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Aerodynamics of Simplified and Detailed Heavy Vehicles
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The wake of the simplified vehicle was mapped with particle image velocimetry to produce a three dimensional representation of the domain of wake fluctuations, showing that the upper and side shear layers contained strong stream-wise fluctuations while the region near the wake closure point was dominated by cross-stream motions. Fourier analysis and Proper Orthogonal Decomposition identified two dominant unsteady processes, a bulk wake pumping at a Strouhal number of 0.08 and a von Karman-like shedding in the lateral direction at a Strouhal number of 0.17. By contrast, the lower region of the wake was found to be quite steady.
Equivalent unsteady motions were identified for detailed heavy vehicles, with the coherence and frequency of the von Karman-like motions generally decreasing for configurations with higher drag. An additional low frequency motion was identified in the lower wake, this was attributed to separation from bluff under-body components.
A comprehensive drag reduction program was applied to the detailed vehicle, illuminating the potential fuel savings achievable by incorporating well-known streamlining techniques to the current heavy vehicle fleet.
The response of the wake to cross-wind and reduced ground clearance was investigated. It was found that in all conditions evidence could be seen of a vortex in close proximity to the lower region of the base, making this structure an important target for drag reduction. Passive flow control devices were implemented to dislocate this vortex with the aim of reducing its low pressure signature on the vehicle's base. A stream-wise splitter plate in-plane with the bottom of the vehicle caused the orientation of the wake to become mirrored in the vertical direction, resulting in a modest drag reduction.