the height exceeds the range of the vehicle’s frontal slope.
Effect of Cab-Trailer Spacing on Aerodynamic Drag
Mean data for the effect of the size of the cab-trailer gap on aerodynamic drag. Click for a larger version
The results of the second experiment prove the clear relationship between the size of the tractor cab to trailer gap and the amount of drag force. The drag force was least when the trailer and cab were closest and grew with additional spacing. There was a roughly 12% increase in drag force when the trailer was moved the equivalent of just 1.25 m (29 mm on the model) from the tractor cab. At a spacing of 3.6 m (86mm), the drag force was approximately 27% higher than when the cab-trailer gap was zero.
The drag force growth between higher cab-trailer spacing intervals was less than the growth between lower spacing intervals. It would be exceedingly impractical, however, for a tractor-trailer to ever have a gap of sufficient size to encounter this; such a gap would likely hamper control and safety.
Effect of Various Tractor-Trailer Configurations on Aerodynamic Drag
Mean data for the drag forces on the tractor-trailer model in each configuration. Click for a larger version
The final experiment tested the effectiveness of two aerodynamic accessories, a cab-trailer sleeve and a flexible rounded trailer balloon (Fig. 2). The former sought to eliminate the drag caused by the cab-trailer gap while still allowing the rotation of the fifth-wheel attaching the trailer to the tractor. The trailer balloon was intended to reduce the drag by creating a tapered aft end but without utilizing a more hazardous steel attachment.
Figure 2: The flexible tail proved to be ineffective in reducing drag
The testing results show that the flexible sleeve (Fig. 3) was very effective in reducing drag: the drag forces on the tractor-trailer were almost equal to the drag forces on the basic tractor without the trailer. The tractor-trailer in its standard configuration created 23% more drag force than in the sleeve configuration.
Figure 3: The connective sleeve was very effective at reducing drag.
The balloon proved to have a negligible effect when used alone and a negative effect when used in conjunction with the sleeve. Given the aesthetic weaknesses and set up costs, it is unlikely this method of drag reduction would be worthwhile for the transporting industry.
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