Quote:
Originally Posted by Phase
Can you explain how their box cavity reduces drag?
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I'm presuming that you're discussing Coventry University's Audi A2 modifications?
If so, then, looking from behind, the rear-extending panels, which follow the perimeter of the cavity, combined with blocking plates, which separate each cell of the cavity, around the periphery, loosely define where an actual boat-tail would terminate, at that distance behind the Audi. 'IF' the newly-created 'contour' is 'streamlined.'
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When the flow separates at the original location, it reattaches by the new trailing edge, capturing a 'locked'-vortex' which continually circulates within the confines of each cell, fed by momentum interchange from the local streamline into the turbulent boundary layer. This constitutes a cannibalistic loss.
The outer 'inviscid' flow 'skips' over the vortex as if it were a phantom surface.
Since the new trailing edge is at a smaller cross-section, the inward rushing flow experiences an acceptable level of deceleration as the streamlines diverge, causing the pressure recovery, which is responsible for all meaningful drag reduction.
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According to Mair's research, where an actual boat-tail would provide a negative Cd 0.099 drag reduction, a box-cavity of the same 'elongation' will provide a negative Cd 0.053 reduction ( 46.4% less efficient ).
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If the box-cavity were to be 'skinned' at a later date, creating an actual 'boat-tail', then the performance would improve by 46.4%.